Evolution 2014: Thermoregulatory Behavior Both Prevents and Promotes Evolutionary Divergence

Anolis cybotes sitting on a rock. Image from Discover Life.

Martha Muñoz presented data on how shifts in behavior constrain evolution of thermophysiology and drive morphological differentiation in the Anolis cybotes complex. The A. cybotes complex occurs across a large altitudinal range on the Caribbean island of Hispaniola and Martha was interested in whether and how lizards are adapted to thermal differences at different elevations. Martha tested whether body temperatures differ between lizards found in the different thermal habitats. Sampling more than 400 individuals, Martha did not find differences in body temperature between the populations. This is surprising because ambient temperature differed by more than 10 degrees Celsius between the low and high elevation localities.

So, if temperatures differ so dramatically among the different altitudinal habitats, how do lizards maintain similar body temperatures? Habitat use data from the two populations show that the high elevation lizards use rocky substrates more often than low elevation lizards, which are mostly found on tree trunks and other types of vegetation. Data obtained by using copper models that measure temperature as a lizard would experience it in a given habitat show that rock habitats are too hot at lower altitudes, but ideal at higher ones. This suggests that lizards keep their body temperatures stable by shifting from arboreal habitat type to rocks in higher elevations.

Martha then asked whether similarity in body temperature was matched with similarity in underlying thermal physiology. In ectotherms such as lizards, the ability to perform a task is dependent on temperature such that is optimized over a narrow range and then drops at lower and higher temperatures until the animal is immobilized. She found that lizards from all populations have similar preferred temperatures, and so their underlying physiologies do not appear to be evolving.

While thermoregulation keeps thermal physiology stable, the shift in structural habitat affects morphology. Martha measured morphological characters such as head shape, limb proportions, and lamella number to test whether habitat use has driven morphological change. Martha found that lizards in high elevations have wider and flatter heads and shorter limbs than populations in lower habitats. Thus, she found that a thermoregulatory behavior impedes physiological evolution while simultaneously driving morphological evolution.