The cold-blooded reptiles go into a deep sleep when the temperature falls into the 40s Fahrenheit (5 to 10 degrees Celsius). Their bodies turn gray, and they lose their grip, experts say.
But it isn't necessarily sudden death: The reptiles usually perk up again when the temperature rises.
Miami MetroZoo officials said the night cold causes the most drops.
"The worst part of the cold comes in the evening, and they literally just shut off," said Ron Magill, communications director for Miami Metrozoo. "Their bodies shut off and they lose their grip on the tree, and they start falling."
Temperatures hit the high 20s Fahrenheit (-3 degrees to -1 degree Celsius) and low 30s Fahrenheit (-1 to +1 degree Celsius) this week in Florida during an unusual cold snap.
While many of the iguanas will wake up, they could face death if low temperatures persist. Iguanas can sustain cold for four to ten hours before they have to wake up, Magill said.
According to Kenneth Krysko, a herpetologist at the Florida Museum of Natural History, losing a few iguanas "is a good thing."
"The [iguana] populations have expanded so drastically [that] when we do experience a really good cold snap, it will kill off a lot of them," Krysko said, adding that "they're not native, and they're considered a nuisance."
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