Wrapping entire islands in netting and tracking lizards' stamina by running them on miniature treadmills may seem like the stuff of science fiction, but these techniques were recently used by two Dartmouth researchers in a recent study. By artifically controlling the numbers of predators on various islands in the Bahamas, post-graduate researcher Robert Cox and biology professor Ryan Calsbeek found that competition between Caribbean anole lizards, rather than the presence or absence of predators, drives their natural selection, the researchers told The Dartmouth.
Cox and Calsbeek's findings were published online by Nature on May 9. The team tested the stamina of 480 male lizards before placing them different islands, Cox said in the interview.
"Our current view of selection in the wild is based almost entirely on correlations," Calsbeek and Cox said in the study. "Here, we simultaneously manipulate predation and competition to disentangle the causal agents of natural selection using an experimental framework in the wild."
The researchers concluded that "competition was more important than predation in driving natural selection on these islands," Calsbeek said in the interview.
Lizards with larger bodies, longer limbs and higher stamina than their competitors survived, while those with less favorable traits died, according to the study.
"The important thing that we're interested in is not who lives and who dies, but the particular traits that determine who lives and who dies," Cox said.
While more lizards died when predators were present, the selection of lizards that died was random, Cox said in the interview.
In order to test the lizards' stamina, each lizard ran to exhaustion on a miniature treadmill, he said.
"If you keep your hand behind them to keep them moving, and they will run until they physiologically can't run anymore," Cox said.
After the lizards stopped running, the researchers turned them on their backs if they could not roll themselves over, then they were thoroughly exhausted, Cox said.
"It's a convenient way of measuring the point at which they're completely exhausted," Cox said.
The lizards regained energy after 10 minutes, and the testing process was not inhumane, he added.
After each lizard was tested, it was identified and marked, according to the study.
The lizards were then divided into groups of 80 and placed onto six different islands with three levels of predation no predators, birds and snakes, and birds only, according to Cox. Two islands were dedicated to each level of predation.
Islands designed to lack predators were entirely wrapped in a plastic no-tangle bird netting, while the islands where birds were allowed to prey were wrapped in the same amount of netting around the perimeters but were left open on top, Cox said. The islands that contained birds and snakes were wrapped the same way as the island that only had winged predators, but the researchers introduced predatory snakes to these islands, Cox said.
Because the snakes were native to the region, introducing them to the experimental islands did not harm the islands' ecosystems, he said.
"It's not that the snakes are not native to the small islands they actually are native. It's usually that there aren't any snakes on there because they'll eat all the lizards and then [the snakes] are out of luck," Cox said.
Natural disasters such as hurricanes often exterminate the snakes and lizards that are native to the islands, he added.
"There's no real large-scale ecological consequence to this," Cox said.
Calsbeek and Cox spent three years studying these lizards, but the preliminary research and experimental design for this project took 10 years to complete, Cox said.
The researchers are currently in the Bahamas working on a new study concerning natural selection in juvenile lizards that have not yet reached sexual maturity, Calsbeek said.
