Modelling Fluctuating Asymmetry in Relation to Stress and Fitness

Fluctuating asymmetry (or FA) refers to directionally random deviations from bilateral symmetry for a character pair of an organism. Character FA has been related to stress imposed during development of organisms and quality or fitness of individuals. However, FA-stress and FA-quality relations often are absent or variable depending on the characters assessed. Some researchers have hypothesized that FA of sexually selected traits may relate to stress and fitness more reliably than other traits, and that genetic predispositions towards asymmetry may confound relations between asymmetry and either stress or quality. Others have suggested that because character FA is generated randomly, it may not relate to fitness or to FAs of other characters within samples of individuals. In this paper, we modelled development of character FA in relation to random developmental noise or perturbations, stress, developmental stability, and individual quality. We concluded that FA of sexually selected traits need not relate to stress or individual quality more than FA of other characters. We also determined that, in the absence of confounding factors such as genetic predispositions toward asymmetry, measurement error, and character FA-size relations, FA of some characters need not relate to stress or quality. Interestingly, leptokurtic asymmetry distributions could be generated using models expected to generate true FA; thus, leptokurtic asymmetry distributions should be assayed for relations with stress and fitness. We also found that unsigned FA could reliably relate to organism-wide developmental stability and quality, even in the absence of organism-wide FA relations among traits. Finally, conclusions about the utility of FA measures will depend strongly on representation of high FA individuals in samples, because FA-quality relations were typically triangular in distribution.