Patterns of fluctuating asymmetry in beetle horns: no evidence for reliable signaling

Negative relations between trait size and levels of fluctuating asymmetry in secondary sexual traits have been claimed to be indicative of honest signaling of male quality. Comparative studies of beetle horns have been used to illustrate the required negative relation between trait size and asymmetry. However, such studies may be confounded by measurement error or sampling bias due to population differences or differences within species in the phenotypic expression of hornedness. We examined the patterns of fluctuating asymmetry within two species of horned beetle. We found that, in agreement with theory, horns exhibit greater asymmetry than naturally selected traits. However, we found a strong positive relation between horn size and asymmetry in Onthophagus taunts, a species with male dimorphisms, and a flat relation in Bubas bison, a species with continuous variation in horn size. We suggest that these differences may reflect functional differences in horns. We conclude that patterns of asymmetry in horned beetles do not support the notion of honest signaling.