Social bonds between unrelated females increase reproductive success in feral horses

Abstract

In many mammals, females form close social bonds with members of their group, usually between kin. Studies of social bonds and their fitness benefits have not been investigated outside primates, and are confounded by the relatedness between individuals in primate groups. Bonds may arise from kin selection and inclusive fitness rather than through direct benefits of association. However, female equids live in long-term social groups with unrelated members. We present 4 years of behavioral data, which demonstrate that social integration between unrelated females increases both foal birth rates and survival, independent of maternal habitat quality, social group type, dominance status, and age. Also, we show that such social integration reduces harassment by males. Consequently, social integration has strong direct fitness consequences between nonrelatives, suggesting that social bonds can evolve based on these direct benefits alone. Our results support recent studies highlighting the importance of direct benefits in maintaining cooperative behavior, while controlling for the confounding influence of kinship.equid ͉ friendship ͉ social structure ͉ alliances ͉ sociality

Evolutionary foundations of human prosocial sentiments

“…In humans, social support is correlated with better physical and mental health as well as lower mortality risks (37)(38)(39)(40). Similar kinds of findings are accumulating for a range of nonhuman species, including rodents (41,42), dolphins (43), wild horses (44), female baboons (45)(46)(47), and male macaques (48).…”

Coping with the cold: predictors of survival in wild Barbary macaques, Macaca sylvanus

“…This is important, as the scarce evidence on the link between sociality and fitness has so far focused on long-term fitness benefits (e.g. lifetime reproductive success: [4][5][6][7][8][9][10]). Taken together, our study and the previous research on this topic support the hypothesis that the capacity to establish and maintain social relationships may have independently evolved in different taxa, any time ecological conditions gave fitness benefits to individuals with a stronger network of relationships [3,22,23].…”

“…In primates, individuals that share stronger and more stable social relationships tend to have greater infant survival [4,5], a longer life expectancy [6] and sire more offspring [7]. In dolphins, horses and marmots, reproductive success has also been linked to the strength of an individual's social relationships [8][9][10]. No data are currently available on whether sociality affects fitness in response to sudden, extreme changes in ecological conditions that can affect animal survival.…”

Urinary oxytocin and social bonding in related and unrelated wild chimpanzees

“…In both cases, these relationships are usually defined in terms of high rates of cooperative behaviours, including grooming [1,2,[5][6][7][8]11]. Although the maintenance of enduring social bonds is associated with fitness benefits, whether these are between kin [1,2] or non-kin [6,11], the underlying mechanism of how such relationships are maintained over time is unclear. Contingent reciprocity, where one remembers a service given by another and then offers a service in return at a later date, offers a possible explanation, although this mechanism is rare in animals [16,17] and has been found among individuals that interact at low rates [18].…”