Sexual segregation in ungulates: A new approach

In many mammals, males and females live solitarily or in separate groups ou tside the breeding season. Sexual segregation is wide-spread in ungulates, but also occurs in whales, seals, monkeys, macropods, elephants, fish and b ird species. What causes segregation by sex is still poorly understood, des pite intense research done mainly on different ungulate species. In most sp ecies studied, males were clearly larger than females. The evolution of sex ual 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 w ith 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 a bility to test alternative hypotheses. More emphasis has to be put on the s tudy of activity budgets, sociality and habitat choice of non-dimorphic spe cies to explain the evolution of permanent territoriality, of long-term bon ds 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 pro posed to explain sexual segregation, discuss alternative outcomes and predi ctions for each hypothesis, suggest alternative explanations for the evolut ion of sexual segregation and mating systems, and indicate new and importan t directions for research. We conclude that a phylogenetic comparison of be havior of a wide range of ungulates and other mammals will be needed to sol ve the enigma of sexual segregation.