Lizards are becoming extinct in many places, and scientists who have studied them say it is 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 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."
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, 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. He said there also are massive extinctions occurring in the Amazonian Basin and equatorial Africa, though researchers do not know the magnitude because not all the species have been described from these areas.
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 in an interview via e-mail.
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.