Adaptive radiation of auricular flags in hippotragin bovids: Hippotragus, Oryx, and Addax, part 2

...continued from https://www.inaturalist.org/journal/milewski/88041-adaptive-radiation-of-auricular-flags-in-hippotragin-bovids-hippotragus-oryx-and-addax-part-1#

SUMMARY OF ANTERIOR AURICULAR FLAGS

Species qualifying for an anterior auricular flag are

• Hippotragus equinus (juveniles),
• Oryx gazella and Oryx beisa (which share a pattern),
• Oryx callotis (which differs in pattern from O. beisa), and
• possibly Addax nasomaculatus (winter pelage).

The clearest depictions so far, of an anterior auricular flag in A. nasomaculatus, are:
https://www.alamy.com/addax-addax-nasomaculatus-hai-bar-reserve-israel-image3637497.html?imageid=81028A0B-4151-454F-A2A3-080D3EED99FB&p=11788&pn=3&searchId=51a17103163bda311280e6d62ad3f212&searchtype=0
https://africageographic.com/stories/critically-endangered-addax-antelope-reintroduced-chad/.
scroll to second photo in https://wildresponse.org/blog/what-species-are-going-extinct/.

DISCUSSION

The bare skin at the centre of the front-of-ear is

• flesh-coloured in Hippotragus,
• blackish (protective against ultraviolet by melanin) in Oryx, and
• unknown (but perhaps flesh-coloured, https://stock.adobe.com/images/curved-horned-antelope-addax-addax-nasomaculatus-it-is-listed-a-critically-endangered-species/296340426?prev_url=detail and scroll in https://www.12images.net/endangered-and-extinct-animals/the-addax/ and scroll to 16th photo in https://fossilrim.org/blog/what-goes-into-antelope-breeding-decisions/) in Addax.

This partly reflects an ecological difference, viz. that Hippotragus is more shade- and water-dependent than Oryx. I have yet to see any photo clearly revealing the degree of pigmentation in the case of A. nasomaculatus, in which - paradoxically for a species extremely adapted to desert - the ear pinna does not seem to function as a thermoregulatory panel.

As in my study of dark flank-bands (https://www.inaturalist.org/journal/milewski/87889-surprising-adaptive-radiation-of-the-dark-pigmented-flank-band-in-hippotragin-bovids#), I find O. callotis to be surprisingly different from O. beisa. This suggests that these are different species, not merely subspecies.

The colouration of the ears in hippotragins has radiated, evolutionarily, to the same degree as the facial colouration, but not necessarily in ways congruent with the latter. The emphases only partly correspond.

This evolutionary radiation involves

• size of ear pinna (largest in H. equinus, smallest in O. dammah)
• shape of ear pinna (sickle-shaped in H. equinus, narrow in H. niger, O. callotis and A. nasomaculatus, broad in O. gazella, nondescript-oval in O. dammah, O. leucoryx and O. beisa)
• degree to which the pelage on the front-of-ear opens to shed body heat under thermoregulatory stress (obvious in H. niger, discernible in O. beisa, yet to be detected in A nasomaculatus)
• apical tufting of ear pinna (only in H. equinus and O. callotis, and individually variable in both spp.)
• density/length/coverage of pelage on the front-of-ear (hairs long and dense in H. equinus and A. nasomaculatus, short and sparse in most spp. of Oryx, intermediate in O. callotis and H. niger)
• visible degree of pigmentation of bare skin on front-of-ear (maximal = blackish in Oryx spp., minimal = flesh-coloured in H. niger)
• pallor (= depigmentation) of pelage on ear pinna (whitish in Hippotragus, A. nasomaculatus, and most spp. of Oryx, medium-tone in O. callotis)
• apical darkness (without tufting) on ear pinna (absent without trace in O. dammah and O. leucoryx, small in H. niger and some individuals of H. equinus, medium-size in O. beisa and O. gazella)
• narrow dark outlining of ear pinna (extensive in O. gazella, restricted and hardly noticeable in O. beisa)
• involvement of base of ear pinna (noticed, so far, only in O. gazella and A. nasomaculatus in winter pelage)
• exaggeration of size of ear pinna in juveniles (noticeable only in Hippotragus, https://es.123rf.com/photo_121615261_hermosa-cabra-con-grandes-cuernos-se-encuentra-bajo-el-sol.html)
• sexual dimorphism (pallor on back-of-ear in H. equinus in mature males, see https://www.inaturalist.org/journal/milewski/88277-adaptive-radiation-of-auricular-flags-in-hippotragin-bovids-hippotragus-oryx-and-addax-part-3#).

