Detailed similarities and differences in the colouration of gerenuk and impala

In my last post I mentioned the puzzle of apparent protective mimicry of the impala by the gerenuk. Here I summarise the similarities and differences in colouration.

Both species have dark fawn on the dorsal surface of the torso, giving way to paler fawn on the flanks and then white in the belly, the borders between the zones being oddly crisply defined. The main difference is that in the gerenuk the back-flank border is so emphasised that it appears like a pale horizontal stripe in its own right. Only the impala possesses a dark spot of bare skin at the stifle-fold, but this is mere punctuation rather than a 'capital letter' in the typeface the animal presents to predators.

The faces have patterns so similar in detail that it seems unlikely that such convergence would have evolved merely by chance. 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 whitish in some lights. However, these differences would not be visible to any predator scanning the stationary figure as the alert antelope faces the intruder.

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. It is true that only the impala has white pasterns; but this is invisible unless the animal is standing on bare ground or the most closely-cropped of lawns.

As for the hindquarters, the patterns are different in detail but give a similar overall impression of inconspicuous vertical bars of whitish near the tail. Only the impala possesses dark pygal stripes, but this presents as a bar-code rather than a flag.

Both the gerenuk and the impala have tails surprisingly unlike those of other antelopes (including other gazelles in the case of the gerenuk) or deer. They differ in that the terminal, insect-swishing tassel is white in the impala vs blackish in the gerenuk. However, this is hard to see when the tails of apprehensive animals are held still and, in the case of the impala, tucked between the legs.

The bottom line is this: there are many detailed differences in colouration between gerenuk and impala, but none large-scale enough to distinguish the two species in an obvious way unless viewed at close quarters. Any human observer with any experience can easily tell them apart even at distance, but this is because the neck of the gerenuk is so long, and its face small to the point of near-absurdity.

So how do wild predators perceive them, and of what adaptive value are the convergences in colouration?

Publicado por milewski milewski, 08 de abril de 2021


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