Dog
The domestic dog (Canis familiaris or Canis lupus familiaris) is a domesticated form of wolf. The dog descended from an ancient, extinct wolf, with the modern wolf being the dog's nearest living relative. The dog was the first species to be domesticated by hunter–gatherers more than 15,000 years ago, prior to the development of agriculture. Their long association with humans has led dogs to be uniquely attuned to human behavior, enabling an abundant cosmopolitan distribution and to be able to thrive on a starch-rich diet that would be inadequate for other canids. 

The dog has been selectively bred over millennia for various behaviors, sensory capabilities and physical attributes. Dogs are subclassified into breeds, which vary widely in shape, size and color.Why are different breeds of dogs all considered the same species? – Scientific American .  Nikhil Swaminathan. Accessed on 28 August 2016. They perform many roles for humans, such as hunting, herding, pulling loads, protection, assisting police and the military, companionship, therapy and aiding disabled people. This influence on human society has given them the sobriquet of "man's best friend."

Taxonomy

In 1758, the Swedish botanist and zoologist Carl Linnaeus published in his Systema Naturae the two-word naming of species (binomial nomenclature). Canis is the Latin word meaning "dog", and under this genus he listed the domestic dog, gray wolf and the golden jackal. He classified the domestic dog as Canis familiaris, and on the next page he classified the gray wolf as Canis lupus. Linnaeus considered the dog to be a separate species from the wolf because of its upturning tail (cauda recurvata), which is not found in any other canid.

In 1999, a study of mitochondrial DNA (mtDNA) indicated that the domestic dog may have originated from the gray wolf, with the dingo and New Guinea singing dog breeds having developed at a time when human communities were more isolated from each other. In the third edition of Mammal Species of the World published in 2005, the mammalogist W. Christopher Wozencraft listed under the wolf Canis lupus its wild subspecies, and proposed two additional subspecies which formed the domestic dog clade: familiaris Linneaus 1758 and dingo Meyer 1793. Wozencraft included hallstromi (the New Guinea singing dog) as another name (junior synonym) for the dingo. Wozencraft referred to the mtDNA study as one of the guides informing his decision. Mammalogists have noted the inclusion of familiaris and dingo together under the "domestic dog" clade, and they debate this classification.

In 2019, a workshop hosted by the IUCN/Species Survival Commission's Canid Specialist Group considered the dingo and the New Guinea singing dog to be feral dogs Canis familiaris and therefore should not be assessed for the IUCN Red List.

Evolution
thumb|180px|Location of a dog's carnassials; the inside of the 4th upper premolar aligns with the outside of the 1st lower molar, working like scissor blades

The Cretaceous–Paleogene extinction event occurred 65 million years ago and brought an end to the dinosaurs and the appearance of the first carnivorans. The name carnivoran is given to a member of the order Carnivora. Carnivorans possess a common arrangement of teeth called carnassials, in which the first lower molar and the last upper premolar possess blade-like enamel crowns that act similar to a pair of shears for cutting meat. This dental arrangement has been modified by adaptation over the past 60 million years for diets composed of meat, for crushing vegetation, or for the loss of the carnassial function altogether as in seals, sea lions, and walruses. Today, not all carnivorans are carnivores, such as the insect-eating Aardwolf.

The carnivoran ancestors of the dog-like caniforms and the cat-like feliforms began their separate evolutionary paths just after the end of the dinosaurs. The first members of the dog family Canidae appeared 40 million years ago, of which only its subfamily the Caninae survives today in the form of the wolf-like and fox-like canines. Within the Caninae, the first members of genus Canis appeared 6 million years ago, the ancestors of modern domestic dogs, wolves, coyotes, and golden jackals.

Domestication

The generally accepted earliest dog remains were discovered in Bonn-Oberkassel, Germany. Contextual, isotopic, genetic, and morphological evidence shows that this dog was clearly not a local wolf. The dog was dated to 14,223 years ago and was found buried along with a man and a woman, all three being sprayed with red hematite powder and buried under large, thick basalt blocks. The dog had died of canine distemper. Earlier remains dating back to 30,000 years ago have been described as Paleolithic dogs but their status as dogs or wolves remains debated, because considerable morphological diversity existed among wolves during the Late Pleistocene.

This timing indicates that the dog was the first species to be domesticated in the time of hunter–gatherers, which predates agriculture. DNA sequences show that all ancient and modern dogs share a common ancestry and descended from an ancient, extinct wolf population which was distinct from the modern wolf lineage. The majority of dogs form a sister group to the remains of a Late Pleistocene wolf found in the Kessleroch cave near Thayngen in the canton of Schaffhausen, Switzerland, which dates to 14,500 years ago. The most recent common ancestor of both is estimated to 32,100 years ago. This indicates that an extinct Late Pleistocene wolf may have been the ancestor of the dog, with the modern wolf being the dog's nearest living relative.

The dog is a classic example of a domestic animal that likely traveled a commensal pathway into domestication. The questions of when and where dogs were first domesticated have taxed geneticists and archaeologists for decades. Genetic studies suggest a domestication process commencing over 25,000 years ago, in one or several wolf populations in either Europe, the high Arctic, or eastern Asia. In 2021, a literature review of the current evidence infers that the dog was domesticated in Siberia 23,000 years ago by ancient North Siberians, then later dispersed eastwards into the Americas and westwards across Eurasia.

