What makes the impala tick? Some initial thoughts...

The impala (Aepyceros melampus) is unusual in its oral allogrooming (mutual grooming by different individuals of the fur). It is well-equipped to self-groom most of its fur by means of specialised, comb-like front teeth plus an additional tooth-like bulge of the gums. The difficulty of reaching its own head and neck helps to explain the allogrooming.

Several other extremes in the nature of the impala may be relevant here.

Firstly, the impala is unusually gregarious for a cover-dependent species with camouflage-like colouration. This helps to explain why the impala is seldom kept in zoos, which are too cramped for a large group. Likewise it has long been known that the impala cannot be reintroduced to game ranches as a few founders in the normal way. Instead, a group-size at least in the dozens is needed right from the start.

Secondly, the impala is the smallest species of host attractive to oxpeckers (Buphagus), which comb through the fur in search of ticks and other parasites, as well as - on far larger hosts such as giraffes and the African buffalo (Syncerus caffer) - blood and pus oozing from wounds.

These three extremes, in allogrooming, gregariousness and complementary grooming by birds, may fit together. We may never fully distinguish cause and effect in the full context of the predator- and parasite-rich habitat of the impala. However, let me start to put together the pieces of the puzzle by pointing out a fourth extreme, in the form of the fur itself.

Some African ungulates have hairs which are simply circular or oval in cross-section. However, many have hairs which are kidney-shaped in cross-section, which may alter the colouration by means of complex sheen-like effects. For example, wildebeests (Connochaetes) are brown up-close but can look black, bluish or car-bonnet silver at a distance, because of how the light plays on the micro-texture of the fur.

Oddly, the hairs of the impala have a triangular cross-section, resembling that of a cricket bat.

Possibly its odd fur helps the impala to blend in as it forages among the bushes by day and rests in patches of open ground by night. But a downside of its hair-shape may be that ticks find it particularly easy to cling to the fur, requiring the extreme compensation by various forms of grooming that we see in the impala.

If so, the various oddities might begin to add up to a coherent adaptive strategy.

Posted by milewski milewski, April 26, 2021 11:06

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