Seals are marine mammals that navigate both aquatic and terrestrial environments. Their distinctive body shape, evolved over millions of years, supports their semi-aquatic existence. This design allows them to thrive in diverse marine habitats, from polar waters to temperate coasts, efficiently pursuing prey underwater and moving on land for resting and breeding.
Hydrodynamic Design for Aquatic Life
The streamlined, torpedo-like or fusiform body shape of seals minimizes drag in water. This sleek contour is maintained by a thick layer of blubber, which provides insulation in cold waters and contributes to buoyancy. Blubber can be up to 6 cm thick over the chest in adult grey seals, helping them retain warmth even in near-freezing temperatures.
Seals propel themselves using their flippers. True seals (phocids) use powerful sideways movements of their hind flippers, spreading webbed toes for propulsion. Their fore flippers assist with steering and stability. Eared seals, such as sea lions, use strong front flippers in a wing-like motion to “fly” through the water. These features enable seals to swim quickly, with harbor seals reaching speeds of up to 19 kilometers per hour (12 miles per hour).
Shape and Terrestrial Movement
Seal body shape and flipper structure influence their movement on land, with distinct differences between true seals (phocids) and eared seals (otariids).
True seals have hind flippers that extend directly behind their bodies and cannot rotate forward. This anatomical limitation results in a characteristic “humping” or “wiggling” movement, as they push themselves forward using chest and abdominal muscles.
Eared seals, including sea lions and fur seals, can rotate their longer hind flippers forward under their bodies. This allows them to use all four limbs for locomotion, enabling them to “walk” or “gallop” on land with greater agility than true seals. Their strong fore flippers also provide support during terrestrial movement.
Diversity in Seal Body Forms
While seals share a generally streamlined shape, their body forms show diversity, reflecting adaptations to specific habitats, prey, and lifestyles. For instance, the massive blubber layer and large size of elephant seals are adaptations for deep diving and insulation in cold ocean waters.
In contrast, the leopard seal, a true seal, has a distinctively long and muscular body, which helps it hunt fast-moving prey. Unlike most true seals, leopard seals have evolved wing-like front flippers, similar to eared seals, for powerful swimming to enhance speed when pursuing swift prey. These variations show how each species’ shape is tuned to its ecological niche, enabling specialized foraging and survival.