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

(Also see https://www.inaturalist.org/journal/milewski/39738-contrary-to-field-guide-books-reedbucks-do-not-flag-the-tail-in-alarm# and https://www.inaturalist.org/journal/milewski/39938-an-easily-overlooked-but-extreme-adaptation-in-the-grey-rhebok#.)

The grey rhebok (Pelea capreolus, https://www.inaturalist.org/taxa/42336-Pelea-capreolus and https://dewetswild.com/tag/vaalribbok/) and the mountain reedbuck (Redunca fulvorufula, https://www.inaturalist.org/taxa/42306-Redunca-fulvorufula and https://dewetswild.com/2015/05/15/mountain-reedbuck/) 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.

In this Post I point out differences, and ponder their adaptively significance.

The ear pinnae are more upright in the grey rhebok than in the mountain reedbuck.

In terms of colouration, the differences are as follows.

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. The mountain reedbuck does have a white caudal flag, but this is activated only in social interactions.

Both species are countershaded with pale on the ventral torso and inner surfaces of the upper hindlegs.

However, 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://www.johnxsafaris.com/wp-content/uploads/2010/02/reed-buck-copy.jpg and 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).

The white on the tip of the inert tail extends narrowly up the sides of the tail in the mountain reedbuck (see https://www.shutterstock.com/nb/image-photo/rare-mountain-reedbuck-ram-surprised-out-23330074 and 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/ and https://www.inaturalist.org/observations/108195178).

By contrast, it does not do so in the grey rhebok.

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.

The two white features of the stationary mountain reedbuck, described above, are noticeable only at fairly close quarters.

However, they distinguish it from both the grey rhebok (see https://www.inaturalist.org/observations/81306076) and the other two species of reedbucks.

One interpretation is as follows:

The white display (caudal flag) of the grey rhebok - which tends to be diurnal - is a dynamic, long-distance one, communicating mainly with predators.

By contrast, the white display (subtle highlighting of flanks and buttocks) 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 would make social sense that the mountain reedbuck - which is more adapted than other reedbucks to exposed grassy slopes - is somewhat more 'intraspecifically showy', for the sake of group-cohesion, than its closest relatives.

A further, small-scale difference is located about the lips, as follows.

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 (third photo in https://www.inaturalist.org/observations/117351318 and 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 (please see comment in https://www.inaturalist.org/observations/141682810).

Posted on June 20, 2021 06:42 AM by milewski milewski


One of the few photos showing the mountain reedbuck running with the tail up is https://www.inaturalist.org/observations/116401815v. However, this is probably in social/sexual display, not in alarm.

Posted by milewski over 2 years ago

Wow! Thanks for the info!

Posted by ludwig_muller over 1 year ago

The following shows the crisply-defined whitish occurring on the inner upper foreleg of the mountain reedbuck: https://www.inaturalist.org/observations/107234005. In the grey rhebok, the paleness on this part of the anatomy is hardly noticeable: https://dewetswild.com/2015/05/24/grey-rhebok/#jp-carousel-7384

Posted by milewski over 1 year ago

Photo showing that white pelage transgresses mere countershading:
Redunca fulvorufula male:

This transgression does not occur in Pelea capreolus, Redunca redunca, or Redunca arundinum.

Posted by milewski about 1 year ago

The following is possibly the clearest illustration on the Web of the buccal semet in the grey rhebok:


Posted by milewski 7 months ago

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