Our study thus aims to fill gaps in this picture of male cooperative and competitive behavior by determining (1) whether male social bonds predict reproductive success and (2) what mechanisms link social bonds to coalition formation and paternity. Yet no study in chimpanzees has demonstrated a link between social relationships and reproductive success. Finally, males with more strong affiliative ties are more likely to rise in rank ( Bray et al., in press), and higher-ranking males sire more offspring ( Boesch et al., 2006 Langergraber et al., 2013 Newton-Fisher et al., 2010 Wroblewski et al., 2009). Additionally, male affiliative behavior can predict coalitionary support in aggressive interactions ( Mitani, 2006 Watts, 2002), and subordinate males with higher direct and indirect centrality in the aggressive coalitions network have higher current and future rank ( Gilby et al., 2013 Watts, 2018) and are more likely to sire offspring in the short term, independent of dominance rank ( Gilby et al., 2013). Subordinate males that support and affiliate with the alpha male have higher mating success ( Bray et al., 2016 Duffy et al., 2007). Previous studies in male chimpanzees provide reason to suspect similar mechanisms at work. Here, we use long-term behavioral and genetic data to investigate the relationship between sociality and reproduction in male chimpanzees. In Assamese macaques ( Macaca assamensis), males that had stronger bonds formed more aggressive coalitions, rose in rank in subsequent periods, and ultimately sired more offspring ( Ostner and Schülke, 2018 Schülke et al., 2010). In wild horses ( Equus caballus), more social mares experienced less frequent male harassment and had higher rates of offspring production ( Cameron et al., 2009). Yet despite growing evidence of fitness consequences of sociality, only two studies in non-human mammal species have provided evidence of the mechanisms by which sociality might lead to higher survival or reproductive success ( Ostner and Schülke, 2018 Schülke et al., 2010 Thompson, 2019). Humans and other animals form differentiated affiliative bonds that are associated with survival and reproduction ( Holt-Lunstad et al., 2010 Ostner and Schülke, 2018 Seyfarth and Cheney, 2012). Together, these results suggest that male bonds evolved in chimpanzees by affording both short- and long-term pathways to reproductive success. In the long term, male ties influenced fitness via improved dominance rank for males that attain alpha status. In the short term, bonds predicted coalition formation and centrality in the coalition network, suggesting that males benefit from being potential allies to numerous male rivals. Independently, males with larger networks of strong bonds had higher siring success. Social bonds increased fitness in several ways: first, subordinate males that formed strong bonds with the alpha male had higher siring success. How, then, did selection favor the evolution of cooperative social bonds? We used behavioral and genetic data on wild chimpanzees ( Pan troglodytes schweinfurthii) in Gombe National Park, Tanzania, to study the mechanisms by which male-male social bonds increase reproductive success. In most male mammals, fitness is strongly shaped by competitive access to mates, a non-shareable resource.
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