INTRODUCTION

To date, McCann's (1933) encounter with a male hoolock (Hylobates hoolock) is the only documented sighting of a wild gibbon male (other than a siamang, H. syndactylus) carrying an infant. McCann shot both the adult male carrying the four-month-old infant, as well as another male, not yet fully grown. No female that could have belonged to this group was heard or seen in the neighbourhood. But, this anecdote aside, there is no other evidence to indicate that males in the genus Hylobates – H. syndactylus excepted – exhibit any measurable amount of direct paternal care in the form of infant carrying (Chivers, 1974; Fischer & Geissmann, 1990). Unless one assumes the unlikely scenario that with the exception of the siamang, direct paternal care was lost secondarily during hylobatid evolution, selective pressures other than the need for paternal care must be considered as elements in the evolution of social monogamy (cf. Komers & Brotherton, 1997; van Schaik & Kappeler, 1997).

In particular, benefits to or constraints on males need to be identified in order to understand the evolution of social monogamy (Clutton-Brock, 1989). The classical constraints on male reproductive strategies are the temporal distribution of fertile females, the spatial distribution of resources, and relationships between females. When females favour a non-gregarious, widely dispersed lifestyle (cf. Sterck et al., 1997) and live in exclusive ranges, males are left with limited options to maximize their fitness.