Biology
thumb|upright=1.25|A lateral view of a dog skeleton

Anatomy

Domestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities and physical attributes. Modern dog breeds show more variation in size, appearance and behavior than any other domestic animal.

Size and weight
Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier that stood only  at the shoulder,  in length along the head-and-body and weighed only . The most massive known dog was a Saint Bernard, which weighed  and was  from the snout to the tail. The tallest dog is a Great Dane that stands  at the shoulder.

Senses

A dog's senses include vision, hearing, smell, taste, touch and sensitivity to Earth's magnetic field. Another study suggested that dogs can see Earth's magnetic field.Magnetoreception molecule found in the eyes of dogs and primates  MPI Brain Research, 22 February 2016

Coat

thumb|Dogs display a wide variation on coat type, density, length, color and composition

The coats of domestic dogs are of two varieties: "double" being familiar with dogs (as well as wolves) originating from colder climates, made up of a coarse guard hair and a soft down hair, or "single," with the topcoat only. Breeds may have an occasional "blaze," stripe, or "star" of white fur on their chest or underside. Premature graying can occur in dogs from as early as 1 year of age; this is shown to be associated with impulsive behaviors, anxiety behaviors, fear of noise and fear of unfamiliar people or animals.

Tail
thumb|A Finnish Spitz with a curled tail
There are many different shapes for dog tails: straight, straight up, sickle, curled, or corkscrew. As with many canids, one of the primary functions of a dog's tail is to communicate their emotional state, which can be crucial in getting along with others. In some hunting dogs, however, the tail is traditionally docked to avoid injuries.

Differences from wolves
Despite their close genetic relationship and interbreedability, there are several diagnostic features to distinguish gray wolves from domestic dogs. Domestic dogs are distinguishable from wolves by starch gel electrophoresis of red blood cell acid phosphatase.Elliot, D.G., and M. Wong. 1972. "Acid phosphatase, handy enzyme that separates the dog from the wolf". Acta Biologica et Medica Germanica 28:957–962 The tympanic bullae are large, convex, and almost spherical in gray wolves, while the bullae of dogs are smaller, compressed, and slightly crumpled. Compared with equally-sized wolves, dogs tend to have 20% smaller skulls and 30% smaller brains. The teeth of gray wolves are also proportionately larger than those of dogs.Clutton-Brock, Juliet (1987). A Natural History of Domesticated Mammals. British Museum (Natural History), p. 24,  Dogs have a more domed forehead and a distinctive "stop" between the forehead and nose. The temporalis muscle that closes the jaws is more robust in wolves. Wolves do not have dewclaws on their back legs unless there has been admixture with dogs that had them.  Most dogs lack a functioning pre-caudal gland and enter estrus twice yearly, unlike gray wolves, which only do so once a year. So-called primitive dogs such as dingos and Basenjis retain the yearly estrus cycle.

Dogs generally have brown eyes and wolves almost always have amber or light-colored eyes. Domestic dogs' skin tends to be thicker than that of wolves, with some Inuit tribes favoring the former for use as clothing due to its greater resistance to wear and tear in harsh weather. The paws of a dog are half the size of a wolf and their tails tend to curl upwards, another trait not found in wolves. The dog has developed into hundreds of varied breeds and shows more behavioral and morphological variation than any other land mammal.

Health

Many household plants are poisonous to dogs, including Begonia, Poinsettia and Aloe vera.

Some breeds of dogs are prone to specific genetic ailments such as elbow and hip dysplasia, blindness, deafness, pulmonic stenosis, cleft palate and trick knees. Two severe medical conditions significantly affecting dogs are pyometra, affecting unspayed females of all breeds and ages, and Gastric dilatation volvulus (bloat), which affects the larger breeds or deep-chested dogs. Both of these are acute conditions and can kill rapidly. Dogs are also susceptible to parasites such as fleas, ticks, mites, hookworms, tapeworms, roundworms and heartworms, which is a roundworm species that lives in the hearts of dogs.

Several human foods and household ingestibles are toxic to dogs, including chocolate solids, causing theobromine poisoning, onions and garlic, causing thiosulphate, sulfoxide or disulfide poisoning,Sources vary on which of these are considered the most significant toxic item. grapes and raisins, macadamia nuts, xylitol, as well as various plants and other potentially ingested materials. The nicotine in tobacco can also be dangerous to dogs. Signs of ingestion can include vomiting of large amounts (e.g., from eating cigar butts) or diarrhea. Some other symptoms are abdominal pain, loss of coordination, collapse, or death. Dogs are susceptible to theobromine poisoning, typically from the ingestion of chocolate. 

Dogs are also vulnerable to some of the same health conditions as humans, including diabetes, dental and heart disease, epilepsy, cancer, hypothyroidism and arthritis.

Lifespan

thumb|left|Mixed-breed dogs such as this terrier mix have been found to run faster and live longer than their purebred parents (see Heterosis)

In 2013, a study found that mixed-breed dogs live on average 1.2 years longer than purebred dogs. Increasing body-weight was negatively correlated with longevity (i.e., the heavier the dog, the shorter its lifespan).

