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How does an animal’s body shape reduce drag while swimming?

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Least tern, Sternula antillarum, flying with wings raised and its yellow-tipped black bill pointed forward. — Locomotion These birds largely reside in coastal areas of North America, although some populations live in the continent's interior along large rivers.

To reduce drag while swimming, many aquatic animals have evolved a fusiform (streamlined) body shape. This shape is highly effective because it minimizes turbulence as the animal moves through the water.1

Drag in water is primarily caused by the viscosity of the fluid, which creates friction as water flows over the animal's surface, and by the formation of eddies behind the animal's tail. A streamlined body allows water to flow smoothly around the animal; if the body is not shaped to allow these water layers to blend gradually, they meet abruptly, creating high turbulence and a strong backward pull.1

Furthermore, fast-swimming animals often concentrate their thrust in the tail and reduce or fold their limbs against their bodies to maintain this efficient, low-drag profile. In fish, a fusiform body facilitates carangiform locomotion, where only the posterior portion of the body flexes, creating minimal lateral displacement and turbulence.23