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Do domesticated animals necessarily live longer than wild animals? counterexample wanted!
Generally domesticated animals do not need to worry about their food and other predators, and they have a lower probability of getting sick.
However, it seems there is no sharp difference between the life expectancy of wild pigs and domesticated pigs. Both can survive more than ten years if they are not killed.
Is it normal for domesticated animals to have a longer life span? Is it necessarily so?
Is there an example of a wild animal that statistically lives longer than its domesticated counterpart?

generally we haven't been selecting for long life, given the amount of inbreeding many species go through and possible trade-offs (milk production in cattle for example, might be stressful for the animal) it is likely that many domesticated species live shorter than their natural counterparts. I think this question may be a) near impossible to answer (its not likely that a properly designed experiment has been conducted) and b) too broad, there's a lot of domesticated species

For a properly designed experiment you would need both sets of animals being kept in the same conditions (food space, climate, stressors, medicines), and these are likely to be further from what one group is adapted to than the other, and given the complexity, requirement of large sample sizes, cost of the experiment & how little there is to be gained from doing such an experiment, it is unlikely to have been done.

I think this will either encourage opinion based answers, or bring to light a paper that looked into this in great detail. Either way, my money is on a pug surviving longer in a Parisian apartment rather than the Chinese wilderness!

Yeah this question will potentially draw many isolated answers. For example, indoor-cats are indeed known to grow older than cats that spend a lot of time outdoors (e.g. feral cats) because the latter contract more disease and get killed by cars, or get baited by nasty folks etc.

guys, can we just make this question simple?  e.g.  wild birds vs. pet birds ?  I just need a counterexample which statistically speaking lives longer than its domesticated counterpart .

@questionhang - that's exactly what we are trying to analyze for you. It is up to you to make this question answerable. If you think you know exactly what you want, why isn't it in the question? The concluding question is simply too broad. I vote to close until edited.

@aliceD  OK, I have revised the question.

I think the heart of the matter is in the "not killed". A domestic animal which slows down due to age often will be cared for by its humans. The same animal in the wild is almost certainly food for another animal.

@jamesqf  On the other hand, there are domesticated animals which do not like their man-made environments and crowded people who stares at them.

@questionhang Consider we raise pigs to eat them, longevity is not a priority, in fact breeding them to get larger faster probably would shorten their normal lifespans. Consider comparing species bred for a purpose other than consumption.


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Are there species that occupy several roles in the food hierarchy?
Are there species that occupy several roles in the food hierarchy?
By role, I am referring to producer, primary consumer, secondary consumer, tertiary consumer, quaternary consumer and so on.
Examples of more specific questions

Are there species that are both primary and secondary consumer?
Are there species that are primary consumer, secondary consumer and a tertiary consumer?
Are there species that are both producer and primary consumer?

I greatly modified the question to generalize and clarify it. Please do not hesitate to roll back if you preferred the original version for any reason.

I thought I remembered answering a very similar question here before, but it turned out to be on Worldbuilding.SE instead.

Food hierarchy and food web
Ecological trophic interactions are better represented by food webs rather than simple hierarchical relationships. As a consequence, the concepts of primary/secondary/tertiary/... consumers sometimes poorly apply to reality.
Obligate and Optional
Many species are able to feed on a various source of nutrients. As a consequence, a species can be a primary consumer under one environmental condition and be a secondary consumer under another. Such species might eventually almost never be both a primary and a secondary consumer at the same time and it renders the classification of species into one or another category a little more arbitrary.
How much does one need to eat to belong to a given category?
Note also that species like the cat, for example, are generally considered as being just carnivorous while in reality they often eat a very small amount of plant material. They also indirectly consume plant materials that are present within the gut of their preys. Such cases make the categorization more arbitrary again
Examples
Primary, secondary and tertiary consumer
This one is pretty easy. Humans, pigs, bears, corvids and catfish are all examples of species that are primary, secondary and tertiary consumer all in the same time. In other words, humans are omnivorous.
I am currently preparing a stew for tonight $\ddot \smile$. There are a number of vegetables and a little bit of pork (which itself is omnivorous). Tomorrow, I'll make some salmon (which is a secondary consumer as it mainly feeds on zooplankton).
Producer and secondary consumer
Carnivorous plants are able to photosynthesize and to capture animals. Most animals they capture are often best described as a primary consumer but some carnivorous plants small fish that feed on phytoplankton or even mice and frogs according to this video.
Producer and primary consumer
Parasitic plants (such as the common holly) are able to both photosynthesize and parasitize other primary consumers (it therefore also is a primary consumer if we equate parasitism to consumerism).
The eastern emerald elysia (a sea slug) feed on algae and is able to photosynthesize (using the chloroplasts of its preys).
Producer, primary consumer and secondary consumer
I cannot think of any examples right now but I would bet that it exists (eventually in a chemoautotroph species and eventually in a species that has a various life cycle).

so can you also be a primary and secondary consumer and a teritary consumer?

Nice answer, but i would argue that parasitism is not equivalent to consumerism

Perhaps you can add a note on whether these food habits are obligate or optional. For example humans can be either secondary or tertiary consumer (I am not sure if humans can totally avoid plant food as meat cannot provide all nutrients). The carnivorous and parasitic plants are strictly both producers and consumers because they cannot produce all the nutrients needed by them and therefore resort to carnivory/parasitism.

Thank you for the comments. I think I addressed them in my edits.

First of all, apart from primary producers, it can sometimes be difficult to assign an exact position or role to organisms in a tropic web or net since trophic levels might be diffuse or tangled. Most obviously, omnivorism is defined as organisms that eat from different food types, which in practice often means they eat from several trophic levels. Besides what is mentioned in the other answers, you also need to add species that shift their trophic role over their life history, which is e.g. the case for many species of fish, where juveniles are primary consumers (of e.g. plancton), while adults are secondary or tertiary consumers. The same situation is found in frogs (tadpoles vs adults) and many other organism groups. The basis for these shifts is often a change in body size (so trophic position is partially a function of body size), and these shifts are often discussed under the label ontogenetic niche shifts (see Werner & Gilliam, 1984). Functionally, such trophic shifts also means that the species can exploit widely different food sources over its life course, which will decrease the amount of intraspecific competition.
Another example of life-history shifts in the feeding hierarchy are species that change their trophic level when they change habitat. This is the case for salmon that migrate between marine and freshwater habitats. A third example is species that have intra-species predation (see e.g. Polis, 1981), which is also called cannibalism. This is, for instance, common in perch (Persson et al, 2000).
See this as a complement to Remi.b's answer.

To follow up with Remi.b's answer, there is an example of an organism that is a combination of  producer + primary consumer + secondary consumer:
Bladderwort (Utricularia spp.) is a carnivorous plant that photosynthesizes and is known to eat insects, fish and tadpoles. In addition, research by Marianne Peroutka suggests these plants also eat algae. (see here and here).
Others have also noticed other carnivorous  plants ingesting plant parts, possibly suggesting that more than one genera of plant fits this combo of trophic roles.

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What is the name of this red plant?
I came across this plant yesterday (14th May) and was wondering what it might be? Would anybody know the scientific name and if it's indigenous to the UK (personally I doubt it is)? The plant grows in an Asian style garden near Cumnock, Scotland.

Close up of leaves:

I keep seeing this in the Hot Network Questions, misreading it as "What is the name of this red planet?" and thinking, "It's Mars, duh."

I'm pretty sure the sun is the only red planet /s

Yes this is Acer Palmatum . We have one in a container in our garden which is twenty years old and does look amazing when in full leaf. It is completely hardy though hard late spring frosts might affect leaves already out.

Red? Something wrong with your photo or your colour vision.

This is the "Acer palmatum" or Japanese maple, which shows a wide variety of different leaf forms (from here):

Specically you found "Acer palmatum dissectum 'Red Dragon'", for more information look here (picture also from this site):

Wow. Thanks for the quick answer. I am well impressed.

Looks like a cultivar of the Japanse maple (Acer palmatum). I assume that is why you have found it in an Asian stlye garden. There are various cultivars that differ by color and shape of the leaves, as you can see in this picture form Wikipedia:
Image source:
https://en.wikipedia.org/wiki/Acer_palmatum#/media/File:Acer-palmatum-ssp.jpg

It's an acer dissectum ( spelling may not be quite right!). It is Japanese and one of a number of similar plants from there. Becoming popular in UK.  Needs slightly acid soil, PH of about 6. semi shade and shelter from winds

Welcome to Biology.SE. Why do you answer a question that has already been answered and provide the same answer (without references, and actually borderline wrong as this is not A. dissectum - which does not exist - but A. palmatum in the dissectum variant) as the accepted answer? Your answer as is might be down-voted or deleted. Please take a look at the tour and the guidelines for good answers. Thanks!

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How do scientists discover a new antigen and its epitope?
I've found some database on the internet that list all discovered antigens and their epitopes.
So how do scientists discover a new antigen? Do they try to inject them into the body to see if it causes an immune response or not? And finally if it's an antigen, how do they know their epitopes?

Okay, you have a few questions building on top of each other (rephrased the questions for clarity):

How do scientists discover a new antigen?

There are multiple ways to this, which are applied for different purposes.
In order to discover a natural antigen, which is recognised by an antibody normally produced by the immune system, one would first have get hold of this specific antibody. This would be an approach to reproduce or understand the effective response of the immune system against a pathogen (e.g. a virus).
In order to get hold of the antibody (and ideally the coding DNA sequence for it), researchers have to use the pathogen, or parts of it and find the antibodies (or B cells) that bind to it. To do this the pathogen (particles) could be fixed to a column and after running blood through the column the respective antibodies and B cells will remain in the column. Then the DNA of these cells can be sequences, or the antibody can be further purified.
In most cases however, scientists want to find a not-yet existing antibody against an antigen, which leads us to:

Do scientists try to inject antigens to the body to see if it causes an immune response or not?

