Adapt or Die

Tracy Langkilde couldn't believe it. The lizard just sat there. With deadly fire ants climbing its back, the three-inch reptile closed its eyes and froze, as though playing dead would make it invisible.

Langkilde, an assistant professor of biology at Penn State, swooped to the rescue, pulling off the attacking insects and ending the trial. "As few as 12 of these little guys can kill an adult lizard in less than a minute," she explains.

Solenopsis invicta, the red imported fire ant, is no ordinary pest. Unwittingly transplanted from Argentina to Alabama in the 1930s, they were first reported near the port of Mobile by a boy naturalist named E.O. Wilson. These aggressive invaders have since colonized the entire southeastern United States, leaving ruined crops and dead livestock in their wake. They are a genuine scourge, a poster pest for invasive species. To Langkilde, that makes them a useful tool for the study of evolution.

Back in her native Australia, she had investigated competition among lizards in a national park habitat and found, not unexpectedly, that the larger species tended to dominate, muscling in on the warmest shelter sites. "But what I also found was that the subordinate species got along fine in spite of this," Langkilde says. Being the loser in competition, in other words, didn't always seem to have ecological costs. "What excited me was the idea that maybe these lizards had successfully adapted to their situation," she says.

Unfortunately, the only way to test that theory was to go back in time—"to have a look at this community at the moment when these species first came into contact with one another," Langkilde says. "As I was trying to figure out how to do that, I thought of invasive species."

The nice thing about such invaders, she explains, "especially really nasty, pesty things," is that they don't go unnoticed for very long. In the case of fire ants, "We've got great county-level records of their first occurrences in all these areas—fantastic records of the rate and route of spread. So we can see at different sites how long invasive fire ants have been interacting with native species." In effect, she could fashion an evolutionary time machine.

Coming to the U.S. in 2007, Langkilde gathered common fence lizards from sites in Alabama and Mississippi where fire ants had been present for 23, 54, and 68 years, respectively, and from one site in Arkansas that is beyond their present reach. Then she took her subjects to a live fire-ant mound to see how they'd respond to being attacked.

As she quickly found out, some of them didn't. "It really stunned me," she says. "Given the relative sizes of ant and lizard, I assumed that any healthy adult lizard would do what you or I would do—just get up and walk away." The freeze response—actually an attempt to blend in—was more suited to an age-old nemesis: birds of prey. "But then these lizards had never encountered ants that could kill them before."

The lizards that had been exposed to Solenopsis took a very different tack. "They do these big whole body twitches, to flick the ants off, and then they run right off the mound," Langkilde reports. This "twitch and flee" response, she found, grows more prevalent with increased time since a population's first encounter with this invader. "The percentage gets higher and higher, until by the time you get to the lizards from the longest-ago-invaded site, it's up to 80 or 90 percent."

What accounts for the change in behavior, Langkilde admits, is not altogether clear. "It could be evolved or it could be learned from experience." Remarkably, however, she found that lizards from invaded sites actually had longer legs than their un-invaded counterparts—the better to beat a retreat on. "It follows the same pattern. The legs get longer the longer ants have been around." And the difference shows up even among newly hatched lizards, she reports, suggesting that "this is probably an evolved trait."

While it may seem extraordinary to see evolutionary change in what amounts to just 40 lizard generations (the equivalent of about 800 human years), it's not unheard of, Langkilde says. "It's consistent with rates of adaptive shifts exhibited by other native species in response to emerging threats."

Tracy Langkilde, Ph.D., is assistant professor of biology in the Eberly College of Science, e-mail tll30@psu.edu. Her research is supported by the Gaylord Donnelley Environmental Fellowship, the Eppley Foundation for Research, the National Geographic Society, the American Museum of Natural History, and the National Science Foundation.

Last Updated December 07, 2010