Diet composition of the collared lizard (Crotaphytus collaris) in west-central Texas.

Abstract. -- Crotaphytus collaris is a species of lizard widely distributed in the southwestern United States, but there are no quantitative studies of its diet from Texas, where the species is widespread and common. Studies of this species from other locations have shown sexual differences in diet, suggesting differential niche utilization between the sexes that may act as a selective force for the sexual dimorphism that is evident in this species. This study was conducted to provide a descriptive and quantitative account of the diet of C. collaris in west-central Texas in general, and to look for sexual differences in diet. Two-sample t-tests revealed no significant difference between sexes in total volume of prey, total weight of prey, number of prey per stomach, and number of kinds of prey per stomach. Levins' niche breadth values were calculated for each sex and found to be similar and low. Discriminant analysis revealed no significant differences in composition of diet. There is a lack of evidence supporting the hypothesis that sexual differences in diet are acting as a selective force driving the evolution of sexual dimorphism in this population.

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Crotaphytus collaris has a broad distribution in the southwestern United States, ranging from eastern Utah and Colorado to southwestern Illinois, and south into Mexico. Its optimal habitat is rocky, limestone outcroppings and rocky areas (Smith 1946; Conant & Collins 1991). It has been the subject of numerous ecological studies, including its diet (see Best & Pfaffenberger 1987 for a complete listing). Diet has been studied quantitatively and anecdotally for this species in New Mexico (Best & Pfaffenberger 1987), Kansas (Fitch 1956), Oklahoma (Blair & Blair 1941), Arkansas and Missouri (McAllister 1985), and Utah (Knowlton 1938), but not in Texas, a state where the species is widespread and relatively common. Other quantitative studies have focussed on this species at the periphery of its range (e.g., McAllister 1985,) or in areas of habitat other than limestone outcroppings (e.g., Best & Pfaffenberger 1987). The general feeding ecology of this species has been thoroughly described, but this study focuses on sites that have optimal habitat for the species, so that a better understanding of the geographic variation in diet can be determined by comparison to other studies.

Crotaphytus collaris is significantly and variably sexually dimorphic in head width and length, with males having larger heads than females (McCoy et al. 1994; McCoy et al. 1997). One hypothesis for the evolution of sexual dimorphism proposes that the sexes differ in …