Sexual segregation in ungulates: A new approach

In many mammals, males and females live solitarily or in separate groups outside the breeding season. Sexual segregation is wide-spread in ungulates, but also occurs in whales, seals, monkeys, macropods, elephants, fish and bird species. What causes segregation by sex is still poorly understood, despite intense research done mainly on different ungulate species. In most species studied, males were clearly larger than females. The evolution of sexual dimorphism in body size has largely been attributed to sexual selection and mating strategies. While the consequences of body-size differences on energy requirements and metabolic rates received most attention, studies on consequences of sexual body-size differences on behavior are lacking. This review emphasizes the importance to study a wide range of social mammals with a different or no degree of sexual segregation and sexual dimorphism in body size; something which has not been done and has greatly limited our ability to test alternative hypotheses. More emphasis has to be put on the study of activity budgets, sociality and habitat choice of non-dimorphic species to explain the evolution of permanent territoriality, of long-term bonds of male-female pairs and of the occurrence of mixed-sex groups (adults), as well as its absence in dimorphic species. We review five hypotheses proposed to explain sexual segregation, discuss alternative outcomes and predictions for each hypothesis, suggest alternative explanations for the evolution of sexual segregation and mating systems, and indicate new and important directions for research. We conclude that a phylogenetic comparison of behavior of a wide range of ungulates and other mammals will be needed to solve the enigma of sexual segregation.