Whale Sex is Driven by the Hips: The Evolution of the Pelvic Bone

First Posted: Sep 10, 2014 09:00 AM EDT
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When it comes to whales and dolphins, certain bones seem to serve little to no purpose. Their hip bones, in particular, look as if they're just evolutionary remnants from when they used to walk on land. Now, though, scientists have found that these pelvic bones may be crucial when it comes to sex.

"Everyone's always assumed that if you gave whales and dolphins a few more million years of evolution, the pelvic bones would disappear," said Matthew Dean, one of the researchers, in a news release. "But it appears that's not the case."

In this latest study, the researchers analyzed cetacean pelvic bones. The muscles that control a cetacean's penis, which has a high degree of mobility, actually attaches directly to the pelvic bones. This means that the pelvic bones could actually affect the level of control over the penis that an individual cetacean has. By examining hundreds of pelvic bones, the researchers were able to tell whether this was indeed the case.

The scientists used a 3D laser scanner in order to create digital models of the pelvic bones. This showed an unprecedented level of detail when it came to their shape and size. In addition, it allowed the researchers to compared different bones. Then they compared the size of the pelvic bones to the size of the animal's testis.

So what did they find? It turns out that the bigger the relative testis, the bigger the relative pelvic bone. This seems to indicate that more competitive mating environments drive the evolution of larger pelvic bones.

"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 Dean. "As a parallel, we are now learning that our appendix is actually quite important in several immune processes, not a functionally useless structure."

The findings are published in the journal Evolution.

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