A comparison of adaptive colouration between lookalikes: grey rhebok and mountain reedbuck

The grey rhebok (Pelea capreolus) and the mountain reedbuck (Redunca fulvorufula) are similar in body size, ecologically related, and partly sympatric. Both species are weakly gregarious, and their plain colourations are so similar that naturalists frequently confuse them. Here I point out differences, however subtle, which could be adaptively significant.

The most obvious is that, although both species have a white underside to the tail, this is displayed while fleeing only in the grey rhebok. This difference is so categorical and consistent that it can be used to identify fleeing figures at distances too great for details of the animals to be observed.

Another difference is that, although both species are countershaded with pale on the ventral torso and inner surfaces of the upper hindlegs, white extends just high enough on the belly of the mountain reedbuck to catch the light in the form of a crisply-defined ruff of fur (see https://biggamehuntingadventures.com/wp-content/uploads/2015/05/mountain-reedbuck-shot-placement-broadside.jpg and https://www.dreamstime.com/royalty-free-stock-photo-mountain-reedbuck-south-africa-image11193305 and http://www.waterberg-bioquest.co.za/Mammal%20spp%20pgs/red_fulv.html). Also, the white on the tip of the inert tail extends narrowly up the sides of the tail (see https://www.dreamstime.com/stock-photos-mountain-reedbuck-reedbok-image15546303 and https://fineartamerica.com/featured/female-mountain-reedbuck-peter-chadwickscience-photo-library.html?product=canvas-print and https://www.castledewildt.co.za/mountain-reedbuck-breeding-parcel/). These two white features of the stationary mountain reedbuck, although noticeable only at fairly close quarters, distinguish it from both the grey rhebok (see https://www.inaturalist.org/observations/81306076) and the other two species of reedbucks.

One interpretation is that the white display of the grey rhebok - which tends to be diurnal - is a dynamic, long-distance one communicating mainly with predators, whereas the white display of the mountain reedbuck - which tends to be crepuscular - is a static one communicating secretively within the species. The caudal flag of the grey rhebok discourages pursuit whereas the subtle highlights of the mountain reedbuck maintain group-cohesion as the animals graze in dim light. It makes social sense that the mountain reedbuck - which is more adapted than other reedbucks to exposed grassy slopes - is somewhat more showy than its closest relatives.

A small-scale difference is located about the lips. The grey rhebok has a darkish vertical mark - reminiscent of various species of deer but unlike other antelopes including reedbucks - below the side of the mouth (see https://www.inaturalist.org/observations/16496842 and https://www.inaturalist.org/observations/11867209). In the mountain reedbuck it is the white front of the upper lips that is more noticeable than in the grey rhebok (see https://www.istockphoto.com/photo/southern-mountain-reedbuck-gm1302140763-393971475 and https://www.zoochat.com/community/media/mountain-reedbuck-berlin-tierpark-9th-september-2011.162739/). I interpret these patterns as buccal semets, the function of which is to facilitate the mutual monitoring of cud-chewing by group members in a species-specific way.

פורסם על-ידי milewski milewski, יוני 20, 2021 06:42 לפנה"צ

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One of the few photos showing the mountain reedbuck running with the tail up is https://www.biolib.cz/en/image/id219771/. However, this is juvenile play behaviour in a zoo.

פורסם על-ידי milewski לפני 6 חודשים (סמן)

The following two photos show the differences nicely: https://www.inaturalist.org/observations/83216658 for grey rhebok and https://www.inaturalist.org/observations/31363246 for mountain reedbuck.

פורסם על-ידי milewski לפני 6 חודשים (סמן)

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