My impression is that variation among hippotragins in the proportional size of the ear pinnae is adaptive more in terms of signalling (facial expression) than in terms of thermoregulation or acuteness of hearing.

The precociality of the ear in Hippotragus

• involves both the size of the ear pinna and its dark apical feature (https://www.alamy.com/stock-photo-roan-antelope-hippotragus-equinus-mokala-national-park-south-africa-125537973.html?imageid=1A4F3945-92B0-4867-8735-35BCC8778461&p=361664&pn=7&searchId=48c5019063f87c658c767ebd85f25bc9&searchtype=0), and
• deserves explanation, in the context of the overall natural history of H. equinus, in particular.

The dark apical tuft of H. equinus shows

• individual and possibly sexual variation (https://www.shutterstock.com/image-photo/roan-antelope-hippotragus-equinus-head-shoulders-217784422), and
• changes with growth from infancy to maturity (https://www.shutterstock.com/image-photo/roan-antelope-498697057) which run contrary to expectations for sociosexual adornments.

In juveniles, the apical tuft may be as large as the horns (https://www.shutterstock.com/image-photo/roan-antelope-498697057). By contrast, in adolescent males it may wane so completely that not even an apical dark spot remains (https://www.alamy.com/stock-photo-roan-antelope-hippotragus-equinus-adult-w-regional-park-w-transborder-59189738.html?imageid=66386DE1-36D7-4F8B-A3A8-783EC28B2A82&p=86574&pn=5&searchId=5ed1e1ea051df95555f6aaed35fb537c&searchtype=0).

My overall conclusion is as follows.

In hippotragins, the tendency for facial conspicuousness is not paralleled by a commensurate tendency for auricular conspicuousness.

Both the face and the ear pinnae seem maximally conspicuous in O. gazella. However, even in this species, the pale pelage on the front- and back-of-ear do not achieve the whiteness of parts of the face.

What I show in this Post is that an ambivalent conspicuousness of the ears results from a surprising variety (= radiation) of configurations in the various spp. of hippotragins. With respect to the ears, no species could be predicted on the basis of a knowledge of the rest of the species.

This suggests that the ear pinnae and their colouration are finely attuned, adaptively, to the particular habitat and niche of each species.

Hippotragus equinus is perhaps the largest animal on Earth that is specifically adapted to moist/dystrophic savannas, with accordingly modified anatomy, nutrition, physiology, immunology, sociosexual organisation, mutualism with other spp,, etc. Its ears are part of a distinctive syndrome, in ways that remain to be explained.

SUMMARY OF MOST SURPRISING FINDINGS

• Hippotragus equinus shows both ontogenetic (precocial, https://www.alamy.com/young-roan-antelope-hippotragus-equinus-running-with-ears-out-to-the-side-busanga-plains-kafue-national-park-zambia-image263015759.html?imageid=E8BB00CB-F743-4F43-A532-D0FF1C1EAB39&p=269487&pn=1&searchId=d3350f0b953b17636829cec591564ee8&searchtype=0) and sexual (masculine, see https://www.inaturalist.org/journal/milewski/88277-adaptive-radiation-of-auricular-flags-in-hippotragin-bovids-hippotragus-oryx-and-addax-part-3) differentiation in its auricular flags
• Oryx dammah, O. leucoryx, and Addax nasomaculatus, although extremely adapted to aridity and heat in the Saharan/Sahelian/Arabian region, have ear pinnae apparently less adapted for thermoregulation (by virtue of size and heat-radiating surface area) than those of Hippotragus and Oryx gazella
• Oryx beisa, O. callotis, and O. gazella, although closely-related enough to constitute a superspecies, differ categorically in the incidence of auricular flags
• the incidence of dark apical tufts on the ear pinnae fails to show either phylogenetic or sexual correlation in hippotragins, and is also remarkably variable individually (https://www.alamy.com/stock-photo-roan-antelope-hippotragus-equinus-portrait-burkina-faso-59452543.html?imageid=EE57A30A-C272-49FD-8B31-305F0CC22778&p=86574&pn=3&searchId=bf54820912fa8b8654b0587c83cd4188&searchtype=0)
• (see also part 3) the pelage of the ear pinnae - remaining whitish throughout the year - is exempt from the otherwise extensive seasonal change in the pelage of A. nasomaculatus.

to be continued in https://www.inaturalist.org/journal/milewski/88277-adaptive-radiation-of-auricular-flags-in-hippotragin-bovids-hippotragus-oryx-and-addax-part-3#...

Publicado el enero 3, 2024 11:23 TARDE por milewski