Surprising adaptive radiation of the dark-pigmented flank-band in hippotragin bovids

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A dark-pigmented flank-band (https://www.flickr.com/photos/42964440@N08/16581370619) occurs in various tribes of Bovidae (https://en.wikipedia.org/wiki/Bovidae).

This feature is most obvious in gazelles (https://www.istockphoto.com/photo/young-gemsbok-at-ozonjuitji-mbari-waterhole-in-etosha-national-park-namibia-gm1405961116-457681462?phrase=gemsbok&searchscope=image%2Cfilm).

However, it occurs also, in subtle and variable form, in hippotragin bovids (https://www.inaturalist.org/observations?place_id=any&taxon_id=846296&view=species).

How does a dark flank-band function, as a product of natural selection?

(As a matter of contemporary interest, this is the unsatisfactory answer given today by Artificial Intelligence, in the form of bard.google. com: https://bard.google.com/chat/af66f2ed750067e8.)

CONCEPTUAL FRAMEWORK

In the adaptive colouration of ruminants, the basic dichotomy is between enhancing inconspicuousness on one hand, and enhancing conspicuousness on the other.

Certain species, inhabiting dense vegetation, are adaptively inconspicuous to onlookers (particularly predators) by virtue of either

plain, countershaded colouration (https://www.flickr.com/photos/rainbirder/8299306177), or
complex patterns designed to disrupt the figure (https://www.zoochat.com/community/media/chobe-bushbuck-tragelaphus-sylvaticus-ornatus.487415/), or
a combination of both.

By contrast, certain other species of ruminants, inhabiting open vegetation, are adaptively conspicuous - a phenomenon underplayed in textbooks.

This seems to result from the following cost-benefit situation:

The lack of cover means that the animals are inevitably apparent to onlookers (particularly predators). So, instead of attempting to evade detection, they rely instead on gregarious vigilance and cursorial escape. Accordingly, 'plains game' optimise for ease of intraspecific communication - particularly in aid of group-cohesion and the advertisement of sociosexual status - at the scale of the whole figure.

A dark flank-band can potentially serve either conspicuousness or inconspicuousness. If bold enough, it might make the whole figure obvious. In a more subtle configuration, it might function more like tiger-striping, tending to hide the figure against a somewhat complicated background.

Given the above conceptual framework:

How can we interpret the various configurations of the dark flank-band that have arisen in hippotragins?

This tribe is particularly intriguing, because - even more than in gazelles - the feature of colouration in question has proven to be surprisingly plastic in evolutionary terms.

Among hippotragins, the dark flank-band can be classified as follows:

categorically absent: Hippotragus equinus, Hippotragus leucophaeus, and Addax nasomaculatus (summer pelage);
so poorly-developed as to be irrelevant w.r.t. the relationship to predators: Oryx leucoryx, Oryx dammah, and Oryx callotis;
sufficiently well-developed to aid overall conspicuousness, in divergent ways: Oryx gazella vs Oryx beisa; and
subsumed into a clear-cut distinction between relatively dark flank and pale ventral pelage on the torso: Hippotragus niger and Addax nasomaculatus (winter pelage).

In no species of hippotragin is the dark flank-band as conspicuous as in typical gazelles. This is because it is situated too low (i.e. too far in a ventral direction) on the torso for either its darkness or its offsetting by the whitish, ventral to it, to raise it to visual prominence.

The species in which the dark-pale contrast on the lower flank is most conspicuous is H. niger, in which the flank-band is hardly recognisable as such.

This contrast is so strong in H. niger that it clearly enhances overall conspicuousness (https://www.shutterstock.com/image-photo/female-sable-antelope-hippotragus-niger-natural-274681958). This applies even to females of Hippotragus niger roosevelti, which always lack the conspicuous overall darkness of males.

DETAILED FINDINGS

In order to illustrate and elaborate the points made above:
first, I present illustrations according to the categorisation above, which crosses generic boundaries, and
second, I present illustrations on a species-by-species basis, in every case choosing views in full profile.