Yes, this is done - but only with animals (mostly rats, rabbits & goats). Polyclonal antibodies can be extracted directly from the blood of these animals, but most often monoclonal antibodies are needed for research or medical purposes. The process required for these is much more complicated, but leads to a cell lines which can then be used to produce the desired antibody in greater quantities.

How do scientists know the corresponding epitope to an antibody?

There are multiple ways to find the exact epitope of an antibody, all of which are commonly described as epitope mapping.
The most common methods are based either on site-directed mutagenesis of the antigen to see which positions (amino acids) are crucial for binding of the antibody, or on peptide fragments of the antigen, which can still be bound by the antibody.

Great answer! Thank you very much. Since "the DNA of these cells can be sequences, or the antibody can be further purified.", we can know its epitope by analysis its DNA or use the purified antibody for epitope mapping.

"The most common methods are based either on site-directed mutagenesis of the antigen to see which positions (amino acids) are crucial for binding of the antibody" . I've never known about this, let me search for it. Thank you very much!

@joe In most cases you would use the purified antibody for epitope mapping. There are people working on computational epitope prediction, but I'm not sure how good/reliable these predictions are so far - this seems to give a recent overview

very responsible answer, thank you very much for sharing your knowledge, keep rocking!

I would like to just add to Nicolai answer.
What is an antigen ?
First, and Nicolai said this, but I just want to make it clear, an antigen is anything that antibodies bind. That is distinct from and immunogen which is a type of antigen that causes your immune system to produce antibodies. But an antigen does not necessarily have to be an immunogen.
Peptides, sugars, nucleic acids and lipids are all common antigens, but peptides are probably what you would be most familiar with. Usually, most sugars and lipids have a hard time producing an immune response on their own and generally are coupled with a protein (peptide). This makes sense because your body naturally produces lots of sugars and lipids and if you made antibodies to all of them, it would end up attacking itself.
How is an antigen discovered?
There are two ways to approach this.

Antibody First Approach

Let's say a person get's infected with some unknown virus but ends up surviving. The next step would be to do two things:

Culture the virus so scientist can study it in the lab. You need
to culture most virus so you can grow an unlimited amount in vitro.
It would be a really tough task to go out and naturally find all the
virus you needed to study.

Genotype the virus. It is necessary to know the viral genome so we can manipulate the genes and see what their effect is.

We can start making changes of the virus changing various components of the genome. We could see how that changes binding to the patient's serum.Since the serum contains antibodies that will bind the virus, the serum will often neutralise or test positive for binding antibodies. Now if we change gene A from the virus and the serum still binds/neutralises, we can assume gene A is not the antigen which made an immune response in the patient. If gene B is mutated and the serum stops binding, we can assume gene B is the antigen of the unknown virus.

Antigen first approach

If we only have the virus and don't have an infected patient, then we would have to inject it into model organisms to find out the antigen. However, since we know an incredible amount of viral species we can probably make a best guess on what the antigen component of the virus is.
For instance, in 2012, when a patient fell ill, they sequenced a virus in them which was found to be a close relative to Coronavirus. Since they know that Coronavirus primary antigen is its spike protein, they rightly assumed that the gene that was closely related to the spike protein of the unknown virus was also the primary antigen. That virus turned out to be Middle East respiratory syndrome coronavirus which the main antigen is the spike protein.

Another great answer, very detailed, thank you very much!


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What is the anatomical term for a two jointed leg?
Allow me to apologize in advance for the layman's terminology.
I'm wondering what the anatomical term for a cat- or a goat-style hind leg is.
Cats, goats, t-rexes, and many many other animals don't have human hind legs (i.e., with one knee / one pivot point). They have two pivot points, with one bone going down, then another going back-ish, then another going down again.
I've googled and googled for the answer, but I'm completely at a loss as to what this is called! Is there any name for that style of leg? I'm sure there's a lot of variation inside the category, but there is a category, correct?

? cats have knees and hips as well... https://animalcorner.co.uk/cat-anatomy/

Welcome to Biology.SE!
I think you are talking about plantigrade, digitigrade and unguligrade.
Please note that the number of joints in mammals does NOT vary, but only the relative length (and shape) of the different parts of the leg.

A Plantigrade walks on the sole of the foot. 'Sole' translates to 'planta' in Latin hence the name.

Examples: Human, squirrel, raccoons, ...
Below is a chimpanzee skeleton

A Digitigrade walks on the digits (the toes).

Examples: dog, cat, hyena, ...
Below is a cat skeleton

An Unguligrade walks on the nails. 'nail' translates to 'ungula' in Latin hence the name.

Examples: cow, reindeer, goat, ...
Below is a cow skeleton

I think the final point you make (about the actual structure being invariant) is a major one and should be moved to the top so it isn't missed. The rest works well as an explanation of how that can be in spite their apparent dissimilarities.

I agree with you. I moved it on top. I thought the same when writing the answer but did not really know how to format. Hope that looks good enough. Feel free to edit to improve the format if you want. Thanks

I'd say that sentence placement, being immediately followed up with a side-by-side comparison chart, is perfect.

WOW I never noticed that they're all technically the same thing! That was more than helpful and extremely interesting! Thanks so much!

So amazing, thank you

Birds are also confusing for a lot of people.

Specifically, they assume that birds only have a two-jointed leg and therefore their knees "bend backwards."

However, like all tetrapods, the leg structure and number of joints doesn't really change a whole lot:

 Copyright Scott Hartman 2011
The confusion is a result of the fact that -- similar to what @Remi.b points out in his answer -- birds are generally digitigrade animals. [see here].

In the case of the "backwards bending" bird knee: that's really the ankle joint.

The bird knee joint between the femur and tibiotarsus points forwards, but is hidden in feathers.
The bird ankle joint between the tibiotarsus and tarsometatarsus points backwards.


text:
Is this a Loranthus Europaeus?
Location: Austria, Vienna Ottakringer Wald
Date: Feb 8
I found something beneath a tree with epiphyta , which I thought to be Viscum album but is vividly yellow instead of white and I found no leaves.

My search of subspecies of Viscum album initially revealed:

Viscum album subsp. austriacum (Wiesb.) Vollmann. Fruit yellow; leaves 2–4 centimetres (0.79–1.57 in). Central Europe. On Larix, Pinus, Picea.

But then I began to think it shouldn’t be V. album. In illustrations of V. album there aren't the thin stalks between berries and branches as in my specimen.

Perhaps Loranthus europaeus?

You are quite right: it's Loranthus europaeus. Vienna was the prime example in our botany lectures where to see that plant without a fuzz.
c/p from the Flora of Vienna:
"Auf Eichen, vorzüglich auf Quercus pedunculata und Qu. sessiliflora, selten. Im Eichenwalde von Schönbrunn, auf dem Gallizin, hinter Neuwaldeck, hin und wieder; viel häufiger auf dem Bisamberge {A}. ... auf den grössern Donau-Inseln z. B. ... in der Lobau {C}."
"Häufigkeit des Vorkommens: häufig bis sehr häufig"
(Confused about the contradicting "rare" and "frequent to very frequent" statements, but anyway.)
Here are some more pictures.

The first „selten“ only applies to the case of quercus. He/she lists „locations“ in order of ascending probability.

I suspect your are right, but I can't be sure. Would have assumed to be the probability/frequency to be in brackets after Q. sessiliflora if it would relate to frequency on the host.

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text:
Why are so few foods blue?
Although blue foods exist, they're rare enough compared to other foods for food preparers to use blue plasters as a convention. The natural colour of a given food is due to pigments that have some biological origin. Is there any evolutionary reason why these are rarely blue?

Fasinating research suggesting that in some cultures, people literally do not perceive blue: http://www.sciencealert.com/humans-couldn-t-even-see-the-colour-blue-until-modern-times-research-suggests

This New York Times article discusses the difficulty of acquiring natural blue dyes for the food industry...So far their best results come from the pigment phycocyanin via "spirulina" made from  the cynobacteria Arthrospira

Obligatory George Carlin link https://m.youtube.com/watch?v=l04dn8Msm-Y

@Tenfour04 There are a lot of popular articles about people in some cultures not perceiving blue, but they grossly overstate the case usually by misstating the evidence. Some (unlike you, props for that BTW) even claim they cannot see blue at all.  People in all cultures can perceive blue, they differ in the speed it takes to differentiate between shades and how language divides the color space.  Matt Yglesias (who is interested in how the Sapir-Whorf Hypothesis is overstated re: political language) has a very well referenced post about this topic at https://www.slowboring.com/p/greeks-blue

Short answer
Blue color is not only rare in edible organisms - Blue color is rare in both the animal and plant Kingdoms in general. In animals, blue coloring is generated through structural optic light effects, and not through colored pigments. In the few blue-colored plants, the blue color is generated by blue pigment, namely anthocyanins. The reason for the scarcity of blue pigments remains unknown as far as I know.
Background
The vast majority of animals are incapable of making blue pigments, but the reason appears to be unknown, according to NPR. In fact, not one vertebrate is known to be able to. Even brilliantly blue peacock feathers or a blue eye, for example, don't contain blue pigment. Instead, they all rely on structural colors to appear blue. Structural colors are brought about by the physical properties of delicately arranged micro- and nanostructures.
Blue morpho butterflies are a great example of a brilliant blue color brought about by structural colors. Morphos have a 6-inch wingspan — one side a dull brown and the other a vibrant, reflective blue. The butterflies have tiny transparent structures on the surface of their wings that scatter light in just the right way to make them appear a vibrant blue. But if you grind up the wings, the dust — robbed of its reflective prism structures — would just look gray or brown.
Similarly, the poison dart frog is blue because of the iridiphores in its skin, which contain no pigment but instead feature mirror-like plates that scatter and reflect blue light (source: By Bio).

