The fucids, i.e. seals, differ from the otariids in the absence of external ears and the inability to tuck their hind flippers under their bodies when on land, hence their characteristic full-body crawl.

Otariids include fur seals and sea lions, which have small but visible ears. Their bodies are covered with hair, except for their fins. They have nails on the three central digits. The long front flippers are used for propulsion in the water, while the hind flippers act as rudders. They are also capable of bringing their hind flippers forward and under their bodies, so that they walk as if they were quadrupeds.

In Californian sea lions, there is an obvious difference between males and females, known as sexual dimorphism in a species. Males weigh three to four times more than females and are up to 1.2 times longer. Males grow to about 2.55 metres and weigh about 523 kg; females, on the other hand, are about 2 metres long and weigh, on average, about 110 kg.