Jan. 28, 2009 -- It takes some effort for fire ants to get under the hard scales of an unsuspecting lizard. When the insects finally penetrate the reptile's fleshy core, the attackers inject a toxin that paralyzes their victim. Then, they tear the lizard to pieces, which they carry back to their nest.
Twelve fire ants can kill a 3-inch lizard in a single minute.
Watch video: Lizards evolve to survive fire ant attacks.
It's an unpleasant way to die, and one that at least one species of lizard is rapidly evolving to avoid. In just 70 years, according to a new study, eastern fence lizards in parts of the United States have developed longer hind limbs and new behaviors that help them escape the clutches of the venomous ants.
The study is one example of how quickly and profoundly an invasive species can shape the way native populations look and act, said Pennsylvania State University ecologist Tracy Langkilde. She's the author of the new study, which might also help wildlife managers better deal with arrival of an aggressive invader.
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Eastern fence lizards (Sceloporus undulatus) live throughout the southeastern United States, a region that was free of fire ants not long ago. Then, starting about 70 years ago, ant colonies started moving in from South America.
Fire ants are such big pests to farms, gardens and people that residents are usually quick to report when the ants first arrive in their area. As a result, Langkilde was able to get records of exactly when ants first showed up in different places.
For her experiments, Langkilde collected lizards from four sites in Arkansas, Mississippi and Alabama. One site was still free of fire ants. The others were invaded 23, 54 and 68 years ago. From each site, she grabbed 20 males and 20 females.
Langkilde convinced each lizard to scurry onto a fire ant mound by tapping its tail. Then she dragged a stick across the mound to get a few ants to come out and see what was happening. The stick-dragging method draws out about seven ants, she said -- enough to get a reaction out of the lizard but not enough to kill it. Langkilde then sat, watched, and recorded everything that happened next.
Lizards do one of two things in the event of a fire ant attack. Either they twitch and shake to fling the bugs off before running away. Or they sit still and hope the ants will leave them alone. The first strategy is far more effective than the second, which usually leads to death.
Results showed that, the longer a lizard population had been exposed to fire ants, the more likely the reptiles were to twitch and flee. Only half of the lizards from the uninvaded site reacted this way, compared with 80 percent of lizards from the site where ants had lived the longest. Lizards from groups with more ant experience also had longer hind legs, which Langkilde suspects, help the lizards scurry away more quickly.
"Individuals that have survived ant attacks probably passed on their genes for doing that behavior," said Laurie Vitt, a zoologist at the University of Oklahoma in Norman. "Lizards that stood there like idiots were killed by ants and didn't pass on their genes. It's as simple as that."
Invasive species are causing problems worldwide. But Langkilde said her study shows that, at least in some cases, creatures can quickly adapt to coexist, even when one species is capable of ripping the other to shreds.
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