Did the wild dromedary break the rule of miniaturisation in Arabia?
In a previous Post (October 02, 2020, https://www.inaturalist.org/journal/milewski/archives/2020/10), I pointed out a pattern which is hard to explain biogeographically or ecologically. This is the remarkably consistent miniaturisation in the wild fauna of large mammals on the Arabian Peninsula.
However, there may be one exception: the extinct wild ancestor of the dromedary (Camelus dromedarius). This occurred on the 'horn' of Arabia where Dubai now stands (https://upload.wikimedia.org/wikipedia/commons/c/cd/United_Arab_Emirates_%28orthographic_projection%29.svg). It seems to have lived not in the inland desert but on a mangrove-edged coastal strip on the Strait of Hormuz (https://www.pnas.org/content/113/24/6707 and https://en.wikipedia.org/wiki/Strait_of_Hormuz).
The reason to think that the wild dromedary was not miniaturised is that its domestic descendent far outsizes any species of ungulate indigenous to Arabia or the Sahara - or for that matter the Sahel (Oryx dammah, adult female body mass less than 140 kg, https://en.wikipedia.org/wiki/Scimitar_oryx).
Consider the average body mass of adult females of the dromedary. This is about 500 kg according to Tibary and Anouassi (1997, pages 2-4 in 'Theriogenology in Camelidae: anatomy, physiology, pathology and artificial breeding'). The figure ranges from 350 kg in parts of Kenya and Sudan to 640 kg in Syria.
The corresponding figures for the bactrian camel (Camelus bactrianus, page 10 of same reference) is about 550 kg, ranging from 480 kg in Mongolia to 650 kg in Kyrgystan.
The fact that the bactrian camel is the more massive is unsurprising because it is adapted to seasonally cold climates (https://en.wikipedia.org/wiki/Bergmann%27s_rule).
This climatic difference applies despite the bactrian camel and the dromedary having both been domesticated in the Iranian region.
Whereas the dromedary was domesticated in a warm coastal semi-desert just south of Iran and across the Strait of Hormuz, the bactrian camel was domesticated in a seasonally cold inland semi-desert in northeastern Iran, about 750 km from this strait.
If the wild ancestor of the dromedary was as massive as the domestic form so closely associated with Arabian culture, then this would break the rule of miniaturisation in Arabia. At about 500 kg, the dromedary is at least fourfold more massive than the next-largest ungulate indigenous to Arabia, namely the extinct Syrian wild ass (Equus hemionus hemippus, https://en.wikipedia.org/wiki/Syrian_wild_ass).
However, it remains possible that the relative gigantism of the dromedary is partly owing to the process of domestication - by a combination of hybridisation and selective breeding in prehistoric times.
It seems that artificial hybridisation with the bactrian camel began from the start of the domestication of the dromedary (https://iranicaonline.org/articles/camel-sotor#:~:text=The%20Iranians%20would%20thus%20have,it%20was%20probably%20not%20numerous.).
This was practicable partly because the bactrian camel had already spread, by that time, to southeastern Iran - where it was separated from the site of domestication of the dromedary by little more than a ride in a boat. It seems to have been easy enough, even four thousand years ago, to transport males of the bactrian camel to what is now the United Arab Emirates across the Strait of Hormuz.
Any systematic hybridisation would have doomed the dromedary in the strict sense of a distinct species. However, it might have facilitated the subsequent selective breeding of increased body size, from early in the process of its domestication.
The incentive for boosting the body size of the dromedary would have been to utilise it farther inland, where viable thresholds of drought-tolerance, mobility, production of milk and capacity for labour all depended on body size. And in view of the flexibility of body size in other species of domestic mammals it seems possible that the body mass could have been doubled within a few centuries.
The Syrian wild ass was diminutive and may also have been domesticated (contrary to Wikipedia), at least partly and for a limited period (see https://books.google.com.au/books/about/Donkey_the_Story_of_the_Ass_from_East_to.html?id=CHIvAQAAMAAJ&redir_esc=y). There is no evidence of any artificial boosting of body size in the case of Equus hemionus. However, its roles in domestication were different from those of the dromedary, the main one being draught of vehicles in warfare.
So, which is more likely for the ancestral, wild dromedary of Arabia: that its adult female body mass was about 500 kg from the start or that it was only about 250 kg until an artificial and rapid enlargement which has left no trace of the original diminutive form?
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Richard W Bulliet states, on pages 148-152 of his book 'The camel and the wheel' (1975, Harvard University Press): "the two-humped camel was actually domesticated in what is today the border region between the northeastern Iranian province of Khurasan and...Turkmenistan...the date of domestication may reach back several centuries before 2500 B.C...A movement of people originating in Iran across the narrow straits separating Iran and Arabia appears certain. Rock drawings found on the Arabian site (the vicinity of the Qatar Peninsula) show that these people encountered the one-humped camel in their new home...early Iranian camel culture was roughly homogeneous from northeastern to southeastern Iran and was without influence from Arabia."
The history of utilisation of camels in Iran is noteworthy, not only because the bactrian camel was domesticated here but also because the dromedary eventually displaced all populations of the bactrian camel in this country.
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