Why the whale kept its hips

Mystice pelvis (whale).png

A bowhead whale’s highly (but not entirely) reduced pelvis. Image via Wikimedia Commons.

Thanks to a remarkably good fossil record, it’s now well established whales and dolphins evolved from land mammals, their forelimbs adapting into flippers, and their hind-limbs almost entirely disappearing. If you’d asked me yesterday what’s going on with that almost—the last vestiges of the hip bones that whales retain, which have no legs to support or direct contact with the rest of their skeletons—I’d have told you they were evolutionary leftovers, and probably going to disappear in another million years or so. I think a lot of other evolutionary biologists (those who aren’t whale specialists, anyway) would’ve agreed with me. But it turns out we’d have been wrong.

As Carl Zimmer describes, a paper recently published in the journal Evolution points out that whales’ hips do have one remaining function, an important one—they anchor muscles that control the penis. And that function is under ongoing sexual selection.

The more promiscuous a [whale] species was, the bigger its pelvis bones tended to be. The scientists also found that as whales evolved to become more promiscuous, their pelvic bones changed shape. These changes weren’t part of some general change to their skeleton, however. The ribs near the hips didn’t show the same patterns of size and shape change.

I strongly recommend Zimmer’s whole article, and you can also read the original research article in Evolution.