No other gazelle is as sexually dimorphic as the blackbuck

In its adaptive colouration, the blackbuck (Antilope cervicapra) of India seems to function as two species in one, and incongruously so. Females and juvenile males lack the conspicuous dark/pale contrast on the hindquarters that is seen in gazelles of the genera Gazella and Eudorcas, despite the fact that the blackbuck is itself a gazelle, phylogenetically and ecologically.

Among gazelles, conspicuous colouration is correlated with gregariousness in open environments, where hiding from predators seems to be less successful than the self-advertisement of alertness and locomotory fitness. The hindquarters tend to have bold patterns of dark (tail-tassel and pygal bands) and pale (buttocks and escutcheon), which stand out in posteriolateral view even when the animals stand still. The display is accentuated by movement of the tail and flaring of white on the buttocks, particularly when the animals stot in demonstration, to any scanning predator, of a current capacity to flee so rapidly and enduringly that pursuit of the individual in question would likely be futile.

The ecology and behaviour of the blackbuck would predict colouration in line with that described above. This species is gregarious and lives in treeless grassland. It stots with vigour and versatility: it leaps high, bounces stiff-legged, or style-trots, and is capable of flaring the white of the buttocks and erecting the tail.

However, similarity to other gazelles has been lost in an extreme sexual dimorphism. Males become more conspicuous than any gazelle as much of their body turns blackish in maturity. This pattern has little relationship with predation and instead functions in masculine rivalry and courtship. For their part, females and juveniles have lost all the noticeably dark features of gazelles (flank-band, pygal bands and tail). And the tail, which has lost its tassel, is usually left inert in gaits and situations in which it would be flicked or erected demonstratively in other gazelles.

In mature males, the effectively black-and-white colouration seems dysfunctional in its seemingly unnecessary conspicuousness to predators. In females and juveniles, the colouration is incongruous in a converse way, because no other gregarious species gazelle living in the open, and frequently stotting and bounding, has such an inconspicuous pattern. Since the sexes live together most of the time, we are left with a puzzle. Even if females do blend into such exposed environments, their presence is likely to be divulged anyway by the outlandish appearance of the mature males among them. So how is the overall colouration of the blackbuck adaptive with respect to predation?

Posted by milewski milewski, March 06, 2021 02:58

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