Why is the rocky mountain goat all-white?

The rocky mountain goat (Oreamnos americanus) is the only ungulate species on Earth which possesses all-white pelage, in both sexes and at all ages. What is the adaptive reason for this extreme specialisation?

The obvious answer is 'nivicolous crypsis', i.e. hiding in the snow and ice in the way epitomised by the polar bear (Ursus maritimus). However, the sheer number of photos of the rocky mountain goat in iNaturalist suggests that this interpretation is incorrect. How could it possibly be that a prey-animal coloured to blend into its typical environment is so easy to spot, and so extremely photogenic?

Instead, it should be clear from the thousands of photos of the rocky mountain goat on the internet that its white pelage advertises it in its normal surrounds, that it is particularly gregarious in winter, that it does not attempt to crouch or 'freeze' when alarmed, and that even the newborns do not hide in the way of most ungulates. The terrain chosen by the rocky mountain goat - even in winter - tends to be rocky and dark rather than snowy and pale, making the animals conspicuous.

The rocky mountain goat is one of the most extreme examples, among ungulates, of the female emulating the male in muscularity, weaponry (deadly horns), adornment (e.g. beard, mane, pantaloons), aggressiveness, and self-advertisement. A similar syndrome occurs in adaptively conspicuous bovids such as wildebeests (Connochaetes) and oryxes (Oryx). However, in the case of the rocky mountain goat the male is so intimidated by the female that his courtship requires juvenile-like postures of submission, and any naturalist familiar with the species knows how hard it can be to distinguish the males in any group.

The rocky mountain goat is so specialised for taking deliberate, sure-footed refuge from predators on cliffs that it has abandoned any attempt to hide, even at night or in infancy, and it has nearly abandoned running when alarmed. It seems to be compatible with this strategy that it has also feminised the bravado usually associated with masculinity in mammals, extending the silhouette by means of furry adornments to make the female figure big and noticeable.

I suggest that it is view of this combination of specialisations - anti-predator reliance on cliffs and maternal defence, going together with extreme 'feminist' self-assertiveness - that the conspicuous monotonal whiteness of the rocky mountain goat can best be understood. This species is all-white not to hide but instead to show off.

פורסם על-ידי milewski milewski, ספטמבר 18, 2020 11:02 אחה"צ

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