Counting the costs: the evolution of male-biased care in the cooperatively breeding banded mongoose

In cooperatively breeding vertebrate societies, male and female helpers commonly vary in how much they invest in cooperative activities. Most explanations for this phenomenon centre on sex differences in the fitness benefits that helpers accrue through providing care, and few studies have explored how sex differences in the costs of helping are likely to influence how much care males and females provide. I investigated helper contributions in the banded mongoose, Mungos mungo, and asked whether differences in the way that helping trades off against male and female reproductive success can explain sex biases in helping effort. Contributions to the two main pup care activities were strongly male biased, and helping resulted in both sexes losing weight in the short term. This weight loss is unlikely to reduce the breeding success of males, as, although heavy males are more likely to gain access to oestrous females, any weight lost through helping can be regained before the next oestrous event. Furthermore, males less than 2 years old rarely gain access to oestrous females regardless of their body weight. Short-term reductions in body weight are likely to be more costly for females as the helping period coincides with conception and gestation of the next litter, and weight loss at this time is likely to reduce their probability of breeding successfully. These results suggest that male-biased care in this species has arisen because males and females differ in the way that helping trades off against their reproductive success. (c) 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.