Why does Roberts' gazelle (Nanger granti robertsi) show perverse change in colouration in males?

There are two dozen species of gazelles, in eight genera (https://en.wikipedia.org/wiki/Gazelle).

Among these, Roberts' gazelle (Nanger granti robertsi, https://www.inaturalist.org/observations?taxon_id=601873), a subspecies restricted to the Serengeti ecosystem, shows a puzzling change in colouration.

I refer to a shift as juvenile males grow into adults.

Juveniles - although individually variable - feature a graphic, dark/pale pattern of complex banding on the flank (https://www.inaturalist.org/observations/15193833 and https://www.inaturalist.org/observations/101600425 and https://www.inaturalist.org/observations/8088622 and https://www.inaturalist.org/observations/185222855 and https://www.inaturalist.org/observations/168801725 and https://www.inaturalist.org/observations/132505883 and https://www.inaturalist.org/observations/67365186 and https://www.inaturalist.org/observations/65556138).

This is suddenly lost, in males, once the horns reach half their mature length (https://www.inaturalist.org/observations/9945347 and https://www.inaturalist.org/observations/175341965 and https://www.inaturalist.org/observations/158741158 and https://www.inaturalist.org/observations/146105752 and https://www.inaturalist.org/observations/33163647).

(Also please see https://www.inaturalist.org/journal/milewski/69402-variation-in-pigmentation-within-genus-nanger#.)

The following shows the colouration, particularly the consistent lack of any dark flank-band, in mature males of Roberts' gazelle (https://www.inaturalist.org/observations/21262806 and https://www.inaturalist.org/observations/193060570 and https://www.inaturalist.org/observations/8507420 and https://www.inaturalist.org/observations/102285295 and https://www.inaturalist.org/observations/102307911 and https://www.inaturalist.org/observations/63674973 and https://www.inaturalist.org/observations/13850159).

(In mature males, the showiest masculine feature - apart from the long dark horns - is instead the sheeny pale of the unusually brawny upper-neck. This is displayed in masculine proud-postures (https://www.inaturalist.org/observations/38785430 and https://www.inaturalist.org/observations/8507420 and https://www.inaturalist.org/observations/122149173). The colouration on the nape is similar in females, but the great enlargement of the nape of mature males makes the sheeny pale conspicuous, https://www.inaturalist.org/observations/38785430.)

At birth, the flank-band is present (https://www.inaturalist.org/observations/9994326), but not as dark as in juveniles.

Roberts' gazelle is also the only form of gazelle which naturally coexisted, throughout its range, with a smaller, far more abundant species of gazelle.

I refer to Thomson's gazelle (Eudorcas thomsoni nasalis, https://www.inaturalist.org/taxa/74321-Eudorcas-thomsonii).

The two species are often seen together in the Serengeti ecosystem (https://www.inaturalist.org/observations/39594586 and https://www.inaturalist.org/observations/78015635 and https://www.inaturalist.org/observations/149765914 and https://www.inaturalist.org/observations/19920657 and https://www.inaturalist.org/observations/20147086 and https://www.inaturalist.org/observations/151393534 and https://www.inaturalist.org/observations/141078112 and https://iago80.files.wordpress.com/2012/11/img_2050.jpg and https://www.inaturalist.org/observations/14282018 and https://www.sciencedirect.com/science/article/abs/pii/S0003347205807845 and https://www.inaturalist.org/observations/180960987 and https://www.inaturalist.org/observations/181794975 and https://www.inaturalist.org/observations/192591863 and https://www.inaturalist.org/observations/177203141 and https://www.inaturalist.org/observations/168727646 and https://www.inaturalist.org/observations/167692596 and https://www.inaturalist.org/observations/163668908 and https://www.inaturalist.org/observations/151676694 and https://www.inaturalist.org/observations/140305597 and https://www.inaturalist.org/observations/9945634).

And, as it happens:

  • there is resemblance in the flank-banding between this coexisting species and juveniles of Roberts' gazelle, and
  • this resemblance in colouration is greatest when the body size of the juveniles approximates that d
    of adults of Thomson's gazelle (https://www.inaturalist.org/observations/44556951).

