A new feature of adaptive colouration in the grey rhebok (Pelea capreolus): the buccal semet

@ludwig_muller @christiaan_viljoen @variani18 @jeremygilmore @simontonge @beartracker @gawie @klauswehrlin @bluehillescape @tonyrebelo

This Post is dedicated to @gigilaidler, whose painstaking photos in Table Mountain National Park and elsewhere have so clearly illuminated the grey rhebok, among other animals.

The grey rhebok (Pelea capreolus) has remarkably inconspicuous colouration (https://www.inaturalist.org/posts/53474-a-comparison-of-adaptive-colouration-between-lookalikes-grey-rhebok-and-mountain-reedbuck#).

However, like various other animals with cryptic or camouflage colouration, it possesses certain features that become conspicuous when activated by movement.

The obvious example, in the grey rhebok, is the white of the ventral surface of the tail. This feature, which becomes extremely noticeable when the animal flees, can be categorised as a caudal flag (https://www.inaturalist.org/journal/milewski/39938-an-easily-overlooked-but-extreme-adaptation-in-the-grey-rhebok#).

However, there is another feature - dark instead of pale - that has previously been overlooked by zoologists.

I refer to what I call a buccal semet, viz. a small-scale feature of dark/pale contrast at the mouth, which is activated by the cud-chewing movements of the jaws (https://www.inaturalist.org/posts/100574-a-new-feature-of-adaptive-colouration-in-the-grey-rhebok-pelea-capreolus-the-buccal-semet#activity_comment_63701963-aaed-462a-b5af-00a3f88dfe6d).

The buccal semet of the grey rhebok consists of dark short pelage on the anterior part of the side of the mandible, between the lower lip and the 'chin' (https://www.inaturalist.org/observations/73018076 and https://thetaxidermystore.com/african-grey-rhebok-shoulder-taxidermy-mount-14576.html and https://uk.inaturalist.org/observations/185412876).

Please focus on the surface below the side of the mouth in the following:

https://www.shutterstock.com/image-photo/one-grey-rhebok-mountain-zebra-national-1061610842

third photo in https://www.inaturalist.org/observations/117351318

https://www.inaturalist.org/observations/16496842

https://www.inaturalist.org/observations/11867209

https://www.inaturalist.org/observations/148981465

https://www.inaturalist.org/observations/242219998

https://www.inaturalist.org/observations/247271379

https://www.inaturalist.org/observations/247162752

https://www.inaturalist.org/observations/242219998

https://www.shutterstock.com/image-photo/grey-rhebuck-pelea-capreolus-natural-habitat-2166233825

https://www.shutterstock.com/image-photo/grey-rhebuck-pelea-capreolus-natural-habitat-2332821615

scroll in https://antelopesg.org/antelopes/

scroll in https://www.ultimateungulate.com/Artiodactyla/Pelea_capreolusReferences.html

DISCUSSION

The buccal semet in the grey rhebok distinguishes it from reedbucks (Reduncini). It supports the assignment of Pelea to its own exclusive tribe, viz. Peleini, within the Bovidae (http://taxonomicon.taxonomy.nl/TaxonTree.aspx?src=957&id=68333 and https://www.nature.com/articles/hdy2013108).

However, this feature of adaptive colouration seems somewhat convergent with that of the unrelated common duiker (Sylvicapra grimmia):

https://www.inaturalist.org/observations/236945215

https://www.inaturalist.org/observations/243638829

https://www.shutterstock.com/image-photo/grey-rhebok-rhebuck-pelea-capreolus-kruger-571314268

https://www.inaturalist.org/observations/249210122

Furthermore, the darkish vertical mark seen in the grey rhebok is vaguely reminiscent of a pattern in various clades of deer (https://www.inaturalist.org/journal/milewski/55804-the-muzzle-ring-as-a-deep-ancestral-marker-in-deer-part-2-the-gape-spot#).

I postulate the following adaptive function of buccal semets in ungulates:

Ruminants spend much time chewing the cud, which interferes with both forms of non-visual vigilance, viz.

• auditory (owing to the 'white noise' of chewing), and
• olfactory (owing to the overwhelming odour of fermented cud).

To compensate for these sensory detractions, individuals place themselves (usually lying down in sternal procumbency) within sight of each other, facing in various directions.

When any individual spots (or smells or hears) a potential predator, it immediately stops chewing and turns the ear pinnae towards the source of suspicion. It is crucial, for gregarious enhancement of vigilance, that others notice this attention immediately.

Thus, the sensory system of ruminants is programmed with an acute search-image for a particular combination of cessation of small-scale motion plus accompanying initiation of small-scale motion. This combination consists of

• cessation of the rhythmic movements of the jaws, and
• sudden synchronous swivelling of the ear pinnae.

The hypothetical function of buccal semets is to enhance this function, by accentuating the visibility of the chewing motion of the jaws. (Certain ruminants also possess auricular semets, viz. patterns of colouration accentuating the motion of the ears, https://stock.adobe.com/video/sambar-deer-rusa-unicolor-phu-khiao-wildlife-sanctuary-thailand-seen-standing-in-the-rain-facing-the-camera-then-turns-its-head-towards-the-right-side-of-the-frame/438778359?prev_url=detail and https://www.inaturalist.org/posts/55694-ten-best-illustrations-of-auricular-semets-in-deer#.)

It is noteworthy that the buccal semet of the grey rhebok is dark, whereas that of the common impala (Aepyceros melampus) is pale (https://www.inaturalist.org/posts/100574-a-new-feature-of-adaptive-colouration-in-the-grey-rhebok-pelea-capreolus-the-buccal-semet#activity_comment_2ea718ad-7d10-4222-a572-5d1cd61ecbe4). This difference may perhaps be explained by a difference in patterns of activity between these two bovids.

Both the grey rhebok and the common impala tend to ruminate in the open (as opposed to under cover of vegetation), where visual vigilance is enhanced during periods of lying down. However, the grey rhebok is cathemeral (https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095555142 and https://treatment.plazi.org/GgServer/html/03F507139908FFB303D5F84BF585F2FE), whereas the common impala tends to lie down only at night (https://www.inaturalist.org/posts/49366-locomotory-and-postural-peculiarities-of-impalas-part-1#).

This suggests that the mutual monitoring of jaw-movements is performed usually in the dark in the common impala, versus often in the bright light of day in the grey rhebok. A pale feature is likely to be more conspicuous than a dark feature, by night.

What emerges from my scrutiny of the grey rhebok is two conspicuous features of colouration, as follows.

The caudal flag is large-scale, strikingly pale, activated by a combination of running and erection of the tail, and directed at potential predators, at distances of up to 0.5 km (https://www.inaturalist.org/observations/204059396 and https://www.inaturalist.org/observations/247276002).

The buccal semet is small-scale, subtlely dark, activated by a combination of cessation of chewing and synchronous swivelling of the ears, and directed intraspecifically, within the group, at distances of only up to 20 m (second photo in https://uk.inaturalist.org/observations/189700627).

Posted on October 19, 2024 04:40 PM by milewski