The typical lifespan of dogs varies widely among breeds, but for most, the median longevity, the age at which half the dogs in a population have died and half are still alive, ranges from 10 to 13 years. Individual dogs may live well beyond the median of their breed.

The breed with the shortest lifespan (among breeds for which there is a questionnaire survey with reasonable sample size) is the Dogue de Bordeaux, with a median longevity of about 5.2 years. Still, several breeds, including Miniature Bull Terriers, Bloodhounds and Irish Wolfhounds are nearly as short-lived, with median longevities of 6 to 7 years.

The longest-lived breeds, including Toy Poodles, Japanese Spitz, Border Terriers and Tibetan Spaniels, have median longevities of 14 to 15 years. The median longevity of mixed-breed dogs, taken as an average of all sizes, is one or more years longer than that of purebred dogs when all breeds are averaged. The longest-lived dog was "Bluey," an Australian Cattle Dog who died in 1939 at 29.5 years of age.Edition included both Bluey and Taffy.

Reproduction

thumb|A female dog nursing newborn puppies
In domestic dogs, sexual maturity happens around 6 months to 1 year for both males and females, although this can be delayed until up to 2 years old for some large breeds, and is the time at which female dogs will have their first estrous cycle. They will experience subsequent estrous cycles semiannually, during which the body prepares for pregnancy. At the peak of the cycle, females will become estrus, mentally and physically receptive to copulation. Because the ova survive and can be fertilized for a week after ovulation, more than one male can sire the same litter.

Fertilization typically occurs 2–5 days after ovulation; 14–16 days after ovulation, the embryo attaches to the uterus and after 7-8 more days, the heartbeat is detectable.

Dogs bear their litters roughly 58 to 68 days after fertilization, with an average of 63 days, although the length of gestation can vary. An average litter consists of about six. puppies,

Neutering
right|thumb|A feral dog from Sri Lanka nursing very well-developed puppies

Neutering refers to the sterilization of animals, usually by removing the male's testicles or the female's ovaries and uterus, to eliminate the ability to procreate and reduce sex drive. Because of dogs' overpopulation in some countries, many animal control agencies, such as the American Society for the Prevention of Cruelty to Animals (ASPCA), advise that dogs not intended for further breeding should be neutered. That way, they do not have undesired puppies that may later be euthanized.

According to the Humane Society of the United States, 3–4 million dogs and cats are euthanized each year.  Many more are confined to cages in shelters because there are many more animals than there are homes. Spaying or castrating dogs helps keep overpopulation down.

Neutering reduces problems caused by hypersexuality, especially in male dogs. Spayed female dogs are less likely to develop cancer, affecting the mammary glands, ovaries and other reproductive organs. However, neutering increases the risk of urinary incontinence in female dogs and prostate cancer in males and osteosarcoma, hemangiosarcoma, cruciate ligament rupture, obesity and diabetes mellitus in either sex.

Inbreeding depression
A common breeding practice for pet dogs is mating between close relatives (e.g., between half- and full siblings). Inbreeding depression is considered to be due mainly to the expression of homozygous deleterious recessive mutations. Outcrossing between unrelated individuals, including dogs of different breeds, results in the beneficial masking of deleterious recessive mutations in progeny.

In a study of seven dog breeds (the Bernese Mountain Dog, Basset Hound, Cairn Terrier, Brittany, German Shepherd Dog, Leonberger and West Highland White Terrier), it was found that inbreeding decreases litter size and survival. Another analysis of data on 42,855 Dachshund litters found that as the inbreeding coefficient increased, litter size decreased and the percentage of stillborn puppies increased, thus indicating inbreeding depression. In a study of Boxer litters, 22% of puppies died before reaching 7 weeks of age. Stillbirth was the most frequent cause of death, followed by infection. Mortality due to infection increased significantly with increases in inbreeding.

Intelligence, behavior and communication
Intelligence

Dog intelligence is the dog's ability to perceive information and retain it as knowledge for applying to solve problems. Studies of two dogs suggest that dogs can learn by inference and have advanced memory skills.  A study with Rico, a Border Collie, showed that he knew the labels of over 200 different items. He inferred the names of novel things by exclusion learning and correctly retrieved those new items immediately and four weeks after the initial exposure. A study of another Border Collie, "Chaser," documented his learning and memory capabilities. He had learned the names and could associate by verbal command over 1,000 words. Dogs can read and react appropriately to human body language such as gesturing and pointing and human voice commands.

A 2018 study on canine cognitive abilities found that dogs' capabilities are no more exceptional than those of other animals, such as horses, chimpanzees, or cats. Various animals, including pigs, pigeons and chimpanzees, can remember the "what, where and when" of an event, which dogs cannot do.

Dogs demonstrate a theory of mind by engaging in deception. An experimental study showed compelling evidence that Australian dingos can outperform domestic dogs in non-social problem-solving, indicating that domestic dogs may have lost much of their original problem-solving abilities once they joined up with humans. Another study revealed that after undergoing training to solve a simple manipulation task, dogs faced with an insoluble version of the same problem look at the human, while socialized wolves do not.

