The term “seal” commonly refers to any marine mammal belonging to the scientific order Pinnipedia, meaning “fin-footed.” These semi-aquatic carnivores are adapted to spend time both in the ocean, where they hunt for food, and on land or ice, where they rest and breed. This diverse group includes true seals, sea lions, fur seals, and the walrus. Scientists currently recognize 34 extant species within this order, each exhibiting specialized adaptations. These species are organized into three distinct families.
Defining the Major Groups of Pinnipeds
The 34 species of pinnipeds are organized into three distinct taxonomic families: Phocidae, Otariidae, and Odobenidae. These families account for all living species of seals, sea lions, and walruses. The largest group is Phocidae, often referred to as the true seals, which comprises approximately 90 percent of all pinniped species.
The second family, Otariidae, includes the eared seals, which are further divided into sea lions and fur seals. The third family, Odobenidae, contains only a single species, the walrus. These three groupings provide the scientific framework used to distinguish between the different types of fin-footed marine mammals.
Characteristics of True Seals
The family Phocidae, known as the true seals or earless seals, is distinguished by the lack of external ear flaps, having only small ear openings visible. These animals possess relatively short fore-flippers and large hind-flippers that are permanently oriented backward and cannot be rotated forward beneath the body. This anatomical structure dictates their movement on land, which is an awkward “belly-flopping” or undulating motion, relying on their abdominal muscles and fore-flippers to pull themselves along.
In the water, true seals are highly streamlined and use powerful, side-to-side movements of their hind-flippers for propulsion. Their fore-flippers function mainly for steering and balance. Notable examples include the massive Southern Elephant Seal, the largest pinniped, and the smaller Harp Seal. Phocids generally have a thick layer of blubber beneath their skin, which provides insulation and energy storage, particularly in cold, polar regions.
Key Differences of Eared Seals and Sea Lions
The family Otariidae, commonly referred to as eared seals, includes all sea lions and fur seals and is defined by several features that contrast with the true seals. The most obvious difference is the presence of small, visible, external ear flaps, or pinnae. Crucially, eared seals can rotate their large hind flippers forward and under their bodies, allowing them to lift themselves up and “walk” or gallop on all four limbs while on land.
This flexibility makes them significantly more agile on rocky coasts and beaches. For aquatic propulsion, eared seals rely predominantly on their long, powerful front flippers, using a sculling motion. Within the Otariidae, a distinction is made between sea lions, which generally have coarser hair and rely more on blubber for warmth, and fur seals, which possess a dense, insulating underfur. Sea lions, such as the California Sea Lion, are also known for being more vocal and forming large, social groups.
The Unique Classification of Walruses
The family Odobenidae contains only one living species, the Walrus (Odobenus rosmarus). The walrus is easily identified by its prominent, elongated upper canine teeth, known as tusks, which grow continuously in both males and females. Walruses share a characteristic with eared seals in that they can rotate their hind flippers forward to walk on land.
However, they lack the external ear flaps of the Otariidae, a trait they share with the Phocidae. Their thick, wrinkled skin is sparsely covered with hair, and the broad muzzle is equipped with hundreds of stiff, sensitive whiskers, or vibrissae, which are used to search for benthic prey on the seafloor. The walrus possesses a unique mix of physical traits and adaptations suited for its Arctic environment.