The sun has just set on the Shirley Basin, that vast prairie between Casper and Medicine Bow. Martin Grenier drives his Game and Fish Department truck at a snail's pace over a bumpy two-track dirt road. He reaches to the dashboard to stop a can of Red Bull from rattling in its cupholder and clicks on a spotlight that's mounted on top of the truck. The high-powered light pierces across the prairie, illuminating sagebrush and small prairie dog mounds. He rotates the light slowly back and forth over the prairie dog colony. The first hour of his nine-hour all-night shift has begun.

It's not easy trying to catch a glimpse of the most endangered mammal in North America, especially since it only comes out at night. But these days the Shirley Basin is the best place in the world to look for them.

Once thought to be extinct until a remnant population was discovered near Meteetsee in 1981, black-footed ferrets now number in the hundreds on the prairie here alongside the two lonely roads that are Highways 77 and 487. The animals are all descendents of seven animals captured from the now-extinct Meteetsee group.

So far, the task for biologists has been helping the Shirley Basin ferrets stay alive and monitoring how they're doing. But with the ferrets thriving, scientists can now become more than caretakers. They can study them. Scientists of the previous century never really had that opportunity.

"The population crashed before they really had a chance to measure their population dynamics. This is really the first opportunity for us to examine a ferret population in the wild," says Grenier.

"We have very little information about what the ferret's role is in the ecosystem. We've got a population that's large enough now that we can probably start to look at things like that."

Things weren't always going so well for the ferret. It's been a long road back from the brink of extinction, and there's still a long way to go.

That's why Grenier and his crew of mammal biologists from Game and Fish are here, slowly driving these dirt roads and walking across the prairie carrying high-powered spotlights. Their month-long effort is an attempt to document how much their population has expanded outside the original study area, and to capture some of the animals to monitor their health and collect data for scientific study.

It's been a miraculous recovery, but the ferret is by no means out of the woods. An outbreak of sylvatic plague or canine distemper could wipe out an entire population. In an article Grenier co-wrote for the Aug. 10 issue of Science magazine, he recommended more widespread releases of captive ferrets, citing the Shirley Basin group as an example of how captive animals can flourish in the wild.

"The population has the potential to grow very rapidly. The success we've had here in the Shirley Basin tells us what good habitat we have here for the species," Grenier says.

Black-footed ferrets are prolific breeders and can quickly populate an area even though they live such short lives in the wild, usually only two to three years. The 2-pound, nocturnal predators rely entirely on prairie dogs for meat and on prairie-dog burrows for shelter.

The search continues

Grenier's spotlight reveals a pair of shining eyes in the distance and the truck grinds to a stop.

"Bunny," he says with a yawn, and the vehicle slowly lurches forward.

Rabbits' eyes shine red in the spotlight. Ferrets' eyes are a super-brilliant emerald green. Antelope eyes have a similar color in a spotlight, but they blink a lot and the animals tend to run away. Ferrets just stare right back, a testament to their curious nature.

Grenier and his crew search the same half-mile plots by vehicle or on foot over and over again, hoping to catch a glimpse of a ferret taking a rare peek above ground. If they do spot one, they'll set a trap in its burrow.

"They can stay underground for nine days and nights at a time. That's why they're so hard to find," he says.

After a couple hours of searching, a call comes on Grenier's radio from mammal biologist Todd Filipi. He's spotted the first ferret of the night and set a trap.

Here in the Shirley Basin, ferrets attack and eat white-tailed prairie dogs, often surprising their prey with a hard bite on the neck while the prairie dog is curled up asleep in its burrow. A ferocious, bloody wrestling match ensues between the two animals - and the ferret doesn't always win. It's fairly common for the crew to capture a ferret that's covered with battle scars.

"It's a hard way to make a living when you're preying on a creature that outweighs you two-to-one," Grenier says.

Ferrets are hunted by coyotes, foxes and great horned owls. A ferret's life is a hard, short-lived existence.

The crew has captured as many as 20 ferrets in a single night this summer. But some nights, like tonight, are slow.

"After a couple nights when it's slow like this you start seeing things. It gets monotonous," he says, rotating the truck's rooftop spotlight back and forth. Soon another call comes in.

"Got him. A juvenile male," Filipi's voice says over the crackling radio. Excited, Grenier turns the truck toward the crew's mobile laboratory a short distance away where Filipi is bringing the captured ferret. It's always a thrill when somebody on the crew catches one.

Ferret No. 095-827-333

Inside the small lab, the two biologists don surgical masks and latex gloves to protect the ferret from germs. They transfer it from the tube-shaped trap to another tube that will be fed with Isoflurane and oxygen to sedate the animal.

Once the animal is unconscious, Grenier gently removes the snoozing ferret from the tube, lays it on the table and fits a cone-shaped oxygen respirator around its head.

"Yep, juvenile male," Grenier says, estimating its age at about 90 days. The younger ones are easier to catch because there's more of them and they're more likely to take a peek out of their burrows.

"The juveniles tend to be a little more adventurous," he says.

The ferret's body bares a few open sores - the marks of some recent scuffles with prairie dogs.

Grenier measures and weighs the ferret, draws a blood sample, takes a hair sample and some DNA. He also measures the animal's teeth to determine its age, pulls off a few ticks with tweezers, gives it a shot of distemper vaccine and then a shot of antibiotics to help its wounds heal.

Finally, Grenier injects a microchip into the ferret's neck so it can be identified later - with a hand-held scanner if it's captured again or with a donut-shaped scanner placed at a burrow exit. Grenier tests the chip with a hand-held scanner that he brushes over the sleeping animal. It beeps and a number pops up on the scanner's screen: 095-827-333.

"It's like a nine-digit Social Security Number for ferrets," Grenier says.

Grenier wraps the ferret in a towel and places it in a small cage. In a few minutes black-footed ferret No. 095-827-333 starts twitching. Soon he's a little spitfire of chirping barks and hisses, bearing his sharp fangs. Only 90 days old and the aggressive little carnivore is ready for a fight.

Grenier covers the cage with a towel to calm the ferret and Filipi drives the animal back to its burrow to let it go.

The ferret had been captured on private ranchlands. Without the cooperation of landowners ferret recovery couldn't be accomplished, Grenier says. About half of the Shirley Basin ferrets are on private lands.

"A lot of the landowners here take pride in having ferrets on their land. It's neat knowing you have one of the most endangered animals in the world right there on your land," Grenier says. "The landowners here are great. They like the ferrets, they tolerate the prairie dogs. It's just a great conservation story."

Grenier climbs back into his truck, starts it up and flicks on the spotlight. Three hours down, six more to go.