Why do seals have flippers instead of legs?

Seals do not have legs in the traditional sense because they are pinnipeds, a group of aquatic mammals that have evolved specialized, winglike flippers to thrive in marine environments.1

While seals are descendants of terrestrial, bearlike ancestors, their anatomy has undergone significant adaptation over millions of years to prioritize efficiency in the water rather than on land. These modifications include:1

• Propulsion: Seals use their flippers to navigate the water with agility and speed. Depending on the family, they may use their forelimbs or hind flippers for movement. For example, true seals (family Phocidae) move their hind flippers side to side like a fish tail to swim.12
• Anatomical Constraints: Unlike sea lions and fur seals, which can rotate their hind flippers under their bodies to walk on land, true seals have hind flippers that cannot rotate. This anatomical limitation forces them to move on land by flopping on their bellies, a motion similar to that of a caterpillar.2

Ultimately, these flippers are highly specialized tools that provide the lift and thrust necessary for these mammals to be swift and graceful hunters in the ocean.13