Nyala as the only ruminant with an erectile crest specialised for courtship

@ludwig_muller @jeremygilmore @tonyrebelo @capracornelius @oviscanadensis_connerties @davidbygott @beartracker @jacqueline_llerena @jakob @tandala @jwidness

Groves and Grubb (2011, http://www.ultimateungulate.com/NewTaxonomy.html) raised the nyala (https://www.shutterstock.com/nb/image-photo/male-nyala-tragelaphus-angasii-walking-against-215233366 and https://www.shutterstock.com/nb/image-photo/nyala-kruger-national-park-south-africa-1149448781 and https://southafrica.co.za/nyala-social-relationships.html#:~:text=The%20brilliant%20coat%20of%20the,to%20access%20to%20oestrus%20cows.) to the status of a monospecific genus.

This is a move that Wikipedia acknowledges, but has not actually adopted (https://en.wikipedia.org/wiki/Nyala).

There may still be many naturalists who doubt that this tragelaphin (https://en.wikipedia.org/wiki/Tragelaphini) is distinct enough to warrant its own genus.
 
I can support the generic distinction, with an aspect of the nyala that has been overlooked.
 
As far as I can see, Nyala is unique among mammals in that the male possesses an erectile organ specialised for sexual adornment (https://www.agefotostock.com/age/en/details-photo/nyala-tragelaphus-angasii-adult-male-displaying-eastern-cape-south-africa/FHR-10840-00058-676 and https://www.shutterstock.com/nb/image-photo/nyala-couple-running-kruger-national-park-1345181534). No other tragelaphin – let alone any other ungulate – seems to resembles Nyala in this way. (However, we need to check the lesser kudu, https://en.wikipedia.org/wiki/Lesser_kudu.)
 
Everyone knows that the masculine tail of Pavo cristatus (https://en.wikipedia.org/wiki/Indian_peafowl) is an erectile organ, restricted to the male, and used mainly for courtship.

The peacock’s tail is so familiar as to seem iconic. However, I cannot think of any mammal that resembles this bird in possessing an analogous erectile organ of sexual display, other than the genus Nyala - and its sole species, Nyala angasii.

I refer mainly to the white dorsal crest (https://www.youtube.com/watch?v=zFQsrgdt8u4 and https://www.shutterstock.com/nb/image-photo/nyala-bull-full-display-2083792402 and https://www.shutterstock.com/nb/image-photo/male-nyala-109644335). This is absent in females (https://www.canstockphoto.com/nyala-antelope-7747867.html).

However, there is a subsidiary fringe of erectile fur (dark, not pale) on the posterior haunch (https://www.istockphoto.com/photo/nyala-grazing-gm1223251377-359259721 and https://www.istockphoto.com/photo/nyala-close-up-gm488392602-74103499). As far as I know, this has no analogue in any other ungulate, because this fringe is erected in courtship, not in anti-predator alarm.

The tail of Nyala is superficially similar to that of other tragelaphins. However, it is extreme in its size and the fact that it is raised in courtship, not in alarm (https://www.agefotostock.com/age/en/details-photo/nyala-tragelaphus-angasi-male-displaying-mkuzi-game-reserve-south-africa/FHR-10840-00039-249 and https://fineartamerica.com/featured/nyala-peter-chadwickscience-photo-library.html and https://www.iol.co.za/news/south-africa/limpopo/nyala-kills-elderly-woman-report-1454853).

I cannot claim that the size and white underside of the tail are, as such, specialised for courtship. This is because females of the nyala also possess a luxuriant (and surprisingly underused) tail (https://es.123rf.com/photo_87269098_female-nyala-antelope-tragelaphus-angasii.html and https://www.alamy.com/stock-photo-nyala-tragelaphus-angasi-female-kenya-111616124.html?imageid=C3EA7836-158B-4C97-A22B-022077969286&p=329545&pn=1&searchId=a90ebf4b2d7e583aef5987e9fbaaf0b0&searchtype=0). However, the erection of the tail in display seems to be exclusively masculine, and exclusively for sexual display.
 
It is important to distinguish between three alternative functions of any erectile organ of display on the body:

• anti-predator exaggeration of body size,
• intimidation in masculine rivalry, and
• enhancement of appearance in courtship.