Behavior

Dog behavior is the internally coordinated responses (actions or inactions) of the domestic dog (individuals or groups) to internal and external stimuli. As the oldest domesticated species, dogs' minds inevitably have been shaped by millennia of contact with humans. As a result of this physical and social evolution, dogs have acquired the ability to understand and communicate with humans more than any other species and they are uniquely attuned to human behaviors. Behavioral scientists have uncovered a surprising set of social-cognitive abilities in the domestic dog. These abilities are not possessed by the dog's closest canine relatives or other highly intelligent mammals, such as great apes, but rather parallel to children's social-cognitive skills.

Unlike other domestic species selected for production-related traits, dogs were initially selected for their behaviors.Serpell J, Duffy D. Dog Breeds and Their Behavior. In: Domestic Dog Cognition and Behavior. Berlin, Heidelberg: Springer; 2014 In 2016, a study found that only 11 fixed genes showed variation between wolves and dogs. These gene variations were unlikely to have been the result of natural evolution and indicate selection on both morphology and behavior during dog domestication. These genes have been shown to affect the catecholamine synthesis pathway, with the majority of the genes affecting the fight-or-flight responseAlmada RC, Coimbra NC. Recruitment of striatonigral disinhibitory and nigrotectal inhibitory GABAergic pathways during the organization of defensive behavior by mice in a dangerous environment with the venomous snake Bothrops alternatus [ Reptilia, Viperidae ] Synapse 2015:n/a–n/a (i.e., selection for tameness) and emotional processing. Dogs generally show reduced fear and aggression compared with wolves.Coppinger R, Schneider R: Evolution of working dogs. The domestic dog: Its evolution, behaviour and interactions with people. Cambridge: Cambridge University press, 1995. Some of these genes have been associated with aggression in some dog breeds, indicating their importance in both the initial domestication and later in breed formation. Traits of high sociability and lack of fear in dogs may include genetic modifications related to Williams-Beuren syndrome in humans, which cause hypersociability at the expense of problem-solving ability.

Communication

Dog communication is how dogs convey information to other dogs, understand messages from humans and translate the information that dogs are transmitting.Coren, Stanley How To Speak Dog: Mastering the Art of Dog-Human Communication, 2000 Simon & Schuster, New York. Communication behaviors of dogs include eye gaze, facial expression, vocalization, body posture (including movements of bodies and limbs) and gustatory communication (scents, pheromones, and taste). Humans communicate to dogs by using vocalization, hand signals and body posture.

Ecology
Population
The dog is probably the most widely abundant large carnivoran living in the human environment.
In 2013, an estimated global dog population was between 700 million and 987 million. Although it is said that the "dog is man's best friend," this refers mainly to the ~20% of dogs that live in developed countries. In the developing world, dogs are more commonly feral or communally owned, with pet dogs uncommon. Most of these dogs live their lives as scavengers and have never been owned by humans, with one study showing their most common response when approached by strangers is to run away (52%) or respond aggressively (11%). Little is known about these dogs, or the dogs in developed countries that are feral, strays, or are in shelters because the great majority of modern research on dog cognition has focused on pet dogs living in human homes.

Competitors and predators
Although dogs are the most abundant and widely distributed terrestrial carnivores, feral and free-ranging dogs' potential to compete with other large carnivores is limited by their strong association with humans. For example, a review of the studies in dogs' competitive effects on sympatric carnivores did not mention any research on competition between dogs and wolves. Although wolves are known to kill dogs, they tend to live in pairs or in small packs in areas where they are highly persecuted, giving them a disadvantage facing large dog groups.

Wolves kill dogs wherever they are found together. One study reported that in Wisconsin, in 1999, more compensation had been paid for losses due to wolves taking dogs than for wolves taking livestock. In Wisconsin, wolves will often kill hunting dogs, possibly because they are in the wolves' territory. A strategy has been reported in Russia where one wolf lures a dog into a heavy brush where another wolf waits in ambush. In some instances, wolves have displayed an uncharacteristic fearlessness of humans and buildings when attacking dogs to the extent that they have to be beaten off or killed. Although the numbers of dogs killed each year are relatively low, it induces a fear of wolves entering villages and farmyards to take dogs and losses of dogs to wolves have led to demands for more liberal wolf hunting regulations.

Coyotes and big cats have also been known to attack dogs. In particular, leopards are known to have a preference for dogs and have been recorded to kill and consume them, no matter what their size. Tigers in Manchuria, Indochina, Indonesia and Malaysia are also reported to kill dogs. Striped hyenas are known to kill dogs in Turkmenistan, India and the Caucasus.

Diet

thumb|left|upright|A Golden Retriever gnawing on a pig's foot

Dogs have been described as omnivores. Compared to wolves, dogs from agricultural societies have extra copies of amylase and other genes involved in starch digestion that contribute to an increased ability to thrive on a starch-rich diet. Similar to humans, some dog breeds produce amylase in their saliva and are classified as having a high starch diet. However, more like cats and less like other omnivores, dogs can only produce bile acid with taurine and they cannot produce vitamin D, which they obtain from animal flesh. Also, more like cats, dogs require arginine to maintain its nitrogen balance. These nutritional requirements place dogs halfway between carnivores and omnivores.

