World Congress Of Herpetology 7: Geographic Variation In Thermal Tolerance

Laura Rubio-Rocha, a masters student at UT Knoxville, presented an anole double-header at Saturday evening’s poster session. In one poster, Laura presented work on thermal physiology of two anole species that occur in the United States: the native green anole (A. carolinensis) and the introduced brown anole (A. sagrei). By sampling each species from a number of localities across a latitudinal transect extending from southern Florida to southern Georgia, Laura was able to test whether populations or species vary in their ability to tolerate cold temperatures. To evaluate each species’ ability to tolerate cold temperatures, she used a simple behavioral assay that diagnosed the lower critical thermal minimum (CTmin) as the temperature at which a species no longer responded to physical stimuli. Although anoles readily recover from this state in captivity, anoles in field aren’t likely to last long when incapacitated.

When CTMin was assessed in the field, Laura found that thermal tolerance largely mirrored the latitude of sampled populations. Within each species, those that occurred further north were better able to cope with lower temperatures than population sampled further south. A strong difference between the two species, with green anoles (A. carolinensis) being able to tolerate much colder temperatures than brown anoles (A. sagrei). The story, however, doesn’t end here.

Recognizing that some degree of variation in a given individual’s cold tolerance might result from acclimation and plasticity, Laura conducted her analyses both in the field immediately after capture and after allowing the animals to acclimate to the conditions of a common garden laboratory environment for several months. When Laura analyzed her laboratory data, she found that the strong difference in cold tolerance between species persisted, but the differences within populations of each species from localities across a latitudinal gradient were no longer evident. Her results suggest that some degree of cold tolerance is genetically determined, but that variation within species along a latitudinal gradient is likely a plastic response.