In dolphins and whales, the pelvis is widely considered to be a good-for-nothing remnant of legged days past. But that pelvic bone didn't become useless when the animals switched from walking to swimming: It may be downright vital for successful reproduction.
A new study published in the journal Evolution reports that the bones have actually continued evolving. No longer adapted to support hind legs, the pelvis has been swayed by sexual selection. The muscles that control the penis attach directly to the pelvic bone, contributing to the crazy amount of control that both whales and dolphins have over the organ.
"People that really know the reproductive biology of whales and dolphins already know and have known that these pelvic bones are an anchor point for reproductive organs," co-corresponding author and Collections Manager of Mammalogy at the Natural History Museum of Los Angeles County Jim Dines said. "But it's not something that they teach you in a marine mammal class." Outside of a small circle of experts in whale anatomy, he said, the common consensus, even among marine biologists, was that the pelvis was a useless bone -- one that would disappear, given a few million years more of evolution.
To give new support to what vintage anatomy texts already claimed, Dines and his co-author Matthew Dean, assistant professor at the USC Dornsife College of Letters, Arts and Sciences, 3D scanned hundreds of bones (many from the Smithsonian) and analyzed them based on size and shape.
Finding evidence of a sexual use was a bit tricky. Whale sex is a particularly enigmatic thing and remains basically unobserved. But we do know that in all animals, larger testes and penises evolve when there's more sexual competition (in other words, more males vying for the same female). If the pelvis evolved to aid in reproduction, the researchers hypothesized, then it would get bigger to support a larger reproductive organ. Sure enough, they observed that trend. As a control, they measured the rib size of each specimen, as well. If the whole skeleton got bigger, the ribs would have been larger, too. But while pelvis and testes were correlated in size, ribs were not.
Dines and Dean also found that the whales and dolphins with the greatest difference in testes size also had the greatest variation in pelvic shape. The researchers can't be sure what this means, but they believe that unique pelvis shapes might allow whales and dolphins to move their reproductive organs in novel ways. In species where males aggressively pursue females, more dexterity might be the key to reproducing.
"It's like someone operating a trick kite, where you pull two strings, and pulling left and right makes it go in a loop-de-loop," Dean said. "That's basically how a whale's penis is working."
"This study represents a tremendous amount of work by Jim Dines and his colleagues, who did a beautiful job of extracting a compelling story on cetacean evolution from a massive dataset of museum specimens," said Maya Yamato, a researcher at the Smithsonian's National Museum of Natural History.
But Dean and Dines did more than just vindicate the whale pelvis. Their use of a 3D scanner to analyze the bones -- which allowed them to measure thousands of coordinates instead of just length, width, and depth -- means that the specimens can now be analyzed anywhere. "I love that they made the 3D laser scanned data publicly available," Yamato said. "It's a tremendous resource for science and outreach."