MIAMI — Alongside runs on hot chocolate and churros, cold-stunned iguanas dropping from trees are one of South Florida's most iconic winter traditions.
When it gets cold, videos of the reptiles sprawled on the ground pop up all over social media, including during the recent Christmas cold snap that plunged temperatures into the 40s near Miami.
But ongoing research suggests Florida's falling iguana phenomenon could be rarer in the future — both due to climbing global temperatures from unchecked climate change and a shift in cold hardiness in the lizards themselves. That's right, the big lizards (cue the sci fi movie music) appear to be adapting.
That's a bummer for anyone hoping that the latest prolonged dip into colder temperatures could help knock back the rapidly growing population of exotic reptiles that rank among the state’s most damaging invasive creatures.
Iguanas are more than a garden and landscape-chomping nuisance in South Florida. They can carry infectious bacteria like Salmonella, devour endangered plants and animals and undermine seawalls and canal banks. On at least one recent occasion, a rogue iguana in search of a snack also knocked out power to an entire city. It wasn't the first time one had fried an electrical system.
When temperatures drop, cold-blooded reptiles like iguanas lose the ability to control their muscles, sending them raining down from the trees they call home or unable to respond to the pokes and prods from curious humans. Once they warm up, they typically snap out of their stupor. But prolonged exposure or freezing temperatures can be fatal and biologists have long pointed to frigid snap has the only realistic hope for curbing the population boom. Recent research suggests it may need to get a lot colder than it did last week. How much and how long is a still-unanswered question.
James Stroud, a postdoctoral research associate at Washington University in St. Louis, found that most of South Florida's most common lizard species are able to withstand slightly lower temperatures than they could even just four years earlier — a drop of about 2 degrees Fahrenheit, according to their 2020 paper published in the journal Biology Letters.
"What we saw is every one of these different types of lizards, they could now move at much colder temperatures than they did before," he said.
Lizards in coolers
For professional iguana hunter Steve Kavashansky, that checks out.
Speaking from Miami Beach, where his company, Iguana Busters, has one of several contracts to eradicate the invasive reptiles, Kavashansky said he's getting fewer calls after a cold snap to deal with dead or stunned iguanas.
"Cooler weather that in years past would have stunned the iguanas, we're not seeing that now," he said. "We used to get calls all the time. Over the years we've seen those calls decrease because they're getting acclimated."
Kavashansky said he's also heard reports of iguanas appearing as far north as Orlando, which could validate researchers' theory that iguana populations may move north as they get used to slightly colder temperatures.
Stroud's study found the magic number for all seven species they looked at was about 44 degrees Fahrenheit. At that point, most South Florida lizards freeze up.
Discovering that number involved packing lizards into an ice-filled cooler and monitoring their internal body temperature over the hour or so it took to cool them down. For the original 2020 study, the coolers were too small for iguanas, so they weren’t included, but Stroud said that they’ve since upgraded to iguana-sized coolers and folded the reptiles into their research.
After the lizards are revived in warmer temperatures, they're tagged and released back into the wilds of Fairchild Botanical Garden so Stroud and his team can run similar tests on them in the future.
But because researchers didn't kill them, they’re not sure exactly what kind of cold is lethal for lizards.
"That's one of the biggest questions we don’t know. We don't know if it's prolonged exposure to these temperatures that's more harmful or one big cold snap," he said.
Acclimation or evolution?
The other outstanding questions in Stroud's ongoing research is how and why, exactly, are these lizards adapting to the cold?
Christian Cox, an assistant professor of evolutionary biology at Florida International University, said the explanation could fall into one of two categories or a combination of both.
One likely explanation could be acclimation, that the animals are simply learning to adapt to their environment and undergoing an individual change. Cox likened it to how people who move to higher altitude places like Denver get used to the environment in a few short months.
On the other hand, the population could be evolving. As cold snaps winnow down the parts of the population that can’t survive them, there’s a possibility that the newer generations are evolving a hardiness to cold that their ancestors didn’t have.
For instance, in South Florida's last serious cold snap in 2010, where temperatures dropped so low that ice formed on shallow water south of Florida City, iguanas and other invasive reptiles like the Burmese python, died off in droves. But an FWC study found the python population recovered quickly, dashing hopes that cold weather alone could contain the problematic snake’s exponential growth.
"What's happening in Florida is really interesting because we have a bunch of species here that have already adapted to a new climate," he said. "They've already gone through a filter that has allowed some species to become really well-established and it’ll be interesting to see how they continue to shift or hit the evolutionary wall."
Cox is currently watching another type of lizard — the Panamanian slender anole — do just that.
To find out, his team plopped a bunch of lizards on the small islands created when the Chagras River valley was flooded to form the Panama Canal. Cox said these hilltops — now islands — are hotter and dryer than the anole's usual habitat deep in the rainforest. Those conditions mimic what the population might see as the world warms from climate change.
Five years later, "we're definitely finding evidence of acclimation and definitely seeing the potential for evolutionary change," he said.