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The Adaptations of Narwhals

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For centuries, the narwhal has been shrouded in mystery and mythology. Dating back to the Middle Ages, teeth of the "sea unicorn" were coveted for alleged medicinal powers. Today, indigenous cultures depend on the animals for their livelihoods. But comparatively little is known of the biology of the beluga's splotchy and elusive cousin. What is clear is that the narwhal, like no other mammal, has uniquely evolved to live in the dark and icy depths of the high arctic.

Basking in an Icy Realm

With 3 to 4 inches of blubber, narwhals are uniquely adapted to the extreme cold of year-round arctic living. Narwhals evolved during the late Pleistocene at roughly the time polar bears diverged from brown bears. During the last glaciation 50,000 years ago, narwhals followed the ice cover as far south as England. As the ice retreated, they followed it northward to their current range of Greenland and northeastern Canada. Like other Arctic-dwelling whales such as the bowhead and their cousin, the beluga, narwhals are about 50 percent fat. Other whales are only 20 percent to 30 percent. Birthed in icy waters, narwhal calves are born nearly one-third the size of their 12-foot long, 2,000-pound parents. While remote and harsh, the narwhals' icy realm provides protection from predatory killer whales, who are unable to navigate their dorsal fins in the dense ice pack, and gives the narwhals nearly exclusive access to bottom-dwelling prey.

Transmitter-Breaking Depths

For more than a decade, scientists have utilized a variety of electronic sensors to study narwhal distribution, migration routes and diving behavior. A pioneer of narwhal tagging techniques, Mads Peter Heide-Jorgensen, lost many of his early transmitters when whales exceeded the devices' depth limits, breaking them. At extreme depths -- more than a mile deep -- that narwhals gorge on their preferred foods of Greenland halibut, Arctic cod and squid. To withstand the pressure, often exceeding 2,200 pounds per square inch, narwhals have evolved flexible and compressible rib cages that can be squeezed as water pressure increases. Their muscles contain an enormous concentration of oxygen-carrying myoglobin, one of the highest levels measured for a marine mammal and nearly eight times the concentration in terrestrial animals. It's an important advantage for prolonged dives and endurance swimming: They can swim longer than 20 minutes without a breath.

Swimming Belly Up

Advances in non-invasive tracking technologies have provided scientists with additional insights into the movements of individual narwhals, including diving and feeding behavior. In a 2007 study, scientists outfitted five free-ranging narwhals with underwater camera pods and/or digital archival tags. Data recorded from the two devices indicated that narwhals spent roughly 12 percent of their time along the bottom of the sea floor. And when on the bottom, the animals were swimming upside down nearly 80 percent of the time. Dietz and his colleagues hypothesize that such behavior may be an adaptation to protecting the animal's lower jaw, which is hollow and thin-boned, probably used for sound reception, and may be facilitated by the animals lack of dorsal fin. Swimming upside down may improve their sonic ranging.

The Elephant in the Room

No discussion of narwhals would be complete without addressing the giant tusks that erupt from some of the animals' upper jaws. During the Middle Ages, they were pedaled as unicorn horn allegedly capable of curing diseases ranging from the plague to rabies. It is in fact a giant, spiraled tooth, filled with dental pulp and nerves, and often covered with algae and sea lice. Most juvenile and adult male narwhals have one, some have two; but only 3 percent of females have a tusk. How or why this feature evolved is unclear. It may serve for foraging, as a weapon or an ice pick; but since the vast majority of females survive without one, the tusk's function must be nonessential. In fact ,most believe it's a secondary sex characteristic -- similar to the antlers on elk -- used by males for establishing dominance hierarchies.

Looking Ahead

In a 2008 special supplement, the Ecological Society of America published analyses by some of the world’s leading researchers on Arctic marine mammals and climate change. Of the seven mammals examined, narwhals -- along with the polar bears and hooded seals -- appear most vulnerable to climate change. As a species uniquely adapted to spending half their lives in dense ice, they may not be flexible enough to survive a warming climate.