Turning a long accepted evolutionary assumption on its head, scientists have discovered that whale pelvic (hip) bones play a crucial role in their mating.
Both whales and dolphins have hip bones, evolutionary remnants from the time their ancestors walked on land more than 40 million years ago.
Common wisdom has long held that those bones are simply vestigial - slowly withering away like tailbones on humans.
New research finds that not only do these pelvic bones serve a purpose - but their size and possibly shape are influenced by the forces of sexual selection.
"Everyone has always assumed that if you gave whales and dolphins a few more million years of evolution, the pelvic bones would disappear. But it appears that is not the case," said Matthew Dean, an assistant professor from the University of Southern California's college of letters, arts and sciences.
Along with Jim Dines from the Natural History Museum of Los Angeles County, Dean examined hundreds of whale pelvic bones.
Using a 3D laser scanner, they created digital models of the curved bones, offering an unprecedented level of detail about their shape and size.
They compared the size of the pelvic bones (relative to body size) to the size of the animal's testis (again, relative to body size).
The results were clear: the bigger the testis, the bigger the pelvic bone - meaning that more competitive mating environments seem to drive the evolution of larger pelvic bones.
"Males from more promiscuous species also evolve larger penises, so larger pelvic bones appear necessary to attach larger muscles for penis control," Dean noted.
"Our research really changes the way we think about the evolution of whale pelvic bones in particular, but more generally about structures we call vestigial. As a parallel, we are now learning that our appendix is actually quite important in several immune processes, not a functionally useless structure," Dean explained.
The paper was published online in the journal Evolution.