Narwhals are best known for their long, unicorn-like tusks. But it is the Arctic whale’s thick muscles that may be a better guide to how the species will cope with climate change. A new analysis of muscle fibers finds that narwhals are world-class endurance swimmers – and that could be a problem in a fast-changing polar environment.

The narwhal (Monodon monoceros) is one of just three whales that live year-round in Arctic waters — the others are the beluga whale (Delphinapterus leucas) and bowhead whale (Balaena mysticetus). Surviving winters in ice-choked waters can be particularly challenging, researchers Terrie Williams and Shawn Noren of the University of California-Santa Cruz, and Mike Glenn of Sea World of San Diego, note in the journal Marine Mammal Science. The whales need to find small cracks in the ice to breathe, for example, limiting their foraging range.

To better understand how the whales cope with such extreme conditions, the researchers staged one of the first detailed studies of narwhal physiology. They measured things like swimming speeds and breathing rates of whales summering off Baffin Island, and took tissue samples from three narwhals killed by Inuit subsistence hunters. Back at the lab, they also analyzed one of the whale’s major swimming muscles, called the longissimus dorsi.

It turns out that if the narwhal were a runner, it would be an elite ultra-marathoner. Narwhals are “built for slow, endurance swimming” and prolonged diving, the researchers write. The longissimus dorsi, for example, is about 80% to 95% composed of “slow twitch” fibers. That’s comparable to human marathoners (sprinters, in contrast, typically have just 25% slow-twitch fibers). “Such a fiber type profile represents the extreme end of the spectrum for locomotory muscles of mammals,” they note.

Being built for the long-haul, however, also has its disadvantages. The narwhal’s physiology, for instance, means it can only forage where ice holes are no more than 1,500 meters (about a mile) apart. In Baffin Bay, that means some 90% of their frozen wintering waters can be off limits, the researchers note. The study also “suggests that narwhals, particularly during challenging winter ice conditions, may already be operating at their maximum performance levels.”

That could spell trouble as global warming rapidly reshuffles Arctic ecosystems. One problem is that narwhals may have “little flexibility to extend dive durations or depths to compensate for the rapid changes in ice cover or prey availability associated with ongoing climate changes.”

The study “demonstrates the power of using basic physiological attributes” to predict how species may respond environmental changes, the authors conclude. And they predict that the narwhal won’t be a “a unique case,” as other “highly adapted” species confront rapid change. – David Malakoff | April 26, 2011

Source: Williams, T., Noren, S., & Glenn, M. (2010). Extreme physiological adaptations as predictors of climate-change sensitivity in the narwhal, Monodon monoceros. Marine Mammal Science DOI: 10.1111/j.1748-7692.2010.00408.x

Image © AlexanderZam