"YOU BOND TO THE TORTOISES YOU FIND." On the edge of Little Shipp Wash in west-central Arizona, the Sonoran Desert is settling in for the night. But the people inside a small trailer ignore the rattling call of a frog in a nearby acacia tree, the scent of moss and wet sand rising up from the arroyo, the twitching of the mesquites in the cool May breeze.

In the kitchen of the mobile home, veterinarian Mark Trueblood and biologist Vanessa Dickinson hover over a desert tortoise, which is balanced on an old coffee can atop a table. The eleven-pound, twenty-year-old male with the no-nonsense name of "Tortoise 500" has a little radio transmitter glued to the top of his hard gray shell. The air is ripe with the vinegary smell of the reptile and the stench of rubber gloves--and worry. Tortoise 500 is futilely kicking while the two humans treat a ragged, bloody gash in one of his scaly legs.

Earlier in the day, Tortoise 500 was accidentally stabbed with a snake hook during an Arizona Department of Game and Fish tortoise study in this isolated stretch of desert 100 miles northwest of Phoenix, just off the highway winding toward Kingman.

The last thing Dickinson needs is to have one of her staffers accidentally stab a desert tortoise while the team is doing trailblazing research on why tortoises in the Southwest are dying.

The guy who wounded Tortoise 500, John Patton, stands helplessly outside the trailer in his dusty Game and Fish uniform. Patton's job of fisheries assistant usually entails counting and studying fish in Arizona lakes. But he loves the desert and had persuaded his supervisor to lend him to Dickinson for a few days of tortoise work.

He didn't mean to stab Tortoise 500 with that snake hook, Patton has miserably said more than once. He had only tried to hook the tortoise's shell so he could pull it out of its rocky burrow deep within the ground. Like a child, Patton crosses his fingers as he waits.

Inside the trailer, Vanessa Dickinson, who is meticulous about her work, tries to hide her annoyance. The reptile was injured stupidly, needlessly. Earlier in the day, when Patton brought the tortoise in from the desert in a bloody pillowcase, Dickinson had controlled herself. "These things happen," she had said. And then she disappeared into the trailer and occupied herself with the routine task of converting the mobile-home kitchen into a science lab, preparing the centrifuge and the test tubes for the long night ahead.

For the most part, Dickinson says she admires her Game and Fish co-workers for what she calls "altruism"--they work long hours of overtime, without pay, because they are committed to helping creatures.

People like John Patton are part of a breed that is becoming as scarce as the desert tortoise--the Marlboro-man Arizonan who still opens doors for ladies and is more at home in the wild Sonoran Desert than on the streets of Phoenix.

"It is almost like Jack London," Dickinson says. "A hundred years ago, these men would have gone to Alaska in search of gold because they love nature. These men love the desert, and are touched by the fragility of it."

That's a partial explanation of why some of Arizona's desert tortoises are crawling around with little radio transmitters glued to their backs. Three times a year--spring, summer and fall--Dickinson and a crew of volunteers hike into five remote study areas of Arizona and southwestern Utah to gather and examine dozens of these tortoises.

Dickinson's five-year study, formally called the "Health Assessment of Desert Tortoises in Arizona and Utah," is aimed at trying to find out why some groups of desert tortoises are dying at alarming rates. It's part of a flurry of federal and state studies into the health and welfare of desert tortoises in the Southwest. (Tortoises are not to be confused with turtles. The main difference is that tortoises, unlike turtles, cannot swim.)

There are two rapidly perishing species of Southwestern tortoises, named after the deserts where they live--the Mojave and Sonoran. The Sonoran tortoises live near Little Shipp Wash and elsewhere in Arizona and northern Mexico.

The Mojave tortoises, which live mostly in the vast, partly urbanized desert east of Los Angeles, are in greater danger of extinction. Dickinson's study overlaps into both species--some Mojave tortoises live in the Arizona Strip, north of the Grand Canyon.

It turns out that Dickinson spends much of her time studying healthy tortoises. One method of unraveling the mystery of dying tortoises is for scientists to find out everything they can about exactly what makes a healthy tortoise. For now, scientists aren't sure.

"I FOUND 500," says Vanessa Dickinson. "You bond to the tortoises you find." She makes his number sound like a nickname, but that's not surprising. Tortoise 500 happens to be one of her very favorites. And Dickinson has had many to choose from; she has spent the past year immersed in tortoises. Thirty years old, with a master's degree in biology, Dickinson has conducted such complex and important projects as health studies of timberwolves in the Midwest. Dickinson earns $25,000 a year from Game and Fish--she could make more working as an environmental biologist for, say, an oil company.

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Terry Greene