Collared lizards, Crotaphytus collaris, live in isolated populations on Missouri glades. Anecdotal observations suggest that another lizard species, Sceloporus undulatus, is rare on glades where C. collaris is present. Possible causes of scarcity of S. undulatus in the presence of C. collaris include competition, predation, and unsuitable thermal conditions. We characterized thermal biology and habitat partitioning in these two species by measuring body and air temperatures, and microhabitat use, at three glades. Sceloporus undulatus maintains lower body temperatures than C. collaris and shifts from open rock perches to shady tree perches during the middle of its activity season. Crotaphytus collaris microhabitats are rockier and more open than those of S. undulatus, which tend to have more branches, leaves, and trees nearby. These data indicate that areas of glades hot enough for use by C. collaris are too hot for S. undulatus. Although we cannot rule out competition or predation, constraints of the thermal environment may be an important factor in the apparent scarcity of S. undulatus on glades.
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