Why does the colouration of the gerenuk resemble that of the impala?

For some strange reason, the gerenuk (Litocranius walleri) and the impala (Aepyceros melampus) have similar colouration (see https://www.flickr.com/photos/helenehoffman/49518633298 and https://www.alamy.com/stock-photo-two-baby-impala-antelopes-in-the-african-bush-29057562.html). This is true despite the fact that the two species are unrelated phylogenetically and differ ecologically, seldom occurring together.

The gerenuk is specialised to forage with upright bipedality, can forgo drinking for years on end, and tends to be solitary. By contrast, the impala has not been observed even to prop its forelegs on a plant stem, must drink nearly daily, and is gregarious. In the narrow zone where the two species share the same landscapes in Kenya, the gerenuk prefers thorn scrub while the impala prefers grassland.

Perhaps the gerenuk has come to mimic the colouration of the impala for protection against predators. The impala tends to be common where it occurs, while the gerenuk is everywhere scarce. Based on the likelihood that the gerenuk is not as enduring a runner as the impala, a naive predator might be misled to turn down hunting opportunities after spotting the gerenuk.

However, there are obvious problems with this explanation. The gerenuk is so much lankier than the impala that the colouration could hardly disguise its identity (see https://megapixl.com/impalas-aepyeros-melampus-and-gerenuk-or-waller-s-gazelle-litocranius-walleri-samburu-park-in-kenya-stock-photo-197574302). Most of the distribution of the gerenuk, on the Horn of Africa, is hundreds or thousands of kilometres from the nearest impala. And there is scant convergence between the two species in their behaviour in alarm. For example, the gerenuk habitually trots (see https://www.istockphoto.com/photo/female-gerenuk-running-gm1143342081-307019225) while the impala is remarkable in how reluctant it is to trot; and the stotting behaviours, in display to scanning predators, are extremely different (see https://www.youtube.com/watch?v=ru-lwzg-rPk and https://www.youtube.com/watch?v=xjb6hStBahg).

Here is a frontier of understanding in Biology, where new hypotheses are needed. I can help by describing in detail the subtle similarities and differences in colouration, which will be the topic of my next post...

Posted by milewski milewski, April 07, 2021 06:33

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