Imagine you are a narwhal. You are cruising through chilly Arctic water when you sense a threat. Most animals, when alarmed, either lash out at their attacker or flee. You, narwhal – the unicorn of the sea – aren't most animals.
You won't fight. Yes, you have a long tusk growing out of your face. Your tusk, a canine tooth that stretches into a spiral five feet or longer, isn't much of a weapon. Narwhal tusks are sensory organs filled with nerves, not dull spears for jabbing at predators or fending off rivals. If an orca swam nearby, you'd slink into deeper water or twist beneath ice floes where the larger whales cannot follow.
This threat is unusual. It's noisy and unfamiliar. Instead of the usual flight response, your body reacts oddly.
You dive, flipping your flippers as fast as they can go. Meanwhile, your heart rate plummets. It's as if your heart wants you to freeze in place, similar to the way young rabbits and deer play possum. (Biologists, borrowing from Greek, call this acting-dead defense "thanatosis.") Yet the rest of you wants to escape. This conflict cannot be good for your cardiovascular health.
The researcher who discovered this reaction almost ignored it. Biologist Terrie Williams of the University of California at Santa Cruz, who studies the physiology of large mammals, spent two summers collecting heart-rate and flipper-activity data from wild narwhals in Greenland.
The whales were stranded or caught in nets. Before cutting the whales loose, scientists outfitted the animals with a monitoring device. Immediately the narwhal bodies showed this conflicting response.
"My first inclination was to throw out the first couple of hours," Williams said. "The animals were doing something weird. It was clear it wasn't a normal dive response." Only later did she realize the weirdness was in the whales' reaction to humans.
Williams had developed the device, a combination EKG monitor, accelerometer and depth meter, to study marine mammals; she first tested it on retired dolphins that had been trained to work with the Navy. The machine was adapted for narwhals, made more rugged for colder and deeper water. Collaborating with Greenland's Institute for Natural Resources, Williams and her colleagues stuck the monitor to wild whales with suction cups.
A few days later, the monitor fell off and floated to the surface, where Williams and her teammates located it via VHF and satellite signals. They repeated the process for a total of nine whales.
This was the first time anyone had measured heartbeats in narwhals, Williams said. As the scientists report in a paper published Thursday in Science, the whales' heart rates plummeted from a resting rate of 60 to about three or four beats per minute.
Meanwhile, despite their sluggish hearts, the narwhals moved their flippers as fast as they could go. Williams likened the conflicting signals to narwhal hearts to the taxing experience of human triathletes: "Stress plus cold water in the face plus exercise." (Triathletes are twice as likely to die during a race as marathoners, at a rate of about 1.5 deaths per 100,000 triathlon participants.)
Williams said it was unclear, at this stage, whether this depressed blood flow plus increased exercise was dangerous to narwhals. She hypothesized that the response probably restricts oxygen to the whales' brains; this might, for instance, explain the disorientation rescuers observe when they try to return beached whales to the sea. The animals are also in danger of overheating, Williams said, if the slow circulatory systems fail to redistribute heat equally around their bodies.
The paper "provides a new angle on the vulnerability of narwhals to anthropogenic disturbance, which is linked to the sweeping environmental changes we are observing across the Arctic," said Kristin Laidre, an ecologist at University of Washington who studies whales and bears in Greenland.
Earth is home to about 123,000 adult narwhals, according to the International Union for Conservation of Nature. Historical threats include killer whales and subsistence hunting by Arctic communities.
Human intrusion and depleted sea ice are looming. "With climate change, we are on a trajectory for a very different Arctic in the coming decades," said Laidre, who was not involved with the Science paper. "This will mean a new reality for narwhals. A better understanding of human impacts is essential for conservation of this species given what the future looks like."
Until recently, sea ice blocked the Arctic from heavy boat traffic and offshore oil and gas development. That's changing.
Narwhals do not move quickly, but they evolved to escape dangers that came from a single source. In a more crowded ocean, polluted by ship noise, "you have novel kinds of threats out there that may not be a point source," Williams said. "Maybe in time evolution will catch up, but it's not there now."