Wet Beast Wednesday: Baikal seal
Welcome to the second annual fresh-uary, the month where I exclusively cover freshwater species for Wet Beast Wednesday. To start this un-salty season, I'm covering the Baikal seal, the world's only seal species that lives exclusively in freshwater. There are other lake-dwelling seals, but they are either populations of species that normally live in the ocean, or are the Caspian seal, which lives in the Caspian Sea, a body of water too salty to be called a lake.
(Image: a Baikal seal basking in the sun in water too shallow to cover it. It is a dark gray and rather rotund seal. The hind flippers are held in the air. End ID)
The Baikal seal (Pusa sibirica), also called the nerpa, is a true seal related to the ringed seal. They are among the smallest seals, reaching about 1.1 to 1.4 meters (3'7 to 4'7) in length and 63 to 70 kg (139-154 lbs) in weight. They display less sexual dimorphism than most pinnipeds, with males being only slightly larger than females. Adults have a dense coating of dark fur. Their backs are silvery-gray and their undersides are lighter. Baikal seals have a unique dentition that allows them to lock their teeth together like a sieve, which they use to strain water through their teeth to trap food. Baikal seals have a greater blood volume than marine seals of a similar size.
(Image: a group of 10 Baikal seals basking on exposed rocks, with more in the water nearby. They are molting, evidenced by the patches of fur on their sides. End ID)
Baikal seals are found only within Lake Baikal in Siberia, though they occasionally swim up the rivers that drain into the lake. They are the only mammals endemic to the lake, though land mammals do visit. As with other seals, Baikal seals are predators. Their diet seems to consist primarily of fish, though recent discoveries indicate that amphipods may also make up a significant part of their diet. Hunting behavior varies throughout the day. During the day, they hunt using vision and target fish. At night, they use their sense of touch to hunt the amphipod Macrohectopus branickii, which is also endemic to Lake Baikal. These amphipods follow diel vertical migration, meaning they stay in the depths of the lake during the day and rise at night. The ability of Baikal seals to lock their teeth together like a sieve lets them take mouthfuls of water and then force it out through their teeth, trapping the amphipods in the seal's mouth. Baikal seals can dive 400 m (1,300 ft) and stay underwater for up to 70 minutes, which is only necessary because Lake Baikal is freakishly deep. The seals can't even reach the very bottom, they still have over 300 meters to go since the lake bottoms out at 1,642 m (5,387 ft).
(Image: a Baikal seal swimming just beneath ice, seen from below. The ice vices the light a greenish tint. End ID)
Lake Baikal freezes over in the winter, especially in the northern waters. The seals prefer to inhabit the northern extent of the lake as their reproduction requires the ice. I'll go over that more in the next paragraph. To survive, they need to make breathing holes in the ice. These holes are formed by breaking up the ice as it starts to form and is still thin and require maintenance all winter to keep them from freezing over. Seals will share holes, which they use to breathe and (in females) haul out to give birth. Unlike other seals, Baikal seals rarely haul out of the water to warm themselves and do not do so at all during the winter, instead spending the whole season underwater. When winter starts to come to an end, the seals will haul out onto the shore, exposed rocks, or ice to molt their fur from the previous year. During this time, they are lethargic and in danger of overheating.
(Image: a Baikal sea hauled out on ice. The camera is close to it, giving a good view of the definition of its fur. End ID)
Mating happens during winter. As the water freezes over, females will form dens in the ice by digging them out from below. These dens have a connection to the water below, but not to the air above. The dens are roughly rectangular and can be very large, reaching up to 5 m (16 ft) long and 2 m (6'7) wide. Mating happens in the water and males will attempt to mate with multiple females. Females are ready to mate soon after giving birth. The gestation period is nine months, but the implantation of the embryo is delayed by a few months, meaning birth usually happens 11 months after mating. Females will often be caring for one pup while carrying next year's offspring. Unlike most seals, Baikal seals can give birth to and successfully raise twins. In most species, twining is either impossible or, should a twin be born, the mother will abandon one of them. Triplets and quadruplets have also been observed, but very rarely. Birthing takes place inside the den and the pup(s) will stay there until spring. The mother will leave the pup to hunt and bring in mouthfuls of air to refresh the oxygen supply. As with other seals, the mother's milk is very fatty, allowing the pup to grow fast and quickly build up protective blubber. The pup will be weaned after 2 - 3.5 months, during which time it can have quintupled its birth weight. The mother will continue to bring food in the form of fish and amphipods until spring. Once spring comes and the ice starts to melt, the pup will be left to fed for itself. Pups are born with white fur, but will replace it with a grey pelt by the time they become independent. Males reach sexual maturity at ages 4 - 7 and females at ages 3 - 6. Baikal seals can live for more than 50 years, which is very long for a seal. Females may cease mating around age 30.
(Image: a Baikal sea pup in its den. It is small and white, with fluffy fur and dark spots above its eyes. The den is a cave in the ice just tall enough for the pup to move around. Watermarks of the logos for BBC of America and AMC+ are in the corners. End ID. Source)
It isn't entirely clear how Baikal seals ended up in the lake. Lake Baikal is over 15,000 km (almost 1,000 mi) from the nearest ocean and there's even a mountain range in the way. They appear to be descended from marine ringed seals, a common species found throughout the Arctic Ocean. The current leading hypothesis is that during the last ice age (or the last glacial maximum if you're being pedantic), glaciers covered large portions of what is now Russia. These formed a network of lakes and rivers that allowed ringed seals to come in from the Arctic Ocean. Ringed seals will travel over land for short distances to move between the sea and lakes. When the glaciers receded and the glacial lakes dried up, the seals were isolated in shrinking bodies of water. In the end, only Lake Baikal was large enough to support a population of seals. Now completely isolated from their oceanic relatives, the Baikal seals evolved into a distinct species through the process of allopatric speciation. Genetic evidence and preserved seal remains found throughout Eurasia support this hypothesis.
(Image: a Baikal seal poking its head out of the water and looking at the camera)
Baikal seals are currently rated as least concern by the IUCN, meaning they are not in danger of extinction. Human activity is their main threat as the only predator of adult Baikal seals is bears, and even that is a rare occurrence. Humans living around the lake have hunted Baikal seals for hundreds of years, targeting them for their skins, meat, and blubber. There are currently legal limits on how many seals can be taken each year. Bigger threats from humans include pollution and global warming. Pesticides and industrial waste enters the lake through washing and industrial dumping. These pollutants accumulate in the bodies of the seals' prey and subsequently enter the seals. They are speculated to weaken the seals' immune systems, leading to more frequent outbreaks of diseases. One of those diseases, by the way, is the actual black death. Fortunately, pollutant levels in the lake have been decreasing since the 90s. Unfortunately, global warming poses a threat. The winter ice is crucial for the seals' reproduction and faster melts could potentially be disastrous. Dens that are exposed to the air through melting threaten the pups by exposing them to eagles and other predatory birds. Should the ice fail to form in enough quantities to make dens, the seals would be unable to pup and would face extinction.
(Image: a Baikal Seal at a breathing hole poking its nose out of the water to take a breath. End ID)
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