Selection in action: attacks favor spike length for lizards.
A quirk in a bird's hunting behavior has given scientists a rare chance to measure an evolutionary force in action in the wild.
When a loggerhead loggerhead: see sea turtle. shrike catches a lizard, the Lizard, The, peninsula, Cornwall, SW England. Its southern extremity (the southernmost point of Great Britain) is called Lizard Point or Lizard Head. The coast has colored serpentine rocks, small coves and bays, wave-hollowed caves, islets (e.g. bird often impales it on a thorn or a spur of barbed wire barbed wire, wire composed of two zinc-coated steel strands twisted together and having barbs spaced regularly along them. The need for barbed wire arose in the 19th cent. and then leaves the carcass hanging, explains evolutionary biologist Edmund D. Brodie III of Indiana University Indiana University, main campus at Bloomington; state supported; coeducational; chartered 1820 as a seminary, opened 1824. It became a college in 1828 and a university in 1838. The medical center (run jointly with Purdue Univ. 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 The University of North Carolina at Chapel Hill is a public, coeducational, research university located in Chapel Hill, North Carolina, United States. Also known as The University of North Carolina, Carolina, North Carolina, or simply UNC . "In 99 percent of all the studies, you don't know Don't know (DK, DKed)
"Don't know the trade." A Street expression used whenever one party lacks knowledge of a trade or receives conflicting instructions from the other party. where the bodies are buried."
Kevin V. Young of Utah State University Utah State University, mainly at Logan; coeducational; land-grant and state supported; chartered 1888, opened 1890. It publishes Utah Science, Western Historical Quarterly, and Western American Literary Journal. 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 Cornell University, mainly at Ithaca, N.Y.; with land-grant, state, and private support; coeducational; chartered 1865, opened 1868. It was named for Ezra Cornell, who donated $500,000 and a tract of land. With the help of state senator Andrew D. . "The reason that the new study is nice is that it actually demonstrates something we all believe."