Detailed similarities and differences in the colouration of gerenuk (Litocranius walleri) and common impala (Aepyceros melampus)

In https://www.inaturalist.org/journal/milewski/48831-why-does-the-colouration-of-the-gerenuk-litocranius-walleri-resemble-that-of-the-common-impala-aepyceros-melampus#, I mentioned the oddly convergent patterns of colouration in the gerenuk (Litocranius walleri) and the common impala (Aepyceros melampus).

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).

In both species

• dark fawn on the dorsal surface of the torso gives way to paler fawn on the flanks, and then to white on the belly, and
• the borders between these zones are 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 (https://stock.adobe.com/cy_en/images/springbok-antelope-kalahari-desert-south-africa/339886722?prev_url=detail).

However, another difference is that only the common 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 common 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, only the common impala possesses a posterior coronal flag (https://www.youtube.com/watch?v=9CGoEJtE9X8 and https://www.inaturalist.org/posts/89191-bleezes-flags-and-semets-in-the-adaptive-colouration-of-the-impala-aepyceros-melampus#).

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 common impala, and
• only the common impala has a pedal flag; its white pasterns are conspicuous when 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).

On the hindquarters, both species give a similar overall impression of inconspicuous vertical bars of whitish near the tail (https://stock.adobe.com/cy_en/images/gerenuk-tsavo-east-national-park/30423210?prev_url=detail and https://commons.wikimedia.org/wiki/File:Impala_ewe_behind.jpg).

However, the patterns differ. For example:

• only the common impala has dark ischial stripes, and
• only the gerenuk can flare the white pelage on the buttocks.

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).

For example:

• in the common impala, the jointed feather-tassel is white, whereas
• in the gerenuk, the tassel is blackish and small.

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 08 April, 2021 04:28 by milewski