The benefits of social capital: close social bonds among female baboons enhance offspring survival

Abstract

Sociality has evolved in many animal taxa, but primates are unusual because they establish highly differentiated bonds with other group members. Such bonds are particularly pronounced among females in species like baboons, with female philopatry and male dispersal. These relationships seem to confer a number of short-term benefits on females, and sociality enhances infant survival in some populations. However, the long-term consequences of social bonds among adult females have not been well established. Here we provide the first direct evidence that social relationships among female baboons convey fitness benefits. In a group of free-ranging baboons, Papio cynocephalus ursinus, the offspring of females who formed strong social bonds with other females lived significantly longer than the offspring of females who formed weaker social bonds. These survival benefits were independent of maternal dominance rank and number of kin and extended into offspring adulthood. In particular, females who formed stronger bonds with their mothers and adult daughters experienced higher offspring survival rates than females who formed weaker bonds. For females lacking mothers or adult daughters, offspring survival was closely linked to bonds between maternal sisters. These results parallel those from human studies, which show that greater social integration is generally associated with reduced mortality and better physical and mental health, particularly for women.

Figure 1.

Effects of maternal kinship on the strength of social bonds among adult females. Means and standard errors of the composite sociality index are given for maternal kin (mothers and daughters: n = 24 dyads; sisters: 28 dyads; aunts and nieces: 40 dyads; cousins: 18 dyads) and ‘nonkin’ (n = 879 dyads). The ‘nonkin’ category includes all dyads for which maternal relatedness is less than 0.0625). Means and standard errors are shown.

Figure 2.

Stability of females’ close social bonds. The duration of close social bonds for dyads composed of mothers and daughters (black bars); maternal sisters (gray bars), and females who are not related through maternal lines (white bars) is shown here. Initial sample sizes for kin categories are given in figure 1. For each type of dyad, the proportion of dyads that maintained close social bonds for a given number of years is plotted on the y-axis. The values were obtained by dividing the number of dyads that maintained a close social bond for at least x years by the total number of dyads that were coresident for x or more years. Mothers and daughters were very likely to maintain close social bonds for as long as they lived together in the group.

Figure 3.

Effects of maternal sociality on offspring survival. For the purposes of illustration here, females were ranked according to the strength of their social bonds to (a) all adult females or (b) to their own mothers and adult daughters, and then divided into three equal groups. Offspring of females with the strongest social bonds are depicted by a solid black line, offspring of females with intermediate bonds by a dashed line, and offspring of females with the weakest bonds by a dotted line. Note that sample sizes decline as longevity increases.