No room for males in caves: Female‐biased sex ratio in subterranean amphipods of the genus Niphargus

Sex allocation theory predicts that the proportion of daughters to sons will evolve in response to ecological conditions that determine the costs and benefits of producing each sex. All else being equal, the adult sex ratio (ASR) should also vary with ecological conditions. Many studies of subterranean species reported female‐biased ASR, but no systematic study has yet been conducted. We test the hypothesis that the ASR becomes more female‐biased with increased isolation from the surface. We compiled a data set of ASRs of 35 species in the subterranean amphipod Niphargus, each living in one of three distinct habitats (surface‐subterranean boundary, cave streams, phreatic lakes) representing an environmental gradient of increased isolation underground. The ASR was female‐biased in 27 of 35 species; the bias was statistically significant in 12 species. We found a significant difference in the ASR among habitats after correction for phylogeny. It is most weakly female‐biased at the surface‐subterranean boundary and most strongly female‐biased in phreatic lakes. Additional modelling suggests that the ASR has evolved towards a single value for both surface‐subterranean boundary and cave stream‐dwelling species, and another value for 9 of 11 phreatic lake dwellers. We suggest that a history of inbreeding in subterranean populations might lower inbreeding depression such that kin selection favours mating with siblings. This could select for a female‐biased offspring sex ratio due to local mate competition among brothers. The observed patterns in sex ratios in subterranean species make them a group worthy of more attention from those interested in sex allocation theory. We tested the hypothesis that the ASR becomes more female‐biased with increased isolation from the surface, and found statistically significant female‐biased ASR in 12 species. After correction for phylogeny, ASR was most weakly female‐biased at the surface‐subterranean boundary and most strongly female‐biased in phreatic lakes. We suggest that a history of inbreeding in subterranean populations might lower inbreeding depression such that kin selection favours mating with siblings. This could select for a female‐biased offspring sex ratio due to local mate competition among brothers.