Pelvic Bone aka The Sex Bone…?

Did whale evolution keep the unconnected “floating” hip bone due to sexual selection? They have not been lost over evolutionary time as would be expected vestigial of bones serving no function so…have they remained to serve some function? Indeed they have believe scientists at the University of Southern California and the Natural History Museum of Los Angeles County.

The cetacean pelvic bones (Image: Wikipedia user Andrew Z. Colvin)

Dines et. al. published their research in Evolution. Through analysis of cetacean pelvic bones they have determined that cetaceans with relatively large testes also have relatively large pelvic bones irrespective of their overall body size. The size and shape of cetacean pelvic bones are evolutionary correlated to relative testes mass. However, their rib bones (used as a control anatomical feature) have not increased in size alongside the pelvic bone increase therefore it is not just a complete bone size increase but a specific correlated increase.

What could be the reason for this?

Firstly large testes are linked to promiscuous species because bigger testes means more sperm which, means greater mating potential and subsequent offspring. More offspring mean that their sexual advantages (large muscles and large testes) are passed on.

A further hypothesis is that the large pelvic bones could provide a mechanism for the enhanced manoeuvrability of relatively large penises by providing a greater surface area for muscles to attach to! These muscles are called the ischiocavernosus and enable enhanced manoeuvrability of the penis from side to side and may also enable the maintenance of an erection. In female cetaceans these muscles insert into the clitoris leading the scientist to speculate that it could be possible that clitoral movements play a role in female mate choice, potentially meaning the female pelvic bone is also subject to sexual selection.

Courtship between a male and female beluga whale. (Photo : Flickr: Brian Gratwicke)

So in essence the large pelvic bones could have been sexually selected for to be larger to support larger muscles and therefore larger male penises in cetacean (with more manoeuvrability…?). Could this be for the pleasure of female cetaceans or is that all being spun around to create media excitement? I guess since we can’t yet ask female cetaceans we shall just have to keep speculating and researching. It would be pretty interesting if these highly social species have sexually selected for highly manoeuvrable penises to please and entice female cetaceans into copulation.

What we do know is that this is probably the first time that scientist have shown sexual selection affecting the internal anatomy that controls the male genitalia of a species.

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