Horse and donkey domestication goes back thousands of years, but researchers believe nature has had the upper hand in our efforts to selectively breed these animals for much of that time.
Researchers, in a special review of evidence of domestication of large herbivores to be published in the Proceedings of the National Academy of Sciences on April 29, believe wild animals interbred with domesticated ones until quite recently.
Fiona Marshall, professor of anthropology at Washington University in St Louis, Missouri, and her colleagues suggest that the role of intentional breeding was not as significant as traditionally thought in the case of donkeys, camels, alpacas, pigs, cattle, sheep, and goats.
The researchers, in evaluating the roles of directed breeding and gene flow in animal domestication, highlighted the issues faced by early horse and donkey owners trying to selectively breed.
The slow breeding rate of equines, as opposed to the likes of pigs, meant wild stock played a major part in the process for millennia.
Marshall and her colleagues noted that evidence for the use of bits, milking, corralling, and size decrease documents domestication by horse-hunters among the Botai in Kazakhstan around 5500 years ago.
Mitochondrial DNA lineages were often initially interpreted in terms of multiple origins, whereas genetic modeling now suggests domestication in a restricted region with later incorporation of many different wild lineages into domestic stocks.
“Horse herds grow slowly and are subject to die-offs in severe storms, so the hardiness of wild horses is advantageous to herders,” they write.
“Accordingly, it has been argued that difficulties in maintaining domestic horse herd sizes during pastoral migrations led directly to restocking through the capture of wild females.”
Humans have relied heavily on horses, donkeys, camelids, and yaks for transport, food, fiber, and ritual practices for thousands of years. These animals enabled mobile herders to survive in cold steppe, desert, and mountainous regions, the researchers said.
“Donkey’s desert adaptations, lack of sociality, long gestation rates, and use by mobile herders for long-distance movement have resulted in particularly low levels of management, little directed breeding, and constant gene flow with their wild and feral relatives, at least within their wild range.”
On some levels, they could, perhaps, not even be considered a domestic animal, they said.
Marshall and her colleagues noted that African wild asses were the ancestors of domestic donkeys. African pastoralists still rely on donkeys for transport and they are rarely slaughtered for food.
“As a result, drought and disease are the principal causes of donkey mortality. Herders value individual animals for strength and hardiness and castrate difficult males, but prefer uncastrated ones for transport use. The presence of multiple breeding males reduces directed selection.
“Because they are challenging to herd, donkeys range widely in search of mates and donkey-owners do little to manage reproduction.”
In regions where wild asses existed historically, continued gene flow resulted from managed or inadvertent breeding of domestic donkeys with wild asses, they said.
“These aspects of the recent past are relevant to understanding ancient processes because they reflect consistent mechanisms, biology, and transport use.
Archaeological and genetic evidence supported the conclusion that donkeys were domesticated in dry environments and bred with a variety of wild populations.
They said the presence of two divergent mitochondrial lineages in donkeys has been interpreted as evidence for more than one domestication, but may be equally consistent with recurrent recruitment of females into domestic herds from genetically divergent Nubian wild ass populations.
The data, they say, suggests donkey breeding has always been laissez faire, characterized by intentional and unintentional interbreeding with wild asses and feral donkeys, as well as by environmental selection for animals that survived in pastoral settlements.
Together, these processes resulted in a prolonged and complicated process of domestication for donkeys.
“If gene flow resulting from breeding between wild and domestic animals was common during domestication and has not ceased until recent historic times, it raises many fascinating questions regarding ways in which behavioral and phenotypic domestication traits were maintained, and just what a domestic population was.
“Instead of assuming strong intentional and directional selection during the early stage of animal domestication, the challenge is to investigate sources of selection more critically, bearing in mind the complex interplay of human and environmental selection and the likelihood of long-term gene flow from the wild.
“These insights on gene flow and unintentional breeding provide new perspectives on early animal domestication, alter current sets of assumptions and questions, and enhance our understanding of domestication as a complex biocultural process.”
Evaluating the roles of directed breeding and gene flow in animal domestication
Fiona B. Marshall, Keith Dobney, Tim Denham, and José M. Capriles.
The full review can be found via a link here.