Alternative mating tactics and male mating success in two species of fiddler crab

The use of alternative male mating tactics can determine the strength of sexual selection on male traits and have implications for sexual dimorphism. We examined size-based mating success in two species of fiddler crabs where males use each of two alternative tactics to obtain matings. In Uca annulipes, larger males were more successful when using the primary mating tactic (burrow mating) but the full size range of males mated when using the secondary tactic (surface mating). In Uca urvillei, both burrow and surface mating males were larger than the average sized male in the population. Standardised directional selection gradients indicated that selection on male size was stronger in U. urvillei than U. annulipes, reflecting the differences between species in male mating success. Our results also showed that sexual size dimorphism was greater in the species with stronger sexual selection on male size than in the species with weaker sexual selection.