Close-Up View of Sea Lion Swimming Underwater

Leftwich says that sea lions generate thrust, or forward propulsion, by bringing their fore-flippers together in big sweeping motions called “claps.” But if you’re imagining flippers coming together with a loud smacking sound, think again. When a sea lion “claps,” it stretches its flippers out to the sides and sweeps them down. Then it tucks its flippers against its body, forming a torpedo shape that glides easily through the water.

The sea lion is the only aquatic mammal that swims this way. Most swimmers—from the tuna fish to the sea lion’s cousin, the seal—generate thrust with the back ends of their bodies, using their tails to propel themselves through water. But sea lions use their fore-flippers. What’s more, they’re very good at it. One clap generates enough thrust to let a sea lion glide through the water, leaving it free to twist or roll with very little additional movement.