ON A COOL OCTOBER DAY, we're standing at 11,500 feet on Mt. Jefferson. Range after range of Nevada mountains sprawl to the east. A herd of bighorn sheep trots by in the gray-brown valley below. It's barely noon, and we've hiked a couple of miles across a broad sagebrush valley, then nearly straight up 3,000 feet to the top of a pass. To dodge the fierce wind, I hunker down in some rubble and look back at biologist "Nifer" (a.k.a. Jennifer) Wilkening, an orange speck gathering data on the snowy ridge. Wilkening, 33, is small and willowy, a habitual smiler, and quick to laugh. For the past three years, she's hiked hundreds of miles each field season to add to scientists' understanding of global warming.

Jennifer "Nifer" Wilkening scans the landscape for evidence of elusive, Mickey Mouse–eared pikas.

Average global temperatures have risen more than one degree Fahrenheit in the past century, and almost 60 percent of the 1,600 species biologists have studied worldwide have moved northward or upward; altered their migration, hibernation, or other patterns; or, in a few cases, simply disappeared.

I've climbed this mountain to learn about the species Wilkening is studying: the pika, or "whistling hare." Next I'll drop down to the Mojave to check on desert tortoises. My plan is to look high and low, in a cold place and a hot one, observing a fast-moving mammal and a slow-moving reptile, to gather clues about wildlife's race against temperatures that are expected to rise an additional 3 to 7 degrees in this century. I think I know the story: As in the original Aesop fable, the determined tortoise has some surprising advantages over the frisky "hare." But then climate change adds some odd hops to the plot.

We are visiting just one of the 25 study sites Wilkening patrols in the Great Basin, an arid, mountainous region between the Rockies and the Sierra Nevada. Scientific archives indicate that pikas lived at each of these locations in the early 20th century. But Erik Beever, a biologist who began revisiting the sites in the mid-1990s, has found pikas gone from a third of them. Wilkening's research shows that the empty sites are considerably hotter than the inhabited ones--one of several pieces of evidence suggesting a link between pika disappearance and global warming.

At our first stop on Mt. Jefferson, Wilkening reads from 1933 field notes describing a pika sighting at a talus, or rock, pile facing west at just about our latitude and longitude. From under a marked rock she pulls a device the size of a round camera battery. For many months, this data logger has been recording the temperature every four hours. Plucking the logger from the crevice as if it were a jewel, she places it in a baggie and installs a new one. Then she searches for a hay pile and listens. Pikas aren't using this talus pile, she decides. But the day is young: We have eight more sites to visit.

An hour or so later, Wilkening hears bleating at 9,900 feet, from one of the lower, warmer locations. There's also a massive hay pile. I notice a sprig of lupine, a plant that is poisonous to livestock. An Into the Wild mistake? Nope. What can kill a sheep doesn't bother the provisioning pika. Also included are other flowers, grass, sagebrush, juniper, and ephedra--treasures that could make a pika fiercely territorial. "When we hear them calling, they seem to be saying, 'Step away from that hay pile!'" Wilkening says.

Relatives of the hare and rabbit, pikas live in talus piles they provision with plants such as sagebrush, juniper, and ephedra.

Up on the windy pass at midday, Wilkening hears another telltale "enk" in the rocks. Heading downhill after that, though, we spend hours visiting one barren location after another. Leaning against a leafless aspen, I consider the setting sun and our still-distant truck. I ask Wilkening if she couldn't put her data loggers in more-accessible places. She looks disappointed in me. "Yes, it would have been easier to put them all by the road," she says. "But it wouldn't have been good science."

By the tenth hour, we are enjoying a rosy sunset in the sagebrush flats. Wilkening says not to worry about the impending darkness--we can find the truck with her GPS. "You should be proud you made it to all the sites," she says. "Did Erik tell you about the time he took out a crew from National Geographic?"

"No."

"It did not go well."

The next morning at camp, Wilkening is up early. Wearing a ski hat and a down jacket, she taps away on her battery-powered laptop, downloading information from the data loggers.

A spate of recent articles suggesting the imminent demise of the pika is misleading, Wilkening says. But the long-term prognosis is bleak. "After only three years, I can see these guys going up the slope," she says. "Sitting on top of these mountains, I can see there's no way they can come down and go somewhere else."

Pikas may be facing similar problems elsewhere. Recent studies show they may have moved hundreds of feet upslope in Yosemite over the past 100 years. In the Rockies, whose many 14,000-foot peaks and cool northern latitudes offer the best habitat for the species in the Lower 48, the pika's status is unknown. University of Colorado biologist Chris Ray, who will begin a study in the Rockies in August, thinks she can anticipate the next plot twist. "Given the trends we are seeing in the Great Basin and the Sierra Nevada, I'd be surprised if we don't see them moving upslope here too."

So this species has become Paul Revere, warning us that consequences are coming. "You know the pika is a small part of the picture, but you start to wonder what else is being affected," Wilkening says.

I ask whether the hare or the tortoise has the best chance of winning the race against global warming.

"The tortoise," she says.

Biologist Ken Nussear with a species that has little use for the golf courses, housing tracts, and casinos that are replacing its native habitat.

A LAND-BASED RELATIVE OF THE TURTLE, the desert tortoise has evolved to withstand heat and drought. It needs rain only once a year. With a maximum shell length of 14 or 15 inches, it's small (compared with, say, a Galapagos tortoise) and can store up to a cup of water in its bladder and live off of it for many months. Instead of wasting water on urination, the tortoise can excrete its wastes in dry lumps. It can vary the date it goes into and out of hibernation by as much as 45 days. Females can lay their eggs anytime between May and mid-July. And because females can store sperm, they are able to reproduce five years or more after mating.

