Seal is a Sneaky Swimmer, The - Science Stories

Terrie Williams

A scientist caught it saving energy.
Terrie Williams was once a swimming instructor teaching people to swim. With practice, some people learn to be good swimmers, zipping through the water with ease.

But Terrie observed that, even with lots of practice, people must work hard and spend a great deal of energy in swimming. That observation led her to study other animal swimmers, from muskrats and otters to porpoises and seals. She has become an authority on marine mammals.

One of Terrie’s favorite subjects is the common harbor seal, which is often seen at aquariums and zoos as a trained performer. Her seals were trained to swim against a current of water pumped through a water channel. For a swimmer, that activity is like a person walking or running on the moving belt of a treadmill. The swimmer stays in one place and can be connected up with wires and tubes to equipment that records the heart and breathing rates of the swimmer.

When people swim in the water channel, their bodies behave just as they do during other forms of exercise. The heart beats faster to get more blood to working muscles. Both the breathing rate and the use of oxygen from the air increase with swimming speed to meet the muscles’ increasing demand for oxygen.

A common harbor seal with her pup.

Seals that swam in the water channel behaved very differently from people. At cruising speed, a seal spent only short spurts of three or four seconds swimming on the surface. While there, the seal would stick up its nose for a few quick breaths. In between breaths, it spent much longer periods—about thirty seconds— submerged and swimming several feet below the surface.

A Sensible Pattern
This swimming pattern makes sense for the animal. A seal spends most of its life hunting for fish and squid far below the surface. Of course it has a problem because it breathes air into lungs just as you do. So a seal must come to the surface to get its air.

You may be wondering, just as I did, why a seal spends most of its time swimming below the surface, even when it is not hunting. The explanation lies in the behavior we see in any moving object. An object’s motion is opposed by a force we call friction. For an object moving in water (or air), that frictional force is called drag. In swimming, that’s the force that seems to be holding you back.

Now, it’s a surprising fact that drag in water becomes greater if the object comes close to the surface. Anything moving on or close to the surface makes waves. And that takes extra energy.

Measuring Drag
Terrie and her team checked out this idea. They measured the drag that a seal must overcome in swimming. To do it, first they trained seals to chomp down on a rubber mouthpiece that could be pulled by a rope. Then they measured the pounds of force on the rope when the seals were towed at different speeds both at the The Seal Is a Sneaky Swimmer surface and submerged in a swimming pool.

As expected, the faster the seal was moving, the greater the drag force that worked against that movement. But even at the seal’s low cruising speed of three miles per hour, drag at the surface was almost two times greater than the drag when submerged. At a higher speed of four and one-half miles an hour the surface drag was almost three times greater than the submerged drag. Terrie concluded that swimming below the surface must be a lot easier. Seals are no dummies. They probably discovered this early in their lives.

The Seal’s Routine
So the seal has a special swimming routine: quick breaths at the surface, then a longer swim underneath. That is possible because of some features of its body machinery. At the surface, the seal’s heart revs up extra fast, to as many as 140 beats a minute. It is pumping blood past the fresh air in its lungs.

The seal can hold a big supply of oxygen. Its body has a lot more blood than yours does. And the blood has more of that red hemoglobin that carries oxygen. Even the seal’s muscles are extra red with a special hemoglobin that gives the muscles an extra oxygen storehouse. Just a few seconds of swimming at the surface loads up the seal’s body with enough oxygen for a long underwater swim.

Terrie Williams has learned how seals can be sneaky swimmers.

Why You Can’t Beat a Seal

Your body can’t store as much oxygen for swimming underwater.
With your knees and elbows sticking out, you have lots more drag than a seal, which is streamlined. So even for slow swimming, you have to spend energy three times as fast as a seal does.