Abstract
Our understanding of sex-biased helping has progressed from a historical emphasis on relatedness differences caused by haplodiploidy to an understanding of the role played by the rarer-sex effect. Theory to date typically assumes that offspring help their mother. We show that an alternative, peer-to-peer cooperation can shed light on the interaction of helpers and the recipients of help. In pine sawfly (Diprion pini) larvae, larval peer-to-peer cooperation takes the form of collective, and individually costly, anti-predator behaviour. Larvae typically occur in mixed-sex groups, but females can lay unfertilized eggs that develop into haploid males, which produce male-only broods. Female-biased sex ratios typically select for female-biased helping, and our model here matches empirical findings. Alternative scenarios provide insight too: (i) if genetic constraints permit no sex-specificity in behaviour beyond haploid males expressing all alleles while helping in females can be recessive or dominant, then the sex difference in helping simply reflects the effects of dominance and (ii) female-biased helping can also emerge under male-biased sex ratios, if males are mostly produced in single-sex broods by unmated mothers. While this last example remains hypothetical for D. pini, it highlights an underappreciated point: the rarer-sex effect impacts solutions not only by modifying fitness prospects of the helper, but also of the recipient of help.