(In Thomson's gazelle, unlike Roberts' gazelle, the flank-band is fully dark already at birth. Please compare https://www.alamy.com/stock-photo-newborn-baby-grants-gazelle-standing-79192809.html and https://www.alamy.com/stock-photo-grants-gazelle-with-her-newborn-baby-79192811.html with https://www.naturepl.com/stock-photo-grant-s-gazelle-nanger-granti-mother-and-newborn-just-after-birth-image01425894.html and https://www.alamy.com/stock-photo-grants-gazelle-gazella-granti-mother-and-baby-serengeti-national-park-13563596.html.)

The result is that naturalists often misidentify the juveniles. Typical examples of photos subject to misidentification are https://www.inaturalist.org/observations/56957658 and https://www.inaturalist.org/observations/41918928.

This is an embarrassing error, because it confuses not just different species, but different genera, of gazelles.

In summary so far, juveniles of Roberts' gazelle possess a dark flank-band that is subsequently lost in adulthhood. As far as I know, this phenomenon does not occur in any other species of antilopin bovid.

How can this puzzle be explained?

One possible explanation is that the flank-band functions as disruptive colouration, helping to hide the animals in their environment.

This, however, is unlikely, because:

An alternative explanation for the ontogenetic shift invokes deceptiveness in the form of adaptive mimicry, as follows:
Juvenile males appease mature males by resembling the 'signature-pattern' of a neutral companion-species, namely Thomson's gazelle, that evokes no masculine antagonism from mature males of Roberts' gazelle.

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.

If this is a form of interspecific mimicry, it perhaps could be called... ?

Roberts' gazelle differs from Thomson's gazelle in that it is sedentary/resident, not migratory. It remains on the Serengeti Plain in the dry seasons, when/where

  • all green matter becomes scarce,
  • drinking water is absent, and
  • the ostrich (Struthio camelus massaicus) competes for food.

I hypothesise that it is these periods of scarcity that are crucial for Roberts' gazelle. This is because particularly intense competition for resources arises between adult males and juvenile males, for the scattered green dicotyledonous plants that contain crucial moisture.

Under these circumstances, any alleviation of antagonism, by means of passive deceptive mimicry on the part of the juveniles, may have been favoured by selective pressure.

The hypothetical mimicry in question is likely to be all the more effective because juvenile females - which tend naturally to be exempt from bullying - share the dark flank-band.

The eastern subspecies, Peters' gazelle (Nanger granti petersi, https://www.inaturalist.org/observations?taxon_id=563632), 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, https://www.inaturalist.org/observations?taxon_id=568581), found in the Laikipia region of central Kenya, shows the darkening on the flank.

However, the effect is less confusing than in Roberts' gazelle. This is because adult females tend to retain the graphic pattern, rather than losing it as happens in Roberts' gazelle.

Overall, the situation seems to make sense. This is because in the Laikipia region, Thomson's gazelle is at a limit of its distribution, and does not outnumber the larger species of gazelle (https://www.inaturalist.org/observations/180622456).

In this northerly population, the value of deceptive appeasement may be reduced by a moderation of the masculinity; mature males of N. g. notata do not grow as massive as do those of Roberts' gazelle.

Posted on June 15, 2021 01:55 AM by milewski milewski

Comments

The following shows that Nanger granti granti achieves conspicuousness through overall pallor, whereas coexisting bovids achieve conspicuousness through overall darkness.

Connochaetes mearnsi:

https://www.inaturalist.org/observations/156222953
https://www.inaturalist.org/observations/66665501

Syncerus caffer:

https://www.inaturalist.org/observations/194808328

The following shows that the overall pallor of N. g. granti is owing to not only depigmentation, but also sheen:

https://www.inaturalist.org/observations/28159337

Posted by milewski 6 months ago
Posted by milewski 6 months ago

The following show clearly the differences between adult females and mature males in Nanger granti granti:

https://www.inaturalist.org/observations/173621306

https://www.inaturalist.org/observations/151474344

Posted by milewski 6 months ago

NANGER GRANTI ADULT FEMALE STOTTING

https://www.inaturalist.org/observations/153185925

Posted by milewski 6 months ago
Posted by milewski 6 months ago

good illustration of colouration of adult female Nanger granti granti near Nairobi:

https://www.inaturalist.org/observations/65740209

Posted by milewski 6 months ago

good illustration of how similar Nanger granti can be to coexisting Eudorcas themsoni:

https://www.inaturalist.org/observations/62023570

Posted by milewski 6 months ago
Posted by milewski 6 months ago
Posted by milewski 6 months ago

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