Female bottlenose dolphins have evolved to the point where they are able to protect themselves from fertilisation by some males.
Certain species of marine animals have extensive vaginal folds that make penis entry more difficult, effectively acting as a reproductive barrier, researchers have found.
Male bottlenose dolphins form strong bonds with two to four others to fend off competitors for their females.
When a female dolphin comes into contact with such a group, she has little choice about who mates with her and may end up mating with each one.
But researchers have found that female bottlenose dolphins have developed a secret weapon to make copulation more challenging for certain males.
“She may not choose who she mates with, but might be able to choose which male or, more precisely, which sperm, fertilises her egg,” Janet Mann of Washington DC’s prestigious Georgetown University, told New Scientist magazine.
The complex vaginal structure of female bottlenose dolphins means that they can make subtle movements during sex to prevent the male from fertilising her successfully, as the penis is sent the wrong way.
Both bottlenose dolphins and common porpoises have evolved these vaginal folds, but the vaginas of the common dolphin and the common seal have developed to allow males access.
Researcher Dara Orbach, of Dalhousie University in Canada, said female bottlenose dolphins may appear passive, but this is not the case.
“Looking at the reproductive anatomy, we’re learning that they have all sorts of cryptic ways to control paternity,” he told the New Scientist.
For the study, Mr Orbach obtained the genitals of cetaceans which had died of natural causes and created silicone moulds of their vaginas, which revealed complicated folds and spirals.
“There’s this unparalleled level of vaginal diversity that we had no idea existed before,” he said.
The scientists, who published their findings in the Proceedings of the Royal Society B, then inflated male cetaceans’ penises to imitate them being erect and used CT scanning to examine how they fitted into females’ vaginas.
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