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Free keywords:
Bateman gradient; body size; genetic mating system; operational sex ratio; opportunity for sexual selection; selection gradient
Abstract:
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 twospotted
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 nestholding
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.
