Otters spend their days foraging, grooming, resting, swimming, and socializing. Their lives revolve around finding enough food to fuel one of the highest metabolic rates of any marine mammal, and nearly every behavior they exhibit ties back to that constant demand for calories. Whether floating on their backs in the ocean or slipping through a river, otters are always working to eat, stay warm, or look after their young.
Foraging and Eating
Sea otters need to eat roughly 25% of their body weight in food every single day. For a 60-pound otter, that’s about 15 pounds of food daily. This enormous appetite means foraging takes up a significant chunk of their time, around 14% of a typical 24-hour period for adult males, though females raising pups likely spend even more time hunting.
What otters eat depends on the species. Sea otters target shellfish: mussels, clams, crabs, snails, and especially sea urchins. River otters have a different menu. Their diet centers on fish (particularly perch-like species) and crayfish, with the occasional salamander, frog, or small mammal mixed in. Coastal river otters lean more heavily on crustaceans, while those living in mountainous areas eat more fish and sometimes catch trout and salmon.
Using Tools to Crack Open Shells
Sea otters are the only marine mammals known to use stone tools. They float on their backs, balance a rock on their chest, and smash shellfish against it to crack open the shell. They also use a second technique: swimming up to large, stationary rocks along the shoreline and striking mussels against points and ridges on the rock surface, all while staying in the water rather than climbing onto land. About 20% of the mussels they eat are opened this way. This kind of problem-solving puts sea otters in rare company among wild animals.
Grooming to Stay Warm
Unlike seals and whales, otters have no blubber. They rely entirely on their fur for insulation, and keeping that fur in working order is essentially a survival task. Adult male sea otters spend about 19% of their day grooming, making it the third most time-consuming activity after swimming and resting.
The goal of all that grooming is to trap a layer of air against the skin. Otter underfur is extraordinarily dense, and the individual hairs interlock tightly enough to prevent water from reaching the skin even during dives. When an otter dives, water pressure compresses the fur and squeezes out some of that trapped air. Each time the otter surfaces, it needs to fluff and rearrange its fur to restore the air layer. Grooming also redistributes natural oils across the skin and hair, which helps maintain the fur’s water-repelling properties. If the fur becomes matted or contaminated (from an oil spill, for example), it loses its ability to trap air and the otter can quickly become hypothermic.
Resting and Rafting
Sea otters rest for about 27% of the day. When they sleep, they float on their backs at the ocean’s surface, often wrapping themselves in strands of kelp to anchor in place and avoid drifting out to sea. Groups of otters resting together are called rafts, and individuals within a raft sometimes hold paws with each other to stay close. Rafts can range from a handful of otters to groups of dozens, and males and females often raft separately.
River otters take a different approach. They den on land in burrows called holts, which might be dug into a riverbank, tucked under tree roots, or built from stacked logs and branches. Holts serve as sheltered resting spots during the day and as safe places to raise pups.
Swimming and Patrolling Territory
Swimming and patrolling together account for about 35% of a male sea otter’s day, making it their single biggest time commitment. Males patrol territories in the water, keeping an eye on boundaries and watching for rival males. River otters are strong swimmers too, capable of covering long stretches of waterway in a single day. Both species mark their territories using scent glands at the base of their tails, which produce a musky secretion. They smear this scent on rocks, logs, and other landmarks. River otters also leave droppings called spraint at prominent spots along their range. These scent signals communicate identity, sex, social status, and reproductive condition to any otter that passes through.
Raising Pups
Otter mothers invest heavily in their young. A sea otter mom needs to provide more than 222,000 calories to raise a single pup from birth to independence. For the first month of life, all of a pup’s nutrition comes from nursing. After that, the pup continues to nurse for about five more months while gradually learning to eat solid food and eventually forage on its own.
Sea otter mothers carry their pups on their bellies while floating, constantly grooming the pup’s fur to make sure it stays buoyant and insulated. Pup fur is less dense than adult fur, sometimes only one-quarter the hair density of an adult pelt, so it needs extra attention. Mothers also wrap pups in kelp at the surface while they dive for food, creating a sort of floating cradle. River otter mothers raise their young inside holts and gradually introduce pups to swimming, sometimes nudging reluctant pups into the water.
Their Role in the Ecosystem
Beyond their daily routines, what otters do has outsized consequences for the ecosystems around them. Sea otters are a textbook keystone species. By eating sea urchins, they prevent urchin populations from exploding and devouring kelp forests. Without otters, urchins overgraze kelp beds and turn them into barren underwater wastelands. Parts of northern California have lost up to 90% of their kelp forests in areas where otter populations are absent or depleted.
When sea otters are reintroduced to an area, kelp forests and seagrass beds recover surprisingly quickly. Those healthy kelp forests then support greater biodiversity, providing habitat for rockfish and dozens of other species. Kelp forests also absorb large amounts of carbon dioxide, which helps reduce ocean warming and acidification. So an otter cracking open a sea urchin on its chest is, in a very real sense, maintaining an entire ecosystem.