A new feature of adaptive colouration in ungulates: the fibular flag

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Please see https://www.inaturalist.org/journal/milewski/70293-the-bambis-part-9-bleezes-flags-and-semets-in-the-bovid-genus-raphicerus#.

INTRODUCTION

Dear reader, here is a glimpse deep into the secret world of intraspecific communication in two
species of bambis.

Members of the antilopin genus Raphicerus are not gregarious. However, they tend to be monogamous.

Furthermore, adult females consort with their offspring, at least until weaning.

Therefore, there is frequent contact among individuals in the 'societies' of Raphicerus, despite the impression that these animals are 'solitary'.

Because bambis are small-bodied enough to hide for most of the time, their intraspecific communications tend to be secretive.

One way to monitor each other's whereabouts, as they forage in the mornings and evenings, is to 'flash' parts of the body that are small and low enough not to be noticed by scanning predators, but pale/bright enough to be conspicuous at fairly close range, at least when moved.

And this brings us to something that zoologists and naturalists may have overlooked.

It seems possible that grysboks (Raphicerus melanotis and Raphicerus sharpei) mediate such social monitoring, at least in part, by means of a certain part of the body that nobody has paid attention to: the inner surface of the hindleg, just above the hock (https://en.wikipedia.org/wiki/Hock_(anatomy)).

The pelage of the inner surface of the upper hindleg is oddly pale in these species.

Raphicerus melanotis:

http://cameratrap.mywild.co.za/p/cape-grysbok.html

https://www.inaturalist.org/observations/26439871

https://www.inaturalist.org/observations/102342423

Raphicerus sharpei:

https://www.inaturalist.org/observations/9522907

https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&filters%5Bcontent_type%3Aaudio%5D=0&filters%5Binclude_stock_enterprise%5D=0&filters%5Bis_editorial%5D=0&filters%5Bfree_collection%5D=0&filters%5Bcontent_type%3Aimage%5D=1&k=steenbok&order=relevance&safe_search=1&limit=100&search_page=3&search_type=pagination&get_facets=0&asset_id=478595701

https://stock.adobe.com/search?k=grysbok&asset_id=466446249

Scroll in https://africawild-forum.com/viewtopic.php?p=234341#p234341

By comparison, this same surface is not anomalously pale in the third member of the same genus, namely Raphicerus campestris.

The following show subspecies campestris of R. campestris, which coexists with R. melanotis but prefers more open vegetation (also see comment below for many more illustrations):

https://www.inaturalist.org/observations/107377965

https://www.inaturalist.org/observations/106332311

https://www.inaturalist.org/observations/50746594

https://www.inaturalist.org/observations/129193891

https://www.inaturalist.org/observations/122610678

https://www.alamy.com/stock-photo-steenbok-raphicerus-campestris-walking-in-shrubbery-south-africa-western-76154029.html?imageid=D54D52A6-FFDA-4FBE-9283-9390D08886ED&p=1142662&pn=1&searchId=b33750fe1700c0af1e7d56e14ee9b7da&searchtype=0https://es.123rf.com/photo_171698571_alert-steenbok-carnero-capturado-temprano-en-la-ma%C3%B1ana-en-el-parque-nacional-karoo-cabo-occidental.html?vti=n84f9ofebt93ncyyzk-1-81

The following shows that, in some individuals of R. campestris campestris, there is white on the inner, upper surface of the hindleg, but this does not extend to the vicinity of the hock: https://www.inaturalist.org/observations/60722940.

COINING A NEW TERM: 'FIBULAR FLAG'

In the context of adaptive colouration, a flag is defined as a relatively small-scale pattern that becomes conspicuous, at the scale of the whole figure, when activated by movement.

A typical location for flags, in ungulates, is the hindquarters, particularly the tail. And the functions of caudal flags include social and sexual signalling, and the announcement of anti-predator alarm to conspecifics, or the potential predators, or both.

What has been previously overlooked is that, in certain secretive species of ruminants with small tails, the location of flags on the posterior of the figure may have been shifted from the tail to another, more subtle and intriguing, location, namely the hind leg above the hock.

By far the most obvious candidate for a fibular flag is Alces alces (https://www.inaturalist.org/taxa/522193-Alces-alces).

