Hips don’t lie. Purpose of a whale’s pelvis? It’s all about sex

Years ago — actually 40 million years ago –whales and dolphins used their hip and pelvic bones to walk on land.

Like humans who still have tailbones that are simply vestigial, it was long believed that the bones of these cetaceans were similar and would slowly wither away.

But new research has now discovered that pelvic bones aren’t just remnants of evolutionary biology but their size and shape are influenced by sexual selection.

Give whales and dolphins a few more years — okay a few more million years of evolution — and the belief had been that pelvic bones would disappear, kind of like our tail bones will.

But Matthew Dean, assistant professor at USC Dornslife college of Letters, Arts and Sciences based in California says that doesn’t appear to be the case.

Dean and co-corresponding author Jim Dines, collections manager of mammalogy at the Natural History Museum in Los Angeles and a former student in Dean’s lab undertook a four-year project to analyze pelvic bones of cetaceans.

A cetacean’s penis has a high degree of mobility (!) and the muscle that controls it is attached directly to its pelvic bones. For evolutionary advantage reasons, control over the penis could have its benefits.

Hundreds of pelvic bones that had been gathered at the NHM and the Smithsonian Institute which has the second and largest respectively collections of marine mammal specimens in North America were examined.

What the penis has in mobility (again !) the pelvic bone in comparison is relatively small and often not collected with the rest of the cetacean’s skeleton.

Dines and Dean combed through hundreds of boxes to find the specific bone they were looking for and used a 3-D scanner to create digital models of the pelvic bone, which has a curved shape.

They then went back to the data gathered from whalers about the size of testis related to body mass in whales. There’s a promiscuity theory in nature–more mating creates a more competitive mating environment and for whales larger testes compared to body mass was one way to outperform the competition.

The two scientists compared the size of the pelvic bone compared to the body size and compared that again to the animal’s testis. Results: bigger the testis, bigger the pelvic bone which meant a more competitive mating environment which appeared to drive the evolution of larger pelvic bones.

Males from more promiscuous species also had larger penises, so larger pelvic bones appear necessary to attach larger muscles for penis control.

Ah, you may be wondering–what if the pelvic bone size was simply a sign of the overall skeletal size?

To offset this possibility, the two scientists compared the testis size to the size of the animal’s ribs. There wasn’t a correlation which led to the interpretation that whale pelvic bones are specifically targeted by selection related to mating system.

And scientists now believe that it should change the way we consider any structure vestigial.

The appendix? It not’s a functionally useless structure after all, in fact, it could be important in several immune processes.


Photo credit: USC Photo/Gus Ruelas

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Peg Fong is also in recovery from newspapers

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