Life History Tradeoffs and Genetic Variation for Social Behaviors in a Wild Primate Population

Abstract

Understanding the genetic and environmental forces that contribute to phenotypic variation is a major goal of evolutionary biology. However, social living blurs the distinction between genes and environments because the social environment is (at least in part) determined by the genes of its members. Therefore, the genes that influence an individual’s phenotype are not limited to his own genes (direct genetic effects) but potentially include the genes of individuals in his social context (indirect genetic effects).Indirect genetic effects are thought to be of particular importance in the evolution of social behavior. Social living is a common phenotype in many animal taxa and is especially well-developed in non-human primates and humans. Social living is believed to be an adaptive response to ecological pressures and indeed, positive correlations between social behaviors and fitness components have been demonstrated in multiple species. However, despite a long-history of theoretical studies investigating the evolution of social behavior, whether social behaviors demonstrate the additive genetic variation necessary for evolution to occur remains poorly understood. No doubt this is due in part to the difficulty of determining the genetic basis of social behavior; social phenotypes are complex and by definition involve interactions between multiple individuals. Genetic variation for social behaviors, therefore, likely results from both direct and indirect genetic effects. Here, I investigated the direct and indirect sources of additive genetic variance in both agonistic and affiliative social behaviors, in a population of wild baboons. In this population (as well as in other populations and species), affiliative social behaviors have been clearly linked to survival, an important fitness component. Therefore, I began with an investigation of the sources of variance in female fitness in our population (Chapter 2). I found that heterogeneity in female quality was a primary driver of the relationship between life history traits, and phenotypic tradeoffs between these traits were apparent only when considering variation in individual quality. In chapter 3, I used a quantitative genetic approach to examine the direct and indirect sources of genetic variation in affiliative social behavior (grooming). I found that both grooming given and grooming received were heritable, and documented a positive genetic correlation between these traits. In chapter 4, I examined the sources of genetic variation in agonistic behavior. This investigation was complicated by the non-genetic transmission of dominance ranks from mothers to daughters. I implemented a novel method to partition social and genetic sources of similarity between relatives and found important roles for both genetic and social variance in the relationship between agonism given and agonism received.