That's one weird tooth: and other bulletins on the elusive narwhal.

What Martin Nweeia noticed first when he encountered narwhals, he says, was the sound. In May 2000, as spring was just reaching Baffin Island in the Canadian Arctic, a famed local hunter took Nweeia out on the ice searching the open water for those tusk-bearing, high-Arctic whales. "I was sitting on a bucket out on the ice doing polar bear watch" he says. At that time of year, daylight lasts around the clock, and at 3 a.m., the gray sky had orange streaks. "The water was like glass, and a light mist was rolling in," he says. "Then, I heard the breathing."

Nweeia soon traced the sound to the dark bodies of at least 10 narwhals that had risen to the surface out in the open water. Their heavy, low-frequency, methodical breaths carried through the night as if "they were breathing in my ear," Nweeia says. It took considerable discipline, he says, to break the spell of their arrival and wake the hunter to go after them.

Many people have searched for narwhals, but Nweeia may be the only Connecticut dentist to have done so. He practices in that state and teaches at Harvard School of Dental Medicine in Boston. And what's there for a dentist not to love about an animal with a 3-meterlong tooth--and the only spiraling tooth ever reported?

Since that first encounter in 2000, Nweeia has spent four field seasons in the Arctic observing narwhals, and he's worked with diverse experts to examine the creatures' tooth structure iu detail. He and his collaborators' most recent finding suggests that the tusk is "exquisitely sensitive," says Nweeia. He proposes that it might sense pressure, temperature, and even changes in water chemistry. General ideas about tooth function are going to have to do some growing to make sense of this new twist.

The narwhal tooth is what collectors in bygone centuries got when they bought a unicorn horn. And narwhals might as well be unicorns, as far as knowledge about their biology goes. Famously hard to study, they don't tolerate captivity, and they spend 6 months of the year in forbidding offshore ice fields where even Inuit hunters seldom venture.

Nweeia is one of several researchers now focusing on narwhals. Advances in tagging whales with instruments that send data to satellites are finally giving scientists a picture of narwhals' migration and hunting habits. Such advances are also revealing these whales' vulnerabilities to humanity's effects on the planet.

EXQUISITE CORPSES Narwhals, which are medium-size whales, typically spend their lives within the Arctic Circle. The world population numbers some 50,000 individuals, many of them living between Canada's Baffin Island and Greenland.

Catching sight of a surfacing narwhal, says whale biologist Kristin Laidre of the Greenland Institute for Natural Resources in Nuuk, usually means seeing a "black blob"--just the rounded back. The whales have rounded heads and a roughly sausage-shaped profile that tapers at the tail end. Females grow to about 4 meters in length. Males tend to grow about half a meter longer, excluding the length of their tusks. The animals can weigh as much as 1.5 tons.

Up close, live narwhals remind Laidre of the wet, smooth inner tubes in which people float down rivers. Narwhal skin is "taut, very slippery," she says. Adults develop a scattering of gray-and-white patches over their dark backs. Whale lore cites the ghoulish mottling of pale grays and blues to explain why narwhals are sometimes called "corpse whales." Laidre, though, says that she finds the skin coloring "really quite beautiful."

To a dentist, the narwhal poses serious questions. "You can't find a more asymmetric tooth," Nweeia says.

Narwhals have only two fully developed teeth, and often just the left one is visible. In males and a few females, that tooth grows into the remarkable tusk. It often becomes more than half as long as the rest of the narwhal's body. The other tooth usually stays embedded within the bone.

The elongating tooth spirals to the left, from a whale's-eye view. In the rare cases of double-tusked narwhals, perhaps 1 out of 500 males, the right tooth doesn't grow in a mirror image. Disdaining the symmetry of other animals' teeth, the narwhal grows another left-handed spiral.

In the years after that first encounter with the animals, Nweeia enlisted several scientific collaborators and collected samples of tusks. He went to much trouble to keep them fresh. For example, Frederick Eichmiller, who works on dental materials at the U.S. National Institute of Standards and Technology in Gaithersburg, Md., advised that tusk samples be preserved in an extremely dilute solution of sodium azide. That's the stuff that explodes to puff out a car's air bag.

