Nobody seems previously to have noticed the intriguing variation in the length of the tail within the Eurasian wild ass (Equus hemionus).
The Mongolian wild ass (Equus hemionus hemionus, https://en.wikipedia.org/wiki/Mongolian_wild_ass) is similar in overall appearance to the Indian wild ass (Equus hemionus khur, https://en.wikipedia.org/wiki/Indian_wild_ass). This makes sense for two subspecies of the same widespread species.
However, for some strange reason the tail of the former (https://www.zoochat.com/community/media/equus-hemionus-hemionus-mongolian-wild-ass-female.209602/ and https://www.youtube.com/watch?v=mAesXpv3aKo and https://www.facebook.com/watch/?v=10155298731989759) is much longer than the tail of the latter (https://www.naturepl.com/stock-photo-indian-wild-ass-equus-hemionus-khur--wild-ass-sanctuary-little-rann-nature-image01643715.html and https://www.youtube.com/watch?v=rvbyM9AmGjs).
Puzzled by this finding, I scrutinised the tails of other ass-like, wild members of the genus Equus, namely the Somali wild ass (Equus africanus somaliensis), Grevy's zebra (Equus grevyi), and the two forms of mountain zebra (Equus hartmannae and E. zebra).
In all of these species, the tail is unremarkable, with the tassel reaching approximately to the hock. The species differ more in whether the tail is dark or pale than in whether the tassel is long or short.
The tail is indeed also unremarkable in the central subspecies of the Eurasian wild ass, namely Equus hemionus onager and E. hemionus kulan: https://www.naturepl.com/stock-photo-nature-image01508093.html and https://fineartamerica.com/featured/1-onager-equus-hemionus-eyal-bartov.html and https://stock.adobe.com/images/onager-also-known-as-hemione-or-kulan-or-asiatic-wild-ass-latin-name-equus-hemionus/241565734 and https://www.dreamstime.com/stock-photo-onager-equus-hemionus-brown-asian-wild-ass-who-famous-inhabitant-israeli-national-nature-reserve-park-near-eilat-image41151402 and https://www.dreamstime.com/royalty-free-stock-images-onager-wild-asian-ass-hai-bar-biblical-nature-reserve-km-north-eilat-israel-image30448629.
What this adds up to is the following: all the ass-like, wild species and subspecies in the genus Equus are similar in the length of the tail, except for the easternmost (tail long) and southernmost (tail short) subspecies of the Eurasian wild ass.
Although this variation remains unexplained in adaptive terms, taxonomists should add it to the list of features distinguishing the Mongolian wild ass from the Indian wild ass.
LONG TAIL
Equus hemionus hemionus
https://mongolia.wcs.org/Wildlife/Asiatic-Wild-Ass-Khulan.aspx
https://news.cgtn.com/news/3d3d414d79417a4d32457a6333566d54/index.html
https://www.zoochat.com/community/media/equus-hemionus-hemionus-mongolian-wild-ass-female.209600/
https://www.zoochat.com/community/media/equus-hemionus-hemionus-mongolian-wild-ass-female.209601/
https://www.biolib.cz/en/image/id374109/
https://zooinstitutes.com/animals/mongolian-wild-ass-dalian-forest-zoo-102717.html
https://www.pbase.com/cokesmith/image/149666436
https://www.fauna-flora.org/news/theres-something-exciting-happening-above-the-gobi-desert
https://twitter.com/TheWCS/status/1195079890514694144/photo/1
https://pixabay.com/photos/onager-mule-zoo-ungulate-horse-1671296/
https://oneworldonehealth.wcs.org/news/ID/13617.aspx
https://www.biolib.cz/en/image/id315682/
https://www.zoochat.com/community/media/equus-hemionus-luteus-gobi-kulan-young-female.209611/
SHORT TAIL
Equus hemionus khur
https://www.jungledragon.com/image/36313/indian_wild_ass.html/zoom
https://www.flickr.com/photos/sarbhloh/33515960704
https://www.alamy.com/stock-photo-asian-wild-ass-equus-hemionus-khur-128390815.html
MEDIUM-LENGTH TAIL
Equus africanus somaliensis
https://www.monaconatureencyclopedia.com/equus-africanus-somalicus/?lang=en
https://zooinstitutes.com/animals/somali-wild-ass-hai-bar-yotvata-2552.html
https://www.naturepl.com/stock-photo-nature-image01508002.html
Equus grevyi
https://www.alamy.com/stock-photo-grvys-zebra-with-her-three-week-old-foal-113614327.html
https://www.inaturalist.org/observations/68061066
https://www.inaturalist.org/observations/53931632
https://www.inaturalist.org/observations/68999700
https://www.inaturalist.org/observations/54908219
https://www.inaturalist.org/observations/38089842
https://www.inaturalist.org/observations/33945931
Equus hartmannae
https://www.inaturalist.org/observations/16019971
https://louisvillezoo.org/animalsandplants/hartmanns-mountain-zebra/
Fifth photo in http://dangerous-wild-animals.blogspot.com/2011/09/mountain-zebra.html
https://dewetswild.com/2018/07/10/hartmanns-mountain-zebra/#jp-carousel-36869
Equus zebra
https://videohive.net/item/grazing-cape-mountain-zebra/22472735
https://wildark.org/species/mountain-zebra/
https://www.dreamstime.com/royalty-free-stock-photography-cape-mountain-zebra-standing-image21533027
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