A first attempt to identify bleezes in the gazelle genus Nanger

I define a bleeze as follows:
any feature of animal colouration that shows so much pale/dark contrast, at such large scale, that it renders the whole figure obvious to scanning predators - even when the figure remains stationary.

Such advertisement is interesting adaptively, because it defies a basic strategy of prey species in avoiding detection.

The gazelle genus Nanger (https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=71873&view=species) exemplifies the complex variation in the incidence/development of bleezes among the species, subspecies, ages, and sexes of ungulates.

Once bleezes are identified, we can begin to weigh the evolutionary costs and benefits of the conspicuous colouration.

All individuals of Nanger granti (https://www.inaturalist.org/taxa/74724-Nanger-granti), including standing infants, show a combination of

  • whitish buttocks, and
  • blackish pygal bands.

I call this an ischiopygal bleeze.

The position of this pattern on the hindquarters means that it is most conspicuous 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 bleeze on the hindquarters. However, this is the only clear case in this genus.

A distinct bleeze also occurs in Nanger dama (https://www.inaturalist.org/taxa/74723-Nanger-dama). However, this refers only to subspecies mhorr, and excludes infants even in this subspecies.

The whitish on the rump and buttocks is more extensive in N. dama than in Nanger granti. However, any bleeze remains somewhat ambivalent. This is 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 (https://www.inaturalist.org/observations?taxon_id=601873) and Nanger granti notata (https://www.inaturalist.org/observations?taxon_id=568581) temporarily develop a blackish flank-band, contrasting with both

This lateral bleeze disappears in all adult males. However, it is retained by some adult female individuals of Nanger granti notata.

Nanger dama mhorr may qualify for a different pattern of lateral bleeze. However, this is ambivalent, because there are no flank-bands.

Nanger soemmerringi (https://www.inaturalist.org/taxa/74725-Nanger-soemmerringii) lacks any posteriolateral or lateral bleezes. This is because no photo shows sufficient dark on the hindquarters or flanks.

However, in at least one subspecies of N. soemmerringi, 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.

Posted on June 17, 2021 06:50 PM by 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/.

Posted by milewski about 3 years ago

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.

Posted by milewski about 3 years ago

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