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 heats 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.
Sinervo said he was "stunned and saddened" by the finding, which he reported in Friday'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."
Five percent of lizard species already have gone extinct, Sinervo said, and his team projects that if the planet continues to heat up at current rates, 20 percent of all lizard species could go extinct by 2080.
In Mexico's Yucatan 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 who collaborated with Sinervo, 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.
This includes locations in Mexico, where a large number of species have evolved in the different volcanic mountain ranges.
Elizabeth Bastiaans, a doctoral student in Sinervo's lab, started studying lizards in a wilderness outside Mexico City near the Aztec pyramids of Teotihuacan where tourists huff and puff up hundreds of stairs in the blazing sun.
"I've been out there doing a lot of sampling over the past few years and you see the lizards in the morning and you see them in the evening. But in the hottest part of day, it's just too hot, you don't see them at all," Bastiaans said.
Some of the spiny lizards with blue bellies she studies went extinct at lower, warmer altitudes. Some moved to higher, cooler ground but, as temperatures continue to rise, that habitat is shrinking.
"If the climate continues to warm, they are going to get pushed off the top of the mountains," Bastiaans said. "There is only so much mountain they can climb."
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. "This will surely have driven some endemic species to the brink of extinction, if not over the precipice," he said.
Sinervo was doing field work in France when he noticed a decline among lizards. While resurveying areas that had been studied in the 1990s, it became clear that lizards were gone from some spots — levels of 30 percent extinction across southern Europe, for example.
He and French researchers contacted colleagues around the world and found similar trends in the United States, Mexico and elsewhere.
"I was surprised at how fast researchers began sending us data," he said. "That's what happens though. When scientists see a problem, with global evidence backing it, they come together."
Funders of the research included the National Geographic Society, University of California Institute for Mexico and the United States; University of California, Santa Cruz; U.S. National Science Foundation; University of Paris; University of Toulouse; National Council on Science and Technology of Mexico; National Scientific and Technical Research Council of Argentina; Academy of Finland; National Autonomous University of Mexico; American Museum of Natural History; Australian Research Council; and Brigham Young University.
The Associated Press and Reuters contributed to this report.