• aggression;
  • body size;
  • courtship;
  • mating competition;
  • resource holding potential;
  • sexual selection

The present study explored how male size relates to mating competition across a natural range of male and female densities in the two-spotted goby Gobiusculus flavescens. Across this range of social environments, large males were more than twice as likely as small ones to chase other males, to become nest-holders, and to court females, but large males were not significantly more likely to engage in agonistic fin displays. Overall, the study showed that large males court and fight more than small ones across a wide, yet natural, span of social environments. Having a large body size appears to confer competitive advantage for males in any social environment of the study species. Further studies are needed to disentangle whether the benefit of large size is mainly in competition over resources, over matings as such, or both.