Interspecific variation in the form of the chestnut among equids, part 1

(writing in progress)

The chestnut is a small patch of callous bare skin on the inner foreleg in equids (https://en.wikipedia.org/wiki/Chestnut_(horse_anatomy)). horny dark pad on the inner foreleg, found in various shapes, sizes and positions in all species of Equidae

The great variation in size and conspicuousness of the chestnut is an intriguing feature of Equus.

In the various species and subspecies of Equus, the chestnut varies in

placement (location), e.g. high in wild asses and low in horses,
shape, e.g. oval in Asian wild asses, teardrop-shaped in plains zebra, narrow in horses,
size, e.g. large in Asian wild asses and plains zebra, small in African wild ass and Grevy's zebra,
darkness, e.g. dark in Asian wild asses, pale in horses,
colouration on surrounding pelage, e.g. plain whitish in wild asses and extinct quagga, striped in mountain zebras, and
degree of individual variation, e.g. most variable in African wild ass/donkey and plains zebra.

The chestnut is conspicuous in those species/subspecies (e.g. Asian wild asses, extinct quagga) in which it is large, dark, and surrounded by plain pale pelage. It is inconspicuous in horses, and mountain zebras, and so inconspicuous as to be negligible in Grevy's zebra.

To the human eye the chestnut suggests a ‘punctuation mark’ but to the eye of Equus it is movement that is most important. Because the chestnut is located high on the leg, and usually in the shade, I suspect that its modest movement during normal walking/milling provides a kind of ‘tracer’ effect facilitating the monitoring of the group intraspecifically./In some taxa, such as all Asiatic wild asses plus the extinct quagga, the chestnut is large and dark, contrasting with the pale legs, and seems to aid conspicuousness. I suggest that its main function is intraspecific: it allows one individual to monitor the positions of others in its group with the merest of glances as the move about slowly.

In this Post I show the variation in the chestnut among the various species and subspecies of Equus.

The fact that some subspecies of Equus quagga have a conspicuous chestnut (and obvious to any observer because the conspicuousness comes from tonal contrast, not hue) while others have an inconspicuous chestnut (because of the stripes interfering with the outline of the chestnut and obliterating the tonal contrast) seems to ‘prove’ that the chestnut cannot be functional as a means of communication, either intraspecifically or interspecifically, in the species as a whole.

This wide-ranging species is just too variable in appearance to make much sense of at the species-level. And if the conspicuous of the chestnut is ‘accidental’ and thus essentially meaningless in E. quagga, this would seem to cast doubts on its meaningfulness in other equids such as Asiatic wild asses (in which it is as prominent as in the extinct quagga, E. q. quagga).

So, although it seems logical to believe that every feature of animals has some adaptive benefit or cost, the inconsistency of the pigmentation of the pelage around the chestnut in the various subspecies of E. quagga does not support an adaptive, i.e. functional, explanation, does it?

EQUUS KHUR

Chestnut conspicuous: large, dark, surrounded by pale pelage (larger than in Equus quagga, and relatively extended in the posterior direction)

http://www.listeningearth.com.au/blog_images/2009_03/42_TheRann_5.jpg

http://3.bp.blogspot.com/-E87xznrzZgQ/UvCMOlqNsJI/AAAAAAAABz4/3hZS4hXg-Lw/s1600/18.jpg

Equus khur is the only equid in which the chestnut is more conspicuous than the tail-tassel. If a similar pattern of colouration in the eland[Footnote: Taurotragus oryx] is any indication, the chestnut may function in intraspecific communication – perhaps to accentuate the motion of the legs - rather than being a merely vestigial feature.

Equus khur: http://farm1.static.flickr.com/716/31599859035_5f87338ba7.jpg

The following clip
https://www.youtube.com/watch?v=nf4L-_JLj0Y
shows a kind of strobe effect as the pale tails, ostensibly swishing to shoo insects, pass back and forth across the dark-pigmented bare skin of the anal and perineal areas. I also notice from this that Asiatic wild asses (of which E. khur is one of the species) have exceptionally small tail-tassels among equids).

In the following clip
https://www.youtube.com/watch?v=s3wOV5Bn2MU you’ll see a pregnant female. Do you think the colouration of the flanks makes her heavy pregnancy more, or less, conspicuous to a scanning predator?

