Evaluating thermal resource partitioning : By sympatric lizards Anolis cooki and A. cristatellus: a field test using null hypotheses.

The field thermal biology of sympatric Anolis cooki and A. cristatellus were evaluated in January and in August in desert scrub forest at Playa de Tamarindo near Guanica, Puerto Rico. Data on randomly positioned copper models of lizards, each equipped with a built-in thermocouple, established null hypotheses about basking frequency and operative temperatures (T ) against which the behavior and body temperatures (T ) of live lizards were evaluated. Both species exhibited non-random hourly basking rates (more marked in cristatellus than in cooki), and cristatellus was virtually inactive during the warm mid-day hours. The relationship between lizards' T and randomly sampled T differed between the species: cristatellus's mean T was 2° to 3° C lower than randomly sampled mean T in both months, whereas cooki's mean T was slightly higher than mean T in January and slightly lower in August. Although cooki's mean T was higher than that of cristatellus in both months, the T 's of the two species overlapped substantially over an annual cycle. Given the similarities in their field active T and the low thermal heterogeneity among microsites at Playa de Tamarindo, these species appear not to partition the thermal environment there in a coarse-grained way. Instead, the relatively small differences in their field active T probably result from small differences in their use of similar microhabitats within their mutually exclusive territories. Thermal resource partitioning by territorial animals is unlikely unless thermal heterogeneity is coarse-grained in relation to territory size.