Seals, Sea Lions, and Walrus: Pinnipeds
Pinnipeds are carnivores that have adapted to an amphibious marine existence. They forage at sea but most come ashore or onto ice at some time of the year to mate, give birth, suckle their young, or to molt. Many of their anatomical features reflect compromises needed to succeed in both marine and terrestrial environments. Externally, pinnipeds share many characteristics with terrestrial carnivores (fissipeds) due to their need for mobility on land.
Pinnipeds have four webbed flippers used to propel their spindle-shaped bodies. Their sensory organs are adapted to function in both air and water: large eyes and well-developed whiskers allow feeding in dimly lit water; tail and external ears are small, limiting drag. Pinnipeds have retained canine teeth but molars are modified for consuming prey whole. All have fur, which is shed or molted annually, but they are insulated primarily by blubber.
Pinnipeds are present in habitats ranging from ice to tropics, coastal to pelagic waters, and may live a migratory or sedentary existence. They are opportunistic feeders and consume their varied prey whole or in chunks. Many pinnipeds are capable of long, deep repetitive dives (to 4500 ft depths and 2 hours). This phenomenal diving ability is possible because of several physiological traits shared by cetaceans, such as high blood volume and reduced heart rate.
Taxonomic Relationships of Alaska Pinniped Species
Family Characteristics of Alaska Pinnipeds
Representatives of all three pinniped families are present in Alaska waters.
Otariidae ("eared" seals)
Otariids have visible external ear flaps (pinnae) and an elongate neck. They swim using their long front flippers for propulsion and hind flippers for steering. On land, otariids are agile and quadrupedal: able to pull their hind flippers up under the body and extend the front flippers for four-legged movement.
Adult otariids are sexually dimorphic: males are nearly twice as large as females, and have thickened necks and pronounced skull crests. Females breed with dominant males that establish and defend territories on breeding beaches or rookeries. Pups are born on the rookeries, and may be dependent on their mothers for a year or more.
Phocidae ("earless" seals)
Phocids have no visible external ear flaps (pinnae). They have short necks and short front flippers, and cannot pull their hind flippers under the body while on land. Although graceful swimmers, phocids are not agile on land and typically "phocidulate"; that is, they move by undulating or heaving the body in a caterpillar-like manner.
Sexual dimorphism is rare in adult phocids (seen only in elephant seals); males and females are generally similar in size and shape. Phocids breed in the water, on shore, or on ice, but (except for elephant seals) don't breed on rookeries. Pups grow at an incredibly fast rate and are weaned young and abruptly after brief lactation during which their mothers probably fast.
The walrus body form combines phocid and otariid traits. They have no external ear flaps (pinnae) but move quadrupedally on land. The upper canines in both sexes elongate and develop into tusks.
NOTE: Figures on this page show general body shape and are not drawn to scale.
Illustrations © Pieter Folkens