Morpho and poison dart frog. sources: Wikipedia & LJN Herpetology
Similarly, in the Kingdom of plants less than 10 percent of the 280,000 species of flowering plants produce blue flowers. In fact, there is no true blue pigment in plants and blue is even more rare in foliage than it is in flowers. Blue hues in plants are also generated by floral trickery with the common red anthocyanin pigments. Plants tweak, or modify, the red anthocyanin pigments to make blue flowers, including pH shifts and mixing of pigments, molecules and ions. These complicated alterations, combined with reflected light through the pigments, create the blue hue (source: Mother Nature Network).
But why the blue pigments are so scarce, seems to be unknown as far as I know (MNN, NPR, Science blogs)
Sources
- MNN
- NPR
- Photobiology

I love it when someone explains something so complex in such a graceful and easily understandable way. Thank you so much :)

so blue pigment is rare in nature, but blue light abounds (so much that we nicknamed our planet after it). in human technology it seems to be just the other way round: blue pigment from minerals has been known to ancient civilizations, while (cold) blue light was engineered only recently after we had red and green luminescence for decades.

Things are not blue because they absorb the blue end of the spectrum. Most plants and animals need ultra-violet light which is at the blue end to photosynthesize either food or vitamin D. Conversely, they try not to bake... so they tend to reflect the infra-red end.

Is Rayleigh scattering also why water looks blue from a distance, but, when up close or in smaller doses, is clear (or at least, not blue)?

This seems to be a good answer except for the references to Rayleigh scattering. Structural coloration seems to be the proper umbrella term for these phenomenon, but the specific physical explanations seem to be mostly thin-film interference or diffraction. None of the references in this answer seem to mention Rayleigh scattering and I would be quite surprised if there actually was an animal whose coloration could be explained by Rayleigh scattering.

@TylerH Rayleigh scattering is the reason that clear skies appear blue. If you are looking at a large body of water outdoors on a clear day, it will be blue partly because it is reflecting the blue light from sky, but also partly because water is intrinsically blue. The intrinsically blue color of water is, however, not caused by Rayleigh scattering, but rather by absorption of shorter wavelengths. This absorption is, however, quite weak, so light has to pass a long distance through the water for this to be noticeable. This is the reason why water in a glass will not appear blue.

I would question the statement about less than 10% of flowering plants having blue flowers, at least if we limit that to plants having conspicuous flowers.  In my garden, and in the mountains & desert hereabouts, blue seems as common as white or yellow, and much more common than red/orange.

I don't have a source nor the knowledge to back it up, but Italian wiki a without a source that the biosynthesis of anthocyanin requires materials that are produced through photosynthesis. Moreover, not even water plants seem to produce them since the required energies are pretty high.

RadioLab did a great story on the scarcity of blue in nature.
http://www.radiolab.org/story/211213-sky-isnt-blue/

If memory serves, the Wikipedia article on Iridescence (or if not, an article it links to directly) has a terrific diagram illustrating the mechanics of the blue light reflected from peacock feathers, butterflies, and blue eyes. I think such a diagram would be a good addition to this answer.

More from NPR on the colour blue.

All of which raises the obvious evolutionary question: Why is one of our eyes three different color cones tuned to see blue?  That is, why can we see blue independently, if nothing actually is independently blue? (Okay, not nothing, but nothing of seeming evolutionary advantage)

A similar question was asked by the YouTube channel It's Ok to Be Smart: Why Is Blue So Rare In Nature?. The video provides some good visuals about blue butterflies (including a rare instance of real blue pigment)!

@RBarryYoung if you look at the natural environment on earth it is composed mostly or browns, greens, and blues. the sky and water, two very important things in your visual field, tend towards blue. without a blue receptor green plants and animals and blue sky would look extremally similar which would be bad. its not really surprising even mammals which lost much of their color vision kept the ability to distinguish sky from everything else. Blue is not rare in nature it is just rare in living things.

The scarcity of blue pigments in living things generally is thought to be due to the difficulty of biosynthesizing organic compounds with the required electronic configurations necessary to appear blue: typically by selectively absorbing orange as well as near orange-reds and yellow light of about 580 to 620nm wavelength.  Reflecting only blue will also do it, but this mechanism requires aborbing even more wavelegths and so encompasses the other mechanism.  I apologize in advance but a description of why this is the case exceeds the allowed comment length so I have to break this in two.

For a molecule to appear a color, an electron in the highest occupied molecular orbital of must absorb the energy corresponding to a particular wavelength of light, promoting it to the lowest unoccupied molecular orbital.  To absorb wavelegths in the 580-620nm range requires that the orbital energy gap be quite small.  In organic molecules, such conditions are obtained with large, complicated networks of alternating pi bonding electrons (mostly C=C bonds).  Thus the necessary compounds are very large, often unstable and require complicated biosynthetic pathways with large energy requirements.

t@DrRadium - you might wish to consider writing an answer with this material, it sounds interesting! Make sure to add a few references/sources

Although @AliceD's answer is a great simple demonstration of the rarity of blue in our natural world, there's likely a more nuanced/technical reason.
Short answer
Blue light was the most available wavelength of light for early plants growing underwater, which likely led to the initial development/evolution of chlorophyll-mediated photosytems still seen in modern plants. Blue light is the most available, most high-energy light that continues to reach plants, and therefore plants have no reason not to continue taking advantage of this abundant high energy light for photosynthesis.
Different pigments absorb different wavelengths of light, so plants would ideally incorporate pigments that can absorb the most available light. This is the case as both chlorophyll a and b absorb primarily blue light. (Absorption of red light likely evolved once plants moved on land due to its higher efficiency).
Pigments appear as whatever color is not absorbed (i.e, they appear as whichever wavelength(s) of light they reflect). Because blue pigment would reflect the majority of light that modern plants rely on for their chlorophyll-mediated photosystems, blue pigments remain scarce in plants.

Photosynthetic organisms would not remain competitive if they failed to absorb the readily available, high-energy blue light, and so therefore evolution has likely very infrequently favored the generation (or propogaiton) of blue pigments.

Long Answer
Atmospheric Transmission
As this page from Humboldt State University demonstrates, blue and green
light pass through the atmosphere to the earth's surface better than almost all other wavelengths of light:

transmission is when electromagnetic energy is able to pass through the atmosphere and reach the surface. Visible light, largely passes (is transmitted)
through the atmosphere.

This means that blue and green light are the most available wavelengths of light.

Note that blue/green are followed closely by the rest of the visible spectrum and NIR (near infrared).

Also note that a large portion of ultraviolet is largely absorbed by atmospheric gases (primarily ozone), and therefore poorly transmitted.

Wave Properties
It's important to understand that (from U. Wisconsin):

More energetic waves have shorter wavelengths while less energetic waves have longer wavelengths.

As a result, blue light (being the most available highest energy wavelength of light), seems to be the optimal wavelength of light for photosynthesis.

Note that although available UV light is more energetic and can power photosynthesis, it's often a tad too energetic and can cause damage to cells. So it's often best for organisms to reflect UV.

More info about the physics behind light energy can be found here.

Photosynthesis
Photosynthesizing organisms contain pigments (typically heme/porphyrin-based chlorophylls and various carotenoids) that absorb light energy. Basically, the energy from a photon of light raises an absorbant pigment to a higher energy state (called an excited state), and then the pigment releases that unstable energy to return to it's ground state -- this excess energy is what powers the biochemical reactions of photosynthesis. See here for more details.
Here's two example graphs (from here and here) showing the absorption spectrum of typical plant pigments:

As you can see, plants have evolved to have pigments that absorb primarily blue light (followed by red light). These pigments reflect green light and so therefore appear green.

Presence of blue pigments (i.e., those that reflect highly abundant blue light), on the other hand, would be in direct contrast to these photosynthetic efforts. As a result, blue pigments remain rare in photosynthetic organisms powered primarily by chlorophyll-driven photosystems.

However a number of sources (e.g., Mae et al. 2000, Brins et al. 2000, and here) suggest that although plants absorb more blue light than other wavelengths, the most efficient photosynthesis does not occur using blue light. Instead, red light results in the highest photosynthetic efficiency.

One of the reasons that they (in this case Brins et al) found was that xanthophylls were dissipating the excess energy associated with blue light causing a rate decrease in blue-light photosynthesis.

This NIH page suggests that high-energy light isn't even necessary for plants:

Chlorophyll a also absorbs light at discrete wavelengths shorter than 680 nm (see Figure 16-37b). Such absorption raises the molecule into one of several higher excited states, which decay within 10−12 seconds (1 picosecond, ps) to the first excited state P*, with loss of the extra energy as heat. Photochemical charge separation occurs only from the first excited state of the reaction-center chlorophyll a, P*. This means that the quantum yield — the amount of photosynthesis per absorbed photon — is the same for all wavelengths of visible light shorter than 680 nm.

However, all of this may be for nought, because there is ample sunlight available to plants. Again from the NIH page:

However, even at the maximum light intensity encountered by photosynthetic organisms (tropical noontime sun, ≈1.2 × 1020 photons/m2/s), each reaction-center chlorophyll a absorbs about one photon per second, which is not enough to support photosynthesis sufficient for the needs of the plant. To increase the efficiency of photosynthesis, especially at more typical light intensities, organisms utilize additional light-absorbing pigments.

In other words, plants are not entirely photosynthetically efficient and do not typically use all of the light available to them. From Wikipedia:

Photosynthesis increases linearly with light intensity at low intensity, but at higher intensity this is no longer the case (see Photosynthesis-irradiance curve). Above about 10,000 lux or ~100 watts/square meter the rate no longer increases. Thus, most plants can only utilize ~10% of full mid-day sunlight intensity.

So in summary:

Blue and green light are the most available wavelengths of light.
Blue light is the most energy-rich of the high-availability wavelengths of light
Plant pigments absorb mostly blue light
BUT plants don't necessarily need the high energy of blue light for efficient photosynthesis.

