The Peanut Butter Solution

The air-conditioning was blasting in a white van zooming across Colorado in late September, despite the fall chill. The driver was bundled up for his four-hour drive to Prowers County in the southeastern part of the state. In the back, cat carriers were stacked one on top of the other, but they weren’t carrying cats.

The frigid conditions ensured the safety of the passengers, some 35 black-footed ferrets. “You really have to be careful not to get them too hot,” says John Hughes, a biologist with the U.S. Fish and Wildlife Service. Ferrets, it turns out, get heatstroke easily.

These weasel relatives were recent graduates of the survival boot camp at the National Black-Footed Ferret Conservation Center, where Hughes works. The captive-bred creatures had spent a month there learning how to kill prairie dogs, their primary prey in the wild. Now, with their newly gained survival skills, they were ready to go free. When the driver reached his destination, a prairie dog town, he would wait until dusk and then, one by one, set them loose.

The release is part of a recovery program put in place after the Fish and Wildlife Service listed black-footed ferrets as endangered 48 years ago. The species has come a long way from the edge of extinction, but its numbers are still below 1,000. To boost the population further, scientists are always on the lookout for new reintroduction sites for these tan animals with black masks—especially areas that aren’t contaminated with the plague (yes, Yersinia pestis, that one).

The bacterium—the same one that can cause bubonic plague in humans, though it rarely does—spreads through rodent populations via fleas. Ferrets can become infected when they invade the burrows of prairie dogs or starve when there’s no prey left to eat. So, to save black-footed ferrets, you first need to save the dogs. And the best way to do that may be vaccinating prairie dogs against plague.

Dual fates

The well-being of prairie dog communities is closely intertwined with the survival of black-footed ferrets. When Americans made their way West and farmers plowed under grasslands to grow crops, prairie dogs simply dug out their dog towns in farmers’ fields instead. Finding the elaborate burrows to be a nuisance, farmers killed off the rodents by the hundreds of thousands, leaving the ferrets hungry. The also left them homeless: Not only do ferrets count on prairie dogs for 90 percent of their diet, but the predators hole up in the dogs’ burrows after they eat them.

Suitable ferret habitat plummeted to just 2 percent of its original range, and populations, which once numbered in the tens of thousands across the Midwest, crashed. The plague took care of the rest.

In the early 1900s, trading ships arriving in California brought plague-infested rats with them from Asia. The deadly bacteria crept east, wiping out prairie dog colonies, and subsequently black-footed ferrets, along the way. By the 1950s, virtually no black-footed ferrets were left. Then, in 1964, biologists found nine in South Dakota and brought them into captivity to breed them. They had no luck, and the species died out. Or so they thought. Nearly two decades later, a Wyoming rancher’s dog brought back a dead black-footed ferret. Suddenly scientists had a new population to work with.

Sadly, the plague, along with canine distemper, was clobbering the Wyoming population, too. So conservationists saved what ferrets they could and brought them inside. In all, they had 18 to work with. To preserve as much genetic diversity as they could, the biologists froze the ferrets’ sperm in liquid nitrogen, then began artificially inseminating some females. Zoos across the country are now breeding the animals, and they give some of the resulting kits to Hughes. This is where the survival school in Colorado comes in.

New hunting grounds

Young ferrets learn to hunt mostly by instinct; they chase prey and clamp down on their throats, suffocating them. To make sure their intuition is right, however, biologists put them in a covered pen and introduce one prairie dog a week. When they’re adept at hunting the dogs, the ferrets are ready for the wild. These preconditioned ferrets survive at a rate 10 times higher than those released without training. Hughes’s team releases them only into plague-free areas with thriving prairie dog populations.

Until recently, all release sites have been on federal lands within 12 states, but last year the Fish and Wildlife Service began paying willing ranchers $15 per acre to monitor and allow prairie dogs and ferrets to take up residence on their property. This has given the scientists a lot more room to work with, says Hughes, since most prairie dog habitat is on private land.

Access to private lands has helped the effort immensely, but plague can strike at any time. And when it does, it can kill up to 90 percent of a prairie dog colony within three weeks. Even if ferrets are vaccinated against the bacterium, as all captive-born ferrets are, they’ll likely starve if the dog colony collapses.

To prevent new plague outbreaks, managers dust dog towns with flea-killing pesticides. The poison works for a while, but eventually the insects develop resistance to the chemicals.

Smooth or crunchy?

Tonie Rocke, a biologist for the USGS National Wildlife Health Center in Madison, Wisconsin, decided on a different approach. She wondered: Why be reactive when you can be preventive? Rocke has been working for the past 15 years on how to inoculate prairie dogs against the plague. “They don’t exactly queue up for a shot,” she says. So Rocke created an oral vaccine, and she’s midway through a two-year field study to test its efficacy.

How do you get a prairie dog to take its medicine? Make it taste like peanut butter.

At 60 dog towns over the summer, Rocke and her team dropped lots of small cubes of hardened peanut butter, dyed fuchsia so the researchers could later determine which rodents ate the snack. Some colonies got cubes containing the vaccine, while others got just a peanut butter treat. A month later, the biologists monitored the colonies for plague and caught as many prairie dogs as possible. They plucked out a few hairs and whiskers and collected some of their fleas in a tube. Back at the lab, the team analyzed the hair and whiskers under a microscope, looking for the telltale fuchsia markings. Though they’re still crunching the numbers, earlier lab tests showed that up to 90 percent of the vaccinated ferrets exposed to plague have survived. If all continues to go well, biologists may inoculate even more wild prairie dog colonies against the disease next summer.

Together, the uptick in reintroduction sites and the possibility of a prairie dog vaccine could be just what black-footed ferrets need to wriggle out of the clutches of extinction. Hughes predicts that within five years, wild populations could be robust enough to seed new ones, if all the pieces fall into place. “It’s genetics, wildlife disease, people management,” he says. “It takes not a village but a city to run ferret recovery.” And cat carriers. Lots of cat carriers.