The pelvic bones of whales and dolphins may have been important to their land-dwelling ancestors, which had legs attached to their hips and walked on Earth millions of years ago.  For these animals that evolved to become seagoing creatures, however, the structure does not seem to have any use at all so evolutionary scientists consider these bones as vestigial that would later on disappear in another few million years.

It appears, however, that the pelvis of whales and dolphins are not as useless as it seems. A group of biologists have found out that the structure still serve a purpose for these marine animals. The pelvic bones anchor the muscles that control these mammals' penises and thus help them better maneuver their penises during sex.

"Contrary to popular belief, these are not vestigial structures. They do have a function. That function is reproduction, and they evolved in response to sexual selection," said Jim Dines, from the Natural History Museum of Los Angeles County in Los Angeles.

For a new study published in the journal Evolution, Dines and colleagues used sophisticated imaging technologies to examine hundreds of cetacean pelvic bones at the Smithsonian Institution and Natural History Museum of Los Angeles County (NHM). They also gathered data on the size of these animal's testes and found that larger testes were associated with larger pelvic bones and the more promiscuous species turned out to have larger penises and pelvic bones.

The monogamous river dolphin franciscana (Pontoporia blainvillei), for instance, was found to have the smallest testes and pelvic bone while the right whale (Eubalaena glacialis), considered as the most promiscuous of the cetaceans examined had the biggest pelvic bone and testes.

"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'," said study author Matthew Dean, from the University of Southern California in Los Angeles. "As a parallel, we are now learning that our appendix is actually quite important in several immune processes, not a functionally useless structure."

The researchers also compared the creatures' testes size with their size to find out if a larger testes has something to do with a bigger animal but they did not find a size correlation suggesting that the pelvic bones of whales and dolphins are the result of sexual selection.

"This study provides evidence that sexual selection can affect internal anatomy that controls male genitalia," the researchers wrote. "These important functions may explain why cetacean pelvic bones have not been lost through evolutionary time. 

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