So what gives?...
Evolution
So given all of that, the question still remains: why absorb mostly blue light and not green light?
The answer, though still somewhat conjectural, is likely due to the light availability of early plants. Early plants evolved, like all life, under water.
It turns out, just like the variability in transmittance of different wavelengths of light through the atmosphere, certain wavelengths of light are more capable of penetrating deeper depths of water. Blue light typically travels to deeper depths than all other visible wavelengths of light. Therefore, the earliest plants would have evolved to concentrate on absorbing this part of the EM spectrum.

However, you'll notice that green light penetrates relatively deeply as well.  The current understanding is that the earliest photosynthetic organisms were aquatic archaea, and (based on modern examples of these ancient organisms) these archaea used bacteriorhopsin to absorb most of the green light.

Early plants grew below these purple bacteriorhopsin-producing prokaryotes and had to use whatever light they could get. As a result, the chlorophyll system developed in plants to use the light available to them. In other words, based on the deeper penetrative ability of blue/green light and the loss of the availability of green light to pelagic prokaryotes above, plants evolved a photosystem to absorb primarily in the blue spectrum because that was the light most available to them.
So why have plants not evolved to use green light after moving/evolving on land? As stated above, plants are terribly inefficient and can't use all of the light available to them. As a result, there is likely no competitive advantage to evolve a drastically different photosystem (i.e., involving green-absorbing pigments). So earth's plants continue to absorb blue light and reflect the green, and blue pigmentation remain uncommon in our world.

Plants likely evolved to produce anthocyanins (the pigments responsible for the reds/blues/purples attributed to blueberries and violets) for reasons other than photosynthesis -- e.g., attraction, UV protection, or even protection against herbivores. See here, here, and here for examples.

So what about non-plant organisms?
According to Wikipedia:

Carotenoids are the most common group of pigments found in nature, including in animals.
Carotenoproteins complexes are responsible for the various colors (red, purple, blue, green, etc.)
Animals are incapable of making their own carotenoids and thus rely on plants for these pigments.

In other words, most pigments in non-plant organisms originate either directly or biochemically from the organism's diet. Without direct ingestion of blue pigments, these chemicals are unavailable or biochemically expensive to produce (see Crustacyanin). As a result, blue pigments are also uncommon in animals.
Though, as AliceD's sources point out, we still don't fully understand why animals don't produce more blue pigments.

That's good as far as photosynthesis goes, but blue (and purple to ultraviolet) pigments are pretty common in fruits & flowers.  One might speculate that this is because flowers and (ripe) fruits have evolved to be conspicuous to their pollinators, or to animals that eat the fruit and so disperse the seeds.  Green flowers are rare, and while immature fruit is green, mature fruit seldom is.  So much so that 'green' has become a synonym for unripe :-)

@jamesqf Yup exactly. See my point (and associated links) about anthrocyanins. However, that's an explanation for why we have any blue foods :p. The rest of my answer is about why we likely don't have more blue pigments in organisms!

I didn't get where exactly do you say anything about Why are so few foods blue?

@J.Manuel: Our food consists of animals and plants. This answer is a huge explanation of why animals and plants aren't blue. What more do you want?

@LightnessRacesinOrbit Forget about the natural world. Blue foods are less common than most others. Even in processed foods. A blue juice will probably be confused with some sort of detergent rather than a juicy drink. The reason is that, people will avoid a blue meal until proven safe

@J.Manuel: And why do you think that is? That's right - because animals and plants don't tend to be blue! It all comes back to that in the end.

Can we have a shorter short answer pretty please ?

@Nikana Plants evolved under conditions of mostly blue light, which led them to develop photosystems that use a pigment that absorbs primarily blue light. Once they moved to land, there was ample (often too much) high energy blue light (and other wavelengths of light, too) to ever necessitate evolving new pigments to become more  competitive. Even if plants were to add new pigments, they would probably not evolve to add more blue pigment b/c that would decrease their ability to photosynthesize using their current chlorophyll-mediated photosystem.

So... "As blue is the most energetic wavelength available to them, plants evolved so as to absorb blue radiations the most, therefore reflecting as little blue as possible. You cannot see blue light coming from a plant because plants eat blue light." Is that roughly correct ?

@J. Manuel: On the contrary, lots of people drink (dark) blue grape & blueberry juices.

Great detailed discussion, but the conclusion...? It seems all this photosyntesis stuff would, if anything, favour plants that can still make some use of the greenish ambient light beneath a forest canopy. Especially since, as you say, plants that are exposed to direct sunlight are basically out of worries anyway, hence no real evolutionary penalty.

@NikanaReklawyks The shortest answer I can think of is "TL;DR: If you don't have the capacity to take 15 seconds to read about a quarter page of text, you don't get to know interesting stuff about things."

I joined just to vote this up.  Wonderfully accessible answer.

This answer makes me wonder, if humans discovered an edible plant on another planet which had evolved blue pigments, cultivated it on earth, and incorporated it into our diets, how our appearances might change!

I'm confused by @theforestecologist's claim that "Blue light was the most available wavelength of light for early plants growing underwater". I'm no expert, but I've heard several times (and read on Wikipedia's Color of Water) that water is blue because it absorbs the red, orange, yellow, and green wavelengths of light and reflects the blue and violet wavelengths. Which seems to imply that blue was the wavelength least available to plants growing underwater.

@Elise van Looij I think your confusion comes from thinking of water as a single mass (i.e. a giant mirror) instead of thinking about it as a bunch of molecules. Each molecule absorbs those colors you list, but they absorb blue light the least effectively and as a result reflect blue light among themselves. In other words, the water molecules act as a massive network of miniature mirrors that continuously pass the color around until enough molecules have slowly absorbed the light (this is the no-light depth). For an extended discussion, see here

If you encounter any further knowledge regarding "why animals don't produce more blue pigments" please update the answer, the depth and details of this answer is really great.

blue pigment in single-celled protist Stentor coeruleus called stentorin (https://www.osti.gov/biblio/7191680-characterization-function-stentorin-photoreceptor-protein-stentor-coeruleus)

We do understand why animals don't produce more blue pigments.  The structural requirements for an organic molecule to produce the selective absorption of light needed for a pigment to appear blue are quite narrow.   It requires very large molecues with extensive networks of alternating pi bonding electrons that are often unstable and biosynthetically taxing with high energy requirements to produce. If blue color  is evolutionarily advantageous, structural color is far less costly to produce. Thus only one animal produces a blue pigment. See my comments to AliceD's answer for more details.

It is not that there are no blue foods, it is that the English language does not like calling foods "blue".
There are no natural borders between "colors" in a colorspace, all colors we name (and learn to distinguish) are culturally defined. So, an important thing to recognize is that, a color somebody calls "blue" can be called "purple", "red" or "maroon" or "green" or something else.
With that in mind, we can take a look at foods. Animal foods on the inside are protein, and cooked protein is usually a pale greyish goo (although muscle cooked medium still has reddish coloring). One of the rare examples would be some eggs with blue shells. In plants, basically all leaves we eat are colored by chlorophyl. So they are out of the equation.
Other plant parts, especially fruits, tend to be colored by one of a few other groups of pigments. And a major group of those pigments, the anthocyanins, ranges from blue to purple to red (they also change with pH). So a ton of foods have a somewhat purplish blue color, and are also named "blue" in other languages. Plums, aubergines, red grapes, red cabbage, and many berries are good examples for that. Then you have some plants which come in many colors, but the major cultivars happen to not be blue - potatoes and maize are the first to come to mind.
Other shades of blue are less common just because they don't happen to be covered by the major pigment groups, but if you go down into shade level, then "royal blue" is just one of many on the spectrum which is missing. Also note that there are still plants which have other shades of blue and are consumed, they are just not commonly thought of as "food". Lavender, chicoree, gentian and pansies all have blue flowers of various shades and are traditionally used in recipes.
So there is no special evolutionary reason why plants or animals should not be blue - many of them are blue, and we eat some of the blue ones. It is simply a cultural or linguistic trend for English speaking people to not see their food as blue.

I knew someone would answer or comment with some smug claim that colours are all completely subjective labels. The reality is that it doesn't matter.

@MilesRout I don't understand your comment - what doesn't matter?

What we call 'blue' in English. You thinking (rightly or wrongly) that we don't apply blue to some things that you think are actually blue isn't relevant to the question.

@MilesRout The question asks "why are there so few foods which are blue". If there are many foods which are blue, it makes no sense to search for reasons. And my answer says that this is the case - there is no lack of blue foods.

@rumtscho But there aren't many foods which are blue.

@MilesRout there are - the answer lists them. They are just not salient when people try to think of them. If you will, I answer the question "why does it seem that there are so few blue foods", which is better applicable to the OP's situation than the literal wording of their question.

@rumtscho No it doesn't. It lists foods that are colours like purple and pink and claims that they're really blue. But they aren't.

@MilesRout Look for example at this: https://commons.wikimedia.org/wiki/File:Close_up_grapes.jpg. I cut out a grape from this picture, went to a color palette creation tool, and picked out colors from it. All of them were blue-based (hue component in the HSV model was between 216 and 220), I would call them blue-greys when seen as colors only. This is why I claim that these foods are blue - because when you photograph or paint them, the color which represents them is blue. The language may call them "red grapes" but this does not make them red any more than a green monkey is green.

@rumtscho they're clearly purple.

Why are there relatively few foods that reflect light in the 450–495 nm wavelength range?

I agree with the critics: this has nothing to do with language. Food isn't called blue in English because it is not really blue. Grapes (like blueberries) look blue from the outside but this is just the “dusty” surface; what you actually eat/drink after washing/processing is not blue. Calling the foods you listed blue (as well as some which are called blue, in particular blue cheese, which is not really blue) would be as inaccurate as calling carrots yellow. Physically speaking, the hue difference between yellow and orange is way smaller than the difference between purple and blue.

