Florida's Falling Iguanas: Cold Weather Turns Invasive Reptiles into Projectiles

The odds of a frozen 10-pound reptile crash-landing on its head in front of you is unlikely, but this time of year in Florida, the chance may be higher than you think. Green iguanas are not native to Florida but were introduced to the Sunshine State in the 1960s. Since then, the large, dinosaur-like reptiles have made themselves at home across the south and central regions, lazing about on backyard fences, moving through canals, and gobbling up many an ornamental garden plant. While the Western Hemisphere’s largest lizard is capable of violence, with sharp teeth and powerful, whip-like tails, these slow-moving animals mostly keep to themselves. That is, except for when a cold snap moves through the region, slamming the brakes on the iguanas’ metabolism and ultimately turning them into scaly projectiles.

“The phenomenon of cold stunning is not unique to green iguanas,” says Natalie Claunch, a wildlife biologist with the USDA Wildlife Services National Wildlife Research Center. “But the phenomenon of observing them falling from trees during these events is due to them being established in a climate outside of their native range and without many of their natural predators.”

Like all reptiles, green iguanas are ectotherms. Ectotherms’ body temperatures are mostly determined by the external environment, rather than creating their own heat like mammals. In turn, an iguana’s body temperature influences its internal processes, such as its metabolism. “During a cold spell, these processes will slow down and can reach a torpor-like state, where their heart rate and breathing rates are slowed down and overall signaling in the body is slowed as a result,” says Claunch.

On the ground, this would mean slower reaction times and perhaps the inability to run away from danger. But for green iguanas stretched out in a backyard tree? A loss of motor function can render them unable to maintain a grip on a branch or tree trunk. And that’s when the lizards start falling out of the sky. Human injuries have been reported, however the state of Florida does not keep an official count.

According to Claunch, cold stunning temperatures can vary based on a reptile’s size, the rate of temperature change, and prior experience with cold events. “For example, a small iguana has a lower surface area and would reach a colder internal body temperature faster compared to a large iguana,” she says. No researchers have studied this specifically under the controlled conditions in a lab. “However, iguanas in Florida have been observed falling into cold stuns when ambient temperatures fall below 50 degrees Fahrenheit,” says Claunch.

Unless the iguana has fallen from a relatively great height or landed poorly, the animal may be better off than you think. Arboreal animals do occasionally misjudge a leap or fall out of their treetop perches, after all, and are often better adapted to survive and minimize the impact of those injuries. Of course, cold stunned iguanas can be easy pickings for predators and even pets. But it’s when cold temperatures persist that the animals start to really incur harm. “Prolonged exposure to cold temperatures can be detrimental to reptiles in that they become more susceptible to infection while their immune system is slowed down,” says Claunch.

Iguanas regain consciousness and resume normal activity faster than you may think. It’s for this reason that Shannon Knowles, communications director for the Florida Fish and Wildlife Conservation Commission, advises against trying to save a green iguana by bringing it into your home or car. “Iguanas can recover from torpor quickly,” she says. “Wild iguanas can be defensive when not able to escape, sometimes using their sharp teeth and nails, and long tails that whip.” It’s also important to keep pets away from the wild animals, lest they wake up and try to defend themselves.

Because green iguanas are not native to Florida, the state does not monitor their harvest. This means that it is legal to remove them from your property and even kill the animals, if done so humanely. For the adventurous-minded, many people swear by an unconventional removal method—lunch. In the Caribbean, where iguanas are native, they are sometimes referred to as "pollo de los árboles," or chicken of the trees. After all, iguana eggs are primarily composed of yolk, with very little albumen, making them a handy omelets ingredient. Green iguana meat is also high in protein, as well as omega-6 and omega-3 fatty acids.

While it may be scary that an animal weighing as much as a bowling ball can fall out of the trees and onto your head, the fact that green iguanas can’t handle the cold is one of the only things keeping them from disrupting ecosystems across the rest of the United States. In this way, iguana falling season may be something to celebrate, too.