Range
As a domesticated or semi-domesticated animal, the dog is nearly universal among human societies. Notable exceptions once included:
 The Aboriginal Tasmanians, who were separated from Australia before the arrival of dingos on that continent
 The Andamanese peoples, who were isolated when rising sea levels covered the land bridge to Myanmar
 The Fuegians, who instead domesticated the Fuegian dog, a different canid species
 Individual Pacific islands whose maritime settlers did not bring dogs, or where dogs died out after original settlement, notably the Mariana Islands, Palau,the Marshall Islands,the Gilbert Islands, New Caledonia, Vanuatu, Tonga, Marquesas, Mangaia in the Cook Islands, Rapa Iti in French Polynesia, Easter Island,the Chatham Islands and Pitcairn Island (settled by the Bounty mutineers, who killed off their dogs to escape discovery by passing ships).

Dogs were introduced to Antarctica as sled dogs, but were later outlawed by international agreement due to the possible risk of spreading infections."Did you know that dogs are banned from Antarctica?", South Pole 1911-2011

Breeds

thumb|Different dog breeds show a range of phenotypic variation

Over the past 200 years, dogs have undergone rapid phenotypic change and were formed into today's modern dog breeds due to artificial selection by humans. These breeds can vary in size and weight from a  teacup Poodle to a  giant Mastiff. Phenotypic variation can include height measured to the withers ranging from  in the Chihuahua to  in the Irish Wolfhound. Color varies from white through grays (usually called "blue") to black, and browns from light (tan) to dark ("red" or "chocolate") in a wide variety of patterns; coats can be short or long, coarse-haired to wool-like, straight, curly, rough or smooth. The skull, body, and limb proportions vary significantly between breeds, with dogs displaying more phenotypic diversity than can be found within carnivores' entire order. Some breeds demonstrate outstanding skills in herding, retrieving, scent detection and guarding, demonstrating the functional and behavioral diversity of dogs. The first dogs were domesticated from shared ancestors of modern wolves; however, the phenotypic changes that coincided with the dog-wolf genetic divergence are unknown.

Roles with humans
thumb|upright|Gunnar Kaasen and Balto, the lead dog on the last relay team of the 1925 serum run to Nome

Domestic dogs inherited complex behaviors, such as bite inhibition, from their wolf ancestors, which would have been pack hunters with a complex body language. These sophisticated forms of social cognition and communication may account for their trainability, playfulness and ability to fit into human households and social situations. These attributes have given dogs a relationship with humans that has enabled them to become one of the most successful animals today.

The dogs' value to early human hunter-gatherers led to them quickly becoming ubiquitous across world cultures. Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and the military, companionship and aiding disabled individuals. This influence on human society has given them the nickname "man's best friend" in the Western world. In some cultures, however, dogs are also a source of meat.

Early roles
Wolves, and their dog descendants, likely derived significant benefits from living in human camps – more safety, more reliable food, lesser caloric needs and more chances to breed. They would have benefited from humans' upright gait that gives them a more extensive range over which to see potential predators and prey and better color vision that, at least by day, gives humans better visual discrimination. Camp dogs would also have benefited from human tool use, bringing down larger prey and controlling fire for various purposes.

Humans would also have derived enormous benefit from the dogs associated with their camps. For instance, dogs would have improved sanitation by cleaning up food scraps. Dogs may have provided warmth, as referred to in the Australian Aboriginal expression "three dog night" (a frigidly cold night). They would have alerted the camp to predators or strangers, using their acute hearing to provide an early warning.

It has been suggested that the most significant benefit would have been the use of dogs' robust sense of smell to assist with the hunt. The relationship between a dog's presence and success in the hunt is often mentioned as a primary reason for the domestication of the wolf and a 2004 study of hunter groups with and without a dog gives quantitative support to the hypothesis that the benefits of cooperative hunting were an essential factor in wolf domestication.

The cohabitation of dogs and humans likely improved the chances of survival for early human groups. The domestication of dogs may have been one of the key forces that led to human success.

Human emigrants from Siberia that came across the Bering land bridge into North America likely had dogs in their company. Although one writerA History of Dogs in the Early Americas, Marion Schwartz, 1998, 260 p., , Yale University Press even suggests that sled dogs' use may have been critical to the success of the waves that entered North America roughly 12,000 years ago, the earliest archaeological evidence of dog-like canids in North America dates from about 9,400 years ago. Dogs were an important part of life for the Athabascan population in North America and were their only domesticated animal. Dogs as pack animals may have contributed to the Apache and Navajo tribes' migration 1,400 years ago. This use of dogs in these cultures often persisted after the introduction of the horse to North America. p. 12

As pets
thumb|upright|A green velvet dog collar, dating from 1670 to 1690
thumb|Siberian Huskies are pack animals that still enjoy some human companionship
thumb|alt= Couple sitting on the lawn with a pet British Bulldog|A Bulldog relaxes at a park

It is estimated that three-quarters of the world's dog population lives in the developing world as feral, village, or community dogs, with pet dogs uncommon.

"The most widespread form of interspecies bonding occurs between humans and dogs" and the keeping of dogs as companions, particularly by elites, has a long history (see the Bonn–Oberkassel dog). Pet dog populations grew significantly after World War II as suburbanization increased. In the 1950s and 1960s, dogs were kept outside more often than they tend to be today(the expression "in the doghouse" - recorded since 1932 - to describe exclusion from the group implies a distance between the doghouse and the home) and were still primarily functional, acting as a guard, children's playmate, or walking companion. From the 1980s, there have been changes in the pet dog's role, such as the increased role of dogs in the emotional support of their human guardians. People and their dogs have become increasingly integrated and implicated in each other's livesto the point where pet dogs actively shape how a family and home are experienced.

