Selection in action: attacks favor spike length for lizards.
When a loggerhead shrike catches a lizard, the bird often impales it on a thorn or a spur of barbed wire and then leaves the carcass hanging, explains evolutionary biologist Edmund D. Brodie III of Indiana University in Bloomington. He and his colleagues compared the length of the horns on dangling remains of horned lizards with horn lengths of lizards still alive. The living lizards typically had slightly longer horns, the researchers report in the April 2 Science.
When studying natural selection, biologists can seldom directly measure the animals that die and that survive. It's "a really creative idea" to use bird behavior, says evolutionary biologist Joel G. Kingsolver of the University of North Carolina at Chapel Hill. "In 99 percent of all the studies, you don't know where the bodies are buried."
Kevin V. Young of Utah State University in Logan, a coauthor of the new study, monitored the population of the flat-tailed horned lizard (Phrynosoma mcalli) in the desert near Yuma, Ariz.
This lizard's multiple pairs of horns, facing backward and sideways from its skull, are "as hard and as sharp as you think they are" says Brodie. When a predator pounces to grab the lizard's body, the reptile rears back its head to stab the attacker.
In Yuma, that attacker is often a shrike, a robin-size bird that hunts prey such as mice and the some-12-centimeter-long lizards. The birds are "quite formidable beasts," says Brodie, and can triumph over lizards despite the horns.
Young measured two kinds of horns on each of 29 dead, speared lizards and 155 live ones collected from the same area. He, Brodie, and their coauthor Edmund D. Brodie Jr. adjusted the data to account for size, and therefore age, differences. The team found that, on average, the living lizards had longer horns than the dead ones. The difference was tiny--2 millimeters for one horn pair and 1 millimeter for another. Brodie points out that this difference represents 10 percent of the horn length.
That's enough for natural selection, he says. The team calculated that the horns' length, under the pressure of bird attacks, could change in as few as 21 generations. Brodie cautions that the research shows only a current selection pressure, not what might have driven earlier evolution.
"There are a lot of studies showing selection in the laboratory but far fewer in the field," comments salamander-evolution specialist Kelly Zamudio of Cornell University. "The reason that the new study is nice is that it actually demonstrates something we all believe."