Lizards that survived a tough hurricane season have larger toe pads, on both front and back limbs, report researchers. The work is first to demonstrate the effects of hurricane-induced natural selection.
The hyperactive 2017 season was one of the worst that the Atlantic Ocean region ever experienced. Hurricane Harvey hit in mid-August 2017, followed just a few weeks later by Hurricane Irma, and Hurricane Maria in September. Each of the storms had winds in excess of 125 mph, with Irma up to 170 mph.
Some tropical anole lizards living on the more remote cays of Turks and Caicos survived—and some didn’t.
Right place, right time
“Hurricanes are in the news, and it seems that they’re becoming more destructive,” says Jonathan Losos, a biology professor at Washington University in St. Louis. “Something like this has never been documented before because it’s so difficult. The timing had to be just right.”
Last September, Colin Donihue, a postdoctoral researcher at Harvard University, was just wrapping up his measurements of endemic Turks and Caicos anole, Anolis scriptus, as a baseline for a long-term invasive species eradication project when Hurricane Irma started to brew.
The hurricane made landfall on September 8, 2017. Donihue read the news of its devastation from his apartment in Boston—and realized his lizard data might have value outside of its original intent.
Donihue had the last eyes on the population before an unthinkable natural disaster. Were the storms a “selective event”—an event with the power to change the course of life not just for the lizards that had weathered them, but potentially for the generations that followed?
The Turks and Caicos government was in recovery mode; clean water, electricity, and shelter were priorities, and basic phone and internet services were still not available across the islands.
But Donihue had made good connections during his recent trip, and one government scientist in particular was sympathetic to his pleas. A new set of biological permits came through, and Donihue and coauthor Anthony Herrel of the Paris Museum were back on a plane.
Back on the experimental islands, it was a mess. Trees were down and tree-limbs strewn about, making it difficult to walk in some places. But there were signs of life.
“Heading back to Pine Cay, we weren’t sure what we’d find, but when we got to the field and saw a few lizards running around, we were eager to get catching and start measuring,” Donihue says.
“We walked exactly the same transects we had the last time. There were definitely fewer lizards. We had to work harder to catch our sample size.”
“…the question is whether hurricanes cause selective mortality: do individuals with certain traits survive better than individuals with different traits?”
The team spent two days collecting just shy of 100 lizards on two separate islands, then measured their forelimbs, hind limbs, and core body lengths, and took pictures of their toe pads.
The vital statistics of the survivors could be compared with the measurements of the general lizard population that had been collected before the storm.
“The prediction was that if we saw any changes, they would be changes in the features that help lizards hold on—they would be related to clinging ability,” Donihue says. “For example, the sticky toe pads on their fingers and toes, maybe they would be larger.”
As reported in Nature, the survivors had proportionately longer fore legs than the initial/pre-hurricane population, while the long bones in between their hips and knees on their back legs (their femurs) were shorter. The survivor population had smaller bodies, too. The observations were statistically significant and consistent at both island sites, the researchers say.
Hurricanes bring death and destruction to those in their paths. There are well-documented recent examples of hurricanes killing off large numbers of individuals from many species as diverse as plants, sea sponges, insects, birds, and monkeys.
“…hurricane-induced natural selection seems like the best explanation for these findings.”
“With regard to evolution, the question is whether hurricanes cause selective mortality: do individuals with certain traits survive better than individuals with different traits? The alternative possibility—that devastation is so massive that mortality is indiscriminate, not favoring some individuals over others—is certainly possible,” Losos says.
The results of this study—that the island populations of lizards both changed significantly, and in the same way from before the hurricanes to after—suggest that natural selection favored individuals with certain characteristics.
Of course, as the authors point out, there are other possibilities.
“Perhaps the hurricane blew in lizards with bigger toepads and shorter hind legs from another island. Or perhaps the act of clinging to the branches in high winds actually caused their forelegs to get longer,” Losos says. “We can’t rule these possibilities out because this study was the result of serendipity, rather than specifically being designed to test the effect of hurricanes.
“Still, hurricane-induced natural selection seems like the best explanation for these findings,” Losos says.
Hold on tight
The missing piece of the story is still a behavioral one. Researchers don’t know what lizards actually do in the middle of a hurricane. Do they abandon their typical tree perches and go to the ground? Or do they try to seek cover in notches or crevices within the trees? Or do they just hang on?
A pilot study exploring wind threshold provides some insight into this aspect of the lizard decision-making process.
When the researchers exposed lizards from the survivor population to hurricane-force winds, the lizards almost uniformly swiveled around their perches to the side opposite from the wind source — and just held on tight. As wind speeds increased, they lost hold with their hind limbs first, and were left hanging by their forelimbs.
It appears that lizards are built to cling, but because of their stance on the perch, their big hind limbs make them vulnerable to getting pushed off by high winds. This could explain the pattern that survivor lizards have longer forelimbs and shorter hind legs after a hurricane, the researchers speculate.
Understanding how storms select for species survival is an important area of future study, worthy of a dedicated study design, the researchers say.
“We know that hurricanes are getting more frequent, and we know that they’re getting more strong,” Donihue says. “So, setting up a network of sites that are actually set up to investigate the question of how hurricanes are changing the evolutionary trajectory of species I think could be really useful.”