With a layer of blubber three or four inches thick, narwhals have no difficulties surviving in the extreme cold. At great depths, where it is often even colder but where they must dive to feed, they are faced with pressures greater than 2,200 pounds per square inch, which they withstand with a flexible and compressible rib cage that can be squeezed without harming them as pressure increases. To carry along enough oxygen to sustain them on deep dives of up to 25 minutes, they have evolved several nifty solutions, including an enormous concentration of oxygen-binding myoglobin in their muscles, more than twice that of most seals and eight times as much as land animals, enabling them to swim under water at speeds of one meter per second for 20 minutes without taking another breath. With muscles better suited for endurance than for speed, narwhals can save even more oxygen by turning off the blood flow to non-critical organs and other body parts.
When they are diving to feed on the bottom, they often do so repeatedly to depths in excess of 1,500 meters. The transit from the surface to the bottom is typically a 30 minute round trip, mostly spent swimming straight down and straight back up. During most of the year, they make that round trip 10-20 times a day every day all day long.
But Jack Orr, a biologist with the Canada Department of Fisheries and Oceans, told me that narwhals may also use several other diving strategies. When they are living in dense ice, they may often use a V dive for feeding and a U dive for finding their next breathing hole. During a V dive, the animals dive straight down to feed at a certain location where they know they can find food, and then they swim straight back up to the surface to a different known breathing hole.“Whereas with a U dive,” Orr said, “they’re going down and then going horizontal for a while and then they’ll start their dive up toward the surface again. What we presume is that they’re getting to a certain depth and then using their echolocation to find their next breathing hole. If you’re 400 meters below the surface and you start looking for a hole and you find one 400 meters away versus 50 meters away, then you can decide whether you have enough breath to go forward to the next hole or, no, I don’t have the air so I have to go back to my original hole. These animals are living in such dense ice over the winter months that they’re coming up in breathing holes that aren’t really even a hole that you would see at the surface. That’s a behavior that they’ve adapted to navigate in dense pack ice.”
Whatever they do down there and however they capture their prey, their extreme diving behavior certainly makes them unique among the world’s cetaceans, and gives me another reason to be in awe of their remarkable lifestyle.