“Big fleas have little fleas upon their backs to bite them,” the old saw says. That may not be true, but some lice can latch on to flies to ride from one species of bird to another, a new study shows. The results explain why some parasites evolve in concert with their hosts, whereas those that can get around more easily evolve more independently.
Pigeons and doves are plagued by two types of lice. Body lice live on the abdomens of the birds, where they feast on downy, insulating feathers. Wing lice dine on the same fluffy feathers but spend most of their time on the wings or tail, where they hide in between barbs on the birds’ large flight feathers. Most species of body lice infest only one species of bird host, whereas each wing lice species generally infests several species of bird. Body lice’s fidelity to their hosts reveals itself in the way the bugs and birds have evolved: Genetic tests show that the evolutionary trees of body lice and their bird hosts often branch at similar positions, implying the two animals’ close relationship influenced when they split into different species. In contrast, the evolutionary trees of wing lice and their hosts poorly match.
But why are body lice so faithful to one species and wing lice so promiscuous? It’s not that the former can’t play the field. Previous work has shown that body and wing lice thrive equally well when scientists placed them on novel host species.
Christopher Harbison and Dale Clayton, evolutionary biologists at the University of Utah in Salt Lake City, wondered whether another bug, the parasitic hippoboscid fly, could explain how body lice get around more. Scientists had known for decades that wing lice occasionally attach themselves to the hippoboscid fly, an insect that sips blood from many different species of birds. Tests have also shown that wing lice can cling to the flies much better than their body lice counterparts. The body lice’s short legs make them poorly adapted to riding the flies, whereas wing lice sport long legs that allow for a good, strong grip, Harbison says. Researchers hypothesized that maybe wing lice more regularly swap host species because they are more adept hitchhikers.
To test this, Harbison and Clayton placed pigeons infested with body and wing lice in sheds together with lice-free pigeons and doves. Plexiglas panels prevented the birds from making physical contact, but one of the sheds also housed flies that could move freely between birds. Every 2 weeks the researchers examined the birds for the sesame-seed-sized lice. The experiments were run over the course of 3 years. “Counting over 50,000 lice took a while,” says Harbison, who is now at Siena College in Loudonville, New York. But the effort bore fruit. In the presence of flies, wing lice travel between host species, whereas body lice do not, the researchers report online today in the Proceedings of the National Academy of Sciences.
“This work raises the bar” for using present-day species interactions to convincingly explain evolutionary patterns that emerged millions of years in the past, says Craig Benkman, a biologist at the University of Wyoming in Laramie. The team used simple, clever experiments to complete the difficult task of linking current processes to evolutionary change, agrees Noah Whiteman, an evolutionary biologist at the University of Arizona in Tucson: “It’s what evolutionary ecologists try to do but rarely accomplish.”