Putting the Heat on Tropical Lizards

Lizards are ectotherms--animals whose body temperatures vary with surrounding temperatures. Ectotherms, which account for the largest population of animals on Earth, are found in the highest concentrations in tropical areas.

Since the 1940s, scientists have known that lizards regulate their body temperatures by moving between sun and shade. Less well understood has been tropical lizards' adaptability to changes in the temperature and availability of shade in their environment.

A study conducted by Raymond Huey of the University of Washington and colleagues was designed to improve our understanding of these variables. The Huey study, funded by the National Science Foundation (NSF), involved:

• Analyzing previous climate data and comparing it to lizard body temperatures in the Amazon rainforest between 1985 and 2005 and in the Caribbean in the 1970s. "In the 1970s a bunch of us were running around the Caribbean with thermometers taking lizard body temperatures for reasons totally unrelated to climate warming," Huey said. "But we can use our data from a third of a century ago as a baseline to now predict how lizards at different latitudes would respond to climate change."
• Measuring the "fitness" of tropical lizards and moderate-climate lizards in the laboratory, i.e., how fast they run at various temperatures.

Huey and his team found that in the laboratory, tropical lizards remained at their peak fitness level within narrow ranges of temperatures that reflected average temperatures in tropical climates. When temperatures exceeded these average temperatures even slightly, the speed at which the lizards ran decreased and their responses became relatively sluggish.

Huey's team also found that the average temperature in the Caribbean forest has risen three and a half degrees, from 80 degrees Fahrenheit to 83.5 degrees F. The rise in temperature has caused these lizards to become heat stressed and not able to function as well. This is important because the more slowly lizards move, the more vulnerable they are to predators; this vulnerability may reduce the sizes of their populations.

Why are tropical lizards apparently more sensitive to temperature increases than moderate-climate lizards? Probably because lizards that live in topical forests, which are consistently hot and humid both day and night almost year-round, are only exposed to slight temperature changes and have not evolved to adapt to significant temperature increases. Therefore, lizards living in tropical forests are relatively sensitive to temperature increases caused by climate change, and their fitness levels decrease as temperatures rise.

By contrast, in many higher latitude climates, lizards experience a wide range of temperatures on a daily and seasonal basis. For example, daytime temperatures in desert regions in the southwest United States may soar well over 100 degrees F, and then dip into the 60s during evening hours. Because lizards living in these areas have evolved to adapt to such relatively extreme temperature changes, they may adapt to increases in temperature caused by climate change relatively easily. Therefore, their fitness levels are less affected by climate warming than are tropical lizards' levels.

Furthermore, because tropical lizards rely on the shade of the forest to help them regulate their body temperature, changes in the canopy structure caused by climate change may indirectly affect their ability to cool down.

Effects of temperature rise will ultimately affect all of us, according to Laurie Vitt, one of Huey's fellow researchers on the project.

"It is very easy to lose contact with the natural world and just assume that losing lizards is no big deal," Vitt said. "But lizards are part of complex ecosystems, and, in addition to tropical lizards, we know from recent work on tropical plants that these also are currently being affected negatively by global warming."

Tropical lizards play a role in the biodiversity humans depend on for nearly everything we use -- from food, materials and pharmaceuticals to the very air we breathe. As such, notes Vitt, biodiversity is a resource we simply can't afford to lose.