http://news.bbc.co.uk/earth/hi/earth_news/newsid_8972000/8972021.stm

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.

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.”

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