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.

Evolutionary theory predicts that mothers of different condition should adjust the birth sex ratio of their offspring in relation to future reproductive benefits. Published studies addressing variation in mammalian sex ratios have produced surprisingly contradictory results. Explaining the source of such variation has been a challenge for sex–ratio theory, not least because no mechanism for sex–ratio adjustment is known. I conducted a meta–analysis of previous mammalian sex–ratio studies to determine if there are any overall patterns in sex–ratio variation. The contradictory nature of previous results was confirmed. However, studies that investigated indices of condition around conception show almost unanimous support for the prediction that mothers in good condition bias their litters towards sons. Recent research on the role of glucose in reproductive functioning have shown that excess glucose favours the development of male blastocysts, providing a potential mechanism for sex–ratio variation in relation to maternal condition around conception. Furthermore, many of the conflicting results from studies on sex–ratio adjustment would be explained if glucose levels in utero during early cell division contributed to the determination of offspring sex ratios.

Although breeding success is known to increase with group size in several cooperative mammals, the mechanisms underlying these relationships are uncertain. We show that in wild groups of cooperative meerkats, Suricata suricatta , reductions in the ratio of helpers to pups depress the daily weight gain and growth of pups and the daily weight gain of helpers. Increases in the daily weight gain of pups are associated with heavier weights at independence and at 1 year of age, as well as with improved foraging success as juveniles and higher survival rates through the first year of life. These results suggest that the effects of helpers on the fitness of pups extend beyond weaning and that helpers may gain direct as well as indirect benefits by feeding pups.

Summary: We examined the relative importance of maternal, environmental and social factors for post‐weaning pup growth and survival in a co‐operatively breeding mammal, the meerkat Suricata suricatta . Pup daily weight gain was primarily influenced by the number of carers per pup and the daily weight gain of those carers. Rainfall and daily temperatures had additional positive and negative effects, respectively, on weight gain of pups born to subordinates. Pup overnight weight loss was primarily influenced by the amount of weight pups gained during the day, and their age. However, pups also lost considerably more weight overnight when temperatures were cold, although such effects were less in large groups. Pup growth rates were positively influenced by the number of carers per pup and carer condition, and negatively influenced by high daytime temperatures. Pup weight at independence was positively associated with weight at emergence and pup weight gain during provisioning, but negatively associated with the extent of overnight weight loss. Pup survival between emergence and independence was related to maternal status, pup sex and overnight weight loss, as well as to group size, daytime temperature and monthly rainfall. Thus, in meerkats, social factors largely, but not wholly, replace the importance of maternal factors that are commonly found to govern reproductive success in non‐co‐operatively breeding social vertebrates.