Fluctuating asymmetry indicates the optimization of growth rate over developmental stability

1. Females from many species assess fluctuating asymmetry (FA; nondirectional deviations from symmetry in bilateral traits) when choosing mates. However, the hypothesis that FA indicates the quality of a potential mate is controversial because of the lack of consistent evidence that FA is conditional or heritable. 2. We present evidence that FA provides information about growth rate in the swordtail fish Xiphophorus multilineatus, and a model for how variation in growth strategies could obscure the relationship between FA and stress. 3. Males from one of the genetically influenced size classes of males in this species (Y-II) are more often asymmetrical for the sexually selected trait vertical bars. We also detected a significant relationship between the absolute difference in otolith ring counts and absolute difference in vertical bar number, supporting the hypothesis that FA in vertical bars indicates developmentally instability. 4. In a diet manipulation study, we found support for an alternative growth strategy and a relationship between growth and FA. The Y-II males that grew faster on a high-quality diet had a relationship between higher growth rates and asymmetry that was not detected in the other genotypes. 5. Our results suggest a hypothesis to explain why FA does not always increase with stress: differences across genotypes in the threshold for environmental stress needed to shift the allocation of resources from growth to developmental quality will obscure the relationship between FA and environmental stress. Our results also provide an explanation for why the larger females of some species of swordtails have been shown to prefer asymmetry: in some conditions, choosing a mate that optimizes growth rate over developmental stability (asymmetry) may be adaptive.