The hurricane effect was much broader than anyone anticipated. The researchers published their results today (Apr. 27) in the Proceedings of the National Academy of Science.
“My role on this paper was to tell Colin not to bother, he was never going to find anything,” said Jonathan Losos, the William H. Danforth Distinguished Professor at Washington University and professor of biology in arts and sciences. Losos is director of the Living Earth Collaborative. “I thought it was extremely unlikely that hurricanes would have a big enough and long-lasting enough effect on the populations that it would show up when you compare populations or species. Of course, I was absolutely wrong and Colin was absolutely right. And the patterns he found are quite exciting.”
The effects are observable at the population level, at the species level, and across a broad region including the Caribbean, Central America and much of South America. The analysis relies on 70 years of NOAA hurricane data from the north Atlantic and north Pacific oceans and hundreds of anole toepad measurements from across their entire range.
“We poked and prodded the data every which way to try to find if there were any holes in it,” Losos said. “And Iʼm convinced that itʼs robust.”
In 2017, Donihue had just finished a detailed survey of Anolis lizards in Turks & Caicos for another research effort when Hurricane Irma — A Category 5 storm with winds exceeding 170 mph —struck the islands directly. Two weeks later, Hurricane Maria scored a second direct hit. Donihueʼs immediate before-and-after comparison showed that hurricane survivors had different physical traits than the population before the storms.
Donihue and colleagues returned to Turks and Caicos one year later and took new measurements. Lizards in the next generation had toepads as large as the hurricane survivors.
The researchers thought that if hurricanes really do affect toepad evolution, then those lizards that live in areas hit by more frequent hurricanes should have larger toepads. But it is not possible to go back and see how the incidence of hurricanes affected toepad size. Instead, as a next step, the researchers looked at many different lizard populations with different histories.
To quantify exposure to hurricanes, collaborator Alex Kowaleski, a postdoctoral scholar in meteorology at Penn State, used NOAA data on historical hurricane paths, primarily tapping the Atlantic Basin Hurricane Database (HURDAT 2) archive of the track and intensity of all North Atlantic cyclones. Even among the first set of 12 island populations that Donihue wanted to compare, there was a large amount of variability in exposure. For example, one population was hit four times in the past 70 years, and one had not experienced a direct hit.
HURDAT 2 contains position and intensity data every 6 hours, but Kowaleski interpolated the track and intensity data to every 15 minutes.
“This was important because it is possible for a hurricane to strike a location between two timesteps,” Kowaleski said. “Interpolation allows us to better capture hurricane strikes at each location.”
“Correcting for things like differences in body size, we found that island populations that had been hit by hurricanes more [frequently] had larger toepads,” Donihue said. “Hurricanes seem to be having some sort of additive effect on the evolution of these lizards — that the more hurricanes you have, the larger toepads you have, on average.
“Toepads might be a key trait for helping lizards hold on tight to the vegetation during storms, but thereʼs probably a tradeoff between the traits that make you really good at surviving a hurricane and the traits that make you really good at being a lizard day in, day out," added Donihue.
“Most of the selective pressure is to just be good at being a lizard: to go catch food, find a mate and avoid predators. These hurricane events are very infrequent and unpredictable, so we expect that there are other selective pressures that are acting on toepads. In other words, over time, these toepads are not going to turn into big snowshoes, or something like that. Thereʼs a balance.”
The results may have implications for other types of animals — not just lizards — and also for other changes under new climate scenarios.
“Our best idea right now is that tropical cyclones will become less frequent globally; however, a higher percentage of them will become intense hurricanes,” Kowaleski said. “Increases in sea-surface temperatures will cause a higher percentage of tropical cyclones that do form to become Category 4 or 5 hurricanes. Precipitation intensity also is expected to increase in tropical cyclones due to climate warming.”
“My best guess is that this isnʼt just a lizard thing,” Donihue said. “For any other species affected by hurricanes where survival is non-random, you would predict this same kind of pattern occurring. Iʼm really hoping that this is going to spark some new analyses of old data — or new data collection going forward — thinking about how hurricanes might be affecting things like the evolution of plants and trees, or snails … or any of the other species affected by hurricanes in this region.”