SIZE-DEPENDENT ASYMMETRY: FLUCTUATING ASYMMETRY VERSUS ANTISYMMETRY AND ITS RELEVANCE TO CONDITION-DEPENDENT SIGNALING

Abstract

Fluctuating asymmetry (FA) has received much recent attention in studies of the evolution of sexual signaling systems. Tests apparently showing that symmetry decreases as individual condition decreases have bolstered the view that FA plays a significant role in the evolution of sexual signals. However, a closer inspection of several examples of bilateral variation as a function of trait size (a correlate of condition) suggests a different pattern of variation. Rather than FA, these traits suggest a pattern of size-dependent antisymmetry (a bimodal frequency distribution of R - L). We introduce some quantitative methods to test for condition- or size-dependent FA. Our analyses reject pure FA for four of the five published datasets involving signals (the fifth is equivocal), but confirm the presence of size-dependent FA in one nonsignaling trait. In the studies not conforming to FA, the data appear to fit more closely a pattern of antisymmetry in individuals with smaller signaling traits. Our results thus suggest that current discussions and conclusions about the role of FA in the evolution of signaling systems should be reconsidered. More specifically, we note that condition-dependent antisymmetry offers a more reliable indicator than condition-dependent FA. We caution, however, that additional work will be needed to determine whether the pattern is general and not an artifact. Our method of analysis could usefully be applied to studies of other continuous factors expected to be correlated with asymmetries, including heterozygosity, inbreeding, and environmental stress. Finally, we suggest that antisymmetry may have commonly been mistaken for FA in a variety of cases dealing with a variety of problems.

Keywords: Quality; sexual selection; signaling; size-dependence.