@rumtscho: Grapes do come in many colors: purple/blue, red, green, yellow, and perhaps others.  Likewise a number of other fruits - plums, elderberries, & mulberries come to mind - of which at least some varieties are blue.  And for those that are blue when growing, the 'dusty surface' is no more important than the fact that some blue animals produce their color by structural manipulation rather than pigment. They still look blue when I pick them :-)

Debating the semantics of the word "blue" is something I would have expected from my fellow pedantic SO users, but @rumtscho, you surprise me.

This is quite possibly the wrong way around, in that it is plausible that one of the influences on languages that had a word what in English is "blue" through the spectrum to what in English is "green" sometimes changed to differentiate them is because green is so common in nature, including food, and blue so rare.

What we call it in the English language is irrelevant. There are only a handful of naturally-occurring, edible things in this world that the majority of people would consider the color English-speakers call "blue".

@rumtscho Even if we accept your position on the semantics of the word 'blue', it's not really an answer to the question, which in this new framing would be asked as "Why are there so few foods with colors that fall in that part of the VL spectrum that the OP so casually used the word 'blue' to refer to?"

@DanHenderson and others - I find that the foods which fall in that part of the spectrum are not at all "few".

What I meant is that when OP said "blue", they were actually referring to a specific subset of the colors that can be called "blue", and that this subset excludes most of your examples. Your answer is not incorrect, but it is framed around your broad application of the word "blue", which, as I'm sure you can recognize, is not the same as what OP is actually asking about.

@Jon Hanna I don't know that there are not numerous foods that refect in the 450–495nm range. To be true blue, you need to not only reflect in this range, but either absorb in the 580-620mn range or absorb everywhere in the visible spectrum except 450-495nm. As I note elsewhere, it is the conditions needed for an organic molecule to absorb at all in the 580-620nm range that are difficult to produce.  Which I suspect strengthens the overall case you seem to be making, there are very few foods that have a color that closely matches the colorspace of 450–495nm light.

@rumtscho why are there so few foods at the center of blue color space?  Is this true of any other color except varieties of blue like cyan?  I also know no other color (except maybe pure indigos and violets but not purples) where pigments of that color require such challenging and energetically disfavored biosynthesis to produce compared to blues.  If it is all just a matter of people's choice of langauge why is the chemistry of blue pigments so much more difficult to achieve?  And since it is, how can the question be approached without addressing this fact?

Summary
I would argue that the more appropriate question to ask is:
Why can we easily identify blue plasters, even though so few foods are blue?
(And the main part of this answer is why this even is a reasonable question.)
And the main answer to that is probably that a sufficiently large food-sparse region in colour space eventually got its own colour term, namely blue.
Terminology
A colour is a region in colour space that speakers of the respective language agree to assign the same name.
Basic colours are colours that you already know in kindergarten and wouldn’t consider a special case of another colour (like aquamarine being a special case of blue).
The evolution of colours
In their evolution, languages refine their segmentation of colour space step by step, usually starting with light-warm vs. dark-cool, then separating red, yellow, and green (further reading).
The fifth segmentation usually splits blue and green – or more precisely: something similar.
After this, English has white, black, red, yellow, green, and blue.
Now, languages split an existing colour when there is a relevant distinction to make within it.
And potential food was the predominant thing for humans to distinguish for most of their history.
If some fruit changing its colour from green to blue indicates that it has become edible, this is not only something relevant to communicate, but having a word for it makes it easier to remember this.
By contrast, humans looked at the sky on a daily basis but whether it was green or blue was nothing they ever needed to communicate.
Consequentially, the colours of foods were a driving factor for how we segment our colour space to begin with.
And here, the distinction between blue/green and purple/red seems to be dominated by exactly this:
Most food that leans on blue just remains within what we call purple (e.g., plums and aubergines) and often, turning from green to purple indicates ripeness.
(And in the case of blueberries, going from purple to blue indicates ripeness.)
However, I am not aware of any food that turns from green directly to blue.
So, there is some reason to assume that our colour space is segmented such that there is little food on the blue side of the blue–purple divide.
Feedback from language to perception
Another weird aspect of this is that language influences our perception:
If we learn a word for a colour from young age, we also become better at distinguishing that colour.
A striking example of this are the Himba people, whose language segments the colour space differently than English (and many other languages).
They find it much more difficult to distinguish certain a certain pair of shades of blue and green (because their language doesn’t) but find it easier to distinguish certain shades of green (because they are different basic colours in their language)¹²³.
So, if the English language didn’t make the blue–green distinction, you likely wouldn’t have asked this question – not only because you would lack the words, but also because you wouldn’t even notice³ that there is a family of shades of green that you find in all sorts of technological products (and the sky), but not in food.
Now, what’s so special about blue?
There are several other colours you will rarely find in food, e.g., fuchsia/magenta, neon green, and brick red.
Why did you not ask the question about them, and why don’t we use these colours for plasters for food preparers?
The answer is probably that blue is a basic colour term but that is only half the truth:
Because blue is a basic colour term, we are better at distinguishing it than e. g. neon green and thus it is more effective as a plaster colour.
This leaves the question:
Why did blue become a basic colour term and continued to exclude food?
The answer to this is naturally hidden within the mists of history.
However, the appearance of some sort of blue–green distinction is a common trait of languages, suggesting that it is more than just chance.
My educated guess it that what we call blue is simply the largest region of colour space that is sparse with food, for the reasons addressed in the answers by AliceD and Theforestecologist.
Mind that a difficulty with this explanation is that it is somewhat subjective how we even measure colour-space volume (and thus define largest) in a way that reflects human perception.
In contrast to other basic colour terms, blue (or more precisely: the blue–green distinction) seems to be caused by the introduction of a respective dye³ and not food.
This suggests in turn that the new colour term started off as an identifier for that particular dye (which is in fact common, see indigo, Prussian blue, etc.).
As such it doesn’t make sense that it covers the colour of something well known including food.
For example, if the new colour term had covered the colour of plums, people would rather call the new dye plum dye and forget about the new term (and the dye) quickly.
As a result, it is not surprising that the emerging colour term would start with a territory that excludes food.
Of course this might change later in a language’s development, but if a colour starts off as a non-food colour, there is some psychological momentum to keep it that way:
You are not used to eating blue stuff, hence you wouldn’t start calling food blue (see also Rumtscho’s answer).
This is paralleled in grey, the other basic colour term that we do not like to use to describe food, even though it is arguably the most fitting colour for meat in some conditions, certain fish, the inside of eggplants, etc.
Rather we tend to describe these by whatever remnant hue the food has or as white or black.
Some reading material

Language Log: It’s not easy seeing green
Gondwana Collection: How do Namibian Himbas see colour?
Business Insider: No one could see the colour blue until modern times

Your analysis fails to address the question, why are there so few foods which have colors in the blue color space regardless of where the margins of that space are placed by a culture.  I also have to note that the some additional reading material regardling blue perception is seriously flawed.  Your reference re: Namibian Himbas is based on a misleading dramatization of the research not the actual research.  See https://languagelog.ldc.upenn.edu/nll/?p=18237 which is an update to the "Not easy seeing green' reference.

The business insider reference also relies upon the inaccurate dramatization of the Himba research as well as claims about ancient Greek perception of blue that are challenged as simplistic assumptions by some scholors of the ancient Greek language, see http://kiwihellenist.blogspot.com/2020/05/ancient-greek-colours.html


text:
Meaning of "auto" in trans autophosphorylation?
Why this process called "auto"?
Is it because each tyrosine kinase receptor subunit of the RTK dimer has the ability to phosphorylate tyrosine or other amino acid residue present in other subunit of the RTK dimer's cytoplasmic tail? Is it because no new set of kinase enzyme is needed to phosphorylate them?

Receptor tyrosine kinases are proteins which float around in the cellular membrane. Upon binding of their ligand, the dimerize (two units form a dimer). This releases the kinase domains and the proteins start phosphorylating themselves over cross (meaning part a phosphylates part b and vice versa). This looks like in this figure below (from here, it is an interesting article):

Besides ATP this process needs no further co-factors so the "auto" refers to the ability of the RTK to phosphorylate itself upon activation. See more about autophosphorylation here.

OP may not see your comment if you don't @mention him... :/

@N00B Instead of thanking me here in a comment you could upvote answers to your questions and accept to show it answers your question. This is a nice gesture towards the people who answer questions (also probably in older questions).

@Chris I did that already


text:
Black spider identification
Location is in Canada / Ottawa
I did a web search, but I can't figure out what species it is.

If it is still there could you take pictures showing its eyes?

This is very likely an araneid (or Orb-weaver spider) in the family Araneidae.
Without further evidence/details, it's hard to narrow down to a species.
One possibility is Larinioides sclopetarius (the bridge spider or gray cross spider).

 Source: Ed Nieuwenhuys

 Source: Eurospiders.com

Description:

Prosoma grey to grey-brown. Opisthosoma grey to grey-brown, dorsally with pronounced, white margined pattern. [Source: University of Bern]

See University of Bern for detailed drawings of anatomical structures.

Size: females = 10-14 mm ; males = 8-9 mm.
Web: orb webs can have diameters of up to 70 cm.
Ecology: often lives on briges or near water; often found on steel structures.
Range: holoarctic including Ottawa, Canada

What it's NOT
Although visually similar, this is not
Nuctenea umbratica (walnut orb-weaver)

 Source

This species is confined to Europe, Asia and Africa.

The specimen is also unlikely Larinioides ixobolus, which generally has a black outline (vs white) on their backs And is constrained to Europe/Asia:

 Source

you posted a similar answer in a different question

Possibly incorrect, but it looks like some kind of orb weaver - from the shape of the abdomen and the posture, it's very similar to a lot of orb weavers and potentially is in the Araneus genus.
I don't think it's this species, but you can perhaps see the similarities;
http://www.spiders.us/species/araneus-diadematus/#more_pictures
It could also be an immature, hard to know :/


text:
Small mottled beetle identification
I live in Chapel Hill, North Carolina, and recently started seeing a bunch of these little guys in my house. They're about 1/8" long (estimate), with brown/black zigzag stripes; it's possible they're babies, but over the last two weeks their size has remained consistent so this seems unlikely.

