Dolphins are one of the most studied animals because they are so much like humans in so many ways. That may well include possibility that they take pleasure in sex.

Canadian biologist Dara Orbach and her co-author, Patricia Brennan, have found that there are remarkable similarities in the structure and physiology of the bottlenose dolphin clitoris, which suggests that it is stimulated in dolphin intercourse the way the human clitoris is stimulated in sex.

Orbach, a biologist and senior lecturer at Mount Holyoke College in Massachusetts, told Quirks & Quarks host Bob McDonald, "What surprised us was how similar the erectile tissue shape was in comparison to human erectile tissue in the clitoris. It suggests female dolphins may experience pleasurable sex."

They presented their research at the recent Experimental Biology conference in Orlando, Florida.

Computer reconstruction of the clitoris of the bottlenose dolphin, which is remarkably similar to the human clitoris in its structure and physiology. (Dara Orbach, Mount Holyoke College)

Dolphins do it for pleasure

Previous research had suggested that sex for dolphins was not just a matter of reproduction, but plays a role in social bonding, just as it does in other social species including bonobos and humans.

"They are animals that mate all year round even though they have a very short period of conception," said Orbach. "So it's highly possible that play and pleasure is a large component of sex."

Computer reconstruction of the bottlenose dolphin clitoris showing the erectile tissue (yellow) embedded in the surrounding muscle and connective tissue (green). (Dara Orbach, Mount Holyoke College)

The clitoris of the female bottlenose dolphin

Orbach and Brennan studied 11 female bottlenose dolphins that had died of natural causes. They used a variety of techniques to explore the structure and function of the clitoris, including CT scans.

The structure suggested them that the clitoris may expand in response to stimulation. The erectile tissue of the clitoris causes it to extend beyond the clitoral hood. Also, the folded skin around the clitoris suggests that it may unfold or expand.

Orbach said another surprising finding was bundles