INTRODUCTION

An individual's lifetime reproductive success is determined by the degree to which it copes with the various challenges that it faces in the different stages of its life. These challenges change constantly from early juvenility until late adulthood (Setchell & Lee, this volume). They require corresponding adaptive variation in the tactics of survival and reproduction, as the animal passes through the successive phases of its life cycle. This dynamic change takes its most dramatic form in those species where an individual goes through one or several distinct larval stages before becoming a reproductively active adult. Apart from such successive changes, there are also variations in fitness-maximising tactics that coexist side by side. In this chapter we deal specifically with such parallel or alternative fitness trajectories in a primate, the orangutan, a species with two adult, sexually mature male morphs. A recent study by Utami et al. (2002) has shown that these two male morphs exist side by side in a natural population and that each morph can and does produce offspring, suggesting that they represent parallel alternative reproductive tactics. Here we review the pertinent evidence.

It has long been an established fact in ethology that interactions with social partners influence an individual's motivational state and vice versa, and, through interactions, its physiological development and condition. For example, the suppression of reproductive processes by the presence of a same-sex conspecific has been documented for many species, including primates.