Sexual segregation in vertebrates: proximate and ultimate causes.

Sexual segregation is very common in vertebrates that live in groups. In this article, I will review proximate and ultimate causes of sexual segregation in social species and in particular in ungulates in which the bulk of research on the topic has been carried out. In most social ungulate species, males and females live in separate groups outside the breeding season, sometimes using different home ranges and types of habitat. In most of these species, males are larger than females. Dimorphism in body size can lead to sexual differences in ecology and behavior making it difficult for the two sexes to stay in the same group. It is important for our better understanding of the evolution of sociality, sexual dimorphism and different mating systems to determine why sexual segregation is so widespread not only in ungulates but also in other vertebrates. In this article, I discuss the ecology of the two sexes by reviewing proximate and ultimate causes of sexual segregation. To do this, I compare a range of studies of ruminants and include explanations for social segregation as well as for habitat segregation by gender. This leads into a review and updates current knowledge of the phenomenon. Although I present a number of different hypotheses, I focus in particular on predation risk, forage selection and activity budget and discuss the social-factors hypothesis. I stress that the key in solving the enigma of sexual segregation lies in clearly separating hypotheses that try to explain social segregation and habitat segregation, as well as in including experiments or model systems. To that end, I present a preliminary study on a test of the activity-budget hypothesis in three-spine sticklebacks and explain why I believe that shoaling fish are useful for analysing the underlying processes and mechanisms that lead to sexual segregation in animals. Lastly, I argue that it is unlikely that a single factor can explain social segregation or habitat segregation but that a model integrating different factors and different levels of segregation might succeed in describing proximate and ultimate causes of sexual segregation.