Dr. Biek, now at Emory, wanted to study how cougar populations in the northern Rockies recovered in recent decades after being nearly wiped out in the first part of the 20th century. One way to do this is to study genetic differences for indications as to whether cougars survived as isolated populations, say, or as one population that spread.

But genetic change in an animal like the cougar comes slowly, so slowly that over 100 years there would be few if any differences to study.

Enter the virus, a feline immunodeficiency virus that is easily passed from cougar to cougar (but does them no apparent harm). Viruses can mutate practically in the blink of an eye, so genetic analysis of the virus from different cougars ought to provide useful information about the recent population dynamics of the animals.

The researchers analyzed virus samples from 98 cougars throughout the northern Rockies and found that they separated into eight lineages, four in a northern group and four in a southern. In general, the lineages were geographically distinct. The findings were reported in Science.

Analysis of the mutation rate in the virus showed that the lineages probably began to evolve separately within the last 20 to 80 years, indicating that the cougars' recovery probably arose from several isolated surviving populations.

Good News for Vultures

For more than a decade, three species of vultures on the Indian subcontinent have been dying in hordes, poisoned when they ate dead livestock that had been given the painkiller diclofenac.

To save these birds from imminent extinction, conservationists have called for a ban on the use of diclofenac. Last year, India agreed to ban the drug eventually. But one key to phasing it out has been finding an alternative that is effective on cattle but safe for vultures.