Social organization in ungulates: Revisiting Jarman’s hypotheses

Ungulates (antelopes, deer and relatives) have some of the most diverse social systems among mammals. To understand the evolution of ungulate social organization, Jarman (1974) proposed an ecological scenario of how distribution of resources, habitat and feeding style may have influenced social organization. Although Jarman's scenario makes intuitive sense and remains a textbook example of social evolution, it has not been scrutinized using modern phylogenetic comparative methods. Here we use 230 ungulate species from ten families to test Jarman's hypotheses using phylogenetic analyses. Consistent with Jarman's proposition, both habitat and feeding style predict group size, since grazing ungulates typically live in open habitats and form large herds. Group size, in turn, has a knock-on effect on mating systems and sexual size dimorphism, since ungulates that live in large herds exhibit polygamy and extensive sexual size dimorphism. Phylogenetic confirmatory path analyses suggest that evolutionary changes in habitat type, feeding style and body size directly (or indirectly) induce shifts in social organization. Taken together, these phylogenetic comparative analyses confirm Jarman's conjectures, although they also uncover novel relationships between ecology and social organization. Further studies are needed to explore the relevance of Jarman (1974) scenario for mammals beyond ungulates.