The Only Freshwater Seal Lives a Thousand Miles From the Sea

Lake Baikal sits in southern Siberia, 1,642 metres deep at its lowest point and roughly 25 million years old. It holds about one-fifth of all the unfrozen fresh water on Earth's surface — more than the five Great Lakes combined. Around 2,600 species live in it, two-thirds of them found nowhere else.

One of those species is a seal. The nerpa (Pusa sibirica) is the only pinniped on the planet that lives its whole life in fresh water, more than a thousand miles from any ocean. The leading hypothesis is that an ancestor swam up a river system from the Arctic about two million years ago, when Eurasia's hydrology was very different, and never left.

It has adapted in strange directions. A nerpa carries about two litres more blood than a marine seal of the same size, which lets it dive to 400 metres and stay under for 40 minutes on a normal feeding run, longer if something is chasing it. Most of what it eats is golomyanka, a translucent, almost-fatless deep-water fish that is also endemic to the lake. In winter and spring, more than 90 percent of the seal's diet is golomyanka, and the seals together eat about 64,000 tonnes of them a year.

The current population is between 80,000 and 100,000 — roughly the lake's carrying capacity. They pup on the ice in late winter, dig breathing holes through it, and have done so since long before the rivers that might have brought them here had reorganised themselves into the modern map.