It’s a long way to go for a warm bath. Each winter, humpback whales travel from the Antarctic to the northern tropics to find warm water in which to raise their young, researchers have discovered. The migration is the longest for any mammal ever recorded.

Kristin Rasmussen at Cascadia Research Collective in Olympia, Washington, US, and colleagues photographed the tails of humpbacks wintering off the Caribbean coast of Costa Rica. From their individual tail patterns they identified seven of the same animals after they had returned to the Antarctic. One mother and calf made the 8300-kilometre (5160-mile) trip in 161 days.

Using satellite data, the team also recorded sea-surface temperatures for the sites where humpbacks spent the winter. “Wintering areas occur where waters with temperatures between 21°C and 28°C are found,” says Rasmussen. This supports the idea that the long migration saves the whales energy in the end.

Some researchers claim that the grey whale holds the record for longest mammalian migration – from Mexico to the Arctic, estimated at about 7600 km (4700 miles). “However, no individual grey whale has been documented travelling the full extent of their migratory range, and it’s possible that no grey whales actually make the entire migration,” says Rasmussen. Only humpbacks have been documented making the full trip.

Rasmussen says that proposals to hunt humpbacks – such as Japan’s decision to catch 50 humpbacks each year as part of its “scientific” programme – makes it important to understand whale migration. “Whales don’t respect political boundaries,” she says. “Killing whales in one area could potentially impact their population half way around the world.”

Journal reference: Biology Letters (DOI: 10.1098/rsbl.2007.0067

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