Why is the gerenuk an unusually striped ungulate?

Stripes, whether pale or dark, can make animals hard to tell from their surroundings. However, horizontal stripes along the torso (as opposed to the classical vertical pattern seen in e.g. the tiger, Panthera tigris) are rare in large mammals. Among ungulates, the gerenuk (Litocranius walleri) - the lankiest of all gazelles and indeed of all antelopes or deer - is a noteworthy exception.

The gerenuk, in both sexes and at all ages from infancy, has a pale horizontal stripe running from the dorsal base of the neck across the flank to the rump. This is the major feature of what is otherwise an unusually plain colouration among gazelles. Presumably the stripe breaks up the figure, helping the gerenuk to hide in the sparsely woody vegetation it inhabits.

The only other gazelles with a similar stripe are the blackbuck (female and juvenile colouration of Antilope cervicapra) and some populations of the goitred gazelle (Gazella subgutturosa), typically in Azerbaijan. In the blackbuck the stripe is outweighed, in subspecies rajputanae, by the conspicuous ventral white which catches the light on the shoulders and above the elbow. Furthermore, the stripe is converted to a series of vague spots as it approaches the rump. In the goitred gazelle, the stripe is merely a local variation of what, in most populations, is a pale band; and either way it is hardly noticeable relative to the bold dark-and-pale of hindquarters which are far more conspicuously marked than in either the blackbuck or the gerenuk.

Horizontal pale stripes occur also in several genera of deer (Dama, Cervus, Axis) and in several species of the antelope genus Tragelaphus. However, in these animals they tend to be mere details of complex patterns of spotting and striping.

This raises the question of what relationship there might be between the unusual striping of the gerenuk and its extreme ability to forage bipedally. Although various species of deer and antelopes can stand on the hindlegs to reach high on plants, only the gerenuk can remain free-standing for minutes at a time without the forefeet being used as props on branches. And only the gerenuk rises bipedally so frequently that this seems to be its main posture in foraging.

What is intriguing is the idea that, in the upright stance, the stripe becomes vertical and thus tends to align with the main stems of the tall shrubs and short trees typical of the habitat of the gerenuk. Perhaps the gerenuk is unusually striped because its torso is so often viewed in the vertical.

Perhaps supporting this explanation is the observation that a slightly less lanky gazelle, the dibatag (Ammodorcas clarkei), lacks any stripe on the torso despite also foraging to some extent bipedally. A difference is the typical habitat of the dibatag is dominated not by 'acacia' (Vachellia species such as tortilis) but instead by sundry tall shrubs in a distinctive vegetation type on sand, called 'gedguwa'. I hypothesise that this form of 'open thicket' tends to be more cluttered with foliage at about one metre above ground than is the case in 'acacia scrub', leading to a subtle difference in visibility and thus in adaptive colouration of the gazelles in question.

Posted by milewski milewski, April 03, 2021 02:11

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