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Simple Summary: Beginning with Darwin, scientists have long considered the reasons for why some species but not others were domesticated. For example, of the equid species, horses and asses, but not the zebra, were domesticated. Many reasons for the 'domesticability' of mammalian species have been proposed (e.g., diet, social organization, response to humans). However, none of these traits have been subjected to empirical investigation. Here, we review past proposals for several domesticability traits and use a novel quantitative and evolutionary analysis to evaluate the explanatory power of these traits for domestication. For hoofed mammals, a heightened physiological response to humans, modeled as vulnerability to capture myopathy, emerged as the only significant obstacle to domestication. This is consistent with the reconstructed evolution of capture myopathy in ungulates, especially equids, and the prehistory of predation on equids, which shows that ancestral horses lost the vulnerability to capture myopathy consequent to a reduction in predation pressure on horses. Zebras, however, retained the vulnerability to capture myopathy as an adaptation to persistent high predation pressure, which would have impeded their domestication. Our results point to the importance of considering an animal's behavior and biology as either an impediment or facilitator of domestication or developing a human-animal symbiotic relationship. Since Darwin, many evolutionary and behavioral researchers have considered the role of phenotypic traits that favor the domestication of nonhuman animals. Among such proposed traits are a species' social structure, level of intra- and interspecific agonistic interactions, sociosexual behaviors, parental strategies, reaction to humans, habitat preference, dietary habits, developmental trajectories, and utility to humans. However, little to no comparative phylogenetic evidence exists concerning the importance of these attributes for the domestication of animals. Moreover, rather than considering domestication as a dichotomous event (non-domesticated vs. domesticated), humans and their potential domesticates encountered numerous socioecological challenges/obstacles during the domestication process before reaching the stage of full domestication. The present study explored the influence of adult body mass, gregariousness, dietary breadth, and reaction to humans on the domestication process of ungulates. The phylogenetic comparative model revealed that capture myopathy (CM), as a proxy for reaction to humans, negatively and significantly influenced the domestication process. The present paper also explored the evolution of CM in equine species in response to the presence of large carnivoran predators during the Pleistocene. Ecologies that preserved most of the large carnivoran predators of equine species also featured more equine taxa with CM (e.g., zebras), which were thus less suitable for domestication. [ABSTRACT FROM AUTHOR]

Since Darwin, many evolutionary and behavioral researchers have considered the role of phenotypic traits that favor the domestication of nonhuman animals. Among such proposed traits are a species’ social structure, level of intra- and interspecific agonistic interactions, sociosexual behaviors, parental strategies, reaction to humans, habitat preference, dietary habits, developmental trajectories, and utility to humans. However, little to no comparative phylogenetic evidence exists concerning the importance of these attributes for the domestication of animals. Moreover, rather than considering domestication as a dichotomous event (non-domesticated vs. domesticated), humans and their potential domesticates encountered numerous socioecological challenges/obstacles during the domestication process before reaching the stage of full domestication. The present study explored the influence of adult body mass, gregariousness, dietary breadth, and reaction to humans on the domestication process of ungulates. The phylogenetic comparative model revealed that capture myopathy (CM), as a proxy for reaction to humans, negatively and significantly influenced the domestication process. The present paper also explored the evolution of CM in equine species in response to the presence of large carnivoran predators during the Pleistocene. Ecologies that preserved most of the large carnivoran predators of equine species also featured more equine taxa with CM (e.g., zebras), which were thus less suitable for domestication.