An unexpected cold snap this week sent thermometers plummeting in Florida and heat-hungry iguanas dropping from tree branches like autumn leaves, scientists and witnesses said.

Passersby in Bill Baggs and Crandon parks in Key Biscayne, south of Miami, were seen picking up the seemingly lifeless lizards from the ground beneath trees and setting them in the sun, where after a brief warm-up, most revived and scampered off into the bushes.

The cold-blooded lizard-with-a-mohawk's comfort level begins at 23 degrees Celsius (73 Fahrenheit) and it positively thrives at 35 C (95 F). But on Wednesday and Thursday, the mercury in south Florida dropped to 4-5 C (39-41 F).

"When the temperature falls below about 60 F (15 C) they become less able to move around. At temperatures below about 40 F (5 C) they become completely immobile and begin to suffer serious stress," said University of Florida wildlife expert Perran Ross.

Since all three iguana species shelter in tree branches and crevices, he added, when the temperature falls low enough, they are unable to hold on and drop to the ground.

"Some of these chilled and cold-stressed iguanas will suffer irreversible damage and die. Others may be able to rewarm themselves by sitting in the sun and may recover," the expert added.

Iguanas are not native to Florida but were brought in by travelers from Mexico, Central and South America.