INTRODUCTION

Primates show a great diversity in their social organizations and mating systems (ranging from single pair bonds to multi-male/multi-female groups), including monogamy, polygyny, and promiscuity (Smuts et al., 1987; Jolly, 1995; Kappeler, 1997; Müller & Thalmann, 2000). Social monogamy is assumed to occur in 15% of all primate species (Kleiman, 1977; Rutberg, 1983; Wright, 1990; Müller & Thalmann, 2000). And even though male care in primates is often associated with pair living, the participation of the male is assumed to be the consequence and not the cause of pair living (Dunbar, 1995; Komers & Brotherton, 1997; Brotherton & Komers, chapter 3; van Schaik & Kappeler, chapter 4). To understand the evolution of monogamy in a certain species, information about cost:benefit ratios of reproductive strategies is necessary. In terms of individual reproductive output, molecular genetic methods are powerful tools for estimating potential costs and fitness benefits associated with different mating strategies (Hughes, 1998). Sociobiological models suggest that sociogenetic monogamy occurs only if fitness benefits (i.e., future reproduction of offspring) can compensate for the costs of missed additional matings (Hamilton, 1967; Trivers, 1985; Mock & Fujioka, 1990; Maynard Smith, 1991). However, paternity analyses, especially in bird species, have revealed that living with a permanent partner does not necessarily imply renunciation of additional matings with extra-pair mates, and that social monogamy masks a diverse range of genetic mating systems (Mock & Fujioka, 1990; Birkhead & Møller, 1992; Stutchbury & Morton, 1995; Reynolds, 1996; Hughes, 1998).