Body Size in Island Lizards: A Cautionary Tale

Samples of lizards of the genus Uta were collected from many of the islands in the Gulf of California. Other species of lizards coexisting with Uta were collected simultaneously. These samples provided data for a detailed analysis of body size and degree of sexual dimorphism in each island uta population and the correlation of body size with numerous independent variables such as the number and abundance of potentially competing species. This study tested the null hypothesis that competiting species had no significant influence on the body size or sexual dimorphism of the utas on particular islands. Various corollaries of the general hypothesis were also tested. The results not only supported the null hypothesis of no competitive effect, but demonstrated that independent variables not obviously associated with competition explained at least much of the variance in Uta body size as did variables directly related to competition. Previous studies on the evolution of body size in insular lizards have become paradigms of a presumed causal relationship between both size and intensity of interspecific competition. Our study invites caution to the indirect type of approach to the study of competition used in these previous studies. The weaknesses of past approaches are discussed at better approaches to the study of competition are suggested. Selective pressures that might have at least as great an effect on body size of organisms as does competition from other species are identified.