The thing about unicorns is, they're hard to find. Legend has it that merely glimpsing the elusive equine requires distant travel to faraway lands, and even then you need luck on your side. Slightly easier would be to locate a narwhal, the toothed whale whose left canine has turned into a unicorn-like tusk protruding from its face. In fact, when Norway's King Frederick III (1609–1670) was regretfully informed that his new throne could not be constructed from unicorn horns as requested, he agreed to narwhal tusks instead. But narwhals are still hard to find, requiring distant travel to the most northern of seas. So, when Terrie Williams set out with her international team of collaborators to investigate the narwhal's physiological stress response, she journeyed more than 6400 km from her University of California, Santa Cruz, USA, base to eastern Greenland's Scoresby Sound.
The team completed the study over two consecutive Augusts, when the weather was agreeable and the indigenous Greenlanders’ annual hunt was on. This ensured the researchers found sufficient narwhal to investigate their behavioural, biomechanical, cardiovascular and energetic responses to escape situations. The narwhal's elusiveness and far-flung locale meant that such measurements had never been made before.
Once there, the team suctioned submersible electrocardiograph–accelerometer–depth recorders to the backs of net-entagled - and thus stressed - narwhal. The data loggers then remained in place for up to 3 days of measurements before sliding from the deep-divers’ backs, ready for retrieval. So, how cool was the narwhal under stress?
In some ways, very cool; in others, not so much. Escaping narwhal rapidly depressed their heart rate by 92% while simultaneously doubling their swimming stroke frequency and elevating their energy expenditure 6-fold compared with normal swimming conditions. This is unlike other whales’ stress responses and is frankly paradoxical. To appreciate the paradox, one must first understand the two types of mammalian stress response: fight or flight, and freezing. A fight or flight response sees the animal's heart, breathing and metabolic rates elevated to ensure about-to-be-used muscles receive sufficient supplies of blood and oxygen. Conversely, a freezing response sees these rates depressed to ensure the animal stays put. These responses are incongruent, and that incongruence travels all the way to the separate regions of the brain that trigger them. Yet, here was the narwhal, simultaneously freezing and fleeing.
The researchers reckon the abnormal stress response is caused by these particular stressful situations, which are novel for the narwhal. Specifically, these situations trigger a complex medley of signals from the narwhal's diving, exercise and neurocognitive fear responses. These signals can oppose one another, and that's not good. For example, diving typically slows heart rate, while exercise elevates it. And in rats, simultaneously inducing these signals under experimental conditions promptly kills them. While the narwhal isn't dying, a response that simultaneously reduces heart rate and elevates energetic demand surely threatens tissue oxygenation and thus homeostasis. And, as rapidly changing Arctic conditions expose the once-isolated species to more predation, hunting by humans, shipping and seismic activity, narwhal will find themselves more frequently in these situations.
In the middle ages, narwhal tusks were seen as entry-level unicorn horns, marketed by Vikings and other northern traders as a practical means of accessing magical powers. One such power was the tusk's ability to purify poisoned water. Whether this also allowed warming water to be magically cooled is unknown, but this power would certainly prove handy for 21st century narwhals. Without it, their adjustment to a changing world – like everyone else's – will hinge on some combination of genetic adaptation, phenotypic plasticity and a bit of luck.
