Female preference for male phenotypic traits in a fiddler crab: do females use absolute or comparative evaluation?

While mate choice is often assumed to be based on an absolute value that may reflect underlying male quality, many mating systems facilitate the comparative evaluation of males. Females can encounter a number of potential mates, either sequentially or simultaneously, before making a mate choice decision. Consequently, the attractiveness of one male may depend on the attractiveness of other males a female has previously encountered. To examine whether female choice in the fiddler crab Uca mjoebergi is based on the comparative evaluation of males, I altered the social context in which a set of males were encountered. Using a robotic crab system to manipulate claw size and wave rate directly, I determined female preference for two male options in the absence and presence of an asymmetrically dominating decoy option. The decoy male option was designed to change the relative attractiveness of the initial two male options. The presence of the decoy significantly altered the absolute preference, but not the relative preference, for the two male options. While this study clearly demonstrates that female U. mjoebergi are not using strict absolute preferences when ranking each male's attractiveness, it does not conclusively support comparative evaluation mechanisms. One possibility is that sexually selected traits in a complex signalling system often vary in how reliably they reflect male quality, and females may use a combination of absolute and comparative measures when assessing male quality.