A beluga whale that was filmed "playing catch" with a group of South African rugby fans may have escaped from a Russian military spy programme, according to researchers.
Experts suggest the whale could be Hvaldimir, a malnourished male beluga who is thought to be a lost "spy" animal trained by the Russian Navy.
Hvaldimir was first reported in April 2019, when he was discovered near northern Norway with a harness attached to his body and a label that said "Equipment of Saint Petersburg".
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The whale has been known to go up to boats to ask for food and play fetch and appears to be tame, suggesting he is used to interacting with humans.
Footage of a whale chasing and returning the rugby ball to a group of men has been viewed more than 19 million times on social media.
"This is likely Hvaldimir, a once captive whale who may have escaped a Russian military programme," Ferris Jabr, a science writer for the New York Times and Scientific American, said of the video.
"Alone, malnourished, and injured, [Hvaldimir] roams the seas, seeking food & attention from people."
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Quad Finn, a researcher of aquatic mammals, and Darren Naish, a zoologist, both supported the theory.
A man in the video can be seen wearing a jacket with the logo of a vessel called "Danah Explorer", which is currently in Norwegian waters.
However, the original source of the video is not known.
The Norwegian Institute of Marine Research and the Hvaldimir Foundation, who help care for the whale, have not yet responded to a request to verify the theory.
Mr Finn has said he believed the animal in the video was Hvaldimir and that the filmed behaviour was "not natural".
"Hvaldimir was taken from the ocean & trained by humans to do tricks like fetching objects in exchange for food," he said.
"That he's malnourished & still dependent on humans for food are likely reasons for what appears to be 'playful' behaviour."
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Earlier this year, another viral video showed Hvaldimir helping to retrieve an iPhone that was dropped into the water near him.
The whale may have become dependent on humans due to hand-feeding and does not appear to be able to successfully hunt and feed for itself, according to the Hvaldimir Foundation.
When he was spotted in early September off the Norwegian coast, the whale also showed signs of injuries from boat propellers.
Both the United States and Russia are known to have military training programmes for aquatic mammals, such as dolphins and whales, who are sometimes trained to detect sea mines and recover inert torpedoes.
A Russian military spokesperson denied in April that Hvaldimir was connected to a training programme but acknowledged that dolphins are sometimes used for military roles.
Morten Vikeby, a former Norwegian consul in Murmansk, has offered an alternative theory that Hvaldimir is a therapy animal from a programme for disabled children near the Russian-Norwegian border.