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

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

24th September 2009
Why do fish have vertical tail fins and whales have horizontal ones? (Getty)

Asked by: Rich Webb, Northampton

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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