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Why do fish have vertical tail fins and whales have horizontal ones? © Getty Images

Why do fish have vertical tail fins and whales have horizontal ones?

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Splish, splash! Fish and sea mammals have very different tails and methods of swimming.

Asked by: Rich Webb, Northampton

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It reflects the evolutionary history of locomotion. Our wormy ancestors slithered on the sea floor, so undulated side-to-side. Fish inherited that movement, for which a vertical tail is best.

Their distant land mammal descendants evolved to run with limbs underneath: an unstable gait allowing rapid direction changes. To extend the stride, their spine flexes up-and-down. Marine mammals kept this movement, for which a horizontal tail is best.

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