DARK FLANK-BAND ABSENT

Examples:
https://stock.adobe.com/au/images/roan-antelope-hippotragus-equinus-in-the-grass-mountain-in-the-background-savuti-namibia-africa-animal-savannah-antelope-in-the-nature-habitat-nature-wildlife/556297380
https://www.alamy.com/israel-aravah-desert-a-herd-of-scimitar-oryx-or-scimitar-horned-oryx-image67892225.html

DARK FLANK-BAND FAINT

Example:
https://www.alamy.com/stock-photo-fringed-eared-oryx-oryx-gazella-cullotis-tsavo-east-national-park-53906727.html?imageid=96530659-FC1E-4F68-ACDF-D70A8616500F&p=22059&pn=1&searchId=4f0dd42eb1a3853d476f41d97adf47e2&searchtype=0

DARK FLANK-BAND SIMILAR TO THAT IN GAZELLES

Examples:
https://www.alamy.com/east-african-oryx-oryx-beisa-in-samburu-national-reserve-kenya-africa-image359795231.html
https://www.sciencephoto.com/media/643368/view/beisa-oryx-herd-in-awash-national-park
https://kakytravel.com/location/samburu-game-reserve-buffalo-spring-national-reserve-and-shaba-game-reserve/

DARK FLANK-BAND RECRUITED TO ANTI-COUNTERSHADING

Example:
https://stock.adobe.com/au/images/gemsbok-oryx-gazella-herd-at-waterhole-kalahari-desert/63594553

DARK FLANK-BAND SUBSUMED INTO FLANK/BELLY CONTRAST

Example:
https://www.alamy.com/small-group-of-sable-antelope-hippotragus-nigersouth-africa-image68948070.html

Now, turning to a species-by-species account:

HIPPOTRAGUS EQUINUS

This is the only extant species of hippotragin in which there is hardly any trace of a dark flank-band (https://www.lategansafaris.com/project/roan-antelope-hippotragus-equinus/ and https://www.reserveafricainesigean.fr/en/animals/roan-antelope/# and https://upload.wikimedia.org/wikipedia/commons/1/10/Roan_antelope_%28Hippotragus_equinus_koba%29.jpg and https://africafreak.com/roan-antelope and https://www.dnazoo.org/post/born-to-roan).

The countershading on the torso in H. equinus conforms to adaptively inconspicuous colouration.

However, even in this species, some individuals (particularly in H. e. koba) retain a relictual flank-band (https://www.inaturalist.org/observations/103681190 and https://www.inaturalist.org/observations/156041605 and https://www.inaturalist.org/observations/143620133 and https://www.inaturalist.org/observations/39846679 and https://www.alamy.com/stock-photo-hippotragus-equinus-roan-antelope-30156559.html?imageid=D0A51919-8339-45E1-84B6-7F5F19351297&p=87775&pn=1&searchId=5b7e805efde1b320f1fde4bd1bddb9a3&searchtype=0).

HIPPOTRAGUS NIGER

Please see illustrations in https://www.inaturalist.org/journal/milewski/87728-is-the-sable-antelope-the-only-ungulate-indeed-mammal-that-advertises-its-penis-with-black-and-white-punctuation#.

There is consistently, in every adult individual of this species, a conspicuously clear-cut boundary between the white pelage on the ventral surface of the torso, and the adjacent ground-colour.

The existence of a flank-band in H. niger niger is revealed ontogenetically.

This is illustrated in Fig. 13 E on page 153 of Grobler (1980, file:///C:/Users/Antoni%20Milewski/Downloads/Body_growth_and_age_determination_of_the_Sable_Hip.pdf).

Juveniles, by seven months old (https://stock.adobe.com/images/sable-calf/221519307?prev_url=detail and https://www.alamy.com/sable-antelope-hippotragus-niger-calf-image376925035.html and https://www.alamy.com/sable-antelope-hippotragus-niger-mother-with-calf-image344167796.html), tend to shed the natal pelage in a horizontal band between the medium-tone pelage of the torso and the ventral whitish of the abdomen. The new hairs are precocially dark.

The ground-colour on the flanks tends to be dark, even in females of a subspecies (H. n. roosevelti) in which females never turn as dark as adult males (https://www.alamy.com/stock-photo-adult-female-sable-antelope-in-the-shimba-hills-reserve-mombasa-kenya-18911057.html).

HIPPOTRAGUS LEUCOPHAEUS

In the absence of information for this extinct species (https://www.researchgate.net/publication/362177022_Blue_turns_to_grey_-_Palaeogenomic_insights_into_the_evolutionary_history_and_extinction_of_the_blue_antelope_Hippotragus_leucophaeus), I assume similarity to H. equinus.

However, mature males are reputed to have been pale enough to resemble pallid spp. of Oryx.

ORYX LEUCORYX

The dark flank-band is faint in most individuals (https://www.flickr.com/photos/dave_irving/32018404501 and https://www.alamy.com/arabian-oryx-oryx-leucoryx-female-with-young-walking-in-sand-image7311051.html and https://www.alamy.com/arabian-oryx-walking-in-the-desert-of-the-middle-east-arabian-peninsula-image336214533.html and https://www.agefotostock.com/age/en/details-photo/arabian-oryx-standing-in-sand-oryx-leucoryx/SSJ-120371).

