Differences in Pelvic Fin Length Represent Sexual Dimorphism in Utah Chub (Gila atraria)

The cyprinid fish Gila atraria Girard (Utah chub) is generally considered a sexually monomorphic species. However, prior observations revealed variation in pelvic fin length within populations that appears sexually dimorphic. We measured the relative pelvic fin length of 419 sexually mature Utah chub from 8 different locations to determine the magnitude and generality of this apparent dimorphism. Pelvic fin length in G. atraria differs between sexes by about 10% on average; males have longer pelvic fins than females. The dimorphism is general across all locations, but it is not related to body size. Magnitude of the dimorphism varies by predation environment - the difference between males and females is slightly greater in low predation environments. We find no evidence for an adaptive function for this dimorphism; how- ever, it does provide an efficient mechanism for determining sex without dissection.