Context-dependent mate choice in relation to social composition in green swordtails Xiphophorus helleri

Comparative evaluation mechanisms of mate choice in relation to social composition of potential mates have not been considered in nonhuman animals. Models of rational decision making suggest that when choice is based on absolute values, the addition of a third option should take choices from the original 2 options in proportion to their original shares and not result in an increase in the absolute preference for any of the 3 options in the set. However, studies of foraging behavior have shown that choice alternatives are often not irrelevant, specifically when preference is based on 2 or more dimensions (multiple cues) and when the third option is an asymmetrically dominated decoy (i.e., it has a lower value than both original options on one dimension but is only lower than one of the 2 original options on the other dimension). Asymmetrically dominated decoys are predicted to increase preference for the option that dominates it on both dimensions if mate choice is context dependent. We tested whether mate choice is dependent on or independent of social context in green swordtails, a species where females commonly use multiple cues in mate choice decisions. Addition of a third, decoy, male to the set of options resulted in females shifting preference away from the phenotype of male that each preferred in binary comparisons. Consequently, although mate choice was context dependent, the asymmetrically dominated decoy effect was not observed. Instead, females showed negative frequency-dependent preference for the rare-male phenotype, which may act to maintain genetic variation under sexual selection.