The optimal sex ratio in cooperatively breeding populations

In some cooperatively breeding species, elder siblings remain within their home ranges to assist their parents in raising their younger siblings by providing protection or food. Previous attempts to model the population sex ratios of such species have assumed that helping is cost-free and only involves benefits to parents, resulting in offspring sex ratio biases towards the helping sex. However, parents may also incur costs in maintaining helpers. We incorporated costs and benefits of helping to predict the sex ratio of offspring in a theoretical model. Our model showed that the evolutionary stable strategy (ESS) sex ratio strongly depends on three parameters: (1) the average number of helpers per female; (2) the benefits that a helper brings to its mother’s offspring production (MOP); and (3) the cost-benefit ratio of helping. When one sex of elder siblings provides help, the ESS sex ratio is biased towards the helping sex if MOP costs are less than the benefits, (i.e., the cost-benefit ratio <1). However, the ESS sex ratio becomes biased towards the non-helping sex if MOP costs exceed the benefits (i.e., the cost-benefit ratio >1). Additionally, our model shows that the ESS sex ratio becomes biased in favor of the more helpful sex when both male and female elder siblings provide help. These results explain why sex ratios may become biased towards the helping sex, as well as the non-helping sex, in some species.