In this large-bodied cervid, the tibial flag covers both the inner and the outer surfaces of the hindleg above the hock. It is individually and seasonally variable, and also depends on illumination.

ILLUSTRATIONS OF FIBULAR FLAG IN ALCES ALCES

https://www.inaturalist.org/observations/135414451

https://www.inaturalist.org/observations/115470987

https://www.inaturalist.org/observations/125843310

https://www.inaturalist.org/observations/121343112

https://www.inaturalist.org/observations/120257251

https://www.inaturalist.org/observations/129646111

https://www.inaturalist.org/observations/119626107

https://www.inaturalist.org/observations/119092478

https://www.inaturalist.org/observations/116521066

https://www.inaturalist.org/observations/134851345

https://www.inaturalist.org/observations/116789745

https://www.inaturalist.org/observations/135324468

https://www.alamy.com/stock-photo-moose-from-behind-142718462.html

https://www.inaturalist.org/observations/131879614

https://www.inaturalist.org/observations/126552983

https://www.inaturalist.org/observations/125993345

Why A. alces is unusual, in possessing a tibial flag, is worthy of further investigation.

However, for now let us return to our bambis, in which any tibial flag is far more subtle.

THINKING LATERALLY IN THE CASE OF RAPHICERUS

In Raphicerus, certain species/subspecies have more countershading (https://en.wikipedia.org/wiki/Countershading) than others.

This countershading is by definition located on the ventral parts of the figure, and the inner surfaces of the upper limbs.

However, on closer examination, there is a significant anomaly.

Countershading - which, by definition, works best when a bright sun is high in the sky- is best-developed in tropical and subtropical subspecies of Raphicerus campestris. It is least-developed in Raphicerus melanotis, which is restricted to the temperate zone

On this basis, one would expect the inner surface of the upper hind leg to be pale in R. campestris, vs not pale in R. melanotis.

However, in reality it is the opposite that is true. Raphicerus melanotis has a pale inner surface on the upper hindleg, whereas R. campestris tends not to have this.

Please scroll to the 11th photo in https://www.africawild-forum.com/viewtopic.php?t=527&start=20 for a particularly clear view of the tibial flag in Raphicerus sharpei.

The fibular flag in R. melanotis and R. sharpei is presumably activated when the animal walks, and it should be particularly visible in the oblique light of evenings and early mornings - when much of the foraging takes place.

The paleness, in R. melanotis, of the inner surface of the hind leg, is puzzling if interpreted simply from the viewpoint of countershading in aid of crypsis. This is because this part of the anatomy

• is paler than the ventral surface of the torso, despite the fact that it
• is not normally visible enough, in the standing figure, to need disguising.

I therefore tentatively suggest that this pale feature functions as a flag, during walking/asymmetrical standing in suitable illumination.

This fibular flag, I hypothesise, aids social monitoring by means of a signal low-profile enough to remain congruous with the furtive habits and overall inconspicuousness of the species concerned.

A similar rationale may possibly apply to an even smaller area of anomalously pale pelage in R. sharpei, which likewise transgresses countershading. This is located on the front surface where the foreleg joins the torso.

The anterior feature can perhaps be called an 'anterior axillary flag'.

https://www.flickr.com/photos/44691276@N06/8250863085

https://es.123rf.com/photo_110620408_sharpe-grysbok-in-kruger-national-park-south-africa-specie-raphicerus-sharpei-family-of-bovidae.html

https://www.alamy.com/stock-image-sharpes-grysbok-raphicerus-sharpei-male-adult-south-africa-mpumalanga-161763316.html?imageid=6332CFDE-0830-42B2-8ED6-8113A09A06AE&p=546796&pn=1&searchId=bb39c60edd2f811fc9c03d99a5112e0b&searchtype=0

https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&filters%5Bcontent_type%3Aaudio%5D=0&filters%5Binclude_stock_enterprise%5D=0&filters%5Bis_editorial%5D=0&filters%5Bfree_collection%5D=0&filters%5Bcontent_type%3Aimage%5D=1&k=steenbok&order=relevance&safe_search=1&limit=100&search_page=3&search_type=pagination&get_facets=0&asset_id=478595701)

Posted by milewski, 27 September, 2022 18:15