Related: What's this black, white and orange beetle?

It's probably an adult varied carpet beetle (Anthrenus verbasci). They are quite small and commonly found in homes. Here's one of a number of images available on Wikimedia:

You'll likely come across more of their larvae than the adults themselves. Their larvae are often found munching on organic matter throughout the home (e.g., bedding, clothing, organic crafts, hair or crumbs in cracks, dead mice in walls, etc.). See the following images showing relative size, actual size, and a zoomed-in view of larvae.

You can contact SE for help fixing this here: http://biology.stackexchange.com/contact


text:
Why does the face turn pale in dangerous situations?
I know what the effects are of a dangerous situation on the brain, i.e., an activation of the hypothalamic-pituitary-adrenal (HPA) axis which eventually results in an increased heart rate and elevated blood pressure. However, I do not understand why enhanced blood circulation would result in a paling of the skin? Do red blood cells actively change direction?

Red blood cells are not equipped with a motor system to propel them through the blood stream. Instead, they are passively transported through the vasculature by the the pumping action of the heart. The effects of dangerous situations on the skin have to do with hormonal effects on the blood vasculature, and not with direct effects on red blood cells.
Dangerous situations trigger the fight-fright-flight response, and is a direct result of adrenaline being released into the bloodstream. Adrenaline prepares the body to fight for its life.
Adrenaline has a variety of effects including enhancing perspiration (or diaphoresis, which prepares the body for the increase in temperature associated with fleeing/fighting), dilation of the pupils (which increases light sensitivity, but reduces acuity), dry mouth (gastric juices and saliva production decreases because blood flow to the digestive system is decreased), enhanced smell and hearing, and a cool, pale skin.
The cool and pale skin is caused by a reduced blood flow to the surface of the body, while blood flow to the arms, legs, shoulders, brain, eyes, ears and nose can be increased. Besides getting ready to run and fight, the body is preparing to think quickly and be aware of threats by hearing, seeing and smelling things better. Pulling blood away from the skin also helps decrease bleeding from cuts and scrapes (Source: Army, Navy & Air Force Australia).
Adrenaline stimulates alpha-adrenoreceptors in blood vessels, which causes the smooth muscles around the vessels to constrict (Bolli et al., 1984)

Adrenaline (epinephrine) released from the bloodstream dilates vessels in the skeletal muscle via beta-receptors, and constricts them in the digestive system and the skin through alpha-adrenergic receptors. Source: Marian University College, Indiana.
Reference
- Bolli et al., J Hypertens (1984); 2(3): S115-8

It has nothing to do with blood cells per se. Blood vessels constrict in areas not necessary for flight-and-fight response (sympathetic nervous system response).
By constricting blood vessels around face, blood is redirected to organs more important in resolving dangerous situations: muscles of legs, spinal cord, and brain. Same way pupils get dilated to capture more light..

Totally agree with the answers already provided: It is directly caused by constriction of blood vessels in the face.
However, from a behavioral perspective, I feel there is an additional function for 'turning pale', that is not yet mentioned:
For social animals (such as humans), the face has great expressive value.
The face turning pale might fulfill the function of warning our fellow species that there is a certain threat. This form of communication might be preferable over, for example, yelling. Yelling might attract to much unwanted attention from predators (if predator was reason why individual is afraid).
If this is true, then we should expect turning pale of the face would more often occur in groups than alone and would also happen more often to the 'weaker' individuals of the group, because stronger individuals that seek help from their weaker friends would not be so advantageous.

Welcome to Biology and thanks for your behavioral perspective. However, the question is not so much on the why, but more on the how.

Ps. Dutch by any chance? :)


text:
How do other primates treat their fingernails?
Do all primates have to trim their fingernails in some way, or do some primates' fingernails wear off through natural use?
Also, is constant nail growth common to all primates?

Both. Many primates bite their nails. Some do not. It's not species specific.
Chimpanzees, for example, usually bite their nails; some prefer to leave them alone. They do grow continuously, and nails not kept trimmed probably break off (looking at the thickness of those nails, I get the impression that this might be rather painful if they break too short.)
The caption for the picture below (taken from a chimpanzee sanctuary) states

Annie likes to pick Missy’s nose. Missy tolerates it but doesn’t seem to enjoy it. I don’t blame her – Annie doesn’t bite her nails down like the other chimps do.

They bite their toenails as well.

The follow-up question then is how do hunter-gatherer societies treat their nails? I'm just curious if continuously growing nails is one of those vestiges of evolution that lost its purpose. It would make more sense if nails grew to a certain length and then stopped (like body hair does). Continuous growth suggests to me that they're like sharks' teeth, they have to be continuously replaced due to wear.

But none of these require grooming (to the point of necessary discomfort when not treated). You don't really have to do anything about your skin or hair, to the point where if you don't trim them they would cause you significant discomfort on day-to-day activity. Also, body hair certainly doesn't continue to go in length, otherwise most people would look like Chubaka.

What's the evolutionary reason for this?

Why some human beings bite their nails? Because, they are primates - it is inherited way to treat our nails, called onychophagia. While normal in some primates (and this is the way they treat their nails), it is considered abnormal in human.

Abnormal or just gross?


text:
What is the cause that angina pectoris is felt in your left arm?
When someone has heart problems sometimes they feel pain in their left arm. But why is the left arm painful?

I know the nerves associated with heart pain are shared with the arm.  A hypothesis (no citation offered) is that the brain has likely never had a need to identify the exact source of heart pain, because those neurons don't fire often enough to make sense of them.  Thus, when the pain neurons in the heart fire, the brain has to make sense of new signals, with little to compare them to, so it does the best it can.

It has mainly to do with the embryonic origin of organs, with the heart being a typically left sided organ, it develops sharing some nerves with the left thorax and left arm.
There is however high variability, typically among patients but also among coronary vessels. For instance, right coronary stenosis may lead to abdominal pain, whereas left circumflex artery stenosis may lead to no pain at all.
References:
Essentials of Medical Physiology
Cardiac Pain. Anatomic Pathways and Physiologic Mechanisms
Changing Features of Anginal Pain After PTCA Suggest a Stenosis on a Different Artery Rather Than Restenosis

Thanks! You can find other references just looking at Google: http://www.healthhype.com/angina-pectoris-heart-pain-meaning-and-different-types.html, and https://books.google.it/books?id=tdODr5fpxEAC&pg=PA633&lpg=PA633&dq=angina+pain+embryonic+heart&source=bl&ots=7dQcfjyzZ2&sig=434B_K1VhNUPCs9x9i_qCKQa0mw&hl=it&sa=X&ved=0ahUKEwiy9-TUju7LAhXGYQ8KHc8aARQQ6AEISzAG#v=onepage&q=angina%20pain%20embryonic%20heart&f=false

Our pain-sensing neurons work in useful ways only when they inform about skin areas or muscles. When nettles sting your left hand, you want to retract your left hand right away. This is in part an educated reaction, because your brain ca re-learn what sensing neuron corresponds to what side of you hand when surgeons stitch together broken nerves.
With internal organs, it is not the same. You can't really retract your heart or your liver, and you can't look inside you to figure what organ is actually struggling. Our brains make sense of pain from internal organs by projecting them onto nearby skin or muscle. This is why doctors never write down "heart pain", but "pain projected to the area anterior to the heart" (in short, "precordial pain"). For them, it is never certain that the heart hurts. It could be even the pectoral muscles. (This is also why hypochondriac types are mocked when the complain about "stomach pain".)
Heart projects into many neighboring places. Some people complain about pain in the hand, but other describe pain in the throat or even the jaw. We feel something is not right, but we can't tell precisely where. The left hand is often the place where we project heart pain, because it is close to the actual trouble.
Pain from the internal organs is often, although not always, projected on the muscles and skin portions covered by the same nerve as the troubled internal organ. A more technical explanation describing which nerves cover the heart, and what skin / muscle structures these nerves are shared with, is given in Medical Physiology by Boron and Boulpaep.

This answer doesn't explain the mechanism of referred pain in any serious way. "The left hand is often the place where we project heart pain, because it is close to the actual trouble." How do we project the pain? The left hand isn't closer than the left scapula, or the jaw, or other sites of referred pain from cardiac ischemia. References would be nice, as would a better explanation.

The association between a heart attack and arm pain appears to be strange since the arm is usually not harmed. The reason for the pain in the arm is that the pericardium is innervated by the phrenic nerve and there may be some neuronal connections to left sided intercostobrachial nerves. During myocardial ischemia, the signals can overlap creating a false impression that the pain is coming from the left arm or jaw. The brain essentially gets confused about the source of the signal and this can often result in the pain from a heart attack being felt as pain in the arm with no sensation of pain in the chest. This is what is known as referred pain. You can check out this post on Wikipedia for more detailed information.


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What Makes an Afrothere an Afrothere?
The mole-like golden moles.
The hedgehog-like tenrecs.
The shrew-like sengis.
The rodent-like hyraxes.
The anteater-like aardvarks.
The whale-seal-hybrid-like sirens.
The tapir-like proboscideans.
All these mammals, diverse in appearance, actually belong to one superorder, called Afrotheria.  Which raises the question — what are the anatomical characteristics common to all afrotheres?

your question is confusing as it is worded/structured. It's fine that you rolled-back my attempted edits, but I suggest that you provide some context at the beginning of your question to help reduce the confusing structure of your question.

I did.  It can't have been any more straightforward.

