A little-known aspect of the erectile bleeze of the pronghorn

The pronghorn (Antilocapra americana) is a ruminant living in open vegetation. The large patch of whitish on its hindquarters, and broadly whitish flanks, make it conspicuous to scanning predators even at a distance (see https://www.worldwildlife.org/photos/pronghorn-antilocapra-americana--2 and https://allthingsforthewin.com/nature-all-around-us/). Upon alarm the fur on the hindquarters is erected to form a disc-like shape, revealing a white so pure that it seems almost luminous (see https://nhpbs.org/natureworks/pronghorn.htm).

What is not well-known is that the neck also contributes to this display. Under certain conditions, a dark patch appears on the back of the neck in conjunction with the piloerection on the hindquarters. The term 'mane' fails to do justice to the complexity of the structures (e.g. see https://www.inaturalist.org/observations/36998216, and my comment on that photo).

The mane of the pronghorn is unlike that of any other ungulate. Its peculiarities remain to be fully investigated, but three are relevant here. Firstly, it is more erectile than expected in ruminants, and virtually disappears when fully folded (see https://www.flickr.com/photos/33403047@N00/4190974216/ and https://www.alamy.com/stock-photo-pronghorn-antelope-young-antilocapra-americana-south-dakota-usa-twins-32554470.html). Secondly, it is proportionately larger in juveniles and females (see https://www.alamy.com/a-young-pronghorn-antelope-antilocapra-americana-buck-with-mane-and-rump-hair-raised-image212343642.html) than in adult males (see https://twitter.com/ironc0re/status/1188562672490438657). And thirdly, it seems to be activated in two different ways, and their intermediates: vertically erected to a tomahawk-shape (see https://www.inaturalist.org/observations/18386792 and https://gramho.com/media/2582497469476833997) or spread sideways into a broad dark patch.

My impression is that the mane of the pronghorn is displayed in different ways by adult males vs by juveniles/females. In the former, it is vertically erected (to a shape most visible in profile) as a minor component of display in certain postures of masculine rivalry and courtship. In juveniles/females, the circumstances of its vertical erection are poorly understood. Instead, the point of particular interest is that it seems to be spread laterally, to a shape most visible from behind. The posteriolateral bleeze is relevant to predators, and the spread mane adds a dark component for contrast (see https://www.nogalesinternational.com/news/41-pronghorn-antelope-have-new-home-in-santa-cruz-county/article_bec4033e-7ebe-11e3-82db-0019bb2963f4.html). This accentuation is consistent with the fact that stotting behaviour is shown more by juveniles than by adults.

If this interpretation is correct, then I would call the pattern seen in stotting juveniles a bimodal erectile posteriolateral bleeze. What are added by the erections/activations are the extension of white on the rump and haunches, and extensive darkness on the back of the neck.

How can a mane take such diverse forms? Is there a skin-fold on the nape of the pronghorn, similar to that on the rump of the springbok (Antidorcas marsupialis) albeit smaller and less noticeable to naturalists?

Publicado por milewski milewski, 20 de mayo de 2021

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