The puma nearly disappeared from the western parts of the U.S. and Canada. Also known as the cougar or mountain lion, North America's biggest cat was nearly wiped out by ranchers and hunters in the early part of the 20th century. Since the 1960s, however, cougar populations have rebounded under new protective laws. To figure out where the resurgent cats have come from, scientists have turned to, of all things, a virus.

Biologist Roman Biek and his colleagues at the University of Montana took blood samples from 352 captured or killed cougars in Montana and Wyoming, and in Canada's Alberta and British Columbia provinces. They then analyzed the genetic information. Despite the samples having come from animals separated by more than 600 miles, the team found little to distinguish the various cats and thus little to reveal the secrets of their migration history.

But the researchers also tested for traces of feline immunodeficiency virus (FIV)--an apparently benign infectious agent passed between cats in close contact--and found that 28 percent of the cougars carried some form of the virus. In fact, they detected eight different strains of the virus in 98 different cougars, each strain tied to specific geographic ranges. By mapping those ranges, the biologists determined how the cougars had spread out to reclaim their ancestral range. They also ascertained that the last common ancestor of the eight viruses existed sometime between 20 and 80 years ago, establishing a strong link between the present diversity of FIV and the bust and boom of its host population. The spread of certain pathogens in humans has been shown to reflect our own species's migrations; now Biek and his colleagues have discovered that the same holds true for cougars.

The technique may prove to be widely applicable because any sufficiently understood host and parasite should reveal similar information. This could allow conservationists to get a handle on other species reacting to the stresses of human encroachment. "Natural populations and ecosystems are currently undergoing changes at unprecedented rates owing to human activities," Biek and his collaborators write in today's issue of Science. "Molecular markers with an equally short [time scale] will be of increasing value to researchers trying to understand and mitigate these changes."