Extreme sexual incongruity in the adaptive colouration of the blackbuck

In its colouration, the blackbuck (Antilope cervicapra) of India seems to function as two incongruous species in one (see https://www.shutterstock.com/nb/image-photo/blackbuck-known-indian-antelope-found-indianepal-751018990).

The blackbuck is a gazelle, phylogenetically and ecologically. However, females and juvenile males lack the conspicuous dark/pale contrast on the hindquarters that is seen in gazelles of the genera Gazella and Eudorcas. At the same time, mature males are more extensively dark than any other gazelle (see https://sustain.round.glass/columns/blackbucks-dung-attract-mates/ and https://www.overdrive.in/news-cars-auto/features/never-stop-discovering-velavadar-blackbuck-national-park/).

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 more-or-less normal colouration for a gazelle. After all, this species is in line with other gazelles in being gregarious and living in treeless grassland. It stots with vigour and versatility: it leaps high (see https://www.saevus.in/velavadar-land-of-blackbucks/ and https://www.india.com/travel/articles/if-you-want-to-spot-blackbucks-in-their-natural-habitat-head-to-rehekuri-wildlife-sanctuary-3564384/ and https://hive.natureinfocus.in/photo_sharing/the-leaping-buck and https://www.shutterstock.com/nb/image-photo/jumping-blackbuck-captured-national-park-velavadar-1800223201), bounces stiff-legged (see https://www.shutterstock.com/nb/image-photo/young-blackbuck-jumping-grassland-talchapar-rajasthsan-1890026977 and https://www.inaturalist.org/observations/30385400 and https://www.shutterstock.com/nb/image-photo/jumping-blackbuck-clicked-velavadae-1630599001), or style-trots, and is capable of flaring the white of the buttocks and erecting the tail (see http://www.thejunglelook.com/gallery2/main.php?g2_itemId=5388 and https://walkthewilderness.net/most-beautiful-indian-antelope-blackbuck/).

However, the patterns typical of gazelles have been lost in an extreme sexual dimorphism. Males become more conspicuous than any gazelle as much of their body turns blackish in maturity (see https://www.shutterstock.com/nb/image-photo/blackbucks-antelope-foraging-vast-grasslands-blackbuck-1696197772 and https://www.flickr.com/photos/cirdan-travels/44441395342 and https://www.francisjtaylor.co.uk/product/blackbuck-group/). This pattern has little relationship with predation and instead functions in masculine rivalry and courtship (see https://www.freeart.com/artwork/art-print/a-young-male-blackbuck-antilope-ce_fa14647235.html and https://www.francisjtaylor.co.uk/product/blackbuck-courtship-display/). For their part, females and juveniles (see https://www.shutterstock.com/nb/image-photo/blackbucks-velavadar-sanctuary-gujarat-india-1678657045 and https://www.shutterstock.com/nb/image-photo/blackbuck-antilope-cervicapra-known-indian-antelope-1771445606) have lost all the noticeably dark features of gazelles (flank-band, pygal bands and tail tassel). And the tail, having lost its tassel, is usually left inert in gaits and situations in which it would be flicked or erected demonstratively in other gazelles.

The only conspicuous aspect of colouration in females and juveniles is the ventral whitish, which extends higher than in any other gazelle on to the elbow region and the ventral surface of the neck (see https://www.inaturalist.org/observations/36571904 and https://www.shutterstock.com/nb/image-photo/blackbuck-antilope-cervicapra-indian-antelope-group-1840517536). This in itself is a puzzling pattern, partly because it has been lost in the southern- and easternmost populations.

In mature males, the effectively black-and-white colouration seems superfluously conspicuous 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 colouration lacking all dark features.

And since the sexes live together most of the time, we are left with an overall puzzle. Regardless of whether the females and juveniles can 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 does the overall colouration of the blackbuck function in a single adaptive strategy with respect to predation?

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

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