Researchers from the U.S. and Russia said that a western gray whale now holds the record for the mammal with the longest-known migration, media reports said Wednesday. According to the latest study, a female western gray whale, Varvara, swam from Russia to Mexico and returned within 172 days, traveling 13,988 miles.

The researchers had placed satellite-monitoring tags on seven western gray whales, including Varvara, living in Sakhalin Island in southeast Russia -- where the mammals feed every year -- to track their migration. Bruce Mate, director of the Marine Mammal Institute at Oregon State University and the lead researcher, said, according to Discovery, that Varvara was the only whale whose tag remained intact through the journey, giving her the record of the longest-known migration. 

“I’ve had to revise my thinking completely,” Mate said, according to the Washington Post, adding: “Needless to say, we’re impressed. ... How she did it remains to be seen."

Until now, the record of the longest-migrating mammal stayed with the humpback whale (Megaptera novaeangliae), which migrates up to 10,190 miles to complete its breeding grounds near the equator and the food-rich waters of Arctic and Antarctic.

The western gray whales are critically endangered and researchers at a time even thought that the mammals were extinct. The Western North Pacific gray whale population is expected to be between 100 and 150. Some researchers speculated that the species travels from Russia to South China Sea in a loop, but the latest study showed that Varvara traveled to Mexico.

“She crossed the Bering Sea, the Gulf of Alaska the lengths of the North American continent to get down to the Baja breeding calf lagoons that are used by eastern North Pacific animals," Mate said, according to Discovery.

Varvara and the other tagged western gray whales also joined a group of some eastern gray whales on the North American coast as Varvara hit three breeding grounds for eastern gray whales. The eastern gray whales are not endangered. 

Mate said that more research about the two varieties of gray whales was required.

He added that it was entirely possible his team of whale might actually be eastern gray whales, which had migrated west to Russia. 

The findings could point toward several possibilities, including one that all the 100 whales may just be the eastern whales that feed near Russia, and that the western gray whales had vanished as a species, the Washington Post reported. Another possibility may be that the 30 trans-oceanic whales are simply mingling with the western stock, which would mean that the western stock of whales still exists.