Despite everything we know about whale migration, why they do it is still somewhat of a mystery. That they summer in cold waters to feed is known, but why they then travel thousands of kilometers to warm tropical waters to breed is less clear. Why travel such feats of round distance at all?

There are in fact many theories on why whales migrate to warmer waters, but no actual consensus amongst scientists. Now, a team led by Oregon State University’s Marine Mammals Institute have a new theory to explain this extreme wandering to throw into the mix: they travel to the tropics to shed their skin.

The team concluded this after tagging and tracking 62 killer whales that migrated from frigid Antarctic waters on some pretty epic roundtrips over eight years. The researchers tracking the orcas tried to determine if they were searching for food, or good birthing grounds, or something else, reporting their results in the journal Marine Mammal Science. Turns out they travel to the tropics to keep their skin looking good. Yes, even killer whales deserve the odd spa break. 

Whales make some of the longest migrations of any animal on Earth. In 2015, a female western gray whale broke the record by swimming from Russia to Mexico and back – 22,511 kilometers (13,988 miles) – in 172 days. Killer whales, in comparison, average around 11,200 kilometers (7,000 miles) on their roundtrips.

Among the theories why whales travel to the tropics is so they can give birth to calves in warm waters, so either the calves or adults or both don't have to expend energy on keeping warm. However, due to whales' size, they should have no problem keeping warm in frigid water. In fact, the researchers photographed newborn orcas in Antarctic waters, proving the whales don't need to travel to warm waters to give birth. There is even a counter-theory that whales expend more energy regulating their temperature in warmer waters to keep cool. 

Instead, they posit that the Antarctic killer whales – and possibly all whales – travel to the tropics to shed their outer skin cells layer that builds up all year round, but doesn't slough off in cold polar waters. 

“I think people have not given skin molt due consideration when it comes to whales, but it is an important physiological need that could be met by migrating to warmer waters,” said lead author Robert Pitman.

All manner of creatures – including us – regularly shed their skin, fur, or feathers. Getting rid of the outer layer of dead skin cells is vital for allowing new cells to regenerate and replace them. The researchers noticed that these killer whales observed in the Antarctic waters had a discolored yellow film of microscopic diatoms (single-celled algae). This suggested they weren't experiencing their normal "self-cleaning" molting. The researchers think that the whales foraging in these freezing waters conserve energy by diverting blood flow away from the skin, reducing the regeneration of new skin cells and essentially halting the sloughing off of dead skin cells. The foraging is so good it makes it worth it, but to revive their skin (and get in some self-care), they then travel to warmer waters to allow the cycle to start up again.

One of the questions surrounding why the whales migrate to warm waters is the lack of food. The colder waters are rich with prey but the warm waters are not so many whales have to fast during their long treks to give birth. The researchers suggest instead of traveling to warm waters for calving, the whales are driven by self-maintenance, and have adapted to having their calves there.

“Basically, the feeding is so good in productive Antarctic waters that the relatively small, warm-blooded killer whale has evolved a remarkable migration behavior," said co-author John Durban. "This enables it to exploit these resources and still maintain healthy skin function.”

Killer whales: because they're worth it.