By David Brown
Friday, May 14, 2010; A03
When it comes to the hazards of global warming, it may turn out that lizards in burrows are the canaries in the coal mine.
In a study to be published Friday in the journal Science, an international team of biologists reports that in more than one-tenth of the places in Mexico where lizards flourished in 1975, the reptiles now cannot be found. The researchers predict that by 2080, about 40 percent of local lizard populations worldwide will have died off and 20 percent of lizard species will be extinct.
The reason for the huge die-off appears to be rising temperatures. But it isn't heat that is killing the lizards directly.
Instead, global warming appears to be lengthening the period of the day when lizards must seek shelter or risk fatal overheating. In the breeding season, that sheltering period is now so long that females of many species are unable to eat enough food to produce eggs and offspring.
Springs that start earlier and are warmer than they once were have been noted in many regions of the world in the past three decades. The new study suggests that the phenomenon may be far more important for the survival of some animals than peak summer temperatures, said Barry Sinervo, an evolutionary biologist at the University of California at Santa Cruz who headed the 26-person research team.
"It is as if something has really happened in world climate and the lizards are telling us that," he said.
The lizard findings also suggest that early stages of global warming may be more than a warning: They may have permanent consequences.
"Many of us have been worried about extinctions in the future," said Raymond B. Huey, a lizard physiologist at the University of Washington in Seattle, who wrote a commentary accompanying the study. "This paper shows that extinctions are already here. I think that will really be surprising to most biologists."
Lizards are cold-blooded. They depend on the environment for the heat necessary to run their bodies -- functions from muscle contraction to digestion and hearing. Some get heat by basking in the sun ("heliotherms"), others by waiting for the air to warm them up ("thermoconformers"). Different species have different optimal temperatures as well as different maximal temperatures they can tolerate.
Several years ago, the research team visited 200 places in Mexico where in 1975 biologists had recorded the presence of basking lizards of the Sceloporus genus, 48 species in all. At 12 percent of the sites, the researchers found no lizards.
The places with none tended to be at southern latitudes and low elevations and in regions where four decades of weather data showed a marked increase in springtime temperatures. Species with lower optimal temperatures -- 90 to 95 degrees, on average -- were also more likely to have gone "locally extinct" than more heat-tolerant ones.
Sinervo hypothesized that springtimes were getting too hot for lizards. He fashioned artificial reptiles out of painted PVC pipe and electronic temperature gauges and put them out in the sun at two places in the Yucatan where the species Sceloporus serrifer survived and two places where it was extinct.
Where the lizards had died out, the average April day had 9.25 hours with temperatures so high that lizards of that species would have had to seek refuge in a cool spot to survive. Where they survived, there were far fewer "hours of restriction."
An analysis of more sites led the scientists to conclude that when Mexican Sceloporus lizards spent more than four daylight hours in burrows out of the sun, extinction was very likely. Females simply wouldn't have enough time to eat.
"The summer maximum [temperature] doesn't matter to them," Sinervo said. "Lizards are fully capable of crawling under a rock and not doing anything for a couple of months. The problem arises for females in the spring who are maximally cranking away for reproduction."
The researchers created a mathematical model linking a lizard's optimal temperature, the maximum outdoor temperature and the hours of restriction to a species' risk of extinction. It correctly "predicted" recent extinctions in South America, Europe, Australia and Africa.
Predictions for 2050 -- local extinction of 16 percent of the world's lizard populations and global extinction of 6 percent of lizard species -- appear unavoidable, the researchers wrote. The more dramatic 2080 die-offs might be avoided if global warming is slowed.