Why is the mountain goat all-white?

The mountain goat (Oreamnos americanus, https://en.wikipedia.org/wiki/Mountain_goat and https://www.youtube.com/watch?v=WneToesxptM) is the only ungulate species on Earth to possess all-whitish pelage, in both sexes and at all ages.

What is the adaptive reason for this extreme specialisation?

The obvious - and obviously incorrect - answer is 'nivicolous crypsis', i.e. hiding in the snow and ice (e.g. see https://www.agefotostock.com/age/en/details-photo/mountain-goat-animal-snow-winter-canada-white/H44-10971090) in the way epitomised by the polar bear (Ursus maritimus, https://en.wikipedia.org/wiki/Polar_bear).

Given the sheer number of photos of the mountain goat in iNaturalist, how plausible is it that a prey-animal so easy to spot (e.g. see https://www.inaturalist.org/observations/56806030), and so extremely photogenic, is coloured to blend into its typical environment?

Instead, it seems from innumerable photos that the mountain goat is advertised by its whitish pelage in its normal surrounds (e.g. https://www.inaturalist.org/observations/48673228).

For clues to the behaviour of this species, see https://www.earthtouchnews.com/natural-world/animal-behaviour/angry-mountain-goat-confronts-a-hiker-on-the-snowy-slopes/ and (with appropriate skepticism for the circumstances) https://www.youtube.com/watch?v=hy3lOaPfDC0.

The mountain goat seems so specialised for taking deliberate, sure-footed refuge from predators on rock-faces that it has abandoned attempts to hide, even at night or in infancy:

  • it is most gregarious in winter;
  • it does not attempt to crouch or 'freeze' when alarmed; and
  • even the newborns do not hide in the way of most ungulates.

The terrain chosen by the mountain goat tends - even in the cold season - to be rocky and dark rather than snowy and pale, making the animals noticeable (e.g. see https://www.inaturalist.org/observations/38942159 and https://www.inaturalist.org/observations/77400383).

The mountain goat is also one of the most extreme examples, among ungulates, of females emulating males in muscularity, aggressiveness, adornment (e.g. beard, mane, pantaloons), and self-advertisement. This species has feminised the macho, extending the silhouette by means of the fur to make even the female figure conspicuous (see https://www.youtube.com/watch?v=VsFlfqlKUk8 and https://www.youtube.com/watch?v=w3rFBxnJJoc).

Any naturalist familiar with the species knows how hard it can be to distinguish the males in any group (https://cpw.state.co.us/Documents/Hunting/HunterEd/MountainGoatGenderIDGuide.pdf).

Female emulation of males occurs also in other adaptively conspicuous bovids such as wildebeests (Connochaetes) and oryxes (Oryx). However, in the case of the mountain goat the horns are designed as sharp weapons rather than adornments or instruments of mere sparring - which differs from most ungulates.

Indeed, in this species the male is so intimidated by the female that his courtship requires juvenile-like postures of submission (see https://academic.oup.com/jmammal/article-abstract/45/4/551/852342?redirectedFrom=PDF and http://juneaunature.discoverysoutheast.org/content_item/mountain-goat-rut/).

The mountain goat has nearly abandoned running when alarmed; and mothers seem capable of defending infants (see https://www.livescience.com/mountain-goat-kills-grizzly-bear).

Surely it is in view of this syndrome - anti-predator reliance on cliffs, 'feminist' self-assertiveness, and maternal defence - that the conspicuous monotonal whitishness of the mountain goat can best be understood?

Is it not plausible that the mountain goat is all-white not to hide but instead to show off?

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

תגובות

Another reason to doubt that the colouration of the mountain goat functions cryptically is that it is not pale enough to blend into a background of snow (e.g. see https://www.inaturalist.org/observations/24793211).

פורסם על-ידי milewski לפני 4 חודשים (סמן)

In these surroundings https://www.inaturalist.org/observations/92626137 and https://www.inaturalist.org/observations/92526236, the mountain goat is certainly inconspicuous. However, they are not representative of the habitat in general.

