Deep in the Russian wilderness, over 3000km from the nearest ocean, 100,000 seals live on a huge frigid lake. They are Baikal seals, known in Russian as nerpas, and are the only seal species found in freshwater.
So how did the Baikal seal end up living in the middle of Siberia? Was it stranded as giant prehistoric lakes retreated millions of years ago or did it reach the frigid lake - Lake Baikal - via rivers after the last ice ages when the lake was more connected to the Arctic?
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How did the Baikal seal reach a lake in the middle of Russia?
The answer is still something of a enigma but it helps to know more about the history of Lake Baikal and the ancestors of its seal in order to unravel the mystery.
Lake Baikal is a waterworld of superlatives and could almost be considered a mini ocean. A 644km long, cobalt-coloured crescent, it is the world's largest lake by volume, containing an astonishing 23 per cent of the world's surface freshwater. It covers an area greater than Belgium and its depths, reaching 1642m, make it the world's deepest lake.
Formed some 25 million years ago, Baikal is also the world's oldest lake. It is essentially a massive crack in the Siberian plateau between plates in the Earth's crust called the Siberian Platform and the North China Plate and it is growing wider as the plates move apart, a few millimetres a year.
But it's thought that Lake Baikal was not always isolated. Some 34 million years ago, it may have been connected to a vast shallow inland sea known as the Paratethys Ocean, which reached from the eastern Mediterranean as far as the southern Arctic. Could a common ancestor with the Arctic ringed seal have made it to Baikal when it was connected to this larger body of water – perhaps two millions years ago – and then become stranded as the waters receded?
Studies of the Baikal seal's ancestry challenge this theory. A paper published in the journal Zoological Science in 2003 titled The Origin and Genetic Relationships of the Baikal Seal (Hiroyuki Sasaki et al), explored the species' mitochondrial DNA. Its findings suggest that the Baikal seal emerged as a separate species from that Arctic ringed seal ancestor just 400,000 years ago.
The subsequent theory proposes that populations of these 'ancestors' reached Lake Baikal from the massive Yenisey River system that flows into the Arctic, became isolated and developed their own characteristics. They eventually became a separate but closely related species of the genus Pusa (the Arctic ringed seal is Pusa hispida hispida).
As well as the Baikal seal (Pusa sibirica), which is significantly smaller than its ringed seal ancestor and lacks its spots, there is another close relative, the Caspian seal (Pusa caspica), that lives solely in the Caspian sea of central Asia – though this is saltwater rather than fresh.
According to the research therefore, the Baikal seal is a relatively recent addition to Lake Baikal fauna. Other studies show that the lake is home to nearly 2,000 animals and plants that are found nowhere else – communities that have established over millions of years of isolation. Such a unique ecosystem has led to Lake Baikal being nicknamed the 'Galapagos of Russia'.
The lake's waters are unusually clear and pure, with visibility to 40m. The reason for this is largely the work of a tiny filter feeding copepod called Epishura baicalensis. Each is no bigger than a poppy seed but in vast numbers, they form an incredibly efficient filtration system. Epishura are also the base of the food chain and form the chief food of endemic fish such as golomyanka – the Baikal oil fish. These strange-looking almost translucent fish form over 50 per cent of the seal's diet. It's been estimated that the seals eat 64000 tonnes of Biakal oil fish a year.
Like many other denizens of the lake, the Baikal seal is perfectly adapted to the local conditions, thriving when ice covers much of the surface in winter by diving for fish through breathing holes. Typical dives last 10 minutes but it can survive underwater for up to 70 minutes, reaching depths of 400m. This is because it has 20 per cent more blood in its vascular system than similarly-sized marine seal species and therefore can store more oxygen.
But being at the top of the food chain also poses problems in the modern age. Recent studies have shown that Baikal seals have pollution loads rivalling those marine seals in the heavily industrialised Baltic Sea. The pollution in Lake Baikal is linked to various industries including a massive lakeside paper mill, as well as some agricultural run off. It is hoped that various conservations measures will continue to protect the lake and its unique inhabitants – and that the mystery of seal's origins will eventually be revealed.
