Otters belong to the Mustelidae family, which also includes weasels, badgers, and ferrets. While all otters share a playful disposition and sleek, aquatic body shape, the sea otter and river otters have evolved distinct characteristics tailored to their vastly different environments. Thriving in either freshwater rivers or the cold saltwater ocean has resulted in profound differences in their physical structures, behaviors, and physiology.
Habitat and Physical Distinctions
River otters, such as the North American species (Lontra canadensis), inhabit freshwater and brackish environments, spending considerable time moving and resting on land. Sea otters (Enhydra lutris) are marine mammals that live almost exclusively in the nearshore Pacific Ocean, rarely hauling out onto land.
This habitat separation results in a significant size difference. A male sea otter can weigh up to 45 kilograms (100 pounds), making it the heaviest member of the weasel family, while a male river otter weighs no more than 14 kilograms (30 pounds).
Their tails reflect their primary mode of aquatic propulsion. River otters possess a long, muscular, and tapered tail used for powerful swimming, while the sea otter has a shorter, flatter, paddle-like tail used mainly for steering and balance. River otters have partially webbed paws equipped with claws for gripping prey and maneuvering on riverbanks and land. Sea otters have large, fully webbed hind feet that are the main source of propulsion. Their forepaws are small and highly dexterous, specialized for handling food and tools.
Hunting Methods and Diet Composition
The available prey has led to specialized hunting techniques and diets. River otters are generalist and opportunistic predators, primarily hunting fish, amphibians, crayfish, and other aquatic invertebrates. They typically hunt alone and use their sensitive whiskers, called vibrissae, to detect prey in the murky water.
Sea otters are specialized predators focusing on hard-shelled marine invertebrates, such as sea urchins, clams, mussels, and crabs. Their foraging behavior is unique as they are known to use stones as tools to crack open these shells. They often use a pouch of loose skin under their foreleg to store collected food and a tool rock before bringing their meal to the surface to eat while floating on their backs. By consuming sea urchins, sea otters play a functional role in maintaining the health of kelp forest ecosystems.
Unique Physiological Adaptations
The most striking difference is the mechanism they use for thermoregulation in cold water. Sea otters lack the insulating layer of blubber found in most marine mammals. Instead, they rely entirely on an extraordinarily dense fur coat, which is the thickest of any mammal, with up to one million hairs per square inch.
This dense fur traps a layer of air against the skin, providing insulation. It requires meticulous, frequent grooming to maintain its waterproofing and insulating properties. Because the sea otter must maintain a high body temperature in the cold Pacific, it has an extremely high metabolic rate, necessitating a daily food intake of 25 to 30 percent of its body weight. River otters have a less dense, two-layered coat and rely more on spending time on land or in sheltered dens to rest and groom.