Researchers studying the pelvic bones of dolphins and whales reveal that these seemingly useless remnants of an ancient landlubber lifestyle actually have a purpose. Turns out, cetacean pelvic bones help control the motion of the ocean, and their size and shape are likely influenced by the forces of sexual selection. This study, published in Evolution this week, provides the first evidence that sexual selection affects the internal anatomy controlling male genitalia. 

Since evolving from land-dwelling ancestors more than 40 million years ago, whales and dolphins have lost their hind limbs and evolved a highly reduced pelvic (or hip) bone, which seems to serve no purpose other than to anchor muscles used for maneuvering the penis. They were considered vestigial leftovers, like our tailbones and appendices. “Everyone’s always assumed that if you gave whales and dolphins a few more million years of evolution, the pelvic bones would disappear,” USC’s Matthew Dean says in a news release. 
    
Male genitalia, on the other hand, are known to evolve rapidly, likely as a result of sexual selection. The cetacean penis has a high degree of mobility, and the muscles that control the dexterous penis attach directly to the curved pelvic bones. "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 explains to Washington Post. If the pelvic bones actually exert control over penis movements, perhaps keeping them offered an evolutionary advantage.

So Dean, Jim Dines from the National History Museum of Los Angeles County, and colleagues analyzed the evolution of the size and shape of pelvic bones from 130 cetaceans spanning 29 species. Check out this picture of dolphin pelvic bones, with researchers for scale: 

They used a 3D laser scanner to create digital models of the bones, then gathered heaps of data about testis size relative to body mass. After all, promiscuous animals are known to develop larger testes to outperform the competition.

By comparing the size of pelvic bones to the size of the testis -- relative to body size -- the researchers found that the bigger the testis, the bigger the pelvic bone. 

Males from species with intense sexual selection (judging by testes size) evolved relatively large penises and pelvic bones compared to their body size; more competitive mating environments seem to drive their evolution. Larger penises presumably require larger pelvic bones for attaching larger muscles for adequate penis control. As a negative control, the team looked for these patterns in the animal’s ribs. If the size of the pelvic bone was simply a reflection of overall skeleton size, there’d be a corresponding correlation in the ribs -- but there wasn’t. 

“We’ll never be able to ask a female whale, ‘was it good for you?'” Dines tells Time. “But it’s plausible that if you can maneuver the penis in a slightly different way, there could be an evolutionary advantage.”

Images: NOAA (top), USC Photo/Gus Ruelas (middle)