Hip bones play key role in whales' reproduction

A new study has revealed that the pelvic (hip) bones in whales were not vestigial, but helped in their reproduction process.

The research from University of Southern California has refuted the assumption of whales and dolphins' pelvic bones being vestigial, which would have slowly withered away like tailbones on humans.

The muscles that control a cetacean's (whale and dolphin) penis, which has a high degree of mobility, are attached directly to its pelvic bones, which could affect the level of control over the penis that an individual cetacean has, perhaps offering an evolutionary advantage.

Researchers, Jim Dines and Matthew Dean, compared the size of the pelvic bones (relative to body size) to the size of the animal's testis (again, relative to body size).

The results clearly show that the bigger the relative testis, the bigger the relative pelvic bone, which meant that more competitive mating environments seemed to drive the evolution of larger pelvic bones.

Males from more promiscuous species also evolve larger penises, so larger pelvic bones appeared necessary to attach larger muscles for penis control.

Dean said that the findings changed the way they thought about the evolution of whale pelvic bones in particular, but more generally about structures called 'vestigial.'

The study is published online by Evolution.