In the following three clips

https://www.youtube.com/watch?v=fxxUjojPNlc and
https://www.youtube.com/watch?v=_jyDO77QTCc and https://www.youtube.com/watch?v=ftD3F4sUcj8 you can see how conspicuous is the dark, large chestnut. As the animals walk and run, the chestnut is one of the most conspicuous features of the whole animal

In all these clips, please note that the overall appearance of E. khur is CONSPICUOUSLY PALE. This species does not blend into its environment, being instead coloured according to the same principles as springbok. Overall the animals are paler than their surrounds, thus standing out from the environment as gleaming objects. And more particularly the vental whitish, as in springbok, has crept so high on the sides of the animals that it catches the sunlight. So the take-home message here, for me, is that this species of equid at least is coloured overall for conspicuousness rather than hiding. I don’t think this applies to zebras in general, which are far less conspicuous in their normal habitats.

EQUUS KIANG
https://www.researchgate.net/publication/262137189_Equus_kiang_Perissodactyla_Equidae

Chestnut conspicuous: large, dark, surrounded by pale pelage

http://thelife-animal.blogspot.com/2012/04/kiang.html
http://thumbs.dreamstime.com/z/tibetan-wild-ass-equus-kiang-38398726.jpg
https://www.dreamstime.com/tibetan-wild-ass-equus-kiang-image193393958
https://fineartamerica.com/featured/kiang-wild-ass-anthony-mercieca.html
https://www.alamy.com/tibetan-wild-ass-equus-kiang-ladakh-india-image362193055.html
https://www.dreamstime.com/tibetan-wild-ass-equus-kiang-image193393958
https://www.dreamstime.com/tibetan-wild-ass-equus-kiang-hanle-jammu-kashmir-state-india-image115719583
https://www.istockphoto.com/photo/kiang-gm153176772-21586609

Shift https://www.flickr.com/photos/fengwei888/15328703313

EQUUS HEMIONUS

Chestnut conspicuous: large, dark, surrounded by pale pelage

https://t1.ftcdn.net/jpg/00/65/62/84/240_F_65628453_8WwoiR21sCAAToluauMcwzF6bDz8oTrn.jpg
http://i.imgur.com/lchMHAc.jpg
https://depositphotos.com/84702218/stock-photo-turkmenian-kulan-equus-hemionus-kulan.html
https://depositphotos.com/127299840/stock-photo-turkmenian-kulan-equus-hemionus-kulan.html
https://depositphotos.com/121228970/stock-photo-young-onager-equus-hemionus.html
https://depositphotos.com/564252958/stock-photo-onager-equus-hemionus-semi-domesticated.html
https://depositphotos.com/563165334/stock-photo-onager-equus-hemionus-semi-domesticated.html

EQUUS AFRICANUS SOMALIENSIS

Chestnut moderately conspicuous: small, dark, surrounded by pale pelage

http://image.shutterstock.com/display_pic_with_logo/55977/236897122/stock-photo-somali-wild-ass-equus-africanus-somaliensis-236897122.jpg
https://mlpforums.com/uploads/post_images/img-3462184-8-somali-wild-ass.jpg
https://upload.wikimedia.org/wikipedia/commons/b/b7/African_wild_Ass-legs.jpg
http://thumbs.dreamstime.com/z/african-wild-ass-9806809.jpg
http://thumbs.dreamstime.com/x/african-wild-ass-eating-10430422.jpg
http://img03.deviantart.net/d480/i/2009/182/e/9/donkey_11_by_jello88.jpg
http://www.findfast.org/images/somali-wild-ass.jpg
https://www.shutterstock.com/nb/image-photo/somali-wild-donkeyequus-africanus-this-species-767862967

EQUUS ASINUS

Chestnut small, dark

http://www.heavencanwaitequinerescue.org/images/tn_whitedonkeymay09a.jpg
https://zaidzamanhamid.files.wordpress.com/2011/09/5129.jpg
https://www.artstation.com/marketplace/p/n7Jx7/israel-white-donkey-animated-vfx-grace
https://www.canstockphoto.com/white-donkey-18378857.html
https://www.everypixel.com/q/white-donkey?image_id=721284373760805855
scroll in http://longearsmall.com/mt/articles/guestcontributor/archives/2006/12/donkey_hoof_ang.html
https://es.123rf.com/photo_128857977_donkey-isolated-on-white-background.html
https://www.pexels.com/photo/donkey-on-grass-lawn-1579989/
https://www.shutterstock.com/nb/image-photo/cute-gray-wild-donkey-walk-through-1074907874
https://www.shutterstock.com/nb/image-photo/grey-donkey-pasture-86325832
https://www.shutterstock.com/nb/image-photo/donkey-ass-burro-cordoba-argentina-equus-87323425
https://www.shutterstock.com/nb/image-photo/cotentin-donkey-breed-domestic-peninsula-equus-2046981290
https://www.shutterstock.com/nb/image-photo/sweet-donkeys-on-their-pasture-2170928655

