Scientists inflate the penises of dead dolphins to get an intimate look at their strange sex lives

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A bizarre new study has provided a glimpse into the strange sex lives of dolphins.

Researchers inflated the penises of dead dolphins, before simulating sex to find out how these organs fit inside the female.

They discovered that the females have unusual folds in their vaginas which the male's penis must navigate to fertilise the egg. 

The study could help researchers discover how evolution shaped the organs into their present forms.

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An image reconstructed from CT scans showing how the penis of the common bottlenose dolphin (red) fits within the intricate folds and turns of a bottlenose dolphin vagina (pink)

Marine mammals have particularly interesting sexual intercourse as they must contend with some unusual constraints, such as making sure sea water does not enter the vagina. 

Dolphins, in particular, have strangely fascinating sex lives due to their elaborate vaginas and a unique type of penis that is almost always erect.

As part of the study, researchers wanted to find out how often dolphins have sex for 'fun'. 

Different species also use various sexual positions.

They also have sex all year round even when they can only conceive for certain periods of the year.

'By looking at how the genitals align, we can now say certain body positions are more likely to lead to successful fertilization than others, which might be for purposes other than reproducing,' Dara Orbach, a marine mammologist at Dalhousie University in Halifax told Science. 

'Is it play? Is it working out hierarchies? Is it establishing dominance? Is it learning? There could be many functions of sex.'

Orbach's team examined the reproductive tracts of dolphins, porpoises and seals that had died naturally in order to find out more about their sex lives.

After blowing up the genitalia they used computed tomography (CT) scans to visualise how deeply the penis penetrates the vagina and where the two touch.

They also created silicone models of the interior of the dolphin vagina to explore how its shape might have co-evolved with the shape of the male penis.

Researchers found that there was an optimal position for the vagina and penis to fit together which varied with overall body positioning.

'The deeper the male is able to extend his penis within the vagina, the shorter a distance his sperm must travel to reach her egg', Orbach told MailOnline.

'In some species it appears belly-to-belly copulation will enhance genital alignment, while in other species it appears a sexual approach from the female's side will result in deeper penetration', she said.  

'There is amazing variation in the number, shape, size, and positioning of vaginal folds across species', she said.  

From looking at the genitalia, scientists could make predictions about what positions the dolphins would use in real life.

'Body positioning during copulation is predicted based on anatomy', Dr Orbach explained.

Scientists simulated sex and used computed tomography (CT) scans to visualise how deeply the penis penetrates the vagina and where the two touch (stock image) 

Most research on genitalia has focused on the penis alone but this study looked at how the penis and vagina fitted together.

'While it may seem intuitive that the penis fits well into the vagina during copulation, the biomechanics and details of the anatomical fit can be quite complex and have seldom been explored,' said Dr Orbach. 

'Whales, dolphins and porpoises have unusual vaginal folds, spirals and recesses that the penis and sperm must navigate through to successfully fertilise the egg', she said.

The dolphin penis contains a large proportion of tough and elastic fibres instead of spongy erectile tissue.

'The penis is extruded prior to copulation, meaning there are hydrodynamic drag forces acting on the penis that likely influence is structure and shape', said Dr Orbach.

'Salt water is lethal to cetacean sperm, and females are expected to have adaptations to keep salt water out of their reproductive tracts', she said. 

'Additionally, cetaceans do not have appendages to hold each other in place during copulation, so the angle of approach is critical'. 

The team found there were particular parts of the vagina that were stimulated during sex which could help captive breeding programmes.  

The findings will be presented at the American Association of Anatomists in Chicago later this month.