Whales Kept Their Hip Bones For Better Sex

Experts have long thought that the pelvic bone that floats unattached to the rest of a whale's skeleton is simply an evolutionary throwback from when their ancestors boasted four flippers. Now, researchers argue that whales actually are using this seemingly useless bone for very specific mating purposes.

Shin bone connected to the knee bone. Knee bone connected to the thigh bone. Thigh bone connected to the ... hip bone?

Wait, wait, no it's not. Not in whales at least.

When we're talking about whale anatomy, the well-known spiritual song "Dem Bones" quickly falls to pieces. And even if we did try to still run with it, we'd have a very strange song on our hands.

That's largely because in whales and dolphins, the hip bone is connected to absolutely nothing at all, leaving experts to assume that the bone is nothing more than an evolutionary throwback without any practical use in modern whales - a lot like the tailbone in modern humans.

However, new research conducted by experts from the University of Southern California (USC) and the Natural History Museum of Los Angeles County (NHM) battles this assumption, arguing that size and even shape of these "useless" pelvic bones are still under the influence of sexual selection.

"Everyone's always assumed that if you gave whales and dolphins a few more million years of evolution, the pelvic bones would disappear," Matthew Dean, a USC researcher, explained in a statement. "But it appears that's not the case."

Dean co-authored a study recently published in the journal Evolution that details how the muscles that control a male cetacean's (whales and dolphins) genitals are attached to the pelvic bone.

Dean and Jim Dines of NHM spent four years examining cetacean pelvic bones from the first and second largest collections of marine mammal specimens in north America (the Smithsonian and NHM, respectively).

They quickly found a very obvious trend. The larger the animal's testes, the larger the pelvic bone, irrespective of the animal's overall size. This makes sense, as a larger reproductive organ requires more muscle to control it. And in nature, bigger is better - at least for testes - as it allows a dominant male to mate with more females in a shorter amount of time, compared to competition with smaller assets.

The authors add that a more muscular penis could simply make cetacean males more preferable to cetacean females, because the sex would be more pleasurable for these highly social animals.

"We'll never be able to ask a female whale, 'was it good for you?'" Dine admitted to TIME magazine, "But it's plausible that if you can maneuver the penis in a slightly different way, there could be an evolutionary advantage."