An example of anti-predator exaggeration is the dorsal crest of the aardwolf (Proteles cristatus, https://www.joelsartore.com/ani062-00221/ and https://m.facebook.com/NowScienceNews/photos/the-aardwolf-is-in-the-same-family-as-the-hyenas-they-eat-insects-mainly-termite/1017630185320382/ and scroll in https://weaselle.tumblr.com/post/643613557844557825/aardwolf and http://wildlifeanimalz.blogspot.com/2014/05/Aardwolfs-Facts.html). This occurs in both sexes and is spectacularly raised in confrontation with predators, serving greatly to increase the apparent body size of the aardwolf.

The dorsal crest of Nyala is different, because this genus neither displays to predators, nor ‘stots’ after the fashion of the springbok (https://www.inaturalist.org/taxa/42283-Antidorcas-marsupialis).
 
An example of intimidation in masculine rivalry can probably be found somewhere in mammals: a crest raised by males in order to impress each other, in contests for territory or hierarchical status, but not displayed in courtship.

It is true that males of Nyala frequently erect the dorsal crest in antagonistic, mutual display. However, the crucial point is that this is seldom if ever accompanied by fighting. It is best viewed as a corollary of courtship in a loose sense, i.e. something more resembling a ‘lekking’ display (https://en.wikipedia.org/wiki/Lek_mating) than part of male-male conflict as such.
 
I do not recall any mammalian example of an erectile structure, other than the penis, that is displayed solely for enhancement of appearance in courtship. Perhaps some species of primate has an erectile ruff of fur, that it uses only for sexual appeal?

There are various sexually dimorphic cervids (https://en.wikipedia.org/wiki/Deer). However, their sexual dimorphism is centred on the antlers (https://en.wikipedia.org/wiki/Antler), rather than pelage or colouration. As far as I know there is no species of deer with a sexually dimorphic mane or dorsal crest.

Among bovids and giraffids, manes are common. However, in most species they are permanently erect.

Furthermore, they tend to be proportionately best-developed in juveniles and females, being proportionately small in mature males, and not particularly shown off in male rivalry or courtship. I refer here to giraffes (https://sciencing.com/baby-giraffes-8632947.html), hippotragins (https://commons.wikimedia.org/wiki/File:Roan_Antelope_%28Hippotragus_equinus%29_%2831281093687%29.jpg), wildebeests (https://www.shutterstock.com/nb/image-photo/blue-wildebeest-connochaetes-taurinus-waterhole-kalahari-205945969), serows (https://en.wikipedia.org/wiki/Serow#/media/File:Serow_Capricornis_sumatraensis.JPG), etc.

Although males of Nyala do possess a mane on the neck (https://stock.adobe.com/hu/search/images?k=nyala&asset_id=230008481 and https://stock.adobe.com/hu/search/images?k=nyala&asset_id=185957261 and https://stock.adobe.com/hu/search/images?k=nyala&asset_id=218075373), this is less noticeable than is the erect dorsal crest. I suspect that the nuchal mane is permanently erect, not erectile.
 
My interpretation of crest-raising in males of Nyala is that this – as in peafowls and a limited range of other birds possibly including certain birds of paradise – is essentially a function of courtship. A reading between-the-lines of Estes (1991) bears this out, even though Estes did not put it that way. Some clues include: males in Nyala do occasionally fight, but the crest does not seem to be raised during fighting.

When males contest for females by displaying their erect crests, the largest male individual, according to Estes (1991), ‘always wins’. And in courtship males always displays the erect crest to females, in what seems like analogy to the Indian peafowl.

Females accept males based largely on how large and healthy the individual looks, with his erectile adornments raised. This is as opposed to accepting whichever individual male wins physical contests. (The nyala does not show territorial behaviour.)
 
In short, I suggest the following:
The genus Nyala is the closest thing in the ungulate world, and possibly in the whole mammalian world, to certain birds in terms of having evolved a specialised dorsal crest, of an erectile nature, that functions mainly for courtship, and only secondarily (or as corollary) in male rivalry.

In being an ‘ungulate peacock’, Nyala shows its uniqueness, and deserves to be placed in its own genus.
 
I suspect that, in the bewildering panoply of mammalian life, there are species I have overlooked in the generalisations I have attempted here. Although various caveats may emerge in due course, I leave readers with this working hypothesis:
 
The genus Nyala is unique among all mammals in possessing, in males, an erectile adornment specialised for courtship.

Posted on July 7, 2022 12:49 AM by milewski