Heritability of three condition surrogates in the yellow dung fly

Condition capture has been proposed as a general mechanism maintaining additive genetic variation, V-a, in sexually selected traits under directional selection. It relies on two main assumptions: condition-dependent trait expression and V-a in condition. Although there is evidence for the former, direct evidence that condition is heritable is scarce, although this is a requirement of most models of handicap sexual selection. We used a parent-offspring, full-sib, two-container laboratory breeding design in the yellow dung fly Scathophaga stercoraria to demonstrate the broad- and narrow-sense heritability of three surrogates of condition commonly used in sexual selection studies: lipid and glycogen reserves (i.e., physiological condition), body size, and fluctuating asymmetry. All three measures are nutrition dependent and have been linked to sexual selection in free-living yellow dung flies. While lipid reserves and body size were heritable, asymmetry and glycogen reserves were not. Moreover, the evolvability of physiological condition was higher than that of the other two traits. Of the three surrogates, physiological condition is most akin to the original definition, but all have their limitations. We conclude that condition is a useful heuristic concept in evolutionary ecology, but its practical value may be limited by the fact that it cannot be measured directly.