The word "Afrothere" comes from Latin Afer (referring to Africa) and Ancient Greek θηρίον (thēríon, meaning “beast”). In other words, "Afrothere" etymologically refers to animals with origins in Africa.
So really, Afrotheria and it's members are referred to based on their origins and not their physical characteristics. In other words, they needn't share morphological characteristics, but rather they all evolved from the same common ancestor from Africa.
From the Afrotheria Wikipedia page:

Afrotheria is a clade of mammals, the living members of which belong to groups that are either currently living in Africa or [are] of African origin...
They share few anatomical features but many are partly or entirely African in their distribution

These animals are all chordates of the class Mammalia, and therefore do share the common "basal" characteristics shared among all members of these higher taxanomic levels -- for example, a notochord (and backbone), hair, three middle ear bones, mammary glands, etc.

Emphasize on the "few anatomical features".  A little clarification is required.

@JohnWDailey The few characteristics they share are those shared with common ancestors. I've listed some of these in my final paragraph.

And what are those "few characteristics"?  That last paragraph answers what makes a mammal a mammal, not what makes an afrothere an afrothere.  And location is very circumstantial.

@JohnWDailey that's right. Because the only characteristics that afrotheres consistently share are those characteristics shared basally by all mammals.

There grouping is based purely on genetics not morphology, although there are a few of those. vertibral count http://www.tandfonline.com/doi/abs/10.1017/S1477200006002258  and placental membranes http://onlinelibrary.wiley.com/doi/10.1002/jez.b.21079/abstract;jsessionid=D30EE1D51D29755F7736702616AB0930.f04t04

@theforestecologist is right, there are very few anatomical synapomorphies (shared derived traits) if any of Afrotheria, and the few there are almost all exhibit homoplasy. But if you want obscure putative anatomical synapomorphies that may not strictly hold:
"
presence of a naviculocalcaneal facet
scattered vomeronasal organ blood vessels, placement of the internal carotid lateral to the anterior pole of pars cochlearis
 four allantoic vessel chambers
 Presence of a small P3 protocone
 presence of well-developed buccal cingula rather than stylar shelves
 increase in lumbar vertebra number from 6 to 8 testicondy (intrabdominal testes)"
Are listed by Seiffert 2007 as possibilities-- but only under certain phylogenetic constructions.
Ref: Seiffert 2007 A new estimate of Afrotherian phylogeny based on simultaneous analysis of genomic, morphological, and fossil evidence. BBC Evolutionary Biology. https://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-7-224


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Gorgonopsid--What's In a Name?

The Gorgon--a terrifying, monstrous female with hairs made of live snakes and a stare so horrifying that it would literally turn you to stone.

The Gorgonopsid--a predatory protomammal that hunted down its prey via saber-like teeth.
Herein lies the problem.  The gorgonopsid looks nothing even remotely like a snake, nor is it likely that it could stare down its own prey into a statue.  So why call this family of predatory protomammals "gorgonopsids", anyway?

If I had to guess, it is because the skull of this animal must have horrified early paleontologist of the 1890s. This was an animal that had a skull that was both reptilian and mammalian. Some kind of horrid in between animal. Neither mammal nor reptile. In the era before the evolution theory was accepted...this animal looked like a defiance to the order of God's creations. There were fish, amphibians, reptiles, birds and mammals. The five kingdoms of animal life. There should not be such a beast, a half reptile, half mammal. Like the gorgon half beast, half human.

Gorgon (via Latin from Greek Gorgō, from gorgos meaning ‘terrible') + opsid (Ancient Greek ὄψις (ópsis) for "appearance”) (often referring to the face).


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Are these birch or aspen trees?
I was just wondering if anybody could help me identify the trees in this image? I am confused as to whether they are aspen or birch, a mix of the two, or whether the brown/reddish trees are some other species I don't know about? Thanks!

pretty sure they're aspen trees (https://upload.wikimedia.org/wikipedia/commons/a/a8/Fall_Colors_at_Kebler_Pass,_Colorado_USA.JPG) rather than birchs. The leave colour of birches is normally a darkish green. To make the identification easier, a geographical description would be helpful, and some close-up photos of leaves, branches and the main stem.

From the growth pattern, I'd say aspens, since they're clonal organisms that grow in groves like that.

Thanks for the quick answers! It's just picture from a google image search, so I have no idea about the geographic location etc. as this information was not provided. So they are probably aspens then! How about the reddish trees? Do some aspen trees get like that?

Jes, some aspens can be somewhat orange to red, even in a stand where most are golden yellow.  In fact, I've never seen a grove that's all orange, and aspens are common in the mountains here, wherever there is sufficient water.

I don't have the vocabulary, but my experience working in forestry alongside professional foresters STRONGLY leans toward aspen. I know that isn't a substantial external justification, but consider it a warning to be sceptical.

The bark texture appears more consistent with aspen than birch, even at this distance it is usually evident.

I think they are birch trees (Betula), based on the white bark with black horizontal patterns (Lenticels). The bark patterns at the base of the trees are also very similar to birch. Either way (relating to one of the comments), it's an autumn picture, where both birch and aspen have yellow leafs (aspen leaves can also be red or reddish, especially at the end of branches).
This page also agrees with me (which doesn't necessarily have to mean that much though), which labels the exact same picture as "birch-tree-wallpaper-3.jpg".
I'm not sure about the exact species, but I know that there are North American species where the bark can shift between white, pink or reddish. One possibe suggestion/pure guess is Betula neoalaskana, but as you can see from the wikipedia page this species can also hybridize with Betula pendula and other species. To me, it doesn't look exactly like the Eurasian Betula pendula or Betula pubescens, which I'm most familiar with (those are the two species found in Sweden).

The name of the .jpg is actually what started my confusion. To me, the bark and the way the leaves are concentrated at the top of the tree makes it look like quaking aspens. However, I don't think I've ever seen the brown/reddish types of aspen before, which makes me think it's birch. Do aspen and birch often grow in the same area? Could it be a mix of the two?

Aspen and birch can definitely grow together. I also think that the distribution of leafs is a bit off compared to what I'm used to in Birch, but I'm not familiar with N. American birch species, or hybrids. The sturdier, shorter tree to the right is extremely similar to a birch though.

The tree with a thicker trunk to the right seems like birch to me too, as well as the other reddish trees. I didn't know that they grow in the same area, so I think I'll conclude that it's a mix of the two. Thank you for all the helpful information!

Taking another look, I'm really unsure though. The overall look of the stand is very aspen-ish. Also, the thinner branches of birch trees are often relatively dark, while aspen branches are usually ligher and more similar to the trunk, which is more similar to the trees in the picture. Both  Populus tremuloides and Populus grandidentata can also be were white with horizontal black stripes.

I also think they look very much like aspen, and the color of the branches is also a good point!

@fileunderwater If you're unsure, you should modify or delete your answer. And I think you're right being unsure, the stand does look very aspen-ish. And btw: Sweden has one more betula: Dvärgbjörk ;-)

Look like aspen to me.

Turns out this is harder than it seems from just the picture...
The three candidates I think one could most argue for are:

Quaking Aspen (Populus tremuloides)
Paper Birch (Betula papyrifera)
Grey Birch (Betula populifolia)

Here is a picture of Populus tremuloides from the USDA:

Notice the smoothness of the above Aspen bark (identical to that of the picture in the OP's question). Now compare it to the peely nature of typical birch bark (here pictured B. papyrifera or paper birch):

Though not all birch species have peely bark: (my emphasis below)

Grey birch (Betula populifolia) typically remains fairly smooth. From Viginia Tech's tree guide:

Bark: Reddish brown with numerous lighter lenticels on very young stems, later turning gray to white and very chalky; remains smooth and generally does not peel.

B. pendula (European white birch) bark is also described as being smooth and only somewhat peely. From Virginia Tech:

Bark: Reddish brown with light lenticels when very young, later turning white, generally smooth but does peel a bit; the base of the tree develops thick bark which splits into narrow, vertical furrows which are nearly black.

However, the branches in the OP's picture are not drooping as is also common for B. pendula (AKA the European weeping birch). So this is not our tree!

Also, some sources describe paper birch as simply being smooth (vs. emphasizing the peeliness). See here for an example.

Here are images of European white birch (left) and gray birch (right) :

But Gray birch remains smooth, even at the base of the tree (though see here)...
Pay attention to the bottom of the trees in the OP's picture. The bark at the base of the trees is becoming furrowed:
Zoomed in OP picture:

This base furrowing is a characteristic of P. tremuloides:

According to the Virginia Tech Tree Guide (my emphasis):

Bark: At first smooth, creamy yellowish-white to very light green; later developing thick furrows and becoming dark, especially near the base.

And according to The Sibley Guide to Trees  (my emphasis):

Bark color from white to greenish, grayish, or bronze. Bark smooth, pale with dark scars that persist for many years; base of trunk becomes coarsely ridged grey on old trunks.

And from the NDSU Agriculture webpage (my emphasis):

Bark Color - Varies from chalky white to olive-gray, and becomes rough furrowed on the lower trunk of mature trees.

[Note: I give all 3 sources to demonstrate the inconsistency in description of P. tremuloides bark color].

Here is a close-up of furrowed aspen bark from the USDA website (yes, this is still aspen):

However, this furrowing can also occur in paper birch.

From V tech (my emphasis):

Bark: Reddish brown with light lenticels on very young stems; later turning chalky to creamy white, peeling in horizontal papery strips; brown to black and may be furrowed at base; orange inner bark.

From Field Guide to Trees of North America (my emphasis):

Bark white papery; older trees have rough black bark near base.

I will comment that I have never seen paper birch with furrowed bark at the base. Further, I was unable to find visual evidence from any reputable sources by searching on Google.

