The dromedary as a humping mule

Everyone knows that mules are a vigorous hybrid between female horse (Equus caballus) and male donkey (Equus asinus), and that the dromedary (Camelus dromedarius, https://en.wikipedia.org/wiki/Dromedary#Relationship_with_humans) is the form of livestock most suited to drought. However, what many may not realise is how similar mules and the dromedary are in various ways.

The first, obvious, similarity is in body mass: both weigh preferably more than 400 kg when adult. This size, combined with hybrid vigour and the natural endurance derived from ancestors adapted to semi-arid conditions, makes both mules and the dromedary excellent beasts of burden.

A far less obvious similarity is that both are interspecific hybrids (see https://pastoralismjournal.springeropen.com/articles/10.1186/s13570-020-0159-3).

The first generation of hybrids between the dromedary and the bactrian camel (Camelus bactrianus) has only one hump but tends to be more vigorous than either parent.

And, as in the case of mules, the hybrids cannot practicably be bred among themselves to perpetuate this vigour. As everyone knows, mules are by definition infertile hybrids. For camels there is no such infertility but the hybrid vigour tends to be lost by the second generation anyway (https://iranicaonline.org/articles/camel-sotor#:~:text=The%20Iranians%20would%20thus%20have,it%20was%20probably%20not%20numerous.).

Because hybridisation between the two ancestral species of camels was practised from the start of the domestication of the dromedary, all populations of what is now classified as the dromedary probably contain genetic modifications from the bactrian camel. The most common sign of this may be long fur on the neck of the dromedary (https://www.inaturalist.org/observations/38474885), which belongs to the bactrian camel rather than the heat-tolerant wild ancestor restricted to Arabia.

Furthermore, as in mules, the hybridisation in camels is most effective with a particular species as the father.

It is best for males of large-bodied breeds of the donkey to be mated with females of the horse, because the mules thus produced are large-bodied. In the case of camels, it is best for males of the bactrian camel to be mated with females of the dromedary. This is because females copulate while lying on their bellies, and the rear hump of the bactrian camel tends to obstruct the mating squat of males of the dromedary.

Both mules and the dromedary are more difficult to breed than most domestic ungulates. And in both cases this is partly because of complications in mating, and partly because the reproductive process is slower than in true ruminants such as oxen (Bos spp.).

In both cases, mating needs to be supervised. The horse, even when in oestrus, finds the donkey so sexually unattractive that some artificial forcing is necessary. In the dromedary, breeding is inconveniently seasonal, rutting males can attack the mother's previous offspring, libido can be lacking, and successful copulation is needed to induce ovulation in the first place (https://www.sciencedirect.com/science/article/abs/pii/0378432087900492 and https://www.researchgate.net/publication/275250659_Male_camel_behavior_and_breeding_management_strategies_How_to_handle_a_camel_bull_during_the_breeding_season and http://www.veterinaryworld.org/Vol.2/February/Reproduction%20in%20Camel.pdf).

Equids have long gestation and a prolonged juvenile period, and this is even more limiting for the dromedary, which gestates for 13 months, first breeds at four years old, and can produce at most one offspring every two years (https://www.sciencedirect.com/science/article/pii/S1658077X20300709).

In compensation for the slow reproduction of equids and particularly camelids, mules and the dromedary are both surprisingly long-lived. The dromedary has a working life fourfold longer than that of the average ox (https://en.wikipedia.org/wiki/Ox), while carrying or pulling twice the load, locomoting more rapidly, working more frequently, and being able to forgo drinking for longer.

Both mules and the dromedary can be kept on food too poor for the horse or oxen. This is partly because the donkey is far better-adapted than the horse to eat fibrous, dead material, and partly because camelids have a digestive system less specialised, and thus less demanding and more versatile, than that of oxen and other true ruminants.

Whether mules or the dromedary were utilised by early civilisations was not necessarily dependent on climate. For example, in ancient Egypt and the Levant of the Old Testament it was mules that were prized for two thousand years, before their roles were largely usurped by the dromedary (https://www.mulemuseum.org/history-of-the-mule.html). It was only at this later stage that the latter species (or, strictly speaking, hybrid) was recruited from nearby Arabia for general service in farming, commerce and warfare in Egypt and the Levant.

Posted by milewski milewski, October 15, 2021 21:03

Comments

Mules and the dromedary are convergent in being docile and obedient. In the case of mules the obedience is largely the result of a calm intelligence in which the donkey is superior to the horse. In the case of the dromedary it is more the result of a relative lack of intelligence in the wild ancestor, which lived in areas naturally remote from predation. Brain size relative to body size has been considerably reduced by selective breeding in most domestic mammals, but in neither the donkey nor the dromedary does it seem likely that this is the case relative to their wild ancestors.

Posted by milewski about 2 months ago (Flag)

The dromedary was adopted in the Levant far earlier than it was in the Maghreb. R W Bulliet, in 'The camel and the wheel' (1975, page 229) states that "Not until the eleventh century...did significant numbers of Arab camel breeders push into the territory of modern Algeria and Morocco". This implies that mules may have been utilised in the semi-arid lands of the Maghreb for four thousand years before some of their roles were usurped by the dromedary.

Posted by milewski about 2 months ago (Flag)

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