Detailed similarities and differences in the colouration of gerenuk and impala

In my last Post I mentioned the oddly convergent patterns of colouration in the gerenuk and the impala. The following list shows that the similarities are enough that, were it not for the obviously long neck and small face of the gerenuk, it would be hard to tell the two species apart at any distance (see https://www.alamy.com/stock-photo-male-gerenuk-walking-past-impala-52531218.html and https://www.alamy.com/impalas-aepyceros-melampus-and-gerenuk-or-wallers-gazelle-litocranius-walleri-samburu-park-in-kenya-image344576107.html).

Both species have dark fawn on the dorsal surface of the torso, giving way to paler fawn on the flanks and then white on the belly, the borders between the zones being oddly crisply defined. The main difference is that in the gerenuk the back/flank border is accentuated enough to look like a pale horizontal stripe in its own right. However, another difference is that only the impala possesses a dark spot of bare skin at the stifle-fold.

The pattern of double-white on the chest is remarkably similar between the gerenuk and the impala, while different from other ruminants (see https://i.redd.it/7zntk0098v221.jpg and https://www.inaturalist.org/observations/6484988 and https://depositphotos.com/44113279/stock-photo-impala.html and https://www.amazon.com/African-Impala-Antelope-River-Journal/dp/1540354911).

The face, like the chest, has patterns too similar in detail to have arisen merely by chance. However, the back of the head differs in that only the impala possesses dark posterior ear-tips and a sheeny-haired crown which switches from fawn to silvery in some lights (https://www.youtube.com/watch?v=9CGoEJtE9X8).

The legs have similar colouration except, for example, that the dark gland-tuft is on the foreleg of the gerenuk, vs the hindleg of the impala. Only the impala has white pasterns, visible when the animal stands on bare ground or close-cropped lawn (https://www.facebook.com/keith.ladzinski/posts/2708823022476328/ and https://depositphotos.com/150252040/stock-photo-young-impala-baby-stands-and.html).

As for the hindquarters, the patterns are different in detail (e.g. only the impala has dark pygal stripes) but give a similar overall impression of inconspicuous vertical bars of whitish near the tail (see https://stock.adobe.com/cy_en/search/images?k=gerenuk&asset_id=30423210 and https://commons.wikimedia.org/wiki/File:Impala_ewe_behind.jpg).

The tails differ in various inconspicuous ways (see https://animalworld.tumblr.com/post/189703937600/gerenuk-male-preening-tail-litocranius-walleri and https://www.alamy.com/stock-photo-impala-aepyceros-melampus-tail-south-africa-krueger-national-park-76137298.html). Whereas the terminal, insect-swishing tassel is white and relatively large in the impala vs blackish and small in the gerenuk, only the gerenuk can flare the white fur on the posterior haunch.

Given that the specialised proportions of the gerenuk should make it easy for predators to recognise, what could the adaptive advantages of the similarities in colouration possibly be?

Posted on Πέμπτη 08 Απρίλιος 2021 04:28:22 UTC by milewski