So what about tree shape and form?
Again, according to The Sibley Guide to Trees:

[P. tremuloides has a] single main trunk with relatively stout , jagged twigs (unlike more slender and graceful twigs of birches).
[B. papyrifera] branches begin lower on trunk than in aspens; more graceful, ascending, and regular with slender straight twigs. Mature tree has oval habit, few strong branches, clumps of fine twigs in the crown.
[B. populifolia has] typically multiple leaning, crooked trunks and narrow crowns. Bark of mature trunks grayish white, nonpeeling.

Citing a different source (Field Guide to Trees of North America) to address different language to differentiate the species:

[P. tremuloides:] Small to medium-sized tree. slender trunk; pyrimidal to round crown of spreading branches with sparse foliage.
[B. papyrifera:] Medium-sized tree. One to several slender, straight trunks; narrow to broad crown; branches upraised in youth, often becoming horizontal or drooping in age
[B. populifolia:] Small tree. sometimes shrubby; One to several trunks; narrow, conical crown.

The smaller trees in the front left of the OP's picture demonstrate a more birch-like architecture and have branches growing along the stem as the description for B. papyrifera describes. These qualities are reduced or eliminated in the larger trees in the OP's picture.
For visual clarification, the typical pyrimidal crown shape of P. tremuloides is shown below (from Oregon State Department of Horticulture). The trees in this image appear more slender (pyrimidal) than the trees in the OP's image, but aspen trees can also have "round" crowns as stated above.

The following photo from Michigan State University Extension's Tree ID Key attempts to demonstrate a difference between paper birch and aspen crown coloration:

The crown branches/twigs visible in the OP's picture seem to be lighter colored, and so this line of evidence points to aspen.

Ok, so what about size? According to Field Guide to Trees of North America the typical sizes of the species under question are fairly similar, but with gray birch being smaller than the other two species:

Quaking Aspen (Populus tremuloides) = 30-70' tall, 1-1.5' diameter
Paper Birch (Betula papyrifera) = 50-70' tall, 1-2' diameter
Grey Birch (Betula populifolia) = 20-30' tall, 0.5-1' diameter

The relative sizes of the trees in the OP's picture don't seem to preclude any of these species.
Both birches and aspens develop yellow leaves in autumn, so this characteristic additionally is not useful for differentiating the two genera (or 3 species).

Though @fileunderwater is correct in stating that birches have horizontal lenticels that are helpful in identifying birch trees, the OP's image is far too zoomed out to make out any lenticels (even when zooming into the OP's picture). Those black spots are not lenticels; they are scars (generally from lost limbs). Regardless, other species (including some aspen species) also develop lenticels.
Here is a close-up of lenticels. The little slits are birch lenticels (here from B. papyrifera):

Names on files and webpages are notoriously incorrect so using that as a line of evidence is dangerous. I saw a number of misidentified species in pictures while I was searching myself.

So thus far it seems as though aspen is a pretty good choice, but paper birch could also be potentially correct. The distance of the picture makes this distinction difficult, but I will point to the two lines of evidence that have convinced me that some of the trees are in fact paper birches.

The girth and shape of the larger trees are more consistent with the growth form of birches vs. the usually slender, single-stemmed aspen.
In the OP's picture, you cannot see peeling or lenticels, but this might be due simply to the distance from the trees. However, you can see a brownish/orangish coloration on a number of the trees, which is consistent with numerous descriptions of paper birch trees that have already started peeling revealing brown/orange bark.

Conclusion = Hard to tell, and likely actually a mix of Paper Birch (Betula papyrifera) and Quaking Aspen (Populus tremuloides). I think you're looking at taller aspen and mostly wider paper birch.


text:
How to anaesthetise mosquitoes during a mass-trapping field study?
Do any of you have experience in anaesthetising mosquitoes during a mass-trapping field study?
Normally, mosquito traps have a little net in which you catch the mosquitoes. The nets can, after collection, be placed in a freezer to knock out the mosquitoes. However, the trap used for my study doesn't have a net, so mosquitoes will fly around inside the trap.
I thought about using Ethyl Acetate to knock out the mosquitoes, however, the trap needs to stay clean of odours, so that's not an option. I think using CO2 would be most convenient, but I will not be able to drag a big CO2 cylinder to all my trapping locations.
Is it maybe possible to use CO2 tablets which can be dissolved in water? I could then use a plastic bottle where the cap is connected to a tube, which I can place in the trap. Would this be efficient? What do you think?
Thanks a lot!

What type of trap are you using?

It's a new prototype from Biogents. Not sure if I'm allowed to give more information on that. I've already started the study and used a big freezer back at the lab where the trap fits as a whole, but that's not very convenient since the traps need to be placed back in the field on the same day.


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Are mutations the cause of alleles?
For example, some leopards have a mutation which causes black fur. So Black fur and normal fur colour (orange-white) are both alternate forms of the same gene, right? So does that mean that mutations cause alleles?
Thanks

That's correct. Alleles are alternate forms of a gene that occur at the same locus and arise by mutation.
Another example might be sickle cell anemia where a SNP (a single base change) is the cause of the disease.

Not correct. Alleles can be identical. They are just copies of the same gene. You get one copy from Mom, and the other from Dad. Alleles need not be alternate forms. Mutations cause variation.

@KarlKjer OED: "Allele: Each of two or more alternative forms of a gene that arise by mutation and which may be found in the same position on a homologous chromosome." Can you point to a source that unambiguously uses 'allele' in the sense that you are suggesting (i.e. as a synonym for 'copy')?


text:
Why do animals have more heavy nitrogen and carbon than plants?

There are two stable Isotopes of nitrogen N14 and N15. The ratio of
  15N/14N tends to increase with trophic level, such that herbivores
  have higher nitrogen isotope values than plants, and carnivores have
  higher nitrogen isotope values than herbivores. Depending on the
  tissue being examined, there tends to be an increase of 3-4‰ at each
  trophic level.
The tissues and hair of vegans therefore contain significantly lower
  percentage of 15N than the bodies of people who eat mostly meat.
  Isotopic analysis of hair is an important source of information for
  archaeologists, providing clues about the ancient diets.

https://en.wikipedia.org/wiki/Isotopic_signature#Nitrogen_isotopes
But how does N14 'disappear' from plant to animal?

Your last line does not seem to match the rest of the question as you did not say anything about N-14 'disappearing' anywhere else in the question. Yet, herbivores have more N-15 than plants and carnivores have more N-15 than herbivores because of bioaccumulation (https://en.wikipedia.org/wiki/Bioaccumulation). This claim is also supported by @Roland's answer.

Organisms accumulate certain isotopes because their metabolic enzymes have a slight preference for molecules containing that isotope. A well-known example is ribulose-1,5-bisphosphate carboxylase (Rubisco) which is responsible for CO$_2$ fixation in photosynthetic plants: this enzyme is somewhat more efficient with $^{12}$CO$_2$ than with $^{13}$CO$_2$, causing plants to accumulate $^{12}$C.
In animals, there is an enrichment for $^{15}$N because the deaminases and transaminases that transfer nitrogen from amino acids to urea (for excretion) tend to favor $^{14}$N. So in this case, $^{14}$N tends to be removed from the body faster than $^{15}$N when breaking down dietary protein. The differences are very small though: the relative enrichment of about 0.4% per trophic level corresponds to 0.367% $^{15}$N in a herbivore, compared to 0.366% $^{15}$N in a plant.
Source: this article.


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How can tree branches grow upwards against gravity?
I saw this tree in front of my room with those branches, and I asked myself why they didn’t grow downward in the direction of gravity since that would be easy for them? And I came to the conclusion that they are seeking sunlight. Is there any research which explain how they can do that?
I found that this mechanism is called Gravitropism (Tree Branches grow against gravity, while roots grow into the soil in the direction of gravity) but I didn’t find a clear explanation.

This review paper might help answer your question: http://www.plantphysiol.org/content/120/2/343.full
It seems that starch-filled organelles called amyloplasts sediment in the cell according to the pull of gravity, and in doing so this gives the cell some form of basic up-down polarisation.  If my memory serves correctly, this can then cause changes to the growth of the cell in the branches, meaning the side of the cell opposite the amyloplasts grows slower and the side closest the amyloplasts grows faster, causing the upwards curvature of the branches seen in your tree.


text:
What is the name of the groove down the middle of an anther?
I thought it had a name along the lines of interlocular groove, but I haven't been able to find that term anywhere.

You probably mean the filament.

There are two longitudinal ridges in a typical anther. An anther has two lobes each with two pollen sacs/thecae. So there is a groove both between the two lobes and also between the two thecae in one lobe. Having a look at the transverse section would help. Although I don't know what they are called, it would be better if you specify which of the two ridges you are refering to.

I think what you are looking for is the connective. Anthers are usually two-lobed and connected by a band of sterile tissue that also connects the anther with the filament - therefore the name, I guess (see Wikipedia article for stamen). It says

Most commonly anthers are two-lobed and are attached to the filament either at the base or in the middle area of the anther. The sterile tissue between the lobes is called the connective.

Each anther lobe usually consists of two pollen sacs (see Wikipedia article for microsporangia). It says:

Each anther lobe develops two pollen sacs. Thus, a two-lobed anther develops four pollen sacs situated at four corners of the anther.

And further (accentuation by me)

The cells of endothecium possess fibrous thickenings. They remain thin-walled and constitute stomium (line of dehiscence) in the shallow groove in between the two microsporangia of the anther lobe.

This groove frequently is called by its orientation, i.e. longitudinal groove, even though I do not think this really is a well-defined technical term (but do a quick Google search for "longitudinal groove" anther and you will find several biology books using the term, e.g. this).

I think that you're talking either about the connective that separates two lobes of the anther,  or the longitudinal groove that I think separates the two theca making it dithecous.

Welcome to Biology.SE! Please take the [tour] and then consult the help pages for additional advice on [Answer] effectively on this site and then either delete or [edit] your answer accordingly. It isn't clear that this adds anything to the existing answer. In addition, answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. Finally, if you include a figure you must document where it came from. Thanks!