Hurricanes may produce lasting evolutionary effects on lizards

Anole lizards in the Caribbean possessed large toepads akin to those that helped their ancestors survive Hurricanes Irma and Maria in 2017. Moreover, numerous species of lizards found in the Neotropics, an area stretching from Brazil through Central America to Florida in the United States that is prone to hurricanes, also have larger toepads, according to a study published in PNAS last month.

This new study builds on an earlier one from the same team that found that Hurricanes Irma and Maria had caused a shift in the anole population (Anolis scriptus) on two Caribbean islands towards members that had larger toepads.

“Our best guess is that these toe pads are helping lizards hold on tight during a storm,” says Dr. Colin M. Donihue, an evolutionary ecologist at Washington University in St. Louis, in the U.S., and a co-author of the study. “And it seems like there’s enough of an advantage to having a large toepad that just allows you to survive and pass on those large toepad genes to your next generation.”

In 1898, an early record of the effect of a short, high-impact event on natural selection was made by Hermon Bumpus at Brown University in the United States. He reported that an unusually harsh storm had favoured the survival of certain house sparrows over others. Some other research groups have noted such rapid evolution events in Galápagos finches due to El Niño, a polar vortex–driven changes in green anoles, and earthquake–guided divergence of freshwater stickleback from their oceanic ancestors. Given the serendipitous nature of recording animal data before and after an often unpredictable natural event, these sorts of studies are not abundant.

It was a result of being “in the right place, at the right time,” says Donihue, who was conducting fieldwork for an entirely different conservation project in 2017 on the Turks and Caicos Islands—specifically, the Pine Cay and Water Cay. Donihue and his colleagues spent their time walking around the islands, ensnaring anoles with the help of two-metre-long poles with little adjustable lassos at the end. They would walk back to their hotel with the lizards in a cloth bag, photograph and measure them, and release the lizards back where they had been found.

Within a week of their leaving the islands, Hurricane Irma hit the Caribbean and parts of the southern United States. Hurricane Maria followed soon after. Since the team had fortuitously collected data before, they were able to go back and analyse the surviving lizard population. The remaining anoles had larger toepads compared to the baseline group that the team had observed before the hurricanes.

With more questions that needed answering, the team returned after 18 months to see if the descendants of the survivors—identified by their smaller body size—had inherited the larger toepad trait. “There was a transgenerational effect that the hurricane had caused these lizards to have large toe pads, which in turn was passed on to the next generation,” adds Donihue.

Next, the team went on to study 12 different populations of the brown anole (Anolis sagrei) and 188 other species of anoles all across the Neotropics. They found that populations of anoles that were more frequently hit by hurricanes going back over a 70-year period were more likely to have larger toepads than others.

“Obviously, the findings of this new study are consistent with the view that the fingerprint of extreme events—even ones going back decades—can still be seen,” says Professor Raymond Huey, an evolutionary biologist at the University of Washington in the U.S. who was a reviewer on the paper, in an email. “Moreover, some models predict the increased frequency of hurricanes in the future, and so we may (unfortunately) have many opportunities for follow-up studies.”

The research team had entertained the idea that other parameters such as temperature, precipitation, vegetation, and tree height, might be responsible for the shift in the toepad trait. But the hurricanes seemed to be the most likely factor. “It's a reasonable assumption to make that hurricanes are selecting on toepads,” says Dr. Seth Rudman, an evolutionary ecologist at the University of Pennsylvania in the U.S.who was not involved in this study. While their results show a strong correlation, no direct experiments have been conducted to show that the hurricanes were selecting for the anoles’ comparatively larger toepads, which Donihue says is next on their agenda.

Further, researchers speculate that the larger toepads could be a reflection of a different feature of the anoles that had changed in response to the hurricane, such as their limb and skeletal proportions, and their limb muscles. “I’m not saying this is the case, but it could be the case that the increased top pad size is a correlate of some other morphological feature of the animal that's actually being selected for,” says Professor James Hanken, a biologist at Harvard University in the U.S. who was not involved in this study.

Toepads seem to prevent the lizards from being carried off with hurricane-force winds. “But it’s possible that toepad size could be correlated with other lizard traits, other morphological traits or behavioural traits that allow them to survive the hurricane,” adds Rudman. The heritability of the trait in question could also be verified with controlled breeding experiments with the anoles, who have short lifespans.

In the future, Donihue and his colleagues intend to focus their attention on conducting wind tunnel experiments to replicate hurricane-like environments in the laboratory. Planning other experiments around hurricanes in the future is also on the table, adds Donihue. 

References: 

1. C. M. Donihue et al., Hurricane effects on Neotropical lizards span geographic and phylogenetic scales. PNAS 117 (19), 10429-10434 (2020). 
2. C. M. Donihue et al., Hurricane-induced selection on the morphology of an island lizard. Nature 560, 88-91 (2018).