A first attempt to identify bleezes in the gazelle genus Nanger

I define the bleeze as any feature of animal colouration showing so much pale/dark contrast, at such large scale, that the whole figure is obvious to scanning predators even when it remains stationary. Such advertisement is interesting adaptively because it defies a basic strategy of prey species in avoiding detection.

The gazelle genus Nanger shows how complex it can be to identify bleezes among the species, subspecies, ages and sexes in ungulates. Once such a classification is made we can begin to weigh the evolutionary costs and benefits of the conspicuous colouration.

All individuals of Nanger granti including standing infants show a combination of whitish buttocks and blackish pygal bands. Its position on the hindquarters means that this pattern qualifies as a bleeze only in posteriolateral view (see https://shootplanet.photoshelter.com/image/I0000WGHanq6qvr8 and https://destinationuganda.com/travel-guide/mammals/grants-gazelle/). Therefore Nanger granti can be classified as a species consistently possessing a posteriolateral bleeze. However, this is the only clear case in this genus.

A posteriolateral bleeze also occurs in Nanger dama, but only in subspecies mhorr and excluding infants even in this subspecies. The whitish on the rump and buttocks is more extensive than in Nanger granti, but the bleeze remains somewhat ambivalent because, in the absence of pygal bands, the darkest adjacent fur is only moderately dark (see second photo in https://www.cbd-habitat.com/en/2019/07/02/the-first-reintroduction-project-for-mhorr-gazelle-into-the-wild/).

Juveniles of Nanger granti granti and Nanger granti notata temporarily develop a blackish flank-band, contrasting with both the whitish ventral torso below and the pale flank-band above (see https://www.zoochat.com/community/media/thomsons-or-grants-gazelle.376153/ and accompanying discussion). This lateral bleeze disappears in all adult males but is retained by some adult female individuals of Nanger granti notata. Nanger dama mhorr may qualify for a different pattern of lateral bleeze, but this is ambivalent because there are no flank-bands.

Nanger soemmerringi lacks any posteriolateral or lateral bleezes because no photo shows sufficient dark on the hindquarters or flanks. However, in at least one subspecies the front of the face in maturity becomes dark enough to contrast with the pale throat and facial stripe (see https://www.istockphoto.com/photo/soemmerrings-gazelle-gm1072202986-286932026 and https://www.biolib.cz/en/image/id359466/ and https://www.flickr.com/photos/timmelling/34068652491 and http://www.arthurgrosset.com/mammals/photos/nansoe46234.jpg). If this pattern is large-scale enough to qualify as a bleeze, it can be called a frontal bleeze.

A frontal bleeze is once again ambivalent in the case of Nanger dama mhorr (see https://www.wikiwand.com/en/Nanger). Although the front of the face becomes whitish in adulthood, the adjacent throat is only moderately dark.

Publicado el junio 17, 2021 06:50 TARDE por milewski milewski


Two more photos showing the frontal bleeze in adult male Soemmerring's gazelle can be seen in https://www.inaturalist.org/observations/68999763 and https://www.flickr.com/photos/ken9244/15751898878/in/gallery-131211772@N08-72157714968071876/.

Publicado por milewski hace casi 3 años

The adult female individual of Nanger dama mhorr shown in https://www.flickr.com/photos/antonio-lorenzana/49764007316/in/gallery-131211772@N08-72157714968071876/ illustrates the ambivalence of both the lateral bleez and the frontal bleeze. The flank and haunch do present a bold pattern of pale and relative dark, but the latter is actually only medium-tone, limiting the conspicuousness of the pattern at distance. The face does present a pattern of pale and dark, but the features are rather small-scale, casting doubt on their conspicuousness at distance.

Publicado por milewski hace casi 3 años

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