Lizard's Tolerance to Cold Stronger than Predicted

Lizards are generally considered a harmless species that emerged millions of years ago from water and evolved to be a part of cold-blooded reptiles.

Since lizards are cold-blooded animals, they adopt exothermic means to control their body temperature. But a unique characteristic has been noticed in the Puerto Rican lizard, Anolis Cristatellus.

Researchers at the Duke University noticed this tropical lizard's tolerance to cold is really stronger than predicted earlier by the scientists. They observed that this tropical lizard had adapted to the cooler winters of Miami. This indicated that they were able to tolerate temperature variations caused by climate change.

It was earlier assumed that being cold-blooded these lizards are unable to adapt to cooler temperatures.

"We are not saying that climate change is not a problem for lizards. It is a major problem. However, these findings indicate that the thermal physiology of tropical lizards is more easily altered than previously proposed," said Duke biologist Manuel Leal, co-author of the study.

Humans, however, introduced Puerto Rican native A. Cristatellus to Miami around 1975. In Miami, the average temperature is about 10 degrees Celsius cooler in winter than in Puerto Rico. The average summer temperatures are similar.

In order to proceed with the finding, A. cristatellus was captured by Leal and his graduate student Alex Gunderson from Miami's Pinecrest area and also from northeastern Puerto Rico.

These animals were shifted back to their North Carolina lab. Here a thermometer was slid into each lizard's cloaca and the temperature was turned cooler. They then noticed how the lizards righted themselves after they had been flipped on their backs.

On doing so they noticed that the lizards from Miami flipped themselves over in temperatures that were 3 degrees Celcius cooler than the lizards from Puerto Rico. 

"Animals that flip over at lower temperatures have higher tolerances for cold temperatures, which is likely advantageous when air temperatures drop," Leal said.

"It is very easy for the lizards to flip themselves over when they are not cold or not over-heating. It becomes harder for them to flip over as they get colder, down to the point at which they are unable to do so," he said.

At the critical temperature minimum the lizards aren't dead. They have no control over their coordination.

"It is like a human that is suffering from hypothermia and is beginning to lose his or her balance or is not capable of walking. It is basically the same problem. The body temperature is too cold for muscles to work properly," he said.

Leal explained that a difference of 3 degrees Celsius is "relatively large and when we take into account that it has occurred in approximately 35 generations, it is even more impressive. Most evolutionary change happens on the time scale of a few hundred, thousands or millions of years. Thirty-five years is a time scale that happens during a human lifetime, so we can witness this evolutionary change."

According to Leal, the lizards' cold tolerance provides a glimpse of hope for some tropical species.

Leal and Gunderson are now working on the heat-tolerance experiments, along with tests to study whether other lizard species can adjust to colder temperatures. 

The study appears in the Dec. 6 issue of The American Naturalist.