פורסם על-ידי milewski לפני בערך חודש (סמן)

Around slopes in this area (southcentral Alaska) support both mountain goats and Dall (thin-horned) sheep. Both similar color, use similar habitats but the sheep have marked sexual dimorphism. Since the female emulation was almost half of your essay but does not apply to sheep could there be another explanation for the white fur?

פורסם על-ידי troydeclan לפני בערך חודש (סמן)

@troydeclan Many thanks for this interesting question. South-central Alaska is at the northern extreme of the range of the mountain goat (https://upload.wikimedia.org/wikipedia/commons/f/fd/Oreamnos_americanus_distribution.svg) but fairly central to the range of the whitish subspecies/ecotype of Dall's sheep (https://upload.wikimedia.org/wikipedia/commons/8/84/Ovis-dalli-map.png). Dall's sheep is, overall, less closely associated with cliffs than is the mountain goat. Dall's sheep differs socially and sexually from the mountain goat in that adult males live separately from females and juveniles for much of the year. What this suggests is that females of Dall's sheep congregate for anti-predator security in much the same way as, and for similar reasons to, typical 'plains game' of environments beyond the mountain meadows of the Arctic and subarctic. Many species of 'plains game' are adaptively conspicuous in the form of bold markings (particularly on the hindquarters), and in a sense the all-white subspecies/ecotype of Dall's sheep has just extended the bleeze to encompass the whole body. So my suggestion is: although it is true that, in your area, there is a coincidence in adaptive colouration between females of Dall's sheep and females of the mountain goat, the crucial adaptive advantages differ: conspicuousness for fugitive gregariousness in Dall's sheep vs conspicuousness for defensive/assertive individuality in the mountain goat. If so, this does raise the question of whether the mountain goat qualifies as aposematic. This is complicated, but I do suggest that the mountain goat can in a broader sense be said to have 'warning colouration'. Your further thoughts?

פורסם על-ידי milewski לפני בערך חודש (סמן)

This is a very interesting take. I can definitely see mountain goats blending in to pallid boulders, but agree that they are not pale enough to be camouflaged in the snow. I also believe that their success in introduced ranges, especially in the Wasatch Range of UT, as well as others throughout the states of UT and NV, is a statement to this. Some introduced populations live in small, isolated parts of the Wasatch on which the snow all but recedes completely during the height of summer (especially in recent years). How do they do so well if they can’t camouflage from mountain lions? This is a question I had not asked until now, but your write-up seems to provide a reasonable explanation. Good read, I certainly hadn’t considered the possible less-obvious reason for the mountain goat’s inconspicuous coat! Without the white coat I doubt I would ever spot them!! It is quite evident that snow is absolutely not a habitat requirement for these bovids. Thanks!

פורסם על-ידי grant_cactus לפני בערך חודש (סמן)

@grant_cactus Many thanks for your comment.

פורסם על-ידי milewski לפני בערך חודש (סמן)

Can't say I've noticed a lot of difference in the propensity to use cliff faces or in flock size between sheep and goats around here. One has to look close if they at a distance to verify which is which. I am not advocating that the white is for camouflage but I will note it is quite effective as such; the individuals need not blend in with snow (or pale rocks) they mimic the small snow patches scattered over the cliff faces.

פורסם על-ידי troydeclan לפני בערך חודש (סמן)

@troydeclan I appreciate the nuances you're bringing to this discussion. The situation in your area is indeed worth thinking more deeply about. Could you please specify the location?

פורסם על-ידי milewski לפני בערך חודש (סמן)

Southcentral Alaska, mostly Anchorage and Kenai Peninsula. Goats are much more restricted than sheep being more coastal. Near Anchorage goats are most easily seen in Crow Pass (where my image was from) and Portage. Sheep are present on most mountains.

פורסם על-ידי troydeclan לפני בערך חודש (סמן)

The following nicely illustrates how conspicuous the mountain goat can be even against a complicated background with variable perspective: https://www.inaturalist.org/observations/49296886.

פורסם על-ידי milewski לפני בערך חודש (סמן)

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