There have been two significant trends occurring within the second half of the 20th century in pet dogs' changing status. The first has been "commodification," shaping it to conform to social expectations of personality and behavior.The second has been the broadening of the family's concept and the home to include dogs-as-dogs within everyday routines and practices.

A vast range of commodity forms aims to transform a pet dog into an ideal companion. The list of goods, services, and places available is enormous: from dog perfumes, couture, furniture and housing to dog groomers, therapists, trainers and caretakers, dog cafes, spas, parks and beaches and dog hotels, airlines and cemeteries. While dog training as an organized activity has operated since the 18th century, it became a high-profile issue in the last decades of the 20th century. Many normal dog behaviors such as barking, jumping up, digging, rolling in dung, fighting and urine marking (which dogs do to establish territory through scent) became increasingly incompatible with a pet dog's new role. Dog training books, classes, and television programs proliferated as the process of commodifying the pet dog continued.

The majority of contemporary dog owners describe their pet as part of the family, although some ambivalence about the relationship is evident in the popular reconceptualization of the dog-human family as a pack. Some dog trainers, such as on the television program Dog Whisperer, have promoted a dominance model of dog-human relationships. However, it has been disputed that "trying to achieve status" is characteristic of dog-human interactions. Pet dogs play an active role in family life; for example, a study of conversations in dog-human families showed how family members use the dog as a resource, talking to the dog, or talking through the dog; to mediate their interactions with each other.

Increasingly, human family-members engage in activities centered on the dog's perceived needs and interests, or in which the dog is an integral partner, such as dog dancing and dog yoga.

According to statistics published by the American Pet Products Manufacturers Association in the National Pet Owner Survey in 2009–2010, an estimated 77.5 million people in the United States have pet dogs. The same source shows that nearly 40% of American households own at least one dog, of which 67% own just one dog, 25% two dogs and nearly 9% more than two dogs. There does not seem to be any gender preference among dogs as pets, as the statistical data reveal an equal number of male and female pet dogs. Although several programs promote pet adoption, less than one-fifth of the owned dogs come from shelters.

A study using magnetic resonance imaging (MRI) to compare humans and dogs showed that dogs have the same response to voices and use the same parts of the brain as humans do. This gives dogs the ability to recognize human emotional sounds, making them friendly social pets to humans.

Work
Dogs have lived and worked with humans in many roles. In addition to dogs' role as companion animals, dogs have been bred for herding livestock (collies, sheepdogs),Dewey, T. and S. Bhagat. 2002. "Canis lupus familiaris, Animal Diversity Web.  hunting (hounds, pointers) and rodent control (terriers). Other types of working dogs include search and rescue dogs,Vikki Fenton, The use of dogs in search, rescue and recovery, Journal of Wilderness Medicine Vol. 3, Issue 3, August 1992, pp. 292–300. detection dogs trained to detect illicit drugsJohn J. Ensminger, Police and Military Dogs: Criminal Detection, Forensic Evidence, and Judicial Admissibility (CRC Press, 2012). or chemical weapons;Philip Shernomay, Dogs Take Their Place in Arsenal Against Chemical Attack, New York Times (13 May 2003). guard dogs; dogs who assist fishermen with the use of nets; and dogs that pull loads. In 1957, the dog Laika became the first animal to be launched into Earth orbit, aboard the Soviets' Sputnik 2; she died during the flight.

Various kinds of service dogs and assistance dogs, including guide dogs, hearing dogs, mobility assistance dogs and psychiatric service dogs, assist individuals with disabilities. Some dogs owned by people with epilepsy have been shown to alert their handler when the handler shows signs of an impending seizure, sometimes well in advance of onset, allowing the guardian to seek safety, medication, or medical care.

Sports and shows

People often enter their dogs in competitions, such as breed-conformation shows or sports, including racing, sledding and agility competitions.

In conformation shows, also referred to as breed shows, a judge familiar with the specific dog breed evaluates individual purebred dogs for conformity with their established breed type as described in the breed standard. As the breed standard only deals with the dog's externally observable qualities (such as appearance, movement and temperament), separately tested qualities (such as ability or health) are not part of the judging in conformation shows.

As food

Dog meat is consumed in some East Asian countries, including Korea, China Vietnam and the Philippines, which dates back to antiquity. It is estimated that 13–16 million dogs are killed and consumed in Asia every year. In China, debates have ensued over banning the consumption of dog meat.  Following the Sui and Tang dynasties of the first millennium, however, people living on northern China's plains began to eschew eating dogs, which is likely due to Buddhism and Islam's spread, two religions that forbade the consumption of certain animals, including the dog. As members of the upper classes shunned dog meat, it gradually became a social taboo to eat it, even though the general population continued to consume it for centuries afterward. Dog meat is also consumed in some parts of Switzerland. Other cultures, such as Polynesia and pre-Columbian Mexico, also consumed dog meat in their history. Dog fat is also reportedly believed to be beneficial for the lungs in some parts of Poland and Central Asia. Proponents of eating dog meat have argued that placing a distinction between livestock and dogs is Western hypocrisy and that there is no difference in eating different animals' meat.

