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A Western gray whale has made the longest known migration by any mammal.

The female, named Varvara, swam 14,000 miles (22,500km) from the east coast of Russia to breeding grounds off the coast of Mexico, and back, without even stopping for a snack when she was nine years old. 

Her return journey across the North Pacific raises questions about the critically endangered creature's conservation status.

A Western gray whale (pictured) has made the longest known migration by any mammal. The female, named Varvara, swam 14,000 miles (22,500) from the east coast of Russia to breeding grounds off the coast of Mexico, and back, reportedly without stopping for a snack

Until now, scientists believed there were two distinct groups of gray whale - the Eastern gray whale, which travels along the west coast of US, and the much rarer Western gray whale, which journeys along the coast of eastern Asia.

The researchers used satellite-monitoring tags to track three Western North Pacific gray whales from their primary feeding ground off Russia's Sakhalin Island across the Pacific Ocean and down the West Coast of the US to Baja, Mexico.

The experts have been monitoring the species living off the coast of Russia for years and tracked the record-breaking journey over five-and-half months between 2011 and 2012.

Gray whales typically don't feed during migration, which has led researchers to believe Varvara didn't eat during the long journey either. 

They found that Varvara, which is Russian for Barbara, visited the three major breeding areas for Eastern gray whales, which are found off North America, according to the study published by the Royal Society in the journal Biology Letters. 

Her return journey across the North Pacific raises questions about the critically endangered creature's conservation status. Experts have been monitoring the species living off the coast of Russia for years and tracked the record-breaking journey between 2011 and 2012. A Western gray whale is shown

'It is very impressive,' Professor Bruce Mate, of Oregon State University, told MailOnline.

'These whales have the same body temperature as you and I, so they are obliged to make these big journeys to get to warmer or cooler waters. But this is a record.'

'Until now we thought they were not very good navigators,' he continued.

'We thought that they just stuck to the coast, keeping the sound of the surf on one side, going up and down the coast until they met their destination.' 

On the way from Russia she is believed to have crossed the northern extremity of the Bering Sea, before hitting Alaska and going south until she reached the warm seas off the Baja peninsula to breed.

But on her way back to Russia, she made a more direct route, cutting across deep water.

'We do not know how she did it but the fact that she took different routes each way means we have to accept that these whales are very accomplished navigators,' Professor Mate continued.   

The study said: 'Her 22,511 km round-trip is the longest documented mammal migration and strongly suggests that some presumed Western gray whales are actually Eastern gray whales foraging in areas historically attributed to Wester gray whales.'

The previous longest journey made by a mammal was held by a humpback whale, who swam more than 11,706 miles (18,839km) in 2011. 

The migration from the feeding grounds was also made by two other whales who were being monitored, called Flex and Agent, who were all tagged seven years ago.

Western gray whales were thought to have gone extinct by the 1970s before a small population was discovered in Russia off Sakhalin Island.

The group is now estimated to be 150 strong and has been monitored by scientists from Russia and the US since the 1990s.

The migration from the feeding grounds was also made by two other whales who were being monitored, called Flex and Agent, who were all tagged seven years ago. Here, experts observe the animals breaching

Like their western cousins, Eastern gray whales were decimated by whaling and listed as endangered, but conservation efforts led to their recovery. There are now thought to be more than 18,000 of them swimming the oceans.

But not all scientists believe that Western gray whales are a separate, distinct species, raising the possibility that only Eastern gray whales survive today.

'The fact that endangered Western gray whales have such a long range and interact with Eastern gray whales was a surprise and leaves a lot of questions up in the air,' said Bruce Mate, director of the Marine Mammal Institute at Oregon State University and lead author on the study.

'Past studies have indicated genetic differentiation between the species, but this suggests we may need to take a closer look.' 

Valentin Ilyashenko of the A.N Severtsov Institute for Ecology and Evolution, who is the Russian representative to the International Whaling Commission and co-author of the study, has proposed since 2009 that recent western and Eastern gray whale populations are not isolated. 

He said that the gray whales found in Russian waters are a part of an eastern population that is restoring its former historical range.

'The ability of the whales to navigate across open water over tremendously long distances is impressive and suggests that some Western gray whales might actually be eastern grays,' Dr Mate said.

'But that doesn't mean that there may not be some true Western gray whales remaining.

'If so, then the number of true Western gray whales is even smaller than we previously thought.'

Protecting the endangered Western gray whales has been difficult, with five whales dying in Japanese fishing nets within the last decade.

Their feeding areas off Japan and Russia include fishing areas, shipping lanes, and oil and gas production, as well as future sites oil sites, while their largely unknown migration routes may include additional hazards.