Eye-catching highlights above the stifle-fold in gazelles

Gazelles vary in the conspicuousness of their colouration in normal surroundings, some species being adapted to hiding from predators and others being so showy even when stationary that they rely on advertisement of fitness to avoid being selected for the chase (e.g. see https://www.dreamstime.com/lion-springbok-antelope-african-dry-habitat-etocha-np-namibia-mammal-africa-evening-back-light-sunset-safari-image144836274).

A contributing feature is the extension of the ventral whitish of the torso to a level above the stifle-fold. The stifle (https://en.wikipedia.org/wiki/Stifle_joint), at the junction of the hindleg and the flank in ungulates, is marked by a fold of skin. Any whitish above the stifle-fold tends to catch the light conspicuously, because it reaches relatively high on the side of the figure, on a surface made convex by the contours of the belly.

The range of variation can be seen by comparing the springbok with the gerenuk. In the springbok (see https://parody.fandom.com/wiki/Springbok?file=Antidorcas_marsupialis.jpg), pure white extends several centimetres above the stifle-fold, to a position halfway up the lateral profile. By contrast, in the gerenuk (see https://www.sciencephoto.com/media/389677/view/male-gerenuk-gazelle and https://www.123rf.com/photo_11078027_gerenuk-litocranius-walleri-male.html), there is virtually no clearance.

The following series exemplifies the intermediate positions.

Western dama gazelle (see https://www.dreamstime.com/dama-gazelle-gazella-dama-mhorr-mhorr-gazelle-species-gazelle-dama-gazelle-gazella-dama-mhorr-mhorr-gazelle-image218403317 and https://www.shutterstock.com/ja/image-photo/lovely-mhorr-gazelle-1021810420)

Females of northwestern subspecies of blackbuck (see https://www.dreamstime.com/stock-photo-wild-antelope-doe-south-texas-blackbuck-female-image60390821)

Sand gazelle (see https://www.inaturalist.org/observations/73853863 and https://www.canstockphoto.com/arabian-sand-gazelle-59326840.html)

Dorcas gazelle (see https://www.dreamstime.com/dorcas-gazelle-gazella-dorcas-dorcas-gazelle-gazella-dorcas-also-known-as-ariel-gazelle-small-common-gazelle-image137827563)

Arabian gazelle (see https://www.biolib.cz/en/image/id171992/ and https://www.biolib.cz/en/image/id171991/)

Thomson's gazelle (see https://www.flickr.com/photos/gmaquedaphotos/50713837128/ and https://alchetron.com/Thomson%27s-gazelle)

Indian gazelle (see https://www.inaturalist.org/observations/56510269 and https://www.downtoearth.org.in/news/wildlife-biodiversity/18-chinkaras-die-within-a-fortnight-in-jaislamer-60977)

Red-fronted gazelle (see http://awwp.alwabra.com/wp-content/uploads/2014/04/Adult_Male_Redfronted_Gazelle_01.jpg and https://www.flickr.com/photos/24544467@N02/14113291712)

Females of southeastern subspecies of blackbuck (see https://www.enidhi.net/2014/07/maidenahalli-jayamangali-blackbuck.html)

This generally corresponds to a decreasing series in the overall conspicuousness of the figures. The springbok is showy in all perspectives, displaying a posteriolateral bleeze, a lateral bleeze (centred on the dark flank-band) and a facial flag. The dama gazelle is so extensively pale elsewhere on the figure as to subsume the pale above the stifle-fold. By contrast, the only conspicuous dark/pale contrast in the red-fronted gazelle is that between the blackish tail and the white buttocks. And in the gerenuk even the pattern on the hindquarters is hardly noticeable at the distances relevant to scanning predators.

Anomalous in this series is Thomson's gazelle. Its ventral white does not reach high on either the flanks or the rump, i.e. the clearance above the stifle-fold is not particularly great and the bleeze on the buttocks ends at the root of the tail. Yet this is one of the most conspicuous species of gazelles, rivalling the springbok by means of a shift in emphasis: a particularly dark flank-band juxtaposed with a particularly well-defined pale flank-band above it.

פורסם על-ידי milewski milewski, יוני 22, 2021 10:21 אחה"צ

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