Sometimes size really does matter. Horned lizards adorned with longer spikes on their heads are more likely to survive predation by lizard-eating birds than their smaller-spiked comrades. The finding is a rare glimpse of natural selection at work in the wild.
Horned lizards are strange inhabitants of Arizona's Sonoran Desert. Because they're so hard to spot, researchers feared their numbers might be dangerously low. To get a better census, graduate student Kevin Young of Utah State University in Logan set up camp in the desert and rose at the break of dawn. He spied wiggly paths in the sand that previous researchers had missed, suggesting that the animals were more prevalent than people thought. But he also found something very disturbing: gruesome scenes in which horned lizard skulls were spiked on creosote bushes, sometimes scores of them in one bush.
The horned lizard graveyards were created by loggerhead shrikes, a raptor wannabe the size of a robin. Lacking talons, the bird grabs its prey in its beak and impales it on branches. Then it picks off the flesh and leaves the bones behind. Young's adviser, Edmund Brodie Jr., and colleague Edmund Brodie III of Indiana University, Bloomington, saw a golden opportunity to study natural selection in the wild. Any armchair Darwinist could tell you the lizards' horns probably evolved to protect it from predators. Sure enough, if you pick up a horned lizard, it jerks its head backwards in defense. And no doubt longer horns would draw more blood. Here was a chance to put that hypothesis to the test.
Young collected 29 skulls and measured the length of their horns compared to their skulls, to control for age differences. He did the same for 155 live lizards. The horns of the living lizards were about 10% longer--by almost a millimeter, on average--than those of the dead ones. Statistical analyses suggest that natural selection could have brought about this change within 20 to 30 lizard generations, the team reports in the 2 April issue of Science.
"The result is a nice corroboration of the adaptive significance of a trait," says environmental biologist Kelly Zamudio at Cornell University in Ithaca, New York. "They were able to quantify something that seems like it should make sense intuitively."