Nweeia therefore had to get some new certifications unusual for a dentist. "I was taking courses with guys who were shipping hazardous waste," he says. Coming home from the field, he avoids flying into U.S. airports with their post-9/11 security procedures. He flies from the Arctic to southern Canada and then tenderly drives his samples across the border.

In December 2005, Nweeia's team released the first findings from its work. The narwhal tusk reverses the arrangement of material in the teeth of most animals, the researchers reported in San Diego at the 16th Biennial Conference on the Biology of Marine Mammals. They found that the narwhal tusk has its harder material, dentin with extra high mineral content, near the core and softer layers of dentin toward the outside.

Also, a scanning electron microscope revealed abundant tiny tubules that run from nerve endings near the tooth's core all the way to its surface. Similar tubules lie inside human teeth, but tooth enamel normally protects them from the outside world. When some dental mishap, such as receding gum tissue, exposes these tubules, the human tooth gets sensitive, making a person wince at the chill of a soda.

Narwhal tusks have a lot of tubules, each about a micron wide, Nweeia reports. Scaling up from the samples that the researchers examined, he estimates that a two-and-a-half meter stretch of narwhal tusk would have more than 10 million of those small structures.

The tubules probably create some kind of sensitivity in narwhals, says Nweeia. "Why would you want that in the Arctic and in cold water, no less?" he asks.

The function of narwhal tusks has inspired plenty of speculation over the years. Some people have proposed that narwhals skewer fish, poke holes in the ice, or jab at the ocean bottom to scare up prey. In recent years, whale biologists have been inclined to link the tusk in one way or another to males finding a mate. They consider the tusk a product of sexual selection, along with the peacock tail. One version of this idea, says Nweeia, has male narwhals dueling with clashing tusks, in the same way that moose butt antlers.

Nweeia says that his predictions about the tooth's great sensitivity argue against the tusk as this kind of dueling tool. "When you have something so exquisite, you don't bring that on the battlefield," he says.

So, could the tusks help narwhals find fish and squid in the deep or sense when ice is closing in around them? "Then why would only adult males have them?" asks Pierre Richard, a narwhal biologist at the Canadian Fisheries and Oceans Department in Winnipeg. Females, too, need to find food and breathing holes in the ice. Richard says that he's often seen females and their young foraging separately from males.

Also, if the tusk were solely a sensor, why would a male need 2 or 3 meters of it? Randall Reeves, a marine mammal biologist in Hudson, Quebec, says, "It's sexually selected all right--you can take that to the bank." However, he notes, that doesn't mean it has to be something for rough-and-ready dueling.

Laidre says that when she's seen males confront each other at tusk point, they don't really bash around. They raise their heads and sway. "It's like a slow-motion ballet," she says. "They do touch their tusks, but they're really gentle."

Nweeia says that it's too early to get attached to explanations for the sensitive, extralong tooth. He and his group plan to look for environmental cues that trigger tusk responses.

WHALE WHEREABOUTS Many things about the narwhal remain a puzzle to scientists. Whales, in general, are hard to examine because they spend so much time underwater. And narwhals specialize in living around sea ice.

The big, Canada-Greenland populations choose a winter haven under Baffin Bay's pack ice, acres of jostling ice chunks. The 24-hour darkness of the Arctic winter and the fast-shrinking bits of open water make this a dangerous place for just about all forms of life.

When spring comes and ice along the coast melts, the narwhals swim toward land. They summer in the bays and fjords that filigree the coasts of Baffin Island and Greenland.

Biologists knew only the broad outlines of narwhals' movements until the 1990s, when scientists began tagging the whales with devices that relay information to polar-orbiting satellites. "We can sit in our offices and download data" Laidre says. "That's revolutionized the study of narwhals."

