Ectotherms rely on interactions with surrounding thermal environments to regulate their body temperature. If their body temperatures get too low or too high, ectotherms may be unable to move effectively or escape dangerous temperatures, potentially leading to death. One plastic physiological response which may help ectotherms avoid the effects of dangerously high body temperatures is heat hardening. Heat hardening is a type of physiological flexibility that entails an organism increasing its heat tolerance after a previous exposure to high temperatures. In areas with high temperatures, differences between ectotherms in their abilities to effectively conduct heat hardening could affect competition between them.
A green anole (Anolis carolinensis) basks at an elevated perch.
Sean Deery, a masters student in the Gunderson lab at Tulane University, chose to investigate heat hardening capacity in two species of anoles, the native green anole (Anolis carolinensis) and the invasive brown anole (Anolis sagrei), both of which are present in New Orleans. As brown anoles have expanded throughout the area, they have displaced green anoles, forcing them higher into vegetation, a pattern repeated in other areas of the southeastern U.S.
Brown anoles are particularly adept at exploiting urban habitats, where temperatures may be considerably higher than surrounding natural areas due to the urban heat island effect. Sean wondered whether the competitive advantage of brown anoles over green anoles might be based in part on a superior heat hardening capacity, which could support their dominance in urban areas.
(a) A male green anole and (b) and a displaying male brown anole in Florida.
To quantify heat hardening in this system, Sean captured green and brown anoles and first measured their upper critical thermal maximum (CTMax) by steadily ramping up their body temperatures until the lizards lost coordination. CTMax represents a temperature that could prove lethal to a lizard as it would be unable to escape these hot conditions. After allowing lizards to recover, Sean measured their CTMax again after periods of 2, 4, and 24 hours. Heat hardening was calculated as the difference between the initial CTMax and the subsequent measurement after exposure to those initial high temperatures.
Sean’s results were surprising: He found that brown anoles showed no evidence of heat hardening at any time after an initial measurement of CTMax. In fact, brown anoles showed a reduction in CTMax, suggesting that the initial testing may have stressed them and reduced their ability to cope physiologically with higher temperatures. Green anoles on the other hand showed a moderate heat hardening response, with significant increases in CTMax just 2 hours after exposure to high temperatures. Sean’s results also suggest that individual lizards with lower initial CTMax values showed greater heat hardening.
For now, it appears that heat hardening is not a factor driving invasions of brown anoles in the southeastern U.S., but the differences between these two species are intriguing. Sean hopes to expand on this work by investigating molecular mechanisms that may support or inhibit heat hardening, such as expression of heat shock proteins.
