The evolution of plastic sex ratio strategies in cooperative breeders

The offspring sex ratio is often biased in cooperative breeders. Two hypotheses explain why this could be adaptive: (1) the local resource enhancement hypothesis; (2) the local resource competition hypothesis. The first poses that offspring of the helping sex should be overproduced, as helpers provide fitness benefits to parents and future siblings. The second poses that offspring of the (nonhelping) dispersing sex should be overproduced, as a reduction in local competition over resources increases the parents’ fitness. Here, we study the evolution of sex ratio strategies using evolutionary individual-based simulations to understand the relative importance of both hypotheses. We use the cooperatively breeding Seychelles warblers, Acrocephalus sechellensis , as inspiration, but our results can be interpreted more widely. Seychelles warblers were hypothesized to adapt their offspring sex ratio to the quality of the territory they reside in, as competition over resources is strong in low-quality territories, while the presence of helpers is beneficial in high-quality territories. We found that an offspring sex ratio strategy based on territory quality evolves readily; as in Seychelles warblers, daughters (the helping sex) are overproduced in high-quality territories, while sons (the nonhelping sex) are overproduced in low-quality territories. However, very similar sex ratio trends evolved when we switched off the sex difference in helping tendency or the sex difference in dispersal. When we removed all sex differences, sex ratio trends based on territory quality (varying in steepness and direction) still readily evolved, which can be explained by the fact that many alternative trends are neutrally evolutionarily stable. We conclude that, in the Seychelles warbler, it is not possible to disentangle the effects of local resource enhancement and local resource competition based on the observed sex ratio trend. More generally, our study suggests that the sex ratio patterns found in nature will often have different explanations, making it difficult to pinpoint the causal determinant of a pattern.