Narwhals are the 'marathon runners' of the oceans, scientists have discovered.
The Arctic-living tusked whales are exceptional endurance athletes, possessing a greater density of slow-twitch fibres in their muscles than any other marine mammal studied.
But this ability also means they are one of the sea's slowest swimmers.
That places them at high risk from climate change, as narwhals will not be able to cope with shifting, highly mobile ice floes caused by warmer seas.
The ice has become highly mobile. That makes icebergs that are too big for these animals to swim beneath, and changes the reliability of known breathing holes
Marine biologist Professor Terrie Williams
Details of the narwhal's extreme endurance ability and susceptibility to climate change are published in the journal Marine Mammal Science.
Narwhals are one of three species of cetacean living in the Arctic, alongside the beluga whale and bowhead whale.
They are also known as the 'unicorns of the sea' due to their long, elongated tusks.
However, narwhals now have a new claim to fame, as the 'marathon runners' of the oceans, say researchers Professor Terrie Williams and Dr Shawn Noren of the University of California, Santa Cruz and Dr Mike Glenn, of Sea World, San Diego, US.
They made the first study of the physiological capacity of narwhals and found that the cetaceans have the highest levels of myoglobin in their muscles of any marine mammal measured to date.
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'Arctic unicorns' in an icy display, filmed by the BBC documentary series Nature's Great Events
High levels of myoglobin allow large amounts of oxygen to be stored.
Narwhals also have the highest percentage of slow twitch muscle fibres within swimming muscles for any marine mammal, the researchers discovered.
In narwhals, around 87% of their swimming muscle fibres are of the slow twitch variety, which respond slowly but do not tire easily.
In contrast, the ratio in dolphins is between 40-50%.
A supremely fast fast land mammal, the cheetah, has less than 20% slow twitch fibres, as the majority of its leg muscles are made of fast twitch fibres that react fast but tire quickly, making them ideal for sprinting.
Narwhals actually have a muscle composition close to that of human endurance runners, as the leg muscles of an elite marathon runner contain more than 90% slow twitch muscle fibres.
"This specialised morphology makes them excellent divers," Prof Williams told the BBC, as their muscles can store and efficiently use large amounts of oxygen.
"But it comes at a cost. They appear to be one of the slowest swimmers out there."
Climate threat
This highly specialised swimming behaviour puts narwhals at significant risk from the effects of climate change, warn the researchers.
Listed as 'near-threatened to vulnerable' on the IUCN list of endangered animals, around 75,000 narwhals are thought to survive, inhabiting the ice-choked waters of the Canadian Arctic and Greenland.
This means narwhals have to be able to predict where holes in the ice are, in order to reach the surface to breathe.
In fact, Prof William's study reveals that narwhals swim so slowly that they can only cover a distance of around 1.4km in any direction before they run out of oxygen, despite their endurance capability.
So to survive, they need to be able to find a break in the ice at least every 1.4km.
"The big surprise is how little of the Arctic waters narwhals can use due to the limitation of breathing," says Prof Williams.
Worse, climate change will reduce the amount of suitable ocean, further endangering the species.
"Warmer temperatures are creating more icebergs and larger floes," says Prof Williams.
"The problem is not the ice is disappearing, rather in the course of disappearing the ice has become highly mobile.
"That makes icebergs that are too big for these animals to swim beneath, and changes the reliability of known breathing holes."
The researchers estimate that just 10% of the water under larger ice floes contains enough breathing holes, or fish to hunt, to be suitable for narwhals.
Shifting winds can also quickly move ice around, making the appearance of breathing holes even more unpredictable.
"A wrong decision [taken by a narwhal] or a shifting wind moving ice could be fatal."
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The ice has become highly mobile. That makes icebergs that are too big for these animals to swim beneath, and changes the reliability of known breathing holes
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