Whales and dolphins still have pelvis bones, left over from their ancestors' days as land-dwelling mammals. For years, scientists thought these bones didn't serve much purpose - like our wisdom teeth or tailbones - but California researchers say they've cracked the mystery of why cetaceans have hips.
It's for sex.
"Everyone's always assumed that if you gave whales and dolphins a few more million years of evolution, the pelvic bones would disappear," says Matthew Dean, an evolutionary biologist at the University of Southern California, in a statement. "But it appears that's not the case."
Among mammals, cetaceans have particularly mobile penises. And by taking 3-D scans of bones at the Natural History Museum of Los Angeles County, the scientists discovered that the cetacean penis muscles have to be anchored to the hip bone to achieve this mobility.
More "promiscuous" species - the ones in a more competitive mating pool - have larger pelvises, the USC press release describes:
The bigger the relative testis, the bigger the relative pelvic bone - meaning that more competitive mating environments seem to drive the evolution of larger pelvic bones. Males from more promiscuous species also evolve larger penises, so larger pelvic bones appear necessary to attach larger muscles for penis control.