Lizards Help Explain Survival of the Not-So-Fittest

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Le Galliard and his colleagues are uncertain how the low-endurance lizards in the food-rich environment were able to catch up with the high-endurance lizards. But they suggest it is related to a trade off between endurance and some other trait.

"We found that individuals that caught up quickly after birth also grew faster, suggesting that they allocated resources [to] traits enhancing endurance," Le Galliard said.

The finding challenges the strong link between running ability and Darwinian fitness by showing that running ability is part of a more complex set of fitness traits that determine which individuals survive, according to Le Galliard.

"I think it is a really nice demonstration of the role of the environment in confounding the potential role of selection," Sinervo said.

Behavioral Influence

Losos and his colleagues wanted to settle a debate over whether species can prevent natural selection by changing their own behavior.

To find out, the team introduced the large, ground-dwelling predatory curly-tailed lizard (Leiocephalus carinatus) to six tiny islands in the Caribbean occupied by the small, ground-dwelling brown anole lizard (Anolis sagrei).

(Losos dismisses any concerns that the introductions would throw off the tiny islands' natural balance. The curly-tails are native to adjacent, larger islands and occasionally colonize the small islands naturally. The introductions thus mimick a natural process, he said. And since hurricanes periodically roll through the area and completely clear off the islands, there are no long-term impacts from these introductions, he added.)

The curly-tails will eat any brown anoles they can catch and fit into their mouths. This led Losos' team to speculate that the brown anoles would do one of two things. They might move into the trees and out of reach of the curly-tails, or they might begin to "select" for larger, harder-to-eat bodies—meaning that smaller anoles would be eaten, with the result that future generations of anoles would be larger than previous generations.

According to the results of the study, both happened. The brown anoles began to spend more time up in the trees. And since they also spent some time on the ground, successive generations had larger bodies and longer legs.

"Our interpretation is that the lizards exhibited a behavioral change to avoid the curly tails, but the change was not sufficient to get them out of harm's way," Losos said.

His team predicts that in the future the brown anoles will move higher and spend more time in the trees in order to avoid the curly tails. This behavioral response should force brown anoles to evolve shorter, more nimble limbs in order to survive on the small branches, he said.

Sinervo said the Losos team's study shows that natural selection resulting from "traits in which behavior is operating are even less straightforward than the physiological avenues demonstrated by Le Galliard."

Losos' research was supported in part by a grant from the National Geographic Society's Committee for Research and Exploration.

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