SEXUAL SEGREGATION IN UNGULATES: A NEW APPROACH by KATHREEN E. RUCKSTUHL 1) and PETER NEUHAUS 2) (LARG, Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK) (Acc. 24-XI-1999) Summary 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, sh 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 ve 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. Keywords: evolution, sexual segregation, mating systems, sociality. 1) Corresponding author, e-mail address: kruckstuhl@hotmail.com 2) We thank Tim Clutton-Brock, Marco Festa-Bianchet, Wendy King, John Gross and Hanna Kokko for suggestions on the manuscript. Critical comments by Rob Slotow and Johan van Rhijn were greatly appreciated. This manuscript was conceived while the rst author was still with the Université de Sherbrooke, Québec. Many thanks for stimulating discussions. c ® Koninklijke Brill NV, Leiden, 2000 Behaviour 137, 361-377