Female fiddler crabs settle for less: the travel costs of mate choice

Females often have strong preferences for the most attractive males that provide the greatest benefits. However, searching for and sampling potential mates is costly. Females are therefore expected to make an optimizing choice whereby the benefits provided by the male outweigh the cost of choosing him. Consequently males should benefit by minimizing costs experienced by sampling females, or by reducing the ability of females to assess these costs. We investigated the economics of female mate choice in the fiddler crab Uca mjoebergi through observation of natural mate-searching behaviour, and by manipulating the costs of choice in a series of two-choice experiments using robotic crabs. Observed females were choosy about their mates, but this selectivity declined when predation risk increased. Experimental females were allowed to choose between males of different attractiveness at a range of distances from the female, with the more attractive male placed further from the female. Females did not travel further to reach a more attractive male except when distances were small. These results suggest that greater attractiveness does not always ensure greater mating success for males, as distant males experience a disadvantage. Male mate-attracting behaviour was then investigated, as male U. mjoebergi often leave their territories to approach sampling females, behaviour that may overcome a distance disadvantage. Males closely approached females regardless of their distance from the female. This behaviour is likely to prevent females from making the optimal choice, as they cannot include travel costs in their assessment of males.