Operational sex ratio bias due to sex‐specific cohort splitting in response to predation

Abstract The operational sex ratio (OSR), that is, the local ratio of fertilizable females to sexually active males at any given time, is of key importance for the strength of sexual selection and the reproduction of populations. We hypothesize that sex‐specific cohort splitting, that is, when one sex mostly metamorphoses while the other mostly enters diapause, may lead to OSR bias in nature. The OSR of an aquatic moth, Acentria ephemerella , has been shown to be strongly male‐biased in situ. Here, we use a mesocosm experiment in which we determine the sexes of active, diapausing, and metamorphosing larvae to test whether the male bias in Acentria is due to sex‐specific mortality or sex‐specific cohort splitting. Fish predation did not result in a strong male bias of the whole population but increased male bias in pupae and female bias in diapausing larvae. The opposite effect of fish on pupal versus diapausing larval sex ratios suggests that fish‐induced sex‐specific cohort splitting, rather than sex‐specific mortality, caused the OSR bias of Acentria observed in situ. Future research needs to study whether the OSR bias is an adaptive response to the presumably higher fish predation pressure on females or a maladaptive byproduct of sex‐specific activity and growth responses to fish presence. Overall, shifts in OSR due to sex‐specific cohort splitting could be a more common component of arthropod life histories than previously thought.