The dawn of primate social complexity: kin selection in asocial mammals
Since Hamilton's ground-breaking theory of inclusive fitness in 1964, kin-biased behavior has been theorized to have played a crucial role in the evolution of mammalian sociality (Hamilton, 1964; de Waal and Tyack, 2003; Chapais and Berman, 2004). Given the amount of attention given to the topic over the subsequent decades, it is surprising that while group-living and social complexity has evolved multiple times in mammals, we still know very little about how this process occurs (Waser and Jones, 1983; Müller and Thalmann, 2000; de Waal and Tyack, 2003). In this section we review what is known about ancestral mammals and how they were the foundation for the evolution of ancestral primates.
Ancestral mammals are believed to have been asocial, as are many extant mammal species (Waser and Jones, 1983; Müller and Thalmann, 2000). Asocial species forage alone and maintain no relationships outside of the mating and infant-rearing seasons (Charles-Dominique, 1974, 1978; Waser and Jones, 1983; Müller and Thalmann, 2000). Interactions between adults, including adult kin, are marked by avoidance and aggression (Charles-Dominique, 1974; Waser and Jones, 1983; Müller and Thalmann, 2000). This is note-worthy because in many species, females typically disperse shorter distances than males, leading to a spatial clustering of female kin (Waser and Jones, 1983; Stoen et al., 2005; Maher, 2009). For many scientists, it is this spatial clustering of kin which is the first step towards increasing sociality (Waser and Jones, 1983; Perrin and Lehmann, 2001; Kappeler et al., 2002; Lutermann et al., 2006; Meshriy et al., 2011; Messier et al., 2012). The transition to group-living is believed to have occurred through solitary foraging (Müller and Thalmann, 2000). Extant solitary foragers forage alone, but, in contrast to the asocial mammalian ancestors, maintain year-round social networks, communicating with conspecifics via scent-marks and vocalizations (Charles-Dominique, 1974, 1978; Zimmermann, 1990, 1995a, 1995b, 2010; Müller and Thalmann, 2000; Nash, 2004; see also Chapter 21). Individuals may interact affiliatively during their active periods and sometimes sleep in social groups, often consisting of matrilineal kin, during the inactive periods (e.g., Radespiel et al., 2001b; Eberle and Kappeler 2006; for review, see Müller and Thalmann, 2000).