In a study published in the September issue of The American Naturalist, A. Kristopher Lappin (Northern Arizona University) and Jerry F. Husak (Oklahoma State University) use the eastern collared lizard (Crotaphytus collaris ), a sexually dimorphic lizard in which the jaws of males function as a weapon in fights, to test the hypothesis that weapon performance (i.e., bite force) is a better predictor of fitness than body size and weapon size.
The study finds that bite-force performance was a strong predictor of reproductive success. However, no size measure was correlated with any estimate of mating success or with potential reproductive output. These results counter the conventional wisdom that bigger is always better, and they support the hypothesis that weapon performance, which is likely to directly influence fight outcomes, has far stronger effects on fitness than size. The strong influence of weapon performance on reproductive success suggests that selection acts on weapon performance, which in turn drives the evolution of weapon morphology. As such, the use of morphology as a proxy for performance and its presumed extensions to fitness should be based, whenever possible, on empirical morphologyperformance relationships.