Rising surface temperatures are threatening biodiversity worldwide, and the impacts of climate change on organisms are hard to predict. Ectothermic species, such as lizards, rely to a large degree on environmental temperatures to maintain their body temperatures and might therefore be especially impacted by the predicted changes. A team of international researchers led by Martha Muñoz, from Duke University, USA, has compared the thermal behavior and physiological characteristics of seven lizard species living in eight different locations and elevations of an Australian rainforest region in order to examine how basking behavior and thermal habitat influence the evolution of thermal physiology such as heat tolerance. This extensive study, published in Evolution, suggests that the ability of a lizard species to tolerate hot environments is dependent not so much on the climate in their habitat, but on their basking behavior, and highlights the high vulnerability of ectothermic species to a warming climate.
The team first identified the thermal microhabitats and basking tendencies of seven lizard species by observing the animals in their natural environments and measuring the temperatures of the surface where the lizards were perching. They then collected lizards from the field and transferred them to the laboratory to conduct a series of tests to classify the physiological traits of each species. First, they flipped the lizards on their back at different temperatures and determined the lowest and highest temperature at which an individual was able to right itself. Following these tolerance tests, Muñoz and colleagues determined the thermal sensitivity of sprinting speeds to establish the temperatures at which sprinting was quickest for each species.
When the team analyzed the data, including information on the phylogenetic relationship of the seven species, they found that the basking species were considerably more heat tolerant than the shade-seeking skinks. Based on this observation, Muñoz and her colleagues concluded that the evolution of heat tolerance likely occurred when the lizards specialized and selected either basking or shade-dwelling as their preferred lifestyle. They also suspect that this led to the differences that they found in heat tolerances even between lizards that live in the same location. Unfortunately, their finding that habitat temperature had no influence on heat tolerance also indicates that these species may not be able to adapt quickly enough to match the pace of climate warming.
However, the lizards’ cold tolerance seems to be shaped mainly by their habitat conditions. The researchers found that all lizards, regardless of their basking behavior, were more cold tolerant when they inhabited cooler habitats. In the cold, animals have no other option than to adjust their physiology to cope better in chilly conditions. Interestingly, lizards from lower elevations, and therefore warmer environments, sprinted optimally at lower temperatures than lizards from cooler areas. The authors concluded that at lower elevations, lizards have a greater risk of overheating and that the lower optimal sprinting temperatures could be caused by an alteration of the lizards’ activity patterns to avoid heat stress.
This study emphasizes the high vulnerability of shade-specialist lizards with low heat tolerance to climate change, and suggests that even closely related species may differ substantially in their ability to tolerate increasing temperatures.