Why do whales swim by moving their tails up and down, instead of side-to-side like a fish?
The ancestors of sea mammals were land mammals. Because air is a very thin medium that does not help support the body, land mammals evolved strong bones and muscles to support themselves against the constant downward pull of gravity. For that reason, all mammals have muscles and bone arrangements that produce strong support and motion in the vertical (up-and-down) plane. These “up-and-down” muscles are much stronger and better developed than those that operate in a horizontal (side-to-side) plane. This means that the land ancestors of whales and dolphins passed on a body musculature best suited for strong vertical motions rather than strong side-to-side motions. It is the vertical motion of the tip of the tail that has developed into the strong swimming motions of the whales, dolphins and porpoises.
