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• Sea otters, unlike seals and whales, do not have subcutaneous fat and rely on fur for heat retention. Their hair density reaches 45,000 per cm², creating an air layer.
• Male sea otters reach sexual maturity at 5–6 years, females at 4 years. Males retain reproductive ability throughout their lives.
• California sea otters use stones up to 3.5 kg to crack hard mollusks. They strike the mollusk on the stone at a rate of about three hits per second.
• A sea otter can dive to a depth of 50 meters, though the absolute diving depth record is likely higher. In times of hunger or danger, it can stay underwater for 5–8 minutes.
• The outer toe on the hind feet of a sea otter is the longest, and the inner one is the shortest. In many mammals, the fifth toe (pinkie) is the shortest. Hind foot claws reach 12 mm.
• Sea otter milk contains 23% fat and only 1% lactose. For the first month of life, the pup feeds exclusively on this milk, which is chemically closer to that of other marine mammals than to that of other mustelids.
• The sea otter is the only non-primate marine mammal using tools, though it is now believed that bottle-nosed dolphins may also use them. The California subspecies is the only one that uses stones in the wild, while other subspecies learn this behavior from them in captivity.
• A female sea otter's pregnancy begins with a delay and includes a latent phase lasting 2–3 months. The embryo does not attach to the uterine wall during this time, a development method shared by about 100 mammal species.
• Sea otters have a unique among mammals adaptation of vision for an amphibious lifestyle. Their eyes allow them to focus on both underwater and above-water objects, and they have emmetropic vision both in water and in air.
• Sea otters are the largest mustelids and one of the smallest marine mammals. In size, they can be compared only to the giant otter (Pteronura brasiliensis).
• Sea otters have unique morphological features that distinguish them from other species. Because of this, their classification has been repeatedly revised.
• Sea otters are peaceful because even small wounds can lead to death from hypothermia due to damage to their fur.
• Modern sea otters have fewer alleles in their genes than their ancestors 300 years ago, when the same genes had dozens of alleles.
• Oil spills pose an exceptional danger to sea otters, as even a small amount of oil can cause the guard hairs to mat and the down layer to become wet. This happens because the lower down layer gets wet and the animals die from hypothermia.
• The sense of touch in sea otters is so well developed that blind individuals can feed using the sensitive pads of their front paws and vibrissae. Even without vision, they find food by analyzing it with their vibrissae.
• Sea otters have a metabolism rate 8 times higher than that of terrestrial mammals of similar size. Sea otters need to eat food amounting to 20–25% of their body weight every day.
• The sea otter's dental structure is unlike that of mustelids and most carnivorous mammals. It has only two pairs of lower incisors (dental formula: 3.1.3/3.1.3).
• The sea otter's lungs account for 6 % of body mass and fit into the thoracic cavity due to an oblique diaphragm. Sea otters have lungs 2–4 times larger than those of pinnipeds.
• The sea otter defeats sea lions despite them being twice as heavy and larger. Hunters from the 18th–19th centuries reported sea otters defeating sea lions when both species were caught in hunting nets.
• The sea otter uses stones as tools when foraging, selecting them by shape and size.
• Sea otters are exceptionally friendly animals towards each other and other animals, except those in their diet. They coexist peacefully with sea lions, seals, and walruses, sometimes sharing resting spots.
• Newborn sea otters are born with 26 already erupted teeth. This number is precisely stated in the formula provided in the text.
• With age, areas of the sea otter's skin not prone to graying darken from reddish tones to dark brown and black, while areas prone to graying, especially the head, lighten.
• A mother sea otter is extremely attached to her pup. In case of danger, she prioritizes caring for it.
• The sea otter hunts fish, but its method of pursuing fish differs from that of other otters. Its gaze is directed downward and sideways, so to catch fish, it must roll onto its back underwater, slowing its movement.
• The sea otter’s nostrils close when diving underwater to prevent water from entering the nose. The elastic walls of the nostrils allow this to happen.
• Sea otters mate with the corpse of a pup many days after its death. In many cases, this occurs after rapes that lead to the death of the pups.
• The sea otter can run on land, arching its back and moving its paws rapidly. For this, it brings its front and hind limbs close together and quickly moves, rapidly moving its paws along the ground.
• The parasite Toxoplasma gondii, carried by cats, is deadly for sea otters. It enters the sea through wastewater containing feces from domestic cats, due to the high density of these animals’ population.
• The sea otter pulls a sea urchin out of the water and cracks its shell against hard objects to reach the eggs. It holds the urchin in its front paws and swims on its back toward rocks or other hard objects.
• The sea otter has strong jaws equipped with powerful muscles and shortened for increased compression — adapted to biting and chewing hard food. The sea otter’s cranial capacity index is 5.80.
• In 99 % of cases, a single pup is born to a sea otter. In rare cases, twins are born, but under normal circumstances, only one pup can survive.
• The sea otter's digestive tract makes up 15.5% of its body weight. Its intestine is long and exceeds the body length more than 10 times.
• The sea otter's molars are broad and flattened, adapted for chewing hard mollusks and crab shells, not for holding slippery fish. In some individuals, the number of teeth reaches 34.
• Sea otters have two fundamentally different ways of moving on land. The main method resembles clumsy crawling, with the belly dragging along the ground.
• Male sea otters do not participate in raising offspring and abandon females a day or two after mating. The mother holds the pup on her belly and grooms it, leaving it in the water only briefly.
• In summer, common sea otters experience hair loss, especially in "Kamchatka" individuals. This phenomenon is particularly observed during the summer period.
• Raising pups in sea otters is described using anthropomorphic terminology due to behavioral similarities with humans. Females caress the young and slap them when they misbehave.
• The sea otter's nasal cavities are greatly enlarged and have a complex structure, related to breathing characteristics during diving. Many internal organs of the sea otter are enlarged compared to other mammals in proportion to body weight.
• During mating, the male sea otter holds the female by the nose with his teeth, and mating ends with a painful bite. Experienced females have characteristic scars on their noses.
• A newborn sea otter can float on its back like a "float," but cannot swim or forage for food. It also cannot groom itself.
• Sea otters depend entirely on their mother for 5 to 15 months, and juvenile mortality is about 30%. One-third of pups die in their first year.
• Sea otters regularly groom their fur to maintain the cleanliness of the underfur. Contamination of this fur can lead to direct skin contact with water and hypothermia.
• The ear flap of a sea otter closes with elastic walls when submerged, preventing water from entering the ear. The elastic walls close the flap, narrowing it toward the outside and forming a tube.
• Sea otters can force orcas to beach when they pursue them. Cases have been observed where orcas beached following fleeing sea otters.
• Male sea otters are larger than females: they are usually 35% heavier and 10% longer. Adult males weigh between 22 and 45 kg.
• Sea otters eat birds and meat from dead animals, especially foxes. During times of scarcity, they may hunt shorebirds.