Why does the donkey have big ears?

Everyone knows that the donkey (Equus asinus) tends to have large ears (https://www.facebook.com/Donkeywhisperer/photos/my-what-big-ears-you-haveoften-we-get-the-sweetest-inquiries-from-children-askin/1672642076086866/). However, who understands the adaptive value of large ears in equids, and why this seems to have been emphasised by domestication?

The ear consist of an outer ear, i.e. the ear pinna, and an inner ear, i.e. the capsule of the skull at the base of the ear-hole - called the auditory (or tympanic) bulla. The inner ear is remarkably small in equids (see http://www.ionhealing.com/sagehillCMK/Equine%20Skull.html), indicating limited hearing.

A small inner ear makes sense because all wild equids - which live in open environments - rely more on vision than on hearing. Their eyes are proportionately even larger than those of coexisting ruminants. And the wild ancestors of the donkey lived in particularly open environments in the semi-deserts of North Africa.

Therefore, if it is adaptively significant that the ear pinnae are larger in the donkey than in e.g. the horse (Equus caballus), this is likely to be in thermoregulation rather than hearing.

And, indeed, the inner ear is similarly small in donkey and horse (https://www.kythera-family.net/en/natural-history-museum/mammals/donkey-skull and https://www.shutterstock.com/nb/image-photo/close-ups-propped-sun-bleached-donkey-1618690138 compared with https://www.deviantart.com/ratrinadragon/art/Trick-the-pony-skull-244132670 and https://www.worthpoint.com/worthopedia/pony-skull-245944474).

So, do the ears of the donkey radiate excessive heat from the body?

It does seem true that, whereas the horse sweats a lot in hot weather, the donkey sweats little. Where the horse relies mainly on evaporative cooling, the donkey may rely partly on radiative cooling.

And, partly because its normal body temperature is only 37 degrees Celsius (compared to 38 degrees Celsius in the horse), the donkey has more leeway than the horse to heat up during the day before cooling down again at night (see https://www.researchgate.net/publication/283248068_Heat_Tolerance_in_Donkeys_and_Mules#:~:text=Compared%20to%20the%20local%20donkeys,absorbs%20more%20incidental%20solar%20radiation.).

But what remains puzzling is that the ear pinnae of the donkey are so furry. Compare Lepus (https://www.alamy.com/close-up-portrait-of-a-black-tailed-jackrabbit-image371183426.html) with an extreme individual of the donkey (https://www.dreamstime.com/stock-photo-donkey-farm-virginia-image10366740).

Semi-deserts tend to be cold at night, and the donkey cannot shelter in burrows, or fold its ears flat, in the way of the fennec fox (https://en.wikipedia.org/wiki/Fennec_fox) or the long-eared jerboa (https://25hournews.com/news/night-loving-mouse-long-eared-jerboa-mouse-family-of-the-dipodidae-3167).

For the furry ear pinnae of the donkey to radiate much heat, the fur would have to be able to part, exposing panels of bare skin. Nobody seems to have observed this - possibly because nobody has had a search-image for it.

The mechanism of piloerection is, however, evident in another ungulate adapted to semi-deserts, the large ears of which are particularly well-photographed: the steenbok (Raphicerus campestris).

In what is presumably cool weather, the fur on the front-of-ear folds down (https://gurushots.com/photo/250bc6ae4df24cd02509bb2ece7850d2 and https://gurushots.com/photo/88519d81993d57aa9d1be3a4066ec609 and https://gurushots.com/photo/42758df999a3df826daf6b46c55ffd8e). But in hot weather, the same fur erects to show the somewhat bloodshot bare skin (https://www.flickr.com/photos/piazzi1969/11253299393 and https://akeke.blog/sydafrika-2016/#jp-carousel-2914 and https://cdn.hungryonion.org/original/3X/f/3/f359c1537c964d5330ec57d3826c7a373b9f86be.jpg). The clearest photo of all can be found by scrolling within https://www.cultafrica.net/home/tours_top/africa_unusial.html.

Can readers imagine a similar mechanism in these ears? https://www.shutterstock.com/nb/image-photo/donkey-outdoors-nature-under-blue-sky-1689196276 and https://www.dreamstime.com/stock-photo-donkey-ears-big-face-forward-to-encourage-us-to-listen-up-image87723778 and https://www.dreamstime.com/two-pairs-donkey-ears-over-blue-sky-image181826262 and https://www.dreamstime.com/beautiful-ears-donkey-head-close-up-donkey-big-ears-thoroughbred-shaggy-sky-image229151207.

And please bear in mind that the ears of the donkey tend to look larger than they really are, relative to body mass.

The compendium of photos below compares the donkey, the Somali wild ass (Equus africanus somaliensis), and Hartmann's zebra (Equus hartmannae, which lives with the steenbok in southern African semi-desert).

