The fitness consequences of plastic responses to adult sex ratio in a paternal care fish

Plastic behavioural strategies may help individuals maximize reproductive success in varying environments, but it cannot be assumed a priori that behavioural responses to environmental changes are adaptive. To quantify the fitness consequences of plastic behavioural strategies in a paternal care fish, we observed the intrasexually competitive and intersexually solicitous behaviours of male fantail darters, Etheostoma flabellare, under varying adult sex ratios (ASR) for six observational periods over the course of two full mating seasons. At the end of each observational period, eggs present in each maleʹs nest were counted. Behavioural and egg count data were used to construct linear and generalized linear mixed models predicting male reproductive success by two proxies: presence and number of eggs in the nest. Lower adult sex ratios (the ratio of males to females) and greater male length positively predicted egg presence. Greater intrasexual aggression increased a maleʹs chances of having eggs in high ASR groups. Intrasexual aggression and solicitation of females positively predicted final egg count regardless of ASR. Our results support the hypothesis that some behavioural responses to changing adult sex ratio are adaptive, and that other traits, such as larger male size and high mate solicitation rates, may increase reproductive success regardless of ASR.