Polar bears have been described as the poster species highlighting the dangers of climate change, but U.S. scientists suggest species living in the tropics may be in graver danger.

Research published in Tuesday's print edition of the Proceedings of the National Academy of Sciences finds that while temperature changes are expected to be more extreme in the Arctic, a warming of just a degree or two could wipe out tropical species.

That's because they are used to living in a climate where the temperature is already close to the highest they can tolerate. Even a small increase could put them beyond their ability to adapt. Protection strategies such as staying out of the sun in the heat of the day or burrowing into the soil are unlikely to save them, says a team led by scientists from the University of Washington.

"There's a strong relationship between your physiology and the climate you live in," Joshua Tewksbury, assistant professor of biology, said in a release.

"In the tropics, many species appear to be living at or near their thermal optimum, a temperature that lets them thrive. But once temperature gets above the thermal optimum, fitness levels most likely decline quickly and there may not be much they can do about it."

While under threat from disappearing sea ice, Arctic species have at least one thing going for them in an increasingly warm climate. They live at temperatures well below their thermal limit and so are not in danger of overheating.

The scientists used global temperature records from 1950 through 2000 and added projections from the Intergovernmental Panel on Climate Change for warming in the first years of the 21st century.

They then compared that information with what is known about the relationship between temperatures and fitness for a variety of tropical insects, frogs, lizards and turtles. Fitness was defined as a measure of population growth rates and physical performance — a measure that shows as the temperature goes up, fitness goes down.

"The direct effects of climate change on the organisms we studied appear to depend a lot more on the organisms' flexibility than on the amount of warming predicted for where they live," Tewksbury said. "The tropical species in our data were mostly thermal specialists, meaning that their current climate is nearly ideal and any temperature increases will spell trouble for them."

The scientists were also are at pains to point out that the tropics hold the large majority of species on the planet.