The cross of the donkey as a social signature

Everyone knows that a typical marking in the colouration of the donkey (Equus asinus) is a cross on the withers ( and and and and and and and and and and

However, who understands the adaptive value of this pattern, which shows individual variation (

All equids have a social habit in which two individuals nibble at each other while facing in opposite directions. There are remarkably many photos of this on the Web, e.g. and Nibbling is centred on the withers, although it extends to the neck and rump.

Mutual nibbling has been assumed to be a form of grooming (, the value of which is partly hygienic for the skin.

However, the following should be considered, particularly in the case of the donkey.

Close observation shows that the nibbling of the skin and fur tends to be repetitive on the same spot, rather than spread-out (see and and

The main method of self-care of the skin and fur in the donkey is by rolling in dust. Unlike the horse (Equus caballus), the donkey does not shake its body after dust-bathing, leaving the fur dusty instead. Another difference is that, unlike the horse, the donkey tends not to respond to seasonal cold by growing a winter coat (

This reliance on dust may be related to the semi-arid climate to which the donkey remains adapted. Whereas the horse relies for thermoregulation in hot weather on sweating, the donkey sweats little even in the heat. Its fur somehow functions with a continual dressing of dust, implying that it would not be helpful for the fur to be cleaned by licking.

All equids have extremely well-developed sight and smell, but the donkey is more playful in adulthood than is the horse. In the horse, mutual nibbling by males is with females, not other males. I suspect (although this needs investigation) that in the donkey there is mutual nibbling between males as well.

Given its differences from the horse, could it be in the donkey mutual nibbling is mainly for social facilitation, not hygiene? And, if so, could the cross on the withers provide visual reinforcement for this mainly tactile and olfactory experience?

For the donkey, caressing a partner is no doubt as pleasant olfactorily as it is in the tactile sense. The olfactory aspect may not be obvious to humans because our sense of smell is poorly developed.

But why would the donkey alone, among equids, have a well-developed visual feature of this kind?

The answer might possibly lie in the social behaviour of the main ancestor of the donkey. The African wild ass (Equus africanus), like all equids, tends to be gregarious. However, it is extremely adapted to stony semi-deserts capable of supporting only sparse populations subject to climatic vagaries which preclude permanent groups.

To the degree that the African wild ass can congregate only in a limited and opportunistic way, its associations among individuals may tend to be transient and promiscuous ( And this species is so long-lived (up to 35 years) that a surviving individual may have to change even its longest-term companions.

When two individuals become temporary companions, the same combination of attraction and antagonism may arise whether they are strangers or former acquaintances. (Re)habituation may depend on a social facilitation.

An extreme example, beyond equids, of social facilitation is known in the bonobo (Pan paniscus), which routinely engages in genital stroking well beyond any sexual function of the genitals (see

The erratic and even promiscuous sociality of the African wild ass may remain relevant in domestication, to the extent that the donkey - particularly useful to the poorest of human societies - has often been owned as a single individual rather than a stable group. Its social life may, in many situations, still be opportunistic rather than continuous.

Is it possible, then, that in the following photos we see not just another case of equids grooming each other, but rather an interaction analogous - in its own way - with kissing and stroking, in which the pleasant sensations are reinforced by the visual cue of the cross on the withers? And in which the individual variation in the pattern facilitates particular personal relationships despite the interruptions inevitable in the ways of life of the African wild ass and the donkey?

Anotado por milewski milewski, 26 de septiembre de 2021 a las 12:11 AM


The cross on the withers is not usually associated with the Asian wild ass or its close relatives, but this individual of Equus khur shows an unusually clear trace of this feature:

Anotado por milewski hace 3 meses (Advertencia)

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