The adaptable tortoise makes the pika look stodgy. While pikas live only on cool mountaintops, tortoises inhabit various kinds of sites in the low and high desert. Pikas need a lot of energy in summer and winter. Tortoises, on the other hand, are masters of energy conservation. They shut down in a state of reptilian hibernation called "brumation" in the winter, never leaving their burrows. If it doesn't rain in the summer, some indulge in an extended nap then too.

But after ten years of studying the tortoises that live in the Mojave Desert, biologist Ken Nussear is not so sure they can survive their current challenges. Their range includes the fringes of Las Vegas, the fastest-growing metropolitan area in the country, with a population that shot up by 83 percent between 1990 and 2000. Scientists don't know exactly how many tortoises live in the Mojave, but on some one-square-mile sample plots, they've found recent declines of up to 85 percent since 1990. The causes are numerous: Tortoises get respiratory diseases. People collect them as pets or run over them in vehicles. Livestock steal their food. Fires and invasions of non-native plants destroy their habitat.

The main threat?

"Pfff!" Nussear snorts. "So many people."

Nussear, 42, is a researcher at the U.S. Geological Survey office just outside Las Vegas. Out in the field, roaming amid the creosote and the cactus in a loose khaki shirt and jeans, he looks as comfortable as a coyote. He grew up in a little town called Apache Junction near Phoenix.

How did he get into biology?

"I was curious about how things work," he says.

Why tortoises?

"They needed my help."

Nussear gets lost as he drives down the freeway. Las Vegas is growing so fast, "they keep changing the exits," he says. I ask him where tortoises like to live. He points to the Strip's gleaming casinos. "That used to be tortoise habitat."

Nussear takes me to a site where tortoises are doing well 30 miles south of the city. A major new airport will be built there soon. Roughly the same distance to the north, bulldozers are razing tortoise country to build a 67-square-mile luxury golf community. Even a new solar power plant near Boulder City, Nevada, sacrificed tortoise plots for megawatts. The Mojave today is not so much a haven for wild creatures as it is a canvas for Americans' dreams.

The Mojave gets less than four inches of rain a year. But its undeveloped stretches are far from a sandy expanse of nothing. Leafy creosote bushes sprawl over one of our stops. Freshened by fall rains, they give the desert a green glow. Nearly every place we scuff a boot, Nussear shows me signs of life: half-moon-shaped tortoise burrows, round kangaroo-rat holes, a bright yellow desert marigold--perfect for a tortoise's lunch.

When a few drops of rain fall from the moody sky, Nussear says, "It's your lucky day."

Tortoises spend up to 95 percent of their time in their burrows. The slightest sprinkle, however, brings them out to drink. The day's warmth is a good sign too. Tortoises are cold-blooded; their body heat rises and falls with the temperature of their surroundings. They're most likely to move, eat, drink, or mate when their temperature is between 80 and 90 degrees.

We don't see any tortoises at our first stop, but Nussear isn't worried. We drive a bit farther to an area cremated by a massive wildfire in 2005. The remaining shrubbery, including skeletal Joshua trees, is still almost completely black. Nussear has attached a radio transmitter to an old male tortoise that lives on the edge of this burn. With a receiver faintly beeping, we ramble across the charred valley. I'm all eyes.

After 15 minutes or so, the beeping grows stronger. I spot a tortoise-like ... rock. We stand still for a couple of minutes, then Nussear smiles and points. Inches from my boot is a dignified, frying-pan-size tortoise, just outside a burrow and so covered with dust that its shell is brown. It follows our motions impassively with one dark eye. If it saw us as a threat, it would pull its head inside its shell or move into the burrow, Nussear says. "It can move faster than you think."

Here on the edge of the old fire, shrubs still provide the tortoise with shade and protection from predators. But there are no such amenities inside the 680-square-mile burn. Old ecology books will tell you that deserts don't have such massive fires; there's not enough fuel. In the past 30 years, however, an invasion of non-native grasses has changed the rules.

Experts differ on whether the increase in these fires is related to global warming. At this stage, they are not even sure whether warming will cause an increase or a decrease in Mojave precipitation. But they do know that, one way or another, climate change will pose challenges. Temperature determines how long it takes tortoise eggs to hatch, for instance, as well as the sex ratios of the young. (Higher temperatures produce mostly females, lower temperatures mostly males.) Tortoises may be one of the species most resistant to climate change, Nussear says. "But," he adds, "rubber bands can stretch pretty far until they don't."

TORTOISE HUGGERS HAVE A CLEAR TO-DO LIST. Secure the species' existing habitat in pieces large enough to support genetically diverse populations. Reduce the fire danger by controlling the spread of non-native grasses. Rigorously enforce laws that protect wildlife and habitat. Reduce livestock grazing and other human intrusions. These measures alone won't save the tortoise from a seriously warming world, but they will give it a fighting chance.

Pika lovers' list is shorter. Pikas' powers of adaptability are minimal. Most of their talus piles are already protected on publicly owned land. "We could relocate them, move them to other mountains," Wilkening says. "But if warming gets bad enough, most of the ranges that could support pikas will already have populations."

Scientists say that even a rise of 3 degrees--on the low side of the range expected over the next century--would put 20 to 30 percent of all the earth's plants and animals at risk.

But there's another scenario. The world could reduce its green house-gas emissions enough to give these species a chance. That's no small task, but it helps to remember this: In the original tortoise and hare tale, perseverance prevailed.

Joan Hamilton, a former editor in chief of Sierra, is a freelance writer based in Berkeley, California.

Photos, from top: Rachel Franklin/Alamy, Nathan Welton/Dreamtime Images, Joan Hamilton, Ed Foster; used with permission.