Even lice are specialized.

Utah scientists have discovered why feather lice that infect birds are the size they are, with large lice on big birds and little species on smaller avian hosts.

The study by Dale H. Clayton, associate professor of biology, and biology Ph.D. student Sarah E. Bush appears in the latest edition of the Proceedings of the National Academy of Sciences. It throws light on the defenses animals have against parasites.

The lice in question are not the blood-suckers that plague some animals. These eat only feathers and debris like dead skin.

The mysteries Clayton and Bush set out to solve are why do so many different species of lice exist and why are they different?

"This group of parasites is among the most host-specific parasites in the world," Clayton told the Deseret Morning News in a

telephone interview. In many cases, "different species of birds have their own species of lice."

Only a third or fewer of the lice species studied were able to infest more than one kind of bird. Even then, the non-host-specific lice could only switch to birds of similar size.

Generally, feather lice do not travel from species to species. They often are not able to survive on a different kind of bird.

"There's a common ancestor for all those birds and all the lice on those birds," he said. According to Clayton, as birds evolved, the lice they carried changed, too.

If you compare the size of their lice to the size of the bird species, Clayton said, "there's a very strong correlation. . . . Big lice are found on big birds, small lice on small birds."

But the reason for this is not obvious. Lice are tiny at their largest. It's not as if a herd of them would be any harder for a little bird to lift than an equal number of smaller lice.

The scientists examined three possible causes: that small lice were better suited to cling to small birds and large lice to large birds; that they were able to feed better on feathers of a matching size; or that a size match meant they were better able to escape from the birds' preening.

Clayton and Bush conducted experiments to see whether lice needed to hang onto feathers of particular sizes. With lice transplanted from different size birds, they "flew pigeons and doves like kites" using fishing line, says a U. press release.

They also taped lice-infected feathers to the blades of a high-speed fan. The feathers twirled at 50 miles an hour in trials lasting from 20 minutes to several hours.

Lice were able to cling to feathers regardless of feather size. "Probably the most surprising result of the study is it's not a problem for them," he said.

Was the size difference due to feeding strategy?

"It's long been thought that lice on the wrong-size feathers can't feed on the feathers," Clayton said. The idea was the insects' mouth parts were the wrong size. "That turns out not to be true." Clayton and Bush discovered that the ability of lice to eat did not depend on the bird's size.

That left defense against bird preening. When a bird preens, it finds lice on its feathers and crushes them in its beak.

When lice were moved from their normal hosts to birds of different sizes, they were not well adapted to hide, and they were more likely to be cracked in the bird's beak.

"Host defense is probably limiting the ability of all sorts of parasites to move between species," Bush said.

"We were actually able to nail down . . . the factor that prevents lice from moving to new host species. It was nice to have such a clean result."

E-MAIL: bau@desnews.com