Why the pronghorn has a striped neck

The colouration of the pronghorn (Antilocapra americana) is strangely incongruous between the hindquarters and the forequarters. On the former, the pattern is simple and identical for both sexes and all individuals. On the latter (see https://www.jasonsavagephotography.com/tag/pronghorn and https://www.inaturalist.org/observations/57653446), there is a complex pattern varying individually and with age and sex.

Furthermore, the horizontal striping on the front of the neck seems incongruous with the habitat. The pronghorn is a gregarious animal of open vegetation, which relies on speed and endurance, rather than hiding, to evade predation. Inasmuch as blending into the grassland remains adaptive in animals subject to inevitable exposure, one might expect the colouration to be plain: a uniform fawn or grey on the forequarters, exemplified by reedbucks (Redunca) and the grey rhebok (Pelea capreolus, see https://animalia.us-east-1.linodeobjects.com/animals/photos/full/original/grey-rhebok-pelea-capreolus-male-.jpg).

Although antelopes and deer can blend into their environments by means of either plain colouration or a complex pattern of stripes and spots, one generally expects the former in open vegetation and the latter in dense vegetation. The colouration on the front of the pronghorn would seem more appropriate in a stalking carnivore (e.g. compare https://depositphotos.com/243807632/stock-photo-close-rare-endangered-ethiopian-wolf.html with https://www.shutterstock.com/ja/image-photo/female-pronghorn-open-meadow-grand-teton-1839926248).

A possible explanation for these incongruities is based on the extreme eyesight of the pronghorn.

Ungulates vary in the size and placement of the eyeballs, with most deer (Cervidae) and cover-dependent bovids having relatively small eyes, placed somewhat forward on the face. Most antelopes of open vegetation, such as gazelles, alcelaphin bovids, and wild sheep and goats, have relatively large eyes, placed on the side of the face with some degree of rear-vision. However, the pronghorn exceeds even the antelopes in its visual specialisation: extremely large eyes, placed so far on the side of the head that they can be clearly seen if the head is viewed from directly behind (see https://www.inaturalist.org/observations/16478652 and photo and discussion in https://forums.bowsite.com/tf/bgforums/thread.cfm?threadid=407206&forum=8).

What this suggests is that, at distances dimming the vision of predators, the pronghorn can still see members of its own species clearly enough to discern the striped pattern on the front of the neck and head, as a kind of bar-code to individual identity. However, this explanation depends on the social system. If, as in the springbok (Antidorcas marsupialis), gregariousness is so indiscriminate that there is no attempt to recognise and monitor the whereabouts of kin and long-term companions, then my explanation would be undermined.

Publicado el mayo 22, 2021 04:40 MAÑANA por milewski milewski

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