In Korea, the primary dog breed raised for meat, the Nureongi (누렁이), differs from those breeds raised for pets that Koreans may keep in their homes.Pettid, Michael J., Korean Cuisine: An Illustrated History, London: Reaktion Books Ltd., 2008, 25. 

The most popular Korean dog dish is gaejang-guk (also called bosintang), a spicy stew meant to balance the body's heat during the summer months. Followers of the custom claim this is done to ensure good health by balancing one's gi, or the body's vital energy. A 19th-century version of gaejang-guk explains that the dish is prepared by boiling dog meat with scallions and chili powder. Variations of the dish contain chicken and bamboo shoots. While the dishes are still prevalent in Korea with a segment of the population, dog is not as widely consumed as beef, pork and chicken.

Health risks to humans

In 2005, the WHO reported that 55,000 people died in Asia and Africa from rabies, a disease for which dogs are the most important vector.

Citing a 2008 study, the U.S. Centers for Disease Control estimated in 2015 that 4.5 million people in the U.S. are bitten by dogs each year. citing  A 2015 study estimated that 1.8% of the U.S. population is bitten each year. In the 1980s and 1990s, the U.S. averaged 17 fatalities per year, while since 2007, this has increased to an average of 31. 77% of dog bites are from the pets of family or friends and 50% of attacks occur on the dog's legal owner's property.

A Colorado study found bites in children were less severe than bites in adults. The incidence of dog bites in the U.S. is 12.9 per 10,000 inhabitants, but for boys aged 5 to 9, the incidence rate is 60.7 per 10,000. Moreover, children have a much higher chance of being bitten in the face or neck. Sharp claws with powerful muscles behind them can lacerate flesh in a scratch that can lead to serious infections.

In the U.K., between 2003 and 2004, there were 5,868 dog attacks on humans, resulting in 5,770 working days lost in sick leave.

In the U.S., cats and dogs are a factor in more than 86,000 falls each year. It has been estimated that around 2% of dog-related injuries treated in U.K. hospitals are domestic accidents. The same study found that while dog involvement in road traffic accidents was difficult to quantify, dog-associated road accidents involving injury more commonly involved two-wheeled vehicles.

Toxocara canis (dog roundworm) eggs in dog feces can cause toxocariasis. In the United States, about 10,000 cases of Toxocara infection are reported in humans each year, and almost 14% of the U.S. population is infected. Untreated toxocariasis can cause retinal damage and decreased vision. Dog feces can also contain hookworms that cause cutaneous larva migrans in humans.

Health benefits for humans
thumb|alt=Small dog laying between the hands|A Doberman Pinscher puppy being cuddled
thumb|A Miniature Poodle puppy begging for cuddles|338x338px

Dogs suffer from the same common disorders as humans; these include cancer, diabetes, heart disease and neurologic disorders. The pathology is similar to humans, as is their response to treatment and their outcomes. Researchers are identifying the genes associated with dog diseases similar to human disorders, but lack mouse models to find cures for both dogs and humans. The genes involved in canine obsessive-compulsive disorders led to the detection of four genes in humans' related pathways.

The scientific evidence is mixed as to whether a dog's companionship can enhance human physical health and psychological well-being. Studies suggesting that there are benefits to physical health and psychological well-being  have been criticized for being poorly controlled. It found that "the health of elderly people is related to their health habits and social supports but not to their ownership of, or attachment to, a companion animal." Earlier studies have shown that people who keep pet dogs or cats exhibit better mental and physical health than those who do not, making fewer visits to the doctor and being less likely to be on medication than non-guardians.

A 2005 paper states "recent research has failed to support earlier findings that pet ownership is associated with a reduced risk of cardiovascular disease, a reduced use of general practitioner services, or any psychological or physical benefits on health for community dwelling older people. Research has, however, pointed to significantly less absenteeism from school through sickness among children who live with pets." In one study, new guardians reported a highly significant reduction in minor health problems during the first month following pet acquisition. This effect was sustained in those with dogs through to the end of the study.

People with pet dogs took considerably more physical exercise than those with cats and those without pets. The results provide evidence that keeping pets may have positive effects on human health and behavior and that for guardians of dogs, these effects are relatively long-term. Pet guardianship has also been associated with increased coronary artery disease survival. Human guardians are significantly less likely to die within one year of an acute myocardial infarction than those who did not own dogs.

The health benefits of dogs can result from contact with dogs in general, not solely from having dogs as pets. For example, when in a pet dog's presence, people show reductions in cardiovascular, behavioral and psychological indicators of anxiety. Other health benefits are gained from exposure to immune-stimulating microorganisms, which can protect against allergies and autoimmune diseases according to the hygiene hypothesis. The benefits of contact with a dog also include social support, as dogs cannot only provide companionship and social support themselves but also act as facilitators of social interactions between humans. One study indicated that wheelchair users experience more positive social interactions with strangers when accompanied by a dog than when they are not. In 2015, a study found that pet owners were significantly more likely to get to know people in their neighborhood than non-pet owners.

