LOS ANGELES, Sept. 9 (UPI) -- Because pelvic bones are vital for walking, but unnecessary for swimming, scientists have long dismissed the isolated pelvic bones of whales and dolphins as a useless relic -- forgotten by time and evolution.
But a new paper shows the pelvis is quite useful, not for walking but for reproductive purposes. The muscles of a male whale's penis attach directly to the pelvis, giving the mammal greater control of its reproductive organs.
"People that really know the reproductive biology of whales and dolphins already know and have known that these pelvic bones are an anchor point for reproductive organs," study author Jim Dines told the Washington Post. "But it's not something that they teach you in a marine mammal class."
Dine, manager of the mammals collection at the Natural History Museum of Los Angeles County, co-authored the paper with Matthew Dean, an assistant professor at the USC Dornsife College of Letters, Arts and Sciences. Their work appeared this week in the journal Evolution.
"Everyone's always assumed that if you gave whales and dolphins a few more million years of evolution, the pelvic bones would disappear. But it appears that's not the case," Dean said in a press release.
A number of studies have already shown that sexual competition promotes the evolution of larger and larger reproductive organs. But the work of Dines and his colleagues go one further by surmising that pelvises in marine mammals, like whales and dolphins, have grown in size, facilitating control of increasingly large testes and penises.
Dines and Dean confirmed their suspicions by first measuring the size of hundreds of pelvic bones from a variety of museum whale and dolphin fossil collections. They also mined recorded data on whales' testes size relative to body size. When they compared the two datasets, they were able to show that the bigger the penis, the bigger the pelvic bone.
The research is evidence that not only is the pelvis not useless, but vital if a male whale wants to maintain control of his growing appendage.
"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, told the Post. "That's basically how a whale's penis is working."