The brown anole lizard in the Bahamas is raising questions about whether some cold-blooded species may be able to adapt to global warming.
Scientists consider cold-blooded species particularly vulnerable to climate change because of their sensitivity to even small temperature shifts. Species like the brown anole lizard maintain their body temperature by moving between sunny and shady areas, in a process called behavioral thermoregulation.
Because these species move between "microhabitats" to avoid overheating, some researchers believe that they don't face adaptive pressures that would favor the survival of individuals with greater heat tolerance. This is bad news for cold-blooded species. If they are unable to adapt, then rising temperatures will lead to their extinction.
"The potential for the organisms to evolve in response to a rapidly changing environment is often downplayed, even though very few studies have examined the extent to which species might adapt," said Michael Logan, who recently received a Ph.D. from Dartmouth College and is now a postdoctoral fellow at the National Science Foundation.
Logan and two other scientists from Dartmouth and the University of Virginia set out to test the adaptive pressures of simulated climate change conditions on brown anole lizards (Anolis sagrei).
The researchers captured and transplanted about 100 male lizards from their forest habitat to a hotter area with greater temperature variability about half a mile away. Then the researchers tested how fast the lizards could sprint at a range of temperatures. The researchers published their results in the Proceedings of the National Academy of Sciences, with Logan as lead author.
Running quickly comes naturally to the brown anole lizards. "They are the Usain Bolts of the lizard world," said study co-author Ryan Calsbeek, an associate professor at Dartmouth, referring to the record-breaking Jamaican sprinter. The lizards need to move quickly to hunt for food and to avoid predators, and previous research has shown that their sprint speed is strongly correlated with survival and reproductive success.
The quick or the dead
Before each sprint, the researchers placed the lizards in an incubator to adjust their body temperature, then had them run as fast as they could along a short track. Each lizard did five sprint trials for each temperature, with a two-hour break between each sprint.
After the initial tests, the lizards were released, and the surviving lizards were recaptured at the end of the breeding season. The researchers then compared the results to another 100 male lizards they used as controls.
"We weren't at all sure whether we would see selection when we started this study. If I had to put money on it, I would have bet against it," Logan said.
Just 22 percent of the transplanted lizards and 45 percent of the control lizards survived the breeding season.
When the researchers analyzed their data, the results showed clear selection pressure in favor of the most heat-tolerant lizards in the transplanted lizards, but no trend among the controls.
"The individuals that ran the fastest at the warmest body temperature and those that ran at the broadest range of temperatures did the best," Logan said of the transplanted group.
Their research is the first time that thermal performance has been used as part of the framework of measuring selection and heritability in cold-blooded species, according to the researchers.
"I was surprised that the selection was as strong as it was. Often when you measure selection in nature, it's at a very low magnitude," said co-author Robert Cox, an assistant professor at the University of Virginia in Charlottesville. "The fact that the expectation and the results match so well gives us assurance that it's actually happening."
The study did not show whether these traits could be passed on to future generations. If the traits are heritable, that could mean the lizards may be able to rapidly evolve to climate change pressures, which would be good news for the species.
An inherited trait?
Cox's laboratory in Charlottesville is now investigating whether the lizards are able to pass on the ability to run fast at higher temperatures and at a broader range of temperatures to their offspring. If they can, then it would appear the lizards could rapidly evolve in response to climate change, according to the researchers.
Testing heritability is easier in laboratory conditions because all the animals are raised in the same environment and the researchers know the relatedness of all the lizards, Cox said.
Calsbeek said he did not want the study's results to be misinterpreted. "This is not us saying that climate change is not a problem," he said. It's also unclear whether other cold-blooded species may also be able to adapt.
"Because there have been no other studies to look at these particular traits, it's hard to tell how broadly applicable it would be," Calsbeek said.
Luke Mahler, a researcher at the University of California, Davis, who studies evolution patterns in Anolislizards, is also skeptical about how broadly the researchers' results could apply even among the nearly 400 other lizard species within the Anolis genus.
"I think it's a stretch to suggest more generally that tropical animal populations may be capable of rapid adaptation to anthropogenic warming," said Mahler, who is also co-chairman of the Anoline Lizard Specialist Group, which studies which anole lizard species are at risk of extinction at the International Union for Conservation of Nature.
"The reason I worry about this suggestion is because Anolis sagrei is hardy, abundant, widespread and tolerant of lots of temperatures. It occurs naturally over a wide range of thermal environments, and has experienced such variety over millions of years. What this means is that the species probably has loads of heritable variation in thermal tolerance for natural selection to work on," Mahler said.
By comparison, phenacosaur anoles living in cloud forests have had very little exposure to temperature variability for over 10 million years and are very much at risk from climate change, he said.
Still, the study does provide convincing evidence that there is natural selection for thermal performance traits.
"It's great to see an answer to the important question of whether behavior always prevents tropical animals from adapting to climate change," he said. "I think in this case, we have satisfying evidence that the answer is 'no.'"
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500