Using dogs and other animals as a part of therapy dates back to the late 18th century, when animals were introduced into mental institutions to help socialize patients with mental disorders.Kruger, K.A. & Serpell, J.A. (2006). Animal-assisted interventions in mental health: Definitions and theoretical foundations, In Fine, A.H. (ed.), Handbook on animal-assisted therapy: Theoretical foundations and guidelines for practice. San Diego, CA, Academic Press: 21–38.  Animal-assisted intervention research has shown that animal-assisted therapy with a dog can increase social behaviors, such as smiling and laughing, among people with Alzheimer's disease. One study demonstrated that children with ADHD and conduct disorders who participated in an education program with dogs and other animals showed increased attendance, increased knowledge and skill objectives and decreased antisocial and violent behavior compared with those not in an animal-assisted program.

Terminology
 The term dog typically is applied to the species (or subspecies) as a whole and any adult male member of the same.
 An adult female is a bitch.
 An adult male capable of reproduction is a stud.
 An adult female capable of reproduction is a brood bitch.
 An immature male or female (that is, an animal not yet capable of reproduction) is a puppy or pup.
 A group of puppies from the same gestation period is a litter.
 The father of a litter is a sire.
 The mother of a litter is a dam.
 A group of any three or more adults is a pack.
 A pack leader is an alpha. Typically a pack will have either an individual alpha, or a male-female alpha pair.
 A pack member subservient to alphas is a beta.
 A pack member subservient to all others is an omega.

Cultural depictions
thumb|Figure of a dog, China 4th century BC, Brooklyn Museum

Mythology and religion
In ancient Mesopotamia, from the Old Babylonian period until the Neo-Babylonian, dogs were the symbol of Ninisina, the goddess of healing and medicine, and her worshippers frequently dedicated small models of seated dogs to her. In the Neo-Assyrian and Neo-Babylonian periods, dogs were used as emblems of magical protection.

In mythology, dogs often serve as pets or as watchdogs. Stories of dogs guarding the gates of the underworld recur throughout Indo-European mythologies and may originate from Proto-Indo-European religion. In Greek mythology, Cerberus is a three-headed, dragon-tailed watchdog who guards the gates of Hades. In Norse mythology, a bloody, four-eyed dog called Garmr guards Helheim. In Persian mythology, two four-eyed dogs guard the Chinvat Bridge. In Welsh mythology, Annwn is guarded by Cŵn Annwn. In Hindu mythology, Yama, the god of death, owns two watchdogs who have four eyes. They are said to watch over the gates of Naraka.

The hunter god Muthappan from the North Malabar region of Kerala has a hunting dog as his mount. Dogs are found in and out of the Muthappan Temple and offerings at the shrine take the form of bronze dog figurines. In Philippine mythology, Kimat, the pet of Tadaklan, the god of thunder, is responsible for lightning.

The dog's role in Chinese mythology includes a position as one of the 12 animals that cyclically represent years (the zodiacal dog). Three of the 88 constellations in Western astronomy also represent dogs:

 Canis Major (the Great Dog, whose brightest star, Sirius, is also called the Dog Star)
 Canis Minor (the Little Dog)
 Canes Venatici (the Hunting Dogs)

In Christianity, dogs represent faithfulness. Within the Roman Catholic denomination specifically, the iconography of Saint Dominic includes a dog, after the hallow's mother dreamt of a dog springing from her womb and becoming pregnant shortly after that. As such, the Dominican Order (Ecclesiastical Latin: Domini canis) means "dog of the Lord" or "hound of the Lord" (Ecclesiastical Latin: Domini canis). In Christian folklore, a church grim often takes the form of a black dog to guard Christian churches and their churchyards from sacrilege.

Jewish law does not prohibit keeping dogs and other pets. Jewish law requires Jews to feed dogs (and other animals that they own) before themselves and make arrangements for feeding them before obtaining them.

The view on dogs in Islam is mixed, with some schools of thought viewing it as unclean, although Khaled Abou El Fadl states that this view is based on "pre-Islamic Arab mythology" and "a tradition to be falsely attributed to the Prophet." Therefore, Sunni Malaki and Hanafi jurists permit the trade of and keeping of dogs as pets.

In China, Korea and Japan, dogs are viewed as kind protectors.

Literature
In Homer's epic poem the Odyssey, when the disguised Odysseus returns home after 20 years, he is recognized only by his faithful dog, Argos, who has been waiting all this time for his return.

 Art 

Cultural depictions of dogs in art became more elaborate as individual breeds evolved and the relationships between human and canine developed. Hunting scenes were popular in the Middle Ages and the Renaissance. Dogs were depicted to symbolize guidance, protection, loyalty, fidelity, faithfulness, watchfulness and love.

 See also 

 Animal track
 Cynanthropy
 Dog–cat relationship
 Dog odor
 Dognapping
 Ethnocynology
 Hachikō – a notable example of dog loyalty
 Pet recovery service
 Domesticated silver fox

 Lists 
 Lists of dogs
 List of fictional dogs
 List of individual dogs

References

Bibliography

Further reading
 

External links

 Biodiversity Heritage Library bibliography for Canis lupus familiaris
 Fédération Cynologique Internationale (FCI) – World Canine Organisation
 Dogs in the Ancient World, an article on the history of dogs
 View the dog genome on Ensembl

Category:Dogs
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Category:Cosmopolitan vertebrates
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Category:Cosmopolitan species
Category:Animal models
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Category:Mammals described in 1758
Category:Taxa named by Carl Linnaeus