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A lizard from Madagascar.
The jeweled chameleon (pictured) is among lizard species under threat on Africa's Madagascar island.

Photograph courtesy Ignacio de la Riva

Three male lizards..

Lizards of the Iguanidae family, said to be threatened by global warming. Photograph courtesy Barry Sinervo.

Ker Than

for National Geographic News

Published May 13, 2010

Lounging in the shade may sound soothing, but it could be the death of many lizards if global warming continues at current rates.

As temperatures inch upward, the reptiles rest more and hunt less. As a result, 20 percent of lizard species could go extinct by 2080, a new study says.

(See "Global Warming 'Marches On'; Past Decade Hottest Known.")

No matter what we do to fight global warming, at least 6 percent of lizard species will go extinct by then, due to the greenhouse gases already in the atmosphere, the study says.

"We've committed ourselves to 6 percent," said lead study author Barry Sinervo, a herpetologist at the University of California, Santa Cruz. "That's our fault, and there's nothing we can do."

And those percentages are conservative, Sinervo said. For example, the study doesn't account for threats such as competition between lizard species or the effects of global warming-caused water shortages.

David Wake, who wasn't involved in the new study, agreed that the findings are far from exaggerated. "It's frightening," the University of California, Berkeley, zoologist said. "But it's reasonable."

(Also see "New Giant Lizard Discovery 'an Unprecedented Surprise.'")

Global Warming to Starve Lizards to Death?

The study team calculated extinction risks for more than a thousand lizard species around the globe for their study, to be published in the journal Science tomorrow.

The research was prompted by the discovery that 50 percent or more of the local populations of certain species had gone extinct in parts of Mexico and Europe.

The team suspected global warming was to blame, because the lizard-population crashes had generally occurred in areas with the warmest springs—when lizards reproduce.

(See "Global Warming Likely Causing More Heat Waves, Scientists Say.")

To test their global warming hypothesis, the researchers created pseudo-lizards out of temperature gauges and painted pipe. The researchers placed the devices at four sites in Mexico's Yucatán Peninsula: two sites where blue spiny lizards are thriving and two where the species has gone locally extinct.

Unlike mammals, lizards and other reptiles are cold-blooded, meaning they can't regulate their body temperatures and so must seek shade when it gets too hot.

Where the blue spiny lizards are still living, the spring temperatures were such that the lizards should have only had to seek shelter from the sun for about four hours a day, the gauges suggested.

But where the lizard had already gone locally extinct, the spring temperature was so high it would have dictated that the lizards spend most of the day hiding from the sun rather than hunting.

That much downtime means the lizards will "pretty much be starving to death" and not laying eggs, said Sinervo, whose study was partly funded by the National Geographic Society's Committee for Research and Exploration. (The National Geographic Society owns National Geographic News.)

(Related: "Heat Triggers Sex Change in Lizards by 'Turning Off' Key Gene.")

Lizard Extinctions Predicted

Next the team set out to determine whether their findings held up at a global scale.

The scientists created a computer model incorporating temperature data back to 1975, global warming prediction data from the Intergovernmental Panel on Climate Change, and the maximum temperatures different lizard species can withstand.

The model calculated the number of hours each day that a lizard species should be incapacitated due to heat as well as the hottest temperatures for different parts of the Earth in the past, present, and future.

The model accurately predicted specific locations where lizard species are known to have gone locally extinct in North and South America, Europe, Africa, and Australia.

Certain Lizards Especially at Risk

Global warming will hit lizards in the Amazon and Madagascar especially hard, the computer model predicted.

"The number of local extinctions we're seeing [in the Amazon], based on the model, is through the roof," Sinervo said. "It's 60, 70, 80 percent even in some places."

And because Amazon lizards are relatively poorly documented, "we don't even know all the different species that we're going to lose," he added.

Another especially vulnerable category is lizards that give birth to live young, including the blue spiny lizard, which Sinervo studied in Mexico.

Live-birth lizards "had to evolve slightly lower body temperatures, because if it's too high, the babies might die or develop developmental abnormalities," Sinervo explained.

Bigger Than Lizards

The new study underscores that global warming isn't just a lizard problem, said Warren Porter, a zoologist at the University of Wisconsin-Madison.

"Many people might consider changes to lizard populations of small importance," said Porter, who wasn't involved in the study.

But, he pointed out, "another major group of cold-blooded animals, the pollinating insects, are also subject to the same constraints that lizards are."

The loss of pollinators, he added, would be devastating to the human food supply. (See "Bee Decline May Spell End of Some Fruits, Vegetables.")

Porter also noted that the team looked mainly at small lizards. The extinction risks for larger lizards, such as the Komodo dragon, could be much higher, he said.

"Larger species of animals are even more vulnerable to heat stress," Porter said, "because they have fewer places to find shade and water on a landscape that is drying out."

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