When the Going Gets Cold, Anoles Get Colder

CTmax, Tb, and CTmin of cybotoid anoles & env. temperature. Modified from Fig 2 in Muñoz et al.

CTmax, Tb, and CTmin of cybotoid anoles & env. temperature. Modified from Fig 2 in Muñoz et al.

AA contributor Martha Muñoz’s work on altitudinal variation in the cybotoid anoles has already netted her the Raymond B. Huey award and of course, been featured on AA. A big chunk of this work, co-first authored with Maureen Stimola, has just been published by the Proceedings of the Royal Society B. If you haven’t read it yet, check it out.

I love this paper. However, in the spirit of full disclosure, I’m completely biased as I happen of be one of the co-authors. But I’m sure I’d love it anyway. Why? In part because it tests a clear hypothesis using multiple lines of evidence and eliminates confounding explanations – characteristics every paper should have. It also has cool (or should I say hot?) results. However, more than this, I think this paper demonstrates the power of combining good ole’ fashioned (yet cutting edge) field work with macroecological and macroevolutionary models, demonstrating how these different approaches can really complement each other.

What did Muñoz and company find? Briefly, they looked at hot and cold tolerance (CTmax and CTmin) of six species of cybotoid anoles on Hispaniola, in relation to elevation. They found far more variation in CTmin than CTmax across species (and populations). By bringing in a little macroecology, they showed that CTmax isn’t correlated with environmental temperature, but CTmin is, i.e. when the going gets cold, the anoles get colder – sort of. The catch is that while CTmin strongly tracks temperature, daytime body temperature does not. This is a neat result in and of itself and fits well with a big, recent, data-mining paper showing similar trends across hundreds of both ecto- and endothermic species. But while it doesn’t have the breadth of that paper, Muñoz et al. were able to go further. Firstly, bringing in a little macroevolutionary analysis, they showed that yes, CTmin has actually evolved significantly faster than CTmax. Neat, but at this point you should be asking yourself, “What about acclimatization?” and “Is this just plasticity?” Muñoz et al. asked the same thing and headed back to the field. A lot of work later and the answer was no. An acclimation experiment rejected this possibility.

At this stage, most macroecological and macroevolutionary analyses would have to stop at the identification of a clear, and intriguing pattern of fast past evolution of cold tolerance along an elevation gradient, but little CTmax evolution. The Discussion of such a paper would suggest potential hypotheses to explain the pattern and that would be that. But Muñoz et al. again went further and, by working in the field to measure perch use and operative temperatures, worked out why . The key result showed that lizards can behaviourally thermoregulate to escape the heat, thus reducing selection on heat tolerance, i.e. the Bogert effect. However, the nighttime cold cannot be escaped (actually, it can, by moving to England where it hasn’t dipped below -2 deg C this winter. Enjoy that polar vortex America!), leading to selection on cold tolerance.

Like I said, very cool results and a real testament to the power of using field experiments and macroevolutionary models to inform each other and go beyond what each approach could do in isolation. So please read it, challenge it, and build on it.

About Adam Algar

Associate Professor at the University of Nottingham (UK). My research focuses on niche limits and dynamics across scales, from individual organisms to the globe. Mostly lizards.

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