Data from: Within-season variation in sexual selection in a fish with dynamic sex roles

The strength of sexual selection may vary between species, among
populations, and within populations over time. While there is growing
evidence that sexual selection may vary between years, less is known about
variation in sexual selection within a season. Here, we investigate
within-season variation in sexual selection in male two-spotted gobies
(Gobiusculus flavescens). This marine fish experiences a seasonal change
in the operational sex ratio from male- to female-biased, resulting in a
dramatic decrease in male mating competition over the breeding season. We
therefore expected stronger sexual selection on males early in the season.
We sampled nests and nest-holding males early and late in the breeding
season and used microsatellite markers to determine male mating and
reproductive success. We first analysed sexual selection associated with
the acquisition of nests by comparing nest-holding males to population
samples. Among nest-holders, we calculated the potential strength of
sexual selection and selection on phenotypic traits. We found remarkable
within-season variation in sexual selection. Selection on male body size
related to nest acquisition changed from positive to negative over the
season. The opportunity for sexual selection among nest-holders was
significantly greater early in the season rather than late in the season,
partly due to more unmated males. Overall, our study documents a
within-season change in sexual selection that corresponds with a
predictable change in the operational sex ratio. We suggest that many
species may experience within-season changes in sexual selection and that
such dynamics are important for understanding how sexual selection
operates in the wild.