The gray whales cruising along California's coast during their annual fall migration are well known to science. But there is another small, mysterious group of related whales off the Russian coast—the western north Pacific gray whale population—that researchers are just now beginning to track.

Researchers have long believed that this critically endangered western group has remained isolated from their eastern Pacific counterparts. But new research is changing that view, and has documented a female in that population who completed the longest migration on record for a mammal.

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The nine-year-old whale, named Varvara, swam from Sakhalin island, Russia, to Cabo San Lucas, Baja California, and back beginning in November 2011. She logged 14,000 miles (22,511 kilometers) during her 172-day trip, researchers report online April 14 in the journal Biology Letters.

In an effort to understand their migratory patterns, Bruce Mate, director of Oregon State University's Marine Mammal Institute, and colleagues used satellite tags to follow seven western Pacific gray whales. Only three tags lasted long enough for researchers to track the migrating animals.

All three headed east from Sakhalin, with two joining the southward migration of eastern Pacific gray whales to Baja California. The third whale's tag stopped working halfway across the Gulf of Alaska, but "I would be extremely surprised if she didn't continue on," says Mate.

Why It Matters

There have been reports of about 30 western gray whales appearing off British Columbia and Baja California over the years, Mate explains. When researchers spotted Varvara off the coast of southern Oregon, she was with a group of eastern Pacific gray whales.

"Usually calves follow their mothers from breeding areas to foraging grounds," Mate says, and they'll stick to those routes as they get older. So the appearance of western gray whales in the eastern Pacific could mean that those whales were born there, he explains.

That could upend the idea of an isolated western gray whale population.

The Big Picture

The question now, Mate says, is whether there is a small group of eastern Pacific gray whales among western gray whales, or if western gray whales are an extension of the eastern population and not a separate group at all.

"The genetics work to date says they are separate," Mate says. But other researchers are analyzing a larger genetic dataset, so "we'll have to wait and see what they find out."