Field and selected body temperatures of the lizards Sceloporus aeneus and Sceloporus bicanthalis

The objective of this study was to evaluate the thermal biology of the Mexican lizards Sceloporus aeneus and Sceloporus bicanthalis (Phrynosomatidae). Sceloporus aeneus is oviparous, and has an elevation range of approximately 2400-3000 m. It is replaced at elevations above 3000 m by its viviparous sister species, S, bicanthalis, whose elevational range extends to at least 4250 m. Selected body temperatures (T-sel) were measured on a laboratory thermal gradient. Mean selected body temperatures of S. aeneus and S. bicanthalis were 35.5 and 34.9 C, respectively. T-sel's for these two high elevation species were thus similar to T-sel of low elevation Sceloporus, an observation that supports the static view of the thermal physiology of Sceloporus lizards. For neither species did selected body temperature vary as a function of sex or reproductive status of females. Body temperatures during activity in the field were not related to the time of year or the reproductive status of females, although male S, bicanthalis at one site had higher body temperatures than females. The mean daily maximum air temperature declined from 22 C at the S, aeneus site at 2800 m, to 19 C at the S, bicanthalis site at 3200 m, and to 8 C at the S. bicanthalis site at 4100 m. Mean body temperatures at these sites were 32, 32, and 29 C, respectively. These species were thus active at body temperatures lower than their "preferred" body temperatures, presumably because of increasingly limited thermal opportunities at high elevations.