A whale's journey across the Pacific Ocean is the longest recorded migration of any mammal, say scientists.
The female gray whale made a round trip of 22,500 km (14,000 miles) from the east coast of Russia to breeding grounds off Mexico and back.
The study, published in Biology Letters, raises questions about the whale's conservation status.
U.S. and Russian biologists say a population living only on the western side of the Pacific may now be extinct.
Until now, it was believed there were two distinct groups of gray whale: the Eastern gray whale, found along the west coast of North America, and the critically-endangered Western gray whale, found along the coast of eastern Asia.
The finding that three of seven whales tagged off Russia crossed the North Pacific to breeding grounds off Mexico, suggests they may be part of the same population.
They include the female, named Varvara by scientists, who made the long migration.
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