Temperature regulation in Puerto Rican Anolis lizards: a field test using null hypotheses

I used null hypotheses about basking behavior and body temperatures ([T.sub.b]) to evaluate the extent and effectiveness of behavioral temperature regulation in Anolis cristatellus and A. gundlachi. I sampled these species in January and August near the extremes of their altitudinal distributions in Puerto Rico. Hollow electroformed copper lizard models, randomly positioned within habitats occupied by the lizards, served as operative temperature ([T.sub.e]) thermometers, providing baseline predictions about basking rates and [T.sub.b] of nonthermoregulating lizards. Comparison of the basking behavior and [T.sub.b] of lizards to the basking rates and temperatures of the models provided unambiguous evidence of nonrandom thermoregulatory activity in some of the lizard populations. Although basking opportunities were similar in the habitats occupied by both species, basking rates differed markedly: A. cristatellus basked more frequently than did randomly positioned models, and its basking rates were higher at high elevation and in January; A. gundlachi used basking sites at random at both elevations in both months. In addition, A. cristatellus limited its activity, particularly at high elevation, to times when operative temperatures were relatively high or basking sites were available (i.e., sunny weather). As a result of its opportunistic basking activity, mean [T.sub.b] of A. cristatellus was often elevated above mean [T.sub.e], and A. cristatellus populations experienced only limited variation of mean [T.sub.b], with elevation and season. In contrast, A. gundlachi mean [T.sub.b] approximated mean [T.sub.e], and the [T.sub.b] of A. gundlachi varied more with elevation and season. Mean [T.sub.b] in A. cristatellus was always higher than the mean [T.sub.b] of A. gundlachi at every elevation and month combination. The data also suggest that the low [T.sub.b] experienced by A. gundlachi at high elevation might dramatically reduce sprinting ability, whereas the [T.sub.b] experienced by A. cristatellus allows a majority of lizards at high elevation to sprint quickly.