Peter Verhoog/Minden Pictures

Svelte porpoises eat twice as much as humans to cope with the cold

Harbor porpoises should be the envy of any dieter. That’s because they eat a tremendous amount given their relatively small size, the equivalent of a 55-kilogram person. Their secret, according to a new study, is very high metabolic rates that let them burn more than twice as many calories per day than the average human. They need all this food to keep themselves from freezing to death in frigid arctic waters. Although that might not help dieting humans, it could help biologists better understand what these animals need to thrive.

For years, biologists have argued over whether porpoises (above) have high metabolic rates. Sea otters and seals, other small mammals that spend much of their life in water, burn energy far faster than their land-lubber relatives. But data on porpoises were mixed, with some suggesting their metabolic rate was not that high, after all.

So, researchers monitored the food intake and activity of three captive harbor porpoises (Phocoena phocoena) for 3 years. They measured the animals’ breathing, determined how much of their fish diet was turned into blubber, and calculated how much energy they use up per breath.

Then, they monitored the movements and sounds made by 13 wild porpoises, which they tagged and tracked from 2012 to 2016. By listening to the porpoises’ breathing, they could calculate respiration rates—and used the captive porpoise energy use rate to determine how many calories they were burning.

The animals took about three breaths per minute, and fewer in the summer, translating into an average burn of about 4300 calories per day, the researchers report today in the Journal of Experimental Biology. (Humans burn about 2000 calories a day.) Porpoises put on a lot of blubber in the summer, they add, likely helping them survive the winter in water that could kill a human in 15 minutes flat. Now we know why they never get fat.