It was an innocent invasion of privacy when Canadian researchers stumbled upon an exclusive retreat for whales — and caught some bowheads that, they determined, were exfoliating their skin in the waters off Nunavut.

The discovery, described in a study published Thursday in the journal Plos One, explains strange behaviour by the whales that has often puzzled Inuit hunters and commercial fishers.

Back in 2014, researchers from the University of British Columbia noticed the whales frolicking around Cumberland Sound, a waterway just off of Baffin Island near Pangnirtung, Nunavut.

The whales danced in the water, twisting and turning and waving their flippers in the air.

Cumberland Sound is like a salt-water spa retreat for bowhead whales. They rub off dead skin on the large boulders in the shallow coastal waters. (Submitted by Ricky Kilabuk )

"We would see whales rolling around… on occasion we would hear really loud vocalizations that sounded almost like a moan," said Sarah Fortune, a PhD student and the study's lead author.

"We were really perplexed," said Fortune, who initially set out to observe whale feeding patterns.

She ruled out mating as an explanation because the whales were scattered throughout the bay. Then they thought the whales were just socializing.

Bowhead whales, the mammal with the longest lifespan, can live up to 200 years. (VDOS Global LLC )

But when they saw large pieces of skin peeling away from the whales' bodies, it clicked. The whales were rubbing off dead skin on boulders in the shallow water. They were exfoliating and Cumberland Sound was, in a way, a giant salt-water spa.

Later, in 2016, the researchers went back. Aerial drone videos confirmed their hypothesis — that the whales were, in fact, engaging in some rock-rubbing action.

"They used these rocks almost like a pumice stone," said Fortune.

Why they rock-rub

Molting — when an animal sheds old skin, hair or feathers — was known in beluga, Southern right whales, and bowhead whales off the coast of Russia. But very little was known about it with bowhead whales in the Eastern Canada and the West Greenland area.

The research indicates that bowhead whales choose to molt, or rock-rub, in shallow waters where it tends to be warmer.

Exfoliating in warm coastal water promotes blood flow, bringing nutrients and hormones that promote skin growth, according to the study.

Canadian researchers recently caught bowhead whales in an intimate act of deep exfoliation in Nunavut waters. 1:04

The findings are important in the long run, says Fortune.

"We know that their habitats are changing rapidly with the decline of sea ice… In order to help with conservation of Eastern Canada, West Greenland bowhead whale population, it's really important to know when, where and why whales go."

The biggest threat to this population is the northward migration of killer whales, due to decreases in sea ice cover, says Fortune.

"So killer whales are the probably the biggest predator of bowhead whales currently."

Pangnirtung, Nunavut is a community of about 1,400 people. Many locals hunt in nearby Cumberland Sound. (Sarah Fortune)

Thickest skin, longest life

Bowhead whales, the mammal with the longest lifespan, can live up to 200 years and were nicknamed "rock-nose whales" by whalers, because they would often rest their chins on large rocks.

Bowheads also have the thickest blubber of any mammal, so they're very well insulated, says Fortune.

The next step is to document the skin conditions of the whales to pinpoint exactly when molting occurs.

"We'd like to work with our local partners … and have them document the skin condition of the whale over time."