Getting the tags on the whales can't be done from an office, though. Some tags have been placed by Inuit harpooners. Researchers including Laidre have tagged narwhals in the Greenland and Canadian summering grounds. The scientists set nets in the water where narwhals might blunder into them. Because an entangled whale could drown if the researchers didn't pull it to the surface soon enough, team members on shore take shifts around the clock watching the inflatable white buoys on top of the net.

When a watcher sees the buoys slip underwater, he or she shouts for action. "You have people stumbling out of their sleeping bags," says Laidre. The crew pulls the panicky whale to the surface. The animal usually stops struggling when it emerges from the water and can breathe. The team members wire or pin tracking devices to the whale's blubber. Richard calls them "body jewelry for whales."

During the past 5 years, Laidre and others have published analyses of their satellite-transmitted data, sometimes combined with satellite measurements of sea ice coverage. Populations of narwhals seem to migrate along the same corridors each year. Each group travels from a specific summering haunt to a particular winter area in Baffin Bay.

Narwhals arrive at their wintering spots when ice covers only some 40 percent of the area, Laidre and polar biologist Mads Peter Heide-Jorgensen of the Greenland Institute of Natural Resources reported in the February 2005 Biological Conservation. By the end of March, as little as 0.5 percent of the area is open water.

The narwhal tags record depths of diving as well as positions. The deepest diver recorded so far reached almost 1,800 meters. Although sperm whales and beaked whales may go slightly deeper, narwhals are among the deepest-diving mammals. "I don't think people had imagined that," says Laidre.

In another part of the research, she put out the word among traditional hunters that she'd appreciate them tucking narwhal stomachs into their freezers until she could pick them up. Laidre examined 121 of these organs. Depending on the animal's size and recent dinners, a narwhal's stomach can weigh between 2 kilograms and 30 kg, she says.

The stomachs' contents and other data are finally telling their story, she says. In the January 2005 Marine Mammal Science, she and Heide-Jorgensen concluded that narwhals reverse the usual whale lifestyle. It's winter, rather than summer, that seems to bring abundant dining, particularly on Greenland halibut, according to the stomach analyses. Halibut hug the depths, where temperatures hover just a few degrees above freezing. Winter-caught narwhals typically have dined recently, but the stamachs of many animals caught in summer are empty.

NARWHAL WORRIES Remote as narwhal territory looks on a map, it's all too close to pollution. As air currents sweep up to the pole, contaminants tend to waft down to Earth there. Among the most recent pollutants to attract attention are polybrominated diphenyl ethers, or PBDEs (SN: 3/26/05, p. 206).

Toxicologist Hans Wolkers of the Norwegian Polar Institute in Tromso reports on a team that was studying marine mammals off the coast of Norway in the late 1990s when several narwhals showed up. The team called a government official from shipboard to get permission to collect narwhal tissue samples.

The researchers then restrained three of the animals, much as the taggers do. Using a retrofitted electric drill, the team bored out a little plug of narwhal blubber from each whale.

Tests found about 170 parts per billion of PBDEs in fat, Wolkers and his colleagues report in the January 2006 Archives of Environmental Contamination and Toxicology. Such concentrations rank among the highest measured in mammals in the area. These are the first PBDEs recorded in narwhals, Wolkers says. "Whales are very poor at breaking down these contaminants," he adds.

Inuit have hunted narwhals for centuries, but recent declines in the populations have inspired Greenland to impose its first quota ever on narwhal hunting by the native people.

Climate studies find a trend toward increased variability in weather conditions in the Arctic, and narwhals may be especially sensitive to such unpredictability. They don't seem to make adjustments in their migrations to avoid hunters or to reach more-abundant fishing grounds.

Laidre is concerned that under some conditions, the ice may freeze over so that the narwhals can't breathe or the fish that they eat may become less abundant. If climate warms, sub-Arctic species may move north and compete with narwhals for food, and killer whales, which attack narwhals, might move into previously ice-covered waters.

To better predict what's in store for these unicorns of the sea, scientists need to know more about their biology, especially what's going on in the winter darkness under the ice. Laidre reports that she and her colleagues have gotten a grant for a winter study and are now figuring out the logistics.

"Call me in 6 months," she says.