Whales and dolphins need their hips, it turns out. The bones that we used to believe were vestigial turn out to be important to reproduction. The muscles that control a cetacean’s penis — which has a high degree of mobility — attach directly to its pelvic bones. As such, it made sense to researchers that the pelvic bones could affect the level of control over the penis that an individual cetacean has, perhaps offering an evolutionary advantage.
“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.
“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.
It took them a while to find the bones, because they tended not to be kept, as they were believed to be vestigial.
Maybe it will turn out that there are hardly any vestigial structures at all … Just as lots of vestigial organs would prove that Darwinian evolution is true, not finding any would also prove that it is true.
Isn’t that the sign of a strong theory?
Male genitalia evolve rapidly, probably as a result of sexual selection. Whether this pattern extends to the internal infrastructure that influences genital movements remains unknown. Cetaceans (whales and dolphins) offer a unique opportunity to test this hypothesis: since evolving from land-dwelling ancestors, they lost external hind limbs and evolved a highly reduced pelvis which seems to serve no other function except to anchor muscles that maneuver the penis. Here we create a novel morphometric pipeline to analyze the size and shape evolution of pelvic bones from 130 individuals (29 species) in the context of inferred mating system. We present two main findings: 1) males from species with relatively intense sexual selection (inferred by relative testes size) have evolved relatively large penises and pelvic bones compared to their body size, and 2) pelvic bone shape diverges more quickly in species pairs that have diverged in inferred mating system. Neither pattern was observed in the anterior-most pair of vertebral ribs, which served as a negative control. This study provides evidence that sexual selection can affect internal anatomy that controls male genitalia. These important functions may explain why cetacean pelvic bones have not been lost through evolutionary time.
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