The monarch butterfuly (Danaus plexippus) is not at all nice to eat.
In fact, its chemical defences are so effective that a predator stands a
good chance of dying a horrible death, sometimes in mere minutes. Despite
this, however, one species – a mouse that lives in Mexico – has adapted
so that it can live exclusively on the poisonous butterflies.
John Glendinning and Lincoln Brower of the University of Florida studied
the mouse, Peromyscus melanotis. They found that in winter it migrates to
the mountainsides in central Mexico where monarchs flock in enormous numbers
at the same time. The mice then dine on the butterflies, which are nutritious
because they are rich in substances known as lipids (Journal of Animal Ecology,
vol 59, p 1091).
Between November and April each year, monarch butterflies, which have
a wingspan of about 12 centimetres, gather in concentrations of about 10
million per hectare. The monarchs are easy prey for the mice because the
low temperatures on the mountainsides – usually below 6 °C – make the
butterflies moribund. They fall from the trees to the forest floor, and
are usually unable to fly or climb back to their roosts.
By feeding on monarchs, say Glendinning and Brower, P melanotis is able
to breed intensively during the winter, something that is impossible outside
the areas where the monarchs gather.
The biologists found that four other local species of mouse, which are
closely related to P melanotis, do not have the ability to live on the butterflies,
and do not migrate to the areas on the mountainsides where the butterflies
The monarch butterfly feeds on milkweed (Asclepias), from which it extracts
chemicals known as cardiac glycosides (cardenolides) and pyrrolizidine alkaloids.
These chemicals, stored in its body tissue, are the basis of defence. The
cardenolides, which act within a few minutes, usually cause vomiting, heart
failure, and neurotoxicity. The alkaloids act over a longer period, and
cause liver and lung damage. Not surprisingly, most would-be predators soon
learn to avoid the monarch butterflies.
So how does P melanotis manage to avoid the toxic effects of the monarch’s
cardenolides? The answer is not yet precisely known. According to Glendinning
and Brower, the mouse does not appear to metabolise the toxins in its stomach.
But the biologists did notice that the mice usually eat only the internal
tissues of the monarchs, which are low in cardenolides. They reject the
cuticles, which are cardenolide-rich.
Glendinning and Brower estimate that the mice eat only about 10 per
cent of the butterflies on the forest floor. There are such huge numbers
of them that the mice can afford to discard the cuticles.
Although the chemical defences of the monarchs have been breached by
one species of mouse, they are still effective against the others, and against
35 out of 37 local birds. This means predators at present post no threat
to the survival of the local monarch population. This could change, however,
if other species evolve the ability to live on monarchs.
The advantage to P melanotis seems to have been considerable because
it is now the most successful species of mouse in the entire area.
Mnyama anayeitwa monarch butterfuly (Danaus plexippus) si mzuri kabisa kula. Kwa kweli, ulinzi wake wa kemikali ni wenye matokeo sana hivi kwamba mnyama-mwitu ana nafasi nzuri ya kufa kifo chenye kuogopesha, nyakati nyingine kwa dakika chache tu. Hata hivyo, spishi moja - panya anayeishi Mexico - imejipatanisha ili iweze kuishi tu kwa kutumia vipepeo wenye sumu. John Glendinning na Lincoln Brower wa Chuo Kikuu cha Florida walichunguza panya, Peromyscus melanotis. Katika majira ya baridi kali, huelekea kwenye milima ya Mexico, ambapo huja kwa wingi. "Mbwa huyo anaitwa ""Mbuzi,"" na ni muhimu kwa sababu ana mafuta mengi (Journal of Animal Ecology, Volume 59, ukurasa wa 109)." Katika kipindi cha kati ya Novemba na Aprili kila mwaka, vipepeo wa kifalme, ambao wana upana wa mabawa ya karibu sentimita 12, hukusanyika kwa kiasi cha karibu milioni 10 kwa hekta. Monarchs ni mawindo rahisi kwa panya kwa sababu joto la chini kwenye miamba - kwa kawaida chini ya 6 ° C - hufanya vipepeo kufa. Wao huanguka kutoka kwenye miti hadi kwenye sakafu ya msitu, na kwa kawaida hawawezi kuruka au kupanda kurudi kwenye makao yao. Kwa kula monarchs, Glendinning na Brower wanasema, P. melanotis ina uwezo wa kuzaliana kwa bidii wakati wa majira ya baridi, kitu ambacho haiwezekani nje ya maeneo ambapo monarchs hukusanyika. Wataalamu wa viumbe wa asili wamegundua kwamba aina nyingine nne za panya, ambazo zina uhusiano wa karibu na P. melanotis, hazina uwezo wa kuishi kwenye vipepeo, na hazihamishi kwenye maeneo ya milimani ambapo vipepeo huishi. Kemikali hizo, zilizowekwa katika tishu za mwili wake, ndizo msingi wa ulinzi. Cardenolides, ambazo hufanya kazi ndani ya dakika chache, kwa kawaida husababisha kutapika, kushindwa kwa moyo, na sumu ya neva. Alkaloids hufanya kazi kwa muda mrefu, na kusababisha ini na mapafu uharibifu. Haishangazi kwamba wengi wa wale wanaotaka kuwa wawindaji hujifunza haraka kuepuka vipepeo hao. Kwa hiyo, P. melanotis hufauluje kuepuka madhara ya sumu ya cardenolides ya mfalme? Jibu bado halijulikani kwa usahihi. Kulingana na Glendinning na Brower, panya haionekani kuchanganua sumu katika tumbo lake. Wataalamu wa viumbe walichunguza wanyama hao na kugundua kwamba kwa kawaida panya hao hula tishu za ndani tu za wanyama hao, ambazo zina kiasi kidogo cha cardenolides. Wao hukataa vifuniko vya ngozi, ambavyo vina kiasi kikubwa cha cardenolide. Glendinning na Brower wanakadiria kwamba panya hula asilimia 10 tu ya vipepeo walio kwenye sakafu ya msitu. Kuna idadi kubwa sana ya panya hao hivi kwamba wanaweza kuepuka kuondoa viungo hivyo. Ingawa ulinzi wa kemikali wa kifalme umevunjwa na aina moja ya panya, bado ni ufanisi dhidi ya wengine, na dhidi ya ndege 35 kati ya 37. Hilo lamaanisha kwamba kwa sasa wanyama hao wa kuwinda hawahatarini uhai wa jamii ya kifalme ya eneo hilo. Hata hivyo, hali hiyo yaweza kubadilika ikiwa spishi nyingine zitakuwa na uwezo wa kuishi juu ya viumbe hao. P. melanotis ni aina ya panya inayopatikana katika eneo hilo.