Watch out for falling iguanas in South Florida this Christmas. Seriously.
The frigid air is also expected to immobilize coldblooded animals. Iguanas sleeping in trees may lose their grip and drop to the ground. Sea turtles may stun and blow ashore from Texas to New England.
“You change the environment, and the organisms that are going to feel it first and hardest are the ectotherms [coldblooded animals] because their entire fitness is thermally dependent,” said Martha Muñoz, an evolutionary biologist at Yale University.
As temperatures drop, wildlife experts are preparing to rescue and observe the effects of the frigid cold on some of the reptiles that could be affected.
It’s raining lizards — at least until they adapt
This weekend, much of Florida is expected to dip into the 30s. Most lizards in Miami, introduced from warmer climates in Central and South America, find it too cold to move once air temperatures dip below about 45 degrees Fahrenheit.
Sea turtles, snakes and other reptiles are known to experience cold-stunning during frosty weather, but raining iguanas are unique to South Florida.
Sometimes, the National Weather Service in Miami issues announcements for “falling iguanas” to emphasize the severe cold and let people know the lizards are not, in fact, dead. But the freeze is temporary most of the time. When temperatures rise, some wake up and resume their normal activities. In the past, people have loaded the frozen iguanas, thinking they were dead, in the back of a car to harvest their meat, only to have the lizards thaw and attack. (So perhaps, don’t do that.)
“Some other lizards are also known to fall out of trees as iguanas do, but they don’t get the publicity that a five-foot lizard does,” said Jonathan Losos, an evolutionary biologist at Washington University in St. Louis.
But researchers and animal experts say the cold spells don’t seem to incapacitate the iguanas like they used to, suggesting that animals are adapting to the chilly weather. People may still see iguanas dropping during the upcoming cold blast, but not as many as two to three decades ago, said Zoo Miami’s Ron Magill.
“With each year when we get a cold streak, I see less and less of those iguanas falling out of trees and being cold-stunned … and it’s not because there are less and less iguanas,” Magill said. “It’s just indicative that these animals are, in fact, adapting. Less and less of them are succumbing to this type of temperature differential.”
Iguanas, like all coldblooded animals, cannot generate their own heat; their internal temperature matches that of their surroundings. To help survive colder weather, they slow down body processes including blood flow and circulation and their heartbeat to the point where it might stop. Magill said they turn from a bright green to a dark gray or black, and their eyes will be sunken in. If they don’t warm up soon enough, they could die.
“When you have a big freeze, the iguanas who don’t survive don’t pass on those genes,” Magill said. “Iguanas that have managed to survive, whether it be by getting into the water or getting underground, figuring out a way to kind of insulate themselves from the cold, they pass that gene on.”
Studies confirm that some lizards are getting better at handling the biting temperatures. In one study, a team of researchers at Washington University in St. Louis led by Losos collected lizards in Miami after Florida experienced its coldest night in a decade in January 2020. The team found multiple lizard species were able to now tolerate much lower temperatures — about four to 10 degrees colder than previously.
Another study looked at thermal tolerances and genomes of green anole lizards that survived an extreme cold event in Texas during the winter of 2013-2014. The team found the southern lizards possessed gene expressions similar to those of northern populations, which experience cold blasts more often. The green anole lizards experienced rapid selection and evolution to tolerate the cold blast.
“It’s not uncommon to observe rapid physiological adaptation to the cold,” Muñoz said. “That doesn’t necessarily mean it’s a universal outcome or a foregone conclusion, but it can and does happen.”
But the rapid adaptations in the iguana population are bad news for Floridians, Magill said. Iguanas, like many invasive species in the state, are pests. They devour native plants and pass on parasites to wildlife and humans. If approached, they can also scratch or tail-whip people. For those reasons, he warned against approaching any stunned lizards.
“A long cold spell is one of the best weapons we have against them,” Magill said. “Let nature take its course.”
Stunned sea turtles are increasing
Other reptiles do not appear to be adapting to the cold temperatures as well as the lizards, though.
In recent years, large quantities of endangered green sea turtles in Texas have washed up on shore during extreme cold blasts. During the historic winter storm in February 2021, around 13,000 turtles were cold stunned, and around 4,000 of those turtles lived. Another 300 cold-stunned turtles were rescued during a cold spell in February.
The sea turtle cold-stunning event this weekend is expected to be at least as bad as February, when hundreds were stunned, but not as bad as the 2021 event that impacted thousands, according to a predictive model by Philippe Tissot at Texas A&M University in Corpus Christi. The event is expected to start Friday and continue through at least Saturday and Sunday, he wrote in an email. Temperatures in the upper Laguna Madre in Texas as expected to dip as low as 40 degrees; the sea turtles start to lose function below 50 degrees.
“They’re not adapting,” said Donna Shaver, chief of sea turtle science and recovery for the National Park Service at Padre Island National Seashore.
Shaver said more stunning events have occurred in recent years but added that could be due to more green turtles in the area. As populations have grown, she said some of the green turtles around Padre Island may also have traveled from farther south, where they likely aren’t adapted to cold blasts.
Shaver and her team are already preparing for this weekend’s potential stunning event. They look along bays where winds can blow the stunned turtles ashore, enlisting the help of other government partners, volunteers, boaters or anyone in the community who sees one. The turtles are brought back to Shaver’s lab to warm up and then are released when safe. She said if the turtles are not found quickly enough, they could be attacked in their stunned state by birds or coyotes. Those that are severely damaged are euthanized and sent for research.
“We just have to go out there and rescue them and recover as many as we can,” said Shaver, who serves as the Texas coordinator of the Sea Turtle Stranding and Salvage Network.