You can see that the ear pinnae are moderately large in both the Somali wild ass and Hartmann's zebra. And many individuals of the donkey have ear pinnae of similar, moderate size relative to the forehead or the body. What makes the ears look outsize in the donkey is the shortening of the legs by domestication (https://www.dreamstime.com/stock-photography-walking-donkey-image16023212 and https://www.dreamstime.com/donkey-standing-sea-animal-four-legs-large-ears-used-loading-mammalian-animal-donkey-standing-sea-image202864961).

In summary:

The ear pinnae of the donkey are not much larger, relative to body mass, than in wild species of Equus which live in semi-deserts with limited water to drink. Their main function is radiation of excessive body heat, rather than particularly acute hearing. Naturalists: please test this by looking for evidence that the fur on the front-of-ear can open up in hot weather to expose panels of somewhat bloodshot bare skin.

Equus asinus

https://www.shutterstock.com/nb/image-photo/beautiful-healthy-young-donkey-head-shot-1689196273

https://www.dreamstime.com/stock-photo-smiling-donkey-pasture-image44455842

https://www.shutterstock.com/nb/image-photo/happy-donkeys-on-streets-maio-island-1774073948

https://wallsheaven.co.uk/wall-murals/burros-A111600542

https://wallsheaven.co.uk/wall-murals/mannar-donkey-in-kalpitiya,-sri-lanka-D842155545

https://wallsheaven.co.uk/wall-murals/donkey-or-ass-on-natural-environment.-C315737899

https://www.dreamstime.com/royalty-free-stock-images-spotted-grey-donkey-image12259539

https://www.shutterstock.com/nb/image-photo/cute-puppy-donkey-white-paws-277371563

https://www.shutterstock.com/nb/image-photo/donkey-153759464

https://wallsheaven.co.uk/wall-murals/donkey-on-natural-environment.-A057620822

Equus africanus somaliensis

https://www.shutterstock.com/nb/image-photo/somali-wild-donkey-equus-africanus-inhabits-417105889

https://www.vecteezy.com/photo/715917-somali-wild-ass-equus-africanus-in-israeli-nature-reserve

https://www.zoochat.com/community/media/equus-asinus-somalicus-somali-wild-ass-male.221844/

https://www.zoochat.com/community/media/equus-asinus-somalicus-somali-wild-ass-male.221848/

https://www.zoochat.com/community/media/equus-asinus-somalicus-somali-wild-ass-male.221846/

https://www.zooborns.com/.a/6a010535647bf3970b019afff0392d970c-popup

https://wallsheaven.co.uk/wall-murals/somali-wild-donkey-(equus-africanus)-in-nature-reserve-of-the-middle-east.-this-species-is-extremely-rare-both-in-nature-and-in-captivity-C345686901

https://www.zooborns.com/.a/6a010535647bf3970b019affeffb27970b-popup

Equus hartmannae

(comparison with Equus quagga burchellii: https://upload.wikimedia.org/wikipedia/commons/b/b0/Equus_quagga_burchellii_%26_Equus_zebra_hartmannae_-_Etosha_2015.jpg)

https://www.gettyimages.com.au/detail/photo/hartmanns-mountain-zebra-equus-zebra-hartmannae-royalty-free-image/508502127

https://www.alamy.com/stock-photo-hartmanns-mountain-zebra-damaraland-namibia-102865344.html

https://animals-are-cool.fandom.com/wiki/Hartmann%27s_Mountian_Zebra?file=Hartmannsmountainzebra1_327w.jpg

https://www.africahunting.com/media/hartmanns-zebra-mountain-zebra.2847/

https://disneyparks.disney.go.com/blog/2020/04/disneymagicmoments-introducing-asha-a-hartmanns-mountain-zebra-foal-at-disneys-animal-kingdom-theme-park/

Equus hemionus

https://wallsheaven.co.uk/wall-murals/the-onager-(equus-hemionus)-is-a-brown-asian-wild-donkey-inhabiting-nature-reserve-park-near-eilat-B692591540

https://wallsheaven.co.uk/wall-murals/the-turkmenian-kulan,-equus-hemionus-kulan,-is-a-rare-asian-donkey-A063742172

Anotado por milewski milewski, 21 de septiembre de 2021 a las 08:29 PM

Comentarios

The following of an albino individual of the donkey shows the bare skin in the ear pinna, which may possibly be bloodshot to radiate heat: https://www.shutterstock.com/nb/image-photo/rare-wild-white-albino-donkey-portrait-1349369381.

Anotado por milewski hace cerca de 2 meses (Advertencia)

Añade un comentario

Entra o Regístrate para añadir comentarios