The gerenuk as 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 unusual in large mammals. Why is the gerenuk (Litocranius walleri) - the lankiest of all gazelles and indeed of all antelopes or deer - odd among ungulates in this way?

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 (see https://www.canstockphoto.com/gerenuk-standing-upright-to-reach-leaves-59579540.html and http://christofftravel.com/Africa/Pages/Gerenuk.html). The colouration is otherwise unusually plain for a gazelle. 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 female and juvenile blackbuck (Antilope cervicapra, see https://www.shutterstock.com/ja/image-photo/profile-female-blackbuck-antilope-cervicapra-known-1435032149) and some populations of the goitred gazelle (Gazella subgutturosa), typically in Azerbaijan (see http://cannundrum.blogspot.com/2017/10/goitered-or-persian-gazelle.html). In the latter, the stripe is merely a local variation of what, in most populations, is a pale band.

Horizontal pale stripes occur also in several genera of deer (Dama, see https://www.inaturalist.org/guide_taxa/889020; Cervus, see https://lightfieldstudios.net/3752748/stock-photo-sika-deer.html; Axis, see https://www.thehindu.com/thread/chasing-chitals-in-chinnar/article28249624.ece; and Rangifer, see https://www.latimes.com/travel/la-tr-tipsheet-where-to-see-reindeer-20181210-story.html) and in several species of the antelope genus Tragelaphus (see https://www.biolib.cz/en/image/id217766/ and https://www.zoochat.com/community/media/sitatunga-whipsnade-01-aug-2020.498179/). However, in these animals they tend to be mere details of complex patterns of spotting and striping.

Perhaps the unusual striping of the gerenuk arose partly because its torso is so often viewed in the vertical. Although various species of deer and antelopes can stand on the hindlegs to reach high on plants, the gerenuk is exceptionally able to remain free-standing for minutes without propping the forefeet on branches. And only the gerenuk rises bipedally so frequently that this seems to be its main posture in foraging. In the upright stance, the stripe tends to align with the main stems of the tall shrubs and short trees typical of the habitat of the gerenuk (https://www.alamy.com/stock-photo-gerenuk-litocranius-walleri-standing-on-the-hind-legs-at-a-shrub-and-76081293.html).

Two slightly less lanky species of gazelles, the dibatag (Ammodorcas clarkei) and the dama gazelle (Nanger dama), lack any stripe on the torso despite also foraging to some extent bipedally (e.g. see https://www.youtube.com/watch?v=psqN8oUycxQ). In the case of the dibatag, the typical habitat 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'. 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. In the case of the dama gazelle, the habitat is seasonally far more open than that of the gerenuk, and the overall colouration is conspicuous, not camouflaging.

Publicado por milewski milewski, 03 de abril de 2021

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