The maximum development is shown in the following, unusual individual (https://www.denverpost.com/2011/10/02/with-arizona-help-fabled-arab-antelope-lives-anew/).

In O. leucoryx, the whole torso is extremely pale, making the figure extremely conspicuous (https://www.thenationalnews.com/uae/oryx-must-be-saved-by-setting-up-wildlife-reserve-1.393069). The flank-band is thus functionally redundant, in convergence with the summer pelage of Addax nasomaculatus.

ORYX DAMMAH

The following (https://a-z-animals.com/animals/scimitar-horned-oryx/pictures/ and https://www.zoochat.com/community/media/scimitar-horned-oryx-oryx-dammah.488627/ and https://riosafari.com/en/scimitar-horned-oryx/ and https://www.alamy.com/stock-photo-scimitar-horned-oryx-oryx-dammah-129676760.html) give the impression that a flank-band is absent in Oryx dammah.

However, in many individuals the feature remains in vestigial form, particularly just posterior to the elbow and near the knee (https://pixabay.com/photos/scimpter-horned-oryx-desert-negev-4764743/ and https://commons.wikimedia.org/wiki/File:Scimitar-horned_Oryx_%28Oryx_dammah%29.jpg and https://www.flickr.com/photos/blmiers2/4939145696 and https://www.mediastorehouse.com.au/uig/animals/outdoors/scimitar-horned-oryx-oryx-dammah-9486465.html and https://www.alamy.com/scimitar-horned-oryx-an-endangered-species-walking-through-the-negev-desert-on-yotvata-hai-bar-nature-reserve-breeding-center-oryx-dammah-israel-image342292910.html and https://www.shutterstock.com/image-photo/very-rare-scimitarhorned-oryx-dammah-extinct-1932362348).

In O. dammah, the flank-band is redundant, because the whole torso is adaptively conspicuousness by virtue of large-scale pallor on the flanks and scapulae (https://www.researchgate.net/figure/Skeleton-of-a-cow-showing-the-anatomical-position-of-the-scapula_fig2_302523940), contrasting with relative dark on the forequarters (https://fineartamerica.com/featured/scimitar-oryx--oryx-dammah-eyal-bartov.html).

ORYX CALLOTIS

The dark flank-band is fairly well-developed in most individuals (https://www.wildlifeofkenya.com/categories/ongulates/oryx-fringe-eared/).

However, it confers negligible adaptive conspicuousness, because the ventral surface of the torso is hardly paler than the flanks (https://www.gettyimages.com.au/detail/photo/fringe-eared-oryx-gazella-beisa-walking-on-the-royalty-free-image/520070640?phrase=fringe+eared&adppopup=true and https://www.inaturalist.org/observations/18775851).

Instead, it is plausible that, in O. callotis, the flank-band tends to reduce conspicuousness, by means of disruptive colouration (https://www.inaturalist.org/observations/132020185 and https://www.inaturalist.org/observations/97548471 and https://eol.org/pages/4447255/media and https://www.shutterstock.com/image-photo/east-african-oryx-beisa-aka-tsavo-2129271320).

Please see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1634902/.

ORYX BEISA

In this species, the flank-band is narrow (https://www.inaturalist.org/photos/60795830).

However, it is dark enough to be conspicuous owing to its contrast with the mainly pale pelage of the torso (https://www.alamy.com/oryx-oryx-gazella-samburu-national-park-kenya-image60253883.html and first photo in https://eol.org/pages/4447253/media?page=4).

Of all hippotragins, O. beisa most resembles the pattern epitomised by gazelles. However, it falls short of most spp. of gazelles, in terms of conferring graphic conspicuousness in full profile (https://www.flickr.com/photos/rainbirder/5523479215 and https://www.flickr.com/photos/bpophoto/40834372585).

ORYX GAZELLA

In this species, the dark flank-band is consistently dark, and broader than in any other hippotragin (https://www.shutterstock.com/image-photo/gemsbok-gemsbuck-south-african-oryx-large-2186304975 and https://naturerules1.fandom.com/wiki/Gemsbok?file=Gemsbok.jpg and https://www.africa-wildlife-detective.com/gemsbok.html).

The following (https://www.istockphoto.com/photo/gemsbok-at-etosha-national-park-in-kunene-region-namibia-gm1412597579-461960331?phrase=gemsbok&searchscope=image%2Cfilm) is particularly instructive.

Note the conspicuousness, even at distance, of a species of gazelle, namely Antidorcas marsupialis, in the background. This is owing to not only the presence of a dark flank-band in this gazelle, but more importantly the high position of this band, allowing the ventral white to catch sunlight in an eye-catching way.

