Summary1. Sociality has been shown to have adaptive value for gregarious species, with more socially integrated animals within groups experiencing higher reproductive success and longevity. The value of social integration is often suggested to derive from an improved ability to deal with social stress within a group; other potential stressors have received less attention. 2. We investigated the relationship between environmental temperature, an important nonsocial stressor, and social integration in wild female vervet monkeys (Chlorocebus pygerythrus), using implanted data loggers to obtain direct measures of core body temperature. 3. Heterothermy (as measured by 24-h amplitude of body temperature) increased, and 24-h minima of body temperature decreased, as the 24-h minimum ambient temperature decreased. As winter progressed, monkeys became increasingly heterothermic and displayed lower 24-h minima of body temperature. 4. Monkeys with a greater number of social partners displayed a smaller 24-h amplitude (that is, were more homoeothermic) and higher 24-h minima of body temperature (that is, became less hypothermic), than did animals with fewer social partners. 5. Our findings demonstrate that social integration has a direct influence on thermoregulatory ability: individual animals that form and maintain more social relationships within their group experience improved thermal competence compared to those with fewer social relationships. 6. Given the likely energetic consequences of thermal benefits, our findings offer a viable physiological explanation that can help account for variations in fitness in relation to individual differences in social integration.

We report the death of 30 wild Barbary macaques, living in two groups, during an exceptionally cold and snowy winter in the Middle Atlas Mountains, Morocco. We examined whether an individual's time spent feeding, the quality and number of its social relationships, sex and rank predicted whether it survived the winter or not. The time an individual spent feeding and the number of social relationships that an individual had in the group were positive and significant predictors of survival. This is the first study to show that the degree of sociality affects an individual's chance of survival following extreme environmental conditions. Our findings support the view that sociality is directly related to an individual's fitness, and that factors promoting the establishment and maintenance of social relationships are favoured by natural selection.

The social environment can affect an individual's wellbeing. This is true for both 2 humans and animals. Here we show that even survival depends on social integration. 3Wild Barbary macaques were more likely to survive an extremely harsh winter when 4 they were part of close affiliative social groups. However, the best predictor for 5 survival was integration in the aggression network -individuals that interacted 6 aggressively with more but less connected partners had the best chances of survival. Morocco. We analyzed the affiliation and aggression networks of both groups in the 22 six months before the occurrences of these deaths, to assess which aspects of their 23 social relationships enhanced individual survivorship. Using only the affiliation 24 network we found that network clustering was highly predictive of individual survival 25 probability. Using only the aggression network we found that individual survival 26 probability increased with a higher number of aggression partners and lower clustering 27 coefficient. Interestingly, when both affiliation and aggression networks were 28 considered together, only parameters from the aggression network were included into 29 the best model predicting individual survival. Aggressive relationships might serve to 30 stabilize affiliative social relationships, thereby positively impacting on individual 31 3 survival during times of extreme weather conditions. Overall, our findings support the 32 view that aggressive social interactions are extremely important for individual 33 wellbeing and fitness. 34 35