Why does the southern Grant's gazelle show such extreme change in colouration in males?

Among the two dozen species of gazelles, in eight genera, the southern Grant's gazelle (Nanger granti granti) shows the most puzzling change in colouration from juvenile males to adult males. Juveniles develop a graphic dark/pale pattern of complex banding on the flank (see https://www.inaturalist.org/observations/8088622), which is suddenly lost when the horns reach half their mature length. In mature males the showiest feature, apart from the long dark horns, is instead the sheeny pale of the unusually brawny upper-neck, which is displayed in masculine proud-postures (see https://www.inaturalist.org/observations/38785430 and https://www.inaturalist.org/observations/8507420).

The southern Grant's gazelle is also the only form of gazelle which naturally coexisted throughout its range with a smaller, far more abundant species of gazelle - the groups readily mixing (see https://iago80.files.wordpress.com/2012/11/img_2050.jpg and https://www.sciencedirect.com/science/article/abs/pii/S0003347205807845). And as it happens there is such resemblance in the flank-banding between juveniles of the southern Grant's gazelle and this coexisting species, namely Thomson's gazelle (Eudorcas thomsoni), that naturalists often misidentify the juveniles - an embarrassing error because it actually confuses different genera of gazelles.

One explanation for this deceptiveness is in adaptive mimicry: juvenile males appease mature males by resembling the 'signature-pattern' of a harmless companion-species. This appeasement hypothetically allows the juveniles to remain secure with their mothers for longer than would otherwise be possible in a society where any hint of masculine precociality is likely to attract bullying ('despotic competition' https://www.sciencedirect.com/science/article/abs/pii/016815919190264X) by the territorial male in the vicinity. The mimicry is all the more effective because juvenile females - naturally exempt from bullying - share the dark flank-band.

The eastern subspecies, Peters' gazelle (Nanger granti petersi), shows little darkening on the flank, even in juveniles. This is possibly because its range is beyond that of Thomson's gazelle. The northerly subspecies (Nanger granti notata), found in the Laikipia region of central Kenya, shows the darkening on the flank but with a less confusing effect because adult females tend to retain the graphic pattern rather than losing it as happens in the southern Grant's gazelle. This perhaps makes sense because Thomson's gazelle is at a limit of its distribution, and does not outnumber the larger species of gazelle, in the Laikipia region. In this population the value of deceptive appeasement may be reduced by a moderation of the masculinity; mature males of the northerly subspecies do not grow as massive as in the southern Grant's gazelle.

Publicado por milewski milewski, 15 de junio de 2021


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