Otters are charismatic semi-aquatic mammals found across nearly every continent except Australia and Antarctica. They belong to the Mustelidae family, which also includes weasels and badgers, and are highly adapted to life in or near water. This specialized existence raises questions about their primary food sources and how they secure them.
Dietary Classification: Carnivores, Not Herbivores
Otters are classified as obligate carnivores, meaning their diet relies almost entirely on animal protein for survival. This places them in contrast to herbivores, which subsist on plant matter. Their digestive system is adapted for a meat-based diet, featuring a relatively short alimentary canal typical of carnivores.
This short digestive tract facilitates the rapid processing of animal tissue, often passing food through in just one to four hours. This rapid digestion supports their high metabolic rate, which is necessary to maintain body temperature in aquatic environments. Their dentition reflects this need, featuring sharp canine teeth for gripping and shearing prey.
Specific Prey Items in Freshwater and Marine Habitats
The specific diet of an otter varies significantly depending on whether the species inhabits freshwater or marine environments. Freshwater species, such as the North American River Otter and Eurasian Otter, primarily consume fish, often preferring slower-moving species like suckers, catfish, and carp. These river otters are generalist predators, supplementing their fish intake with crayfish, crabs, frogs, and other aquatic invertebrates. They opportunistically prey on small mammals, birds, and bird eggs when available.
Marine species, most notably the Sea Otter, have an invertebrate-heavy diet. Sea Otters dive to the ocean floor to retrieve hard-shelled prey.
Marine Invertebrates
Sea otters consume a variety of hard-shelled prey:
- Sea urchins
- Crabs
- Clams
- Mussels
- Abalones
Fish represent a smaller portion of the sea otter’s diet compared to river otters. Individual sea otters often specialize in only a few types of available invertebrates, which may help reduce competition within the population.
The Asian Small-Clawed Otter feeds mainly on crabs, other crustaceans, molluscs, and frogs, with fish being less significant. This unique diet is reflected in their teeth. Sea otters and African Clawless Otters have broad molar teeth adapted for crushing shellfish, while European otters have narrower teeth better suited for slicing fish.
Specialized Feeding Behaviors and Tool Use
Otters employ specialized techniques to hunt and consume their prey, utilizing their physical adaptations and intelligence. Their sensitive whiskers, known as vibrissae, allow them to detect the movement of prey in the water, even in murky conditions or darkness. River otters and Giant Otters use this tactile sense to locate fish and crustaceans before capturing them with their teeth.
The Sea Otter is one of the few non-primate mammals known to use tools for foraging. They use rocks or other hard objects as anvils to crack open the tough shells of clams, mussels, and sea urchins while floating on their backs. This behavior allows them to access larger, harder prey items and helps reduce tooth damage.
Female sea otters are more likely to use tools, enabling them to consume prey that is up to 35% harder than that eaten by males. This strategy is beneficial for females meeting the high caloric demands of raising pups. Some otters, such as the Cape Clawless Otter, have also been observed using rocks to break open mussel shells, a behavior that is learned and passed down through generations.