Why is no mammal coloured green?

(writing in progress)

"We see the world not as it is, but as we are"

Many mammals have colouration adapted for inconspicuousness.

Seasonal changes in the colouration of the pelage are common in mammals. Why not green in the green season?

Furthermore, certain species of mammals are colour-polymorphic. A green morph would seem to be a particularly good idea for many mammals.

Mammals in which it is particularly puzzling that the pelage is never green, at least in part, include

  • Giraffa,
  • spotted forms and growth-stages of deer (particularly infants, and small species in summer pelage, e.g. Capreolus capreolus),
  • striped and spotted bovids such as Tragelaphus, and
  • diurnal rodents in marshes, e.g. Otomys,
  • voles (e.g. Microtus) in summer pelage, and
  • sloths.

Equids are generally surrounded by green, and have eyesnthat are prooirtiinately the largest of any land animal. Yet they are unable to see green and are not coloured to blend into green backgrounds. Instead, equids see the wavelengths known to humans as blue and yellow.

The answer is partly that

  • there is no green pigment in animals
  • most mammals do not see green (viz electromagnetic wavelength 500-550 nanometres)
  • many mammals are nocturnal, active when all colours are difficult to see
  • most mammals have a strong sense of smell, so hardly need to see all the colours as we do
  • most mammals are so active that crypsis is of limited value.

The key to understanding the lack of green in mammals is that the failure to see green is more than made up by other wavelengths of light that mammals can see, but humans cannot see.

For example, ultraviolet is probably visible to many mammals other than humans. Mammals are preoccupied with colours other than free, and they may even see some of these as green.

We could investigate this, beginning with 'colour-blind' persons. Some human individuals are born with dichromatic vision. It is easy to assume that dichromats see less than trichromats, but perhaps they just see differently.

So, most mammals may as well be green, because to mammalian predators they blend into the green background anyway, and to avian and reptilian predators they will be detected anyway, by other means.

(writing in progress)

Posted on 26 December, 2022 05:52 by milewski milewski


After a talk on this topic that I presented to the Western Australia Naturalists' Club (Northern suburbs of Perth) on 21 March 2006, an elderly lady in the audience reported that, at the height of net grieving after a bereavement, she lost all colour-vision, for several days. Has this ever been recorded in the annals of medicine?

Posted by milewski over 1 year ago

The idea that large eyes are associated with nocturnal vision, although fairly anchored among naturalists, May be misleading. Mice and rats have a nocturnal-type retina (e.g. many rods, few ganglion cells, etc.), but small eyes. By contrast, certain birds and reptiles, with diurnal-type visual systems, have extremely large eyes. The latter may not seem large, but their true size is revealed on dissection. Are the eyes of nocturnal birds and reptiles even larger?

Posted by milewski over 1 year ago

A crucial concept is that there is room for only so many capacities in single species or body. The animal cannot do everything, but must choose a strategy. Most mammals have 'traded in the capacity to see green for more useful capacities, e.g. the ability to see clearly at night, times ultraviolet, to see panoramically, to detect the slightest movement, etc. Their world is presumably just as 'colourful', in its own way, as ours.

So, the question can be rephrased: Does the failure of most mammals to see green means a smaller sensory scope than in humans, or is it the human emphasis on green that is limited?

The short answer may be that most mammals can probably see (or at least sense) more than humans can, so that our ability to see green is, as it were, a 'red herring'.

Posted by milewski over 1 year ago

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