Affiliative social behaviors are linked to fitness components in multiple species. However, the role of genetic variance in shaping affiliative social behaviors remains largely unknown, limiting our understanding of how these behaviors can respond to natural selection. Here, we employed the animal model to estimate both environmental and genetic sources of variance and covariance in grooming behavior in the well-studied Amboseli baboon population in Kenya. We found that grooming given, grooming received, and total grooming all are similarly heritable (h2=0.22, h2=0.16, and h2=0.26 respectively), and that rank and the presence of kin contribute to environmental variance. We detected small but measurable indirect genetic effects of partner identity on the amount of grooming given within dyadic grooming partnerships. The genetic correlation between grooming given and grooming received was exceptionally strong and positive (R=0.94 +/- 0.12), and the indirect and direct genetic effects for grooming given were also strongly positively correlated (R=0.86 +/- 0.06). Our results provide insight into the evolvability of affiliative behavior in wild animals, including the possibility for correlations between direct and indirect genetic effects to accelerate the response to selection. As such they provide novel information about the genetic architecture of social behavior in nature, with important implications for the evolution of cooperation and reciprocity.

In many taxa, adverse early-life environments are associated with reduced growth and smaller body size in adulthood. However, in wild primates, we know very little about whether, where, and to what degree trajectories are influenced by early adversity, or which types of early adversity matter most. Here, we use parallel-laser photogrammetry to assess inter-individual predictors of three measures of body size (leg length, forearm length, and shoulder-rump length) in a population of wild female baboons studied since birth. Using >2,000 photogrammetric measurements of 127 females, we present a cross-sectional growth curve of wild female baboons (Papio cynocephalus) from juvenescence through adulthood. We then test whether females exposed to three important sources of early-life adversity (drought, maternal loss, or a cumulative measure of adversity) were smaller for their age than females who experienced less adversity. Using the animal model, we also test whether body size is heritable in this study population. Prolonged early-life drought predicted shorter limbs but not shorter torsos (i.e., shoulder-rump lengths). Our other two measures of early-life adversity did not predict any variation in body size. Heritability estimates for body size measures were 36%-58%. Maternal effects accounted for 13%-22% of the variance in leg and forearm length, but no variance in torso length. Our results suggest that baboon limbs, but not torsos, grow plastically in response to maternal effects and energetic early-life stress. Our results also reveal considerable heritability for all three body size measures in this study population.