River otters are agile semi-aquatic mammals found throughout North America. They inhabit aquatic environments, including freshwater rivers, streams, lakes, and even coastal estuaries and marine waterways. Their sleek, streamlined bodies, short legs with webbed feet, and powerful, tapered tails are well-suited for life both in and out of the water. A dense, water-repellent fur coat helps keep them insulated and dry. They typically weigh between 10 and 33 pounds and can measure 2.5 to 5 feet in length, including their tails.
Underwater Endurance
River otters can stay submerged, which supports their survival and hunting. They can hold their breath for up to eight minutes underwater. Most of their underwater excursions are shorter, typically lasting between one to four minutes. They routinely reach 60 feet or more below the surface.
Physiological Mechanisms
Their underwater abilities are supported by several biological adaptations. When a river otter dives, its heart rate slows significantly, a process known as bradycardia. This reduction can be substantial, decreasing from a resting rate of around 170 beats per minute to as low as 15 to 20 beats per minute, which conserves oxygen. Concurrently, their bodies redistribute blood flow, directing it away from less oxygen-sensitive areas like the limbs and towards organs such as the brain and heart. This ensures sufficient oxygen during the dive.
River otters have a larger lung capacity, allowing them to take in more air. This larger lung volume functions as an internal oxygen reservoir. Their muscles contain a higher concentration of myoglobin, a protein that stores oxygen, enabling muscles to function longer without a fresh supply. A transparent third eyelid, called a nictitating membrane, slides across their eyes to protect them and maintain clear underwater vision.
Life Beneath the Surface
River otters use their underwater endurance. Hunting is a major activity; they mainly consume fish, but their diet also includes crustaceans, amphibians, and occasionally small mammals or birds. They use various hunting strategies, sometimes working in groups to herd fish into shallower waters for easier capture. Their sensitive whiskers detect movement in murky water, aiding in locating prey.
Beyond foraging, otters use their underwater skills to avoid predators and navigate their environment. Their agility and speed in the water allow them to escape threats by diving, twisting, and turning. The streamlined body, powerful tail, and webbed feet facilitate efficient movement through complex underwater landscapes.
Playful behaviors are also a common sight beneath the surface, including diving, sliding, and aquatic acrobatics like twirls and somersaults. These activities help young otters develop hunting skills, strengthen social bonds within groups, and practice defensive maneuvers.