The pelvic bones of whales and dolphins, long thought of as leftover remnants from when their ancestors were land creatures, still have an important function -- and it's all about sex, researchers say.

The pelvic area of cetaceans -- all whales and dolphins -- has continued to evolve to serve to anchor muscles that give male cetaceans exquisite control over the penis, researchers report.

And far from dwindling away, the pelvic bones seem to be continuously evolving for ever-better levels of "performance," they say.

"We'll never be able to ask a female whale, "was it good for you?'" says Jim Dines of the Natural History Museum of Los Angeles County.

"But it's plausible that if you can maneuver the penis in a slightly different way, there could be an evolutionary advantage," says Dines, co-author of a study appearing in the journal Evolution.

Dines and study co-author Matthew Dean from the University of Southern California created 3-D scans of hundreds of cetacean pelvic bones, analyzing the shape and size.

Whale sex remains enigmatic because it's been predictably unobserved, but in nature larger testes and penises generally evolve with increased sexual competition among many males vying for a single female, the researchers say.

The unique pelvis shapes found in whales and dolphins may give them the capability of moving their reproductive organs in complex ways, and in animal species where many males are in aggressive pursuit of pursue females, increased dexterity might be the key to success, they say.

In whales, the researchers suggest, the pelvic bones are involved in that dexterity, since they anchor two powerful muscles that can contract with different tensions to control the action of the penis.

"I think of it like a trick kite, controlled by two strings and capable of complex motion," Dean says.

The findings have changed the thinking on structures once considered "vestigial" and destined for the evolutionary scrapheap, like the tailbones on humans, he says.

"Everyone's always assumed that if you gave whales and dolphins a few more million years of evolution, the pelvic bones would disappear," he says. "But it appears that's not the case."

The evolution of the pelvic bones of whales and dolphins from hipbones to their new improved function likely was a relatively quick process, the researchers say.

"Male genitalia evolve much faster than other parts of the body," because even slight mating advantages can provide an individual animal with more offspring, Dean says.