It was August 2017, a few days before a catastrophic hurricane would sweep through the small Caribbean island archipelago of Turks and Caicos, and some residents were woefully unprepared. While islanders were busy equipping houses or flying out of the country, many of the endemic island lizards lacked the evolutionary chops to deal with the coming deluge.
Colin Donihue, a National Science Foundation post-doctoral fellow at Harvard University, and his colleagues had just wrapped up a study in which they filmed, captured and measured Turks and Caicos anoles in an effort to see whether a planned eradication of invasive black rats on the islands would have any effect on the lizards’ behavior and bodies.
All went according to plan until four days after Donihue left. Hurricane Irma hit Turks and Caicos before moving northwest to strike the Florida mainland then its devastating trail was followed about two weeks later by Hurricane Maria, which would go on to wreak havoc on Puerto Rico and Dominica.
The researcher was immediately concerned about colleagues working on other aspects of the rat eradication project. But after the initial threats died down, his thoughts turned to science—the lizard survey he had helped conduct days before presented a rare opportunity to determine not just what the storms’ effects were on the lizard population, but what traits best equipped lizards to survive the storm.
“We realized we were in a unique position, having had last eyes on these lizards,” he says. Six weeks after Donihue left, he was back on the island repeating his lizard measurements.
He and his colleagues found that many of the lizards did not survive the storm, and those which had likely did so thanks to key differences in their body proportions.
While other research has looked at the effects hurricanes have had on animals like birds, frogs and primates, they mostly deal with the aftermath since it is difficult for researchers to predict the path of future storms. Donihue says this new study, published today in the journal Nature, is a first-of-a-kind look at the before and after physical traits of a population.
“What we’ve documented is a strong case of natural selection due to hurricane,” Donihue says.
In both trips, they conducted the research by walking transects through two small islands in Turks and Caicos—Pine Cay and Water Cay. They would catch lizards along these trails using long poles equipped with slip knots on the end. In the first trip, they took about 70 lizards back to the lab and measured their various body parts.
On the return trip, Donihue says he witnessed significant devastation, both to human structures and to trees and wilderness, with roofs blown off houses, trees overturned and vegetation stripped of its leaves. The researchers worked significantly harder to catch enough lizards due to a relative shortage of the anoles, but eventually ended up with more than 90.
On average, they found the survivors were smaller, and had different physical characteristics.
The toe pads on their front legs were about 9 percent larger while their back toe pads were about 6 percent larger. Donihue says larger toe pads would be critical as the gecko-like appendages would allow them an improved grip on branches or leaves in the face of hurricane-level winds.
The scientists found the survivors’ front legs were about 2 percent longer on average, but they were surprised to find the hind legs were about 6 percent shorter.
To discover why, they ran another experiment on the captured anoles to see their survival strategies. They let loose hurricane-level winds using a leaf blower on capture anoles on a wooden perch similar to a tree branch. The lizards first placed the perch between themselves and the wind then streamlined their body to get the least exposure to the artificial gale. They tucked their arms close to their body and tucked their heads down but the researchers noticed that their back legs were conspicuously exposed to the full force of the wind.
“As the wind speed increased more and more, these back legs would catch more and more wind, kind of acting as sails, until eventually the hind limbs were blown off the perch,” Donihue says.
(No lizards were harmed in this experiment and all were released to their point of original capture.)
The anoles would hang on with their front limbs for a while before being blown off and into a padded wall behind. “No lizards were harmed, everybody was returned back to their point of capture,” Donihue stresses, but adds that during the actual hurricane, lizards with larger back legs would have likely been blown out to sea.
He says that it’s possible that some lizards with these traits were blown into Pine and Water cays during the storms, but it’s unlikely. “This is natural selection in action,” he says.
Martha Muñoz, a biology professor at Virginia Polytechnic Institute and State University who studies lizards and salamanders, says that the study is exciting and “really cool” as it shows an example of evolution occurring in real time—something the scientific community didn’t think possible with vertebrates until a study published in 2006 showed how drought might affect natural selection among Darwin’s finches.
“I think it enriches our understanding of how organisms respond and are impacted by contemporary selective pressures,” says Muñoz, who was not involved in Donihue’s study.
Donihue says that further research must be undertaken to see if this natural selection will extend to the next generation of Turks and Caicos anoles, but Muñoz would not be surprised to see an improved clinging capacity in the lizards moving forward.
“Evolution shocks us with how fast it can be,” she says. “You only need one generation to observe evolution occurring even in vertebrates.”
Donihue believes it’s possible that anoles may regain their larger hind legs since in day-to-day life, this trait helps them jump to avoid predators or catch prey.
“They’re in this constant feedback in their environment,” he says, adding that those adapted to some conditions may not be able to survive others.
David Spiller, a project scientist at the University of California, Davis who was involved in a study on the effects of hurricanes Irene in 2011 and Sandy in 2012 on population levels of Cuban brown anoles (anolis sagrei) on small islands in the Bahamas, worries that climate change may negatively impact this feedback if hurricanes become more frequent and increasingly destructive as predicted.
“If disturbances get even more frequent, species are not going to be as adapted as well to their environment,” says Spiller, who was not involved in Donihue’s study. “It keeps them out of equilibrium.”
Muñoz agrees: “What if the traits that allow you to survive a major impact are in conflict with the traits that are normally under selection for other reasons?”