Back in January, Florida had a serious cold snap. Meteorologists warned of temperatures dipping down into the 30s and 40s. But the National Weather Service forecast didn’t stop there, it also cautioned residents to watch out for large green lizards falling from the treetops.
As predicted, temperatures dropped as did a large number of reptiles. Around that time, evolutionary ecologist James Stroud of Washington University in St. Louis received a photo from a friend in Florida showing a nearly two-foot-long iguana lying flat on its back in the middle of the sidewalk.
“When air temperatures drop below a critical limit, lizards lose the ability to move,” Stroud tells Science News' Charles Choi. Many lizards sleep in trees, and If temperatures slip below this critical limit the scaly critters may lose their grip.
The big freeze has hit Miami! When temperatures drop below a critical limit allowing lizards to function...sleeping lizards lose grip & drop out of trees. To an evolutionary biologist: natural selection in action?— James T. Stroud (@jamesTstroud) January 22, 2020
Thanks for pic @BrettinFlorida! #scicomm #FloridaWinter #Florida pic.twitter.com/GlfR8acFJ6
The photo and the phenomenon it depicted gave Stroud an idea to use these lizards to understand how animals might be responding to extreme weather events. Stroud and his colleagues rushed out into the field and collected 63 lizards representing six species, five of which are tropical species not native to Florida, around Miami, reports Katie Hunt for CNN.
Researchers transported the lizards back to a lab at the University of Miami and proceeded to chill the animals inside coolers full of ice. As thermometers attached to each animal recorded its falling body temperature, Stroud and his colleagues periodically prodded the lizard until it stopped responding. Once the lizard stopped being able to react to being poked or flipped on its back, the researchers recorded its body temperature from the thermometer and labeled that as the animal’s lower temperature limit, per Science News.
Stroud and the team were able to compare these results to a similar study he and his colleagues conducted on lizards’ cold tolerance in 2016. The comparison revealed that the lizards around Miami were now able to tolerate temperatures 1.8 to 7.2 degrees Fahrenheit colder than they could in 2016, the researchers reported last month in the journal Biology Letters.
In 2016, there was a lot of variation among the abilities of these six lizard species—which are also quite different from one another in terms of body size and lifestyle—to tolerate cold. Some species, such as the Puerto Rican crested anole were able to function until their bodies reached 46 degrees, while the much larger brown basilisk was stunned by the time it cooled to just 52 degrees, according to a statement.
The hardy lizards that had survived the chilly crucible last January were not just able to tolerate colder temperatures than the 2016 cohort, all six species’ newfound tolerance had converged on roughly the same temperature: 42 degrees. Stroud tells CNN that this convergence was “a major unexpected result of this study.”
As for how the lizards managed to toughen up so quickly, the researchers aren’t yet sure.
“What we now need to find out is how this was accomplished,” says Jonathan Losos, an evolutionary biologist at Washington University and the paper’s senior author, in the statement. “Is this evidence of natural selection, with those lizards that just happened to have a lower cold tolerance surviving and others freezing to death, or was it an example of physiological adjustment—termed ‘acclimation’—in which exposure to lower temperatures changes a lizard’s physiology so that it is capable of withstanding lower temperatures?”
To get at this question of acclimation versus natural selection, Stroud tells Science News he hopes to measure the cold tolerance of individual lizards immediately before and after a cold snap. Alex Pigot, an ecologist at University College London who was not involved in the research, tells Science News the paper suggests some species may be more able to swiftly evolve or acclimate than we expect, perhaps conferring ecosystems added resilience to extreme climate events.”
Pigot adds that it would be interesting to learn whether there might be similar resilience to extreme heat events, telling Science News that “previous evidence has suggested that species’ upper thermal limits may be less flexible than their lower thermal limits.”