The important inference is that the flank-band of O. gazella does not function in the same way. Instead, when viewed in full profile, it seems to serve mainly to provide dark underscoring to the medium tone of the torso (https://safaripartner.com/blog/facts-about-beisa-oryx).

What this amounts to is a particularly odd permutation of colouration ('anti-countershading') in a large mammal, as follows:

The white ventral panel - with its intrinsic potential for either countershading or highlighting - remains fully intact. However, it is overridden by the 'false-shading' effect of the relatively ventrally-located dark flank-band.

Thus, my interpretation of the function of the flank-band is that it enhances conspicuousness, in contrast to the closely-related O. callotis, and in a different way from the closely-related O. beisa.

ADDAX NASOMACULATUS

The following (https://factanimal.com/addax/ and https://www.superstock.com/asset/addax-addax-nasomaculatus-captive-specimen-addax-also-known-white-antelope/1990-12893722) show the absence of pattern on the flank in summer pelage.

In A. nasomaculatus, a flank-band is, in a sense, redundant, because the pallor of the whole torso is already extremely conspicuous.

The following (https://www.dreamstime.com/royalty-free-stock-photo-addax-1-image18005195 and https://pixabay.com/photos/addax-negev-desert-desert-dweller-4764789/) show how crisp the distinction can be, in winter pelage, between the medium-tone of the flank and the white of the belly.

This feature is adaptively ambivalent. This is because

the flank is not dark enough to produce dark-pale contrast, as in H. niger; in A. nasomaculatus. any conspicuousness of the torso tends to be eclipsed by the crisply-defined pale of the hindquarters (https://www.peakpx.com/en/hd-wallpaper-desktop-ghxfo and scroll to first photo in https://fossilrim.org/blog/masters-project-aims-to-aid-captive-addax/ and https://www.rzss.org.uk/news/article/21834/genetic-research-boosts-conservation-efforts-for-critically-endangered-antelope/), and
the pattern on the torso conforms approximately to countershading (which by definition is inconspicuous).

DISCUSSION

A principle in adaptive colouration, underplayed in the literature, is that a given feature of design can have different - even contrary - effects depending on 'design-context'.

For example, the same band/stripe/panel, located in the same position on the figure, can have opposite effects, according to other design-features around it and elsewhere on the figure.

The following are two examples of this principle.

Firstly:

The belly is whitish/white in both Oryx beisa and Oryx gazella (as in most other hippotragins, as well as most gazelles). However,

in O. beisa the ventral whitish tends to offset the (narrow) dark flank-band, enhancing conspicuousness, whereas
in O. gazella the ventral white effectively 'disappears'; the (broad) dark flank-band prevails in darkly defining the ventral silhouette of the figure, thus eclipsing the white below it in plain sight.

Secondly:

Please closely examine the following of O. gazella (https://www.dreamstime.com/stock-photo-isolated-side-profile-gemsbok-oryx-lone-standing-dry-dusty-plains-etosha-image65067398) and H. niger (https://www.shutterstock.com/image-photo/female-sable-antelope-cow-standing-african-729877612).

In both cases, there is a distinct boundary-line running from the elbow (https://afs.ca.uky.edu/livestock/beef/skeletal/elbow-joint and https://afs.ca.uky.edu/livestock/beef/parts/elbow) to the stifle-fold (https://www.agric.wa.gov.au/beef-cattle/welfare-decisions-sheep-and-cattle?page=0%2C1 and https://afs.ca.uky.edu/livestock/beef/skeletal/stifle-joint).

The important differences are that, in O. gazella

the dark lies ventral to this boundary, whereas in H. niger the dark lies dorsal to this boundary, and
the white of the belly is restricted inconspicuously to the shaded ventral surface, whereas in H. niger the white of the belly extends conspicuously on to the sunlit ventral part of the flank.

My interpretation is that a single ancestral boundary, between a dark flank-band and a white belly, has evolved contrarily in these two spp., as follows:

in O. gazella, the shift of the flank-band in a ventral direction has given it a function of conspicuously dark underscoring of the torso - which can be thought of as 'anti-countershading', whereas
in H. niger, the shift of the flank-band in a dorsal direction (together with its seamless blending with the ground-colour on the flank) has given it a function of conspicuously pale underscoring of the torso.

By the same token, a single ancestral feature, viz. white ventral countershading, has become redundant in O. gazella, whereas it has become the most noticeable feature on the whole torso in H. niger.

In summary:

In hippotragins, adaptive radiation in the patterns studied here has arisen