shift https://es.123rf.com/photo_737696_a-lovely-grey-donkey-in-a-farmyard.html

EQUUS GREVYI

Chestnut extremely inconspicuous: small and disrupted by striping

shift https://www.istockphoto.com/photo/gr%C3%A9vys-zebra-equus-grevyi-gm164326657-23474911?phrase=grevys%20zebra
shift https://www.istockphoto.com/photo/grevys-zebra-equus-grevyi-group-eating-grass-at-samburu-park-in-kenya-gm1253829983-366288450?phrase=grevys%20zebra
shift https://www.istockphoto.com/photo/stripy-grevys-zebra-gm147655000-6691970?phrase=grevys%20zebra
shift https://www.istockphoto.com/photo/butt-of-a-gr%C3%A9vy-or-imperial-zebra-community-gm178188420-25104551?phrase=grevys%20zebra

EQUUS ZEBRA

Chestnut inconspicuous: large, dark, but disrupted by striping

https://stock.adobe.com/search?k=%22cape+mountain+zebra%22&asset_id=350723172
https://s3.amazonaws.com/photos.safaribookings.com/library/southafrica/xxl/Camdeboo_National_Park_006.jpg
https://www.gettyimages.ae/detail/photo/hartmanns-mountain-zebra-at-a-waterhole-etosha-royalty-free-image/1370760385?adppopup=true
https://www.alamy.com/stock-photo-cape-mountain-zebra-equus-zebra-zebra-endangered-species-mountain-48785092.html
https://www.picfair.com/pics/011341189-cape-mountain-zebra-in-natural-habitat
http://previews.123rf.com/images/zambezi/zambezi1007/zambezi100700036/7362338-Una-cebra-de-monta-a-Equus-zebra-pastoreo-en-la-monta-a-Parque-nacional-Zebra-Sud-frica--Foto-de-archivo.jpg
https://www.petesafaris.com/cape-mountain-zebra/
https://commons.wikimedia.org/wiki/File:Cape_Mountain_Zebra_%28Equus_zebra_zebra%29_%2833007816806%29.jpg
http://worldwidequest.com/resources/images/news_articles/uploads/5.Cape_Mountain_Zebra__Pete_Read.JPG
https://www.twenty20.com/photos/411776d3-870c-48b5-87fd-c2ce38e32a7f

EQUUS HARTMANNAE

Chestnut inconspicuous: large, fairly dark, but usually disrupted by striping

https://www.gettyimages.ae/detail/photo/hartmanns-mountain-zebra-royalty-free-image/1402956812?adppopup=true
https://www.gettyimages.ae/detail/photo/hartmanns-mountain-zebras-damaraland-namibia-africa-royalty-free-image/80278999?adppopup=true
https://www.gettyimages.ae/detail/photo/hartmanns-mountain-zebra-equus-zebra-hartmannae-royalty-free-image/89197762?adppopup=true
http://www.ovisume.eu/gallery/gl_hartmann1.jpg
https://stock.adobe.com/search?k=%22cape+mountain+zebra%22&asset_id=45129232
https://www.sciencephoto.com/media/561285/view/hartmann-s-mountain-zebra
http://www.biodiversityexplorer.info/mammals/perissodactyla/equus_zebra_hartmannae.htm
http://www.tswalu.com/blog/wp-content/uploads/2013/02/Shades-of-Grey.jpg
https://www.agefotostock.com/age/en/details-photo/hartmann-s-mountain-zebra-equus-zebra-hartmannae-damaraland-namibia-africa/WR1501902
https://disneyparks.disney.go.com/blog/2013/10/wildlife-wednesdays-rare-hartmanns-mountain-zebra-born-at-disneys-animal-kingdom-lodge-at-walt-disney-world-resort/
https://pixels.com/featured/hartmanns-mountain-zebra-equus-zebra-konrad-wothe.html?product=puzzle&puzzleType=puzzle-18-24

