Researchers looked at the key role whales' pelvic bones play in the process of reproduction.
Whales and dolphins have hip bones leftover from their land-roaming ancestors that existed 40 million years ago. In the past researchers believed these bones were slowly withering away, but new research suggests these pelvic bones actually do serve a purpose and influence sexual selection, the University of Southern California reported.
"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," said Matthew Dean, assistant professor at the USC Dornsife College of Letters, Arts and Sciences, and co-corresponding author of a paper on the research that was published online by Evolution on Sept. 3.
The muscles that control a cetacean's penis attach directly to the pelvic bone, which could affect its mobility. To test this idea the researchers looked at hundreds of pelvic bones from the Natural History Museum of Los Angeles County (NHM) and Smithsonian Institution.
"Cetacean skeletons are stored as boxes of bones on warehouse shelves, with each box containing an individual specimen. You have to comb through each box looking for the specific bone you need. The pelvic bones are comparatively small and aren't always collected with the rest of the skeleton, but after the first couple hundred boxes we got very good at finding them when they were present," said corresponding author Jim Dines, Collections Manager of Mammalogy at NHM and one-time a graduate student in Dean's lab.
The researchers looked at data on whale testes size dating as far back as the time of the whalers. They also used a 3D laser scanner to create digital models of the curved bones, allowing them to manipulate them.
Larger testes have been linked to promiscuity in whale populations, meaning males most likely develop larger testes to outperform others. The researchers compared the size of the pelvic bone relative to body size with the testes, and found the larger the testes the larger the pelvic bone, meaning a more competitive mating environment most likely drives the evolution of larger pelvic bones.
"Our research really changes the way we think about the evolution of whale pelvic bones in particular, but more generally about structures we call 'vestigial.' As a parallel, we are now learning that our appendix is actually quite important in several immune processes, not a functionally useless structure," Dean said.
