Cape Town - Yes, size really does count, especially when you’re a whale and you need to swing those hips to… well, get into the groove.
This is the finding of new research that appears to have turned on its head a long-standing assumption about the evolution of cetaceans (whales and dolphins): that far from being just vestigial – that is, atrophied to the point of being completely functionless, like humans’ tailbones – these creatures’ small “free floating” pelvic bones play a key role in their reproduction.
The research, by scientists from the University of Southern California (USC) and the Natural History Museum of Los Angeles County, appears in the latest online edition of the journal Evolution.
Both whales and dolphins have pelvic (hip) bones, evolutionary remnants from when their ancestors walked on land more than 40 million years ago, say the authors in a media release.
It’s been long believed that those bones are simply vestigial, slowly withering away, but the new research finding directly contradicts that assumption. Not only do the pelvic bones serve a purpose, but their size and possibly shape are influenced by the forces of sexual selection, they argue.
“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 co-author and assistant professor at USC, Matthew Dean.
Dean collaborated with co-author Jim Dines, collections manager of mammalogy at NHM and one-time a graduate student in Dean’s lab, on a painstaking four-year project to analyse cetacean pelvic bones.
The muscles that control a cetacean’s penis – which has a high degree of mobility – attach directly to its pelvic bones, they say. Therefore, it makes sense that these pelvic bones can affect the level of control over the penis that an individual cetacean has, perhaps offering an evolutionary advantage.
To test their hypothesis, they examined hundreds of pelvic bones found in more than 10 000 boxes – first at the Los Angeles museum which has the second-largest collection of marine mammal specimens in North America, and then at the Smithsonian Institution in Washington DC, which has the largest.
“Cetacean skeletons are stored as boxes of bones on warehouse shelves, with each box containing an individual specimen,” said Dines. Using a 3D laser scanner, they created digital models of the curved bones, offering an unprecedented level of detail about their shape and size, as well as giving them the option to computationally manipulate them for comparison purposes.
Then, they gathered data from as far back as the whaling era, looking at testis (testicle) size relative to body mass in whales.
Throughout nature, more “promiscuous” animal species – that is, those with females who mate with several males, creating a more competitive mating environment – develop larger testes relative to their body mass as a way of out-performing the competition, they explain.
Finally, they compared the size of the pelvic bones to the size of the animal’s testis, both relative to body size.
The results were clear, they say: 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.
“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’.”
Cape Argus