WASHINGTON - Sometimes it can be too darn hot even for a lizard.

Cold-blooded creatures that have to soak up the rays to get going might seem like the last animals you would expect to be threatened by global warming.

Well, you would be wrong, researchers say.

It turns out lizards are going extinct in many places, and scientists who have studied them say it's because of rising temperatures. The heat affects reproduction.

"The results were clear. These lizards need to bask in the sun to warm up, but if it gets too hot they have to retreat into the shade, and then they can't hunt for food," said Barry Sinervo of the Department of Ecology and Evolutionary Biology at the University of California, Santa Cruz. He said he was "stunned and saddened" by the finding, reported in today's edition of the journal Science.

"This is an extinction alert for all areas of the globe and for all the various species of lizards," Sinervo said.

Lizards are an important part of the food chain because they are major consumers of insects and in turn are eaten by birds, snakes and other animals.

"This is just the tip of the iceberg," Sinervo said. "It heralds that we have entered a new age, the age of climate-forced extinctions. Extinctions are not in the future. They are happening now."

In Mexico's Yucatán region, scientists found that the time lizards could be out foraging had disappeared. "They would barely have been able to emerge to bask before having to retreat," Sinervo said.

Jack Sites, a biology professor at Brigham Young University, said high temperatures during the reproductive cycle prevent the animals from eating enough to have the energy to support a clutch of eggs or embryos. "The heat doesn't kill them. They just don't reproduce," he said. "It doesn't take too much of that and the population starts to crash."

According to Sinervo, the extinctions are concentrated in what biologists call hot spots of biodiversity, where there are lots of species.

That includes locations in Mexico, where a large number of species have evolved in volcanic mountain ranges. He said there also are massive extinctions occurring in the Amazon Basin and equatorial Africa.

In Madagascar, the Indian Ocean island off the southeastern coast of Africa, the estimate is that one-fifth of all the local lizard populations are now extinct, Sinervo said.

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