The pelves of whales and dolphins have a particular purpose when making babies, said Erik Otárola-Castillo, a fellow in David Pilbeam's paleoanthropology lab in the Department f Human Evolutionary Biology who co-authored a study about this topic.

Promiscuous mating behavior in cetacean species is often linked to the intensity of the competition among males for females. Because of the high demand, female cetaceans become choosier and only mate with males they see are better than others.

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In the wild, a female whale surfaces with its belly up to avoid mating with males it does not desire.

Otárola-Castillo, working alongside Matthew Dean and Jim Dines from the University of Southern California, found that males bypass this problem by evolving longer reproductive organs relative to their body size. Researchers also discovered that the pelvic bones of males anchor the muscles that control the movement of the whale's penis.

Otárola-Castillo also compared and quantified the shape of the pelves of a number of sister-species that had high divergent promiscuity levels. He discovered that the divergence in the shape of the pelvis was extremely high.

The more convoluted and less simple the shape of the pelvis, the more promiscuous the mating system.

"At the less promiscuous end of the spectrum, the pelves look quite straight, possessing little curvature," he said. "On the other hand, the pelvis bones of the more promiscuous species are much more curved and contorted."

As to the cause for the divergence between closest relatives, Otárola-Castillo thinks that the control over sexual organs might be specifically important.

"In some species, females may mate with multiple males in a relatively short time," he said. "I know of at least one occasion when a female whale was observed mating with more than one male at the same time.

"Only one male will win this competition by inseminating the female and ensuring a chance to pass his genes along. This degree of competition creates a sort of sexual-organ arms race."

TagsWhale, Whale pelvis, Promiscuity, Whale promiscuity, whales, Whale penis, evolution