"I had provided a baseline data set with precise localities where the lizards were common," Sites explained. "But Mexican ecologists were going back every few years, and pretty soon the lizards were hard to find, and then they weren't seeing any. These are protected areas, so the habitat's still there. So you start to think there is something else going on."
Using Sites' field notes for comparison, Sinervo and collaborators resurveyed 48 species of spiny lizards (Sceloporus) at 200 sites in Mexico where the lizards had been studied between 1975 and 1995. They found that 12 percent of the local populations had gone extinct.
They later connected the lizards' decline to climate records and studied the effect of rising temperatures on lizard physiology and behavior. For example, cold-blooded lizards can't forage for food when their bodies get too hot – they must seek shade because they can't regulate their own temperature. The researchers found that the hours per day when the temperature allowed foraging dropped significantly.
Sites said that when the temperature increase hits during a critical month of the reproductive cycle, the lizards don't get enough energy resources 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."
But for the phenomenon to be linked to climate change, the pattern would need to be seen globally. Sites connected Sinervo with researchers in Chile and Argentina, where Sites has been working for a decade. Sinervo also worked with researchers who documented lizard declines in Africa, Australia, and Europe.
"To get this kind of pattern, on five continents in 34 different groups of lizards, that's not random, that's a correlated response to something big," Sites said, adding that the effect appears to be happening too fast for the lizards to adapt.
Sites finds no joy in being part of such a significant study. "It's a terrible sinking feeling – when I first saw the data, I thought, 'Can this really be happening?' It's important to point out, but it sure is depressing."
Sites says the model now needs detailed testing on all five continents, with a standardized protocol on lizard species that are widespread.