EQUUS PRZEWALSKII

Chestnut inconspicuous: pale, narrow, placed extremely low, disrupted by dark adjacent (carpal) pelage, and usually hardly visible

https://www.shutterstock.com/nb/image-photo/przewalskis-horse-equus-ferus-przewalskii-known-1257519664
https://c4.staticflickr.com/4/3697/10175540443_64959b8c3b_b.jpg
http://www.monartozoo.com.au/wp-content/uploads/sites/3/animals/PHorse-350x310.jpg
https://simplymarvelous.files.wordpress.com/2007/12/przewalski.jpg?w=470
https://upload.wikimedia.org/wikipedia/commons/a/ac/Przewalskipferd_cologne.jpg
https://www.kevinpepperphotography.com/blog/2016/10/13/today-i-wanted-to-introduce-you-to-the-przewalski-horse-in-mongolia
https://en.wikipedia.org/wiki/Przewalski%27s_horse#/media/File:Przewalskis_horse_02.jpg
https://www.researchgate.net/figure/Two-Przewalskis-horse-bachelor-males-sparing-in-the-Great-Gobi-B-Strictly-Protected-Area_fig2_263895425
https://es.123rf.com/photo_120671951_przewalski-s-horse-or-taki-as-it-s-commonly-known-a-true-wild-horse-seen-walking-across-the-plains.html
https://depositphotos.com/2160747/stock-photo-przewalski-horses.html
https://depositphotos.com/15616741/stock-photo-young-przewalski-horse-in-steppe.html

shift http://chinaplus.cri.cn/photo/china/18/20190525/293725_2.html

EQUUS CABALLUS

Chestnut small, placed low, pale

https://commons.wikimedia.org/wiki/File:White_horse_portrait.jpg
http://www.publicdomainpictures.net/pictures/50000/t2/cheval-blanc-qui-marche.jpg
http://www.bigviewwalkinghorses.com/images/horses/Sires/Finals%20Goldwell.gif

In at least some individuals of the appaloosa horse, the chestnut is virtually invisible. You can see a faint grey speck located just above the carpal joint.
http://www.hiddenwappys.com/images/rainruns_000.jpg

Arabian
http://41.media.tumblr.com/tumblr_maka86ZbFR1rqpuxfo1_500.jpg
Arabian
http://www.examiner.com/images/blog/replicate/EXID37198/images/Arabian_Horse_by_Milan_M_CCL.jpg
Arabian
https://www.arabianhorses.org/images/424279.jpgBoulonnaishttp://www.infobarrel.com/media/image/46159_max.jpg

Taurotragus oryx:
http://3.bp.blogspot.com/-DX3MqwiVp84/U5Js4CdvljI/AAAAAAABv4o/5pyWPA7nCQA/s1600/eland+2.jpg

http://farm8.static.flickr.com/7420/12692185905_a80909bbe1_m.jpg

This variation is in placement, size, and tonal contrast with the surrounding pelage.

In terms of size, the pattern is that Grevy’s zebra and the African wild ass (illustrated by the Somali subspecies) have small chestnuts, too small to be conspicuous, particularly in view of the striping on the legs in both species. All the other species have large chestnuts.

In terms of contrast with the surrounding pelage, the chestnut is rendered inconspicuous in the two species of mountain zebras because it is not clearly separated from the striping on the inner foreleg. The chestnut in mountain zebras is large and dark enough to be potentially conspicuous but the effect is neutralised by striping. In Grevy’s zebra the chestnut is slightly separated from the striping but is too small to be noticeable. In Asian wild asses (Equus hemionus, E. khur and E. kiang) the large dark chestnut is set in whitish pelage, making for a conspicuous appearance.

So now we know that the chestnut – so easy to dismiss as a ‘relictual’ or ‘vestigial’ or ‘ancestral’ feature meaningless in its present form – is surprisingly variable among equids at all levels: the subgeneric, species, subspecies, and individual. Why would a redundant anatomical feature show so much variation, if it has no real function?

In E. przewalskii the chestnut is a nondescript, greyish ‘horn colour’ similar to that of the hooves of the same species, whereas in the plains zebra (E. quagga) the chestnut is essentially black (although it may appear grey in some photos owing to reflection of the light from what is a smooth horny surface).

to be continued in https://www.inaturalist.org/journal/milewski/68124-part-2-surprising-variation-in-the-chestnut-in-various-subspecies-of-the-plains-zebra#...

(writing in progress)

Posted on 11 de julho de 2022, 10:46 PM by

milewski

Interspecific variation in the form of the chestnut among equids, part 1

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