Physiological Adaptation On Ecological Timescales – New Research By Alex Gunderson And Manuel Leal

Anolis cristatellus from Puerto Rico. Photo taken by Liam Revell

Anolis lizards are a model system for studies of evolutionary ecology because they are remarkably adaptable creatures. We know from long-term studies conducted by Jonathan Losos, Dave Spiller, Tom Schoener, and others that anoles can rapidly adapt their behavior and morphology over ecological timescales. For example, the presence of a ground-dwelling predator (Leiocephalus carinatus) forged a strong selective gradient in favor of A. sagrei with longer hindlimbs within a single generation. Interestingly, in a follow-up study the long-term effect of this predator is that A. sagrei evolves shorter hindlimbs, as they will tend to perch higher off the ground, where the perch diameter is narrower than near the ground. These studies of rapid morphological evolution puts anoles in the a very exclusive club with the likes of stickleback fishes, Peromyscus beach mice, guppies from Trinidad, Galapagos finches, and few others, as vertebrate systems in which evolutionary change on ecological timescales has been confidently demonstrated.

A notable exception to Anolis ‘evolvability,’ however, is thermal physiology. The thermal physiology of reptiles is generally evolutionarily conserved – taxa separated by millions of years and found in very different thermal environments will often share similar physiological patterns. But recent research has suggested that some physiological metrics may not be as static as previously thought, and that Anolis invasions provide an excellent opportunity to see how labile physiology actually is.

Although native to the balmy isle of Puerto Rico, A. cristatellus has made a permanent home in Florida. This species is now well established in South Florida, and even on some of the offshore islands. Although Florida may seem perpetually warm to a denizen of the Northeast like me, it is actually much colder than Puerto Rico. In a previous study, Jason Kolbe and colleagues demonstrated that the thermal niches (i.e., available thermal habitat) are quite divergent between Puerto Rico and Florida, and showed a rapid evolutionary response in acclimation to thermal environment in one of the invasive populations. This sets up the possibility that invading anoles will evolve their thermal physiology to match their new environment.

Figure 1. Maximum air temperatures (top lines) and minimum air temperatures (bottom lines) for Miami (solid) and Puerto Rico (dashed) since the A. cristatellus invasion.

Enter Manuel Leal and Alex Gunderson from Duke University. In their most recent paper, they examine whether physiology has evolved in the invasive populations of A. cristatellus from Miami. As Manuel explains on his blog, anoles can use behavior to stay warm during the day, but cannot escape the cold at night. Indeed, the maximum temperatures appear to vary little between Miami and Puerto Rico (Fig. 1). The minimum temperatures, however, show appreciable variation, particularly in winter, where they are separated by about 10°C. He posited that there was possibly a strong selective pressure for adapting to the lower nighttime temperatures through a lower cold tolerance (critical thermal minimum, or CTmin). Alex, however, was more skeptical in light of the overwhelming evidence that physiological evolution is predominantly conserved.

Figure 2. Box plots showing the critical thermal minimum (CTmin) values of A. cristatellus from Miami (gray) and Puerto Rico (white) measured in the spring and fall.

For their cold tolerance experiments, Alex and Manuel examined A. cristatellus from Miami in the spring and in the fall, and from Puerto Rico in the fall (Fig. 2). Despite tangibly different ambient air temperatures in spring and fall, CTmin was not significantly different among Miami lizards. However, mean CTmin was lower by about 3°C in the Florida populations than in the Puerto Rican population. This difference in CTmin between invasive and native populations is quite dramatic in light of the fact that thermal physiology is highly conserved among reptiles and this difference evolved over the course of about 35 generations. Certainly this is compelling evidence for physiological adaptation in anoles over ecological timescales.

Something that struck me while reading this paper is that, although thermal physiology is predominantly conserved among lizards, anoles are a notable exception to this rule. Rodolfo Ruibal, Stan Rand, Paul Hertz, and Frederica van Berkum, among others, have documented extensive variation in thermal physiology among anoles. It is also true that physiological lability is most obvious among closed canopy anoles, and A. cristatellus is an open-habitat species – perhaps its physiology is consequently not expected to be as labile. However, that such strong physiological adaptation can occur over such a short timescale is quite remarkable, even if anoles generally show evolutionary lability in physiological traits.

An important implication for this research is that it may alter our perception of how reptiles will respond to climate warming. Many forecasts predict that tropical reptiles, particularly those in closed canopy forests, will be especially susceptible to climate warming because of their physiological preference for cooler temperatures and the entrenched idea that thermal physiology is evolutionarily inert. Leal and Gunderson demonstrate that dramatic physiological evolution can occur over short timescales, and can potentially aid organisms, at least in the short run, to buffer the effects of environmental warming. The eventual effect of warming, however, is that neither physiology nor behavior will be able to fully mitigate the hotter thermal environment, and that an alarming number of reptiles are at risk of extinction.

ResearchBlogging.orgManuel Leal & Alex R. Gunderson. (2012). Rapid change in the thermal tolerance of a tropical lizard. The American Naturalist. 180: 815-822. DOI: 10.1086/668077

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About Martha Muñoz

Martha is a PhD candidate at Harvard University studying the evolutionary ecology and thermal physiology of anoles. She works in Jonathan Losos' lab and focuses on the cybotoid anoles from the Dominican Republic. Website: www.marthamunoz.weebly.com

9 thoughts on “Physiological Adaptation On Ecological Timescales – New Research By Alex Gunderson And Manuel Leal

  1. It could be enlightening to replicate this work with the (at least) 8 other invasive anoles in Miami. Because those anoles come from different islands and lineages, we could get a feel for whether this physiological adaptation is common across the Anolis tree. I also wonder how much founder effects could have played a role in this result.

  2. Martha, muchas gracias. Your summary is very good, much better than our paper. I agree that anoles might be an exception to the rule with regards to thermal physiology. But as you nicely pointed out, the time scale is what got me excited about our results. The previous studies are most likely looking at a scale of hundreds of thousands or possibly millions of years. So although my “anole brain” was not surprised by the findings, (if is cold for me, it should be cold for them) it was nice to see that it was true.

  3. Is it known or are there speculations about whether CTmax or CTmin is more labile? As in, might it be easier to adapt to colder temperatures than warmer temperatures?

    1. That is an interesting question. There does seem to be more geographic variation in CTmin than CTmax among populations and species of ectotherms, especially along altitudinal and latitudinal gradients. Part of the reason for this may be that minimum environmental temperatures vary more with geography than maximum temperatures (see Figure 1 above, for example). Or, as we also posit, behavior may be less able to buffer lizards from cold conditions, especially at night. Whether or not CTmin is more labile then CTmax with all else equal, I’m not sure there’s an answer to that yet, although laboratory selection on some ectotherms like Drosophila have shown shifts in CTmax. Ray Huey is apparently working on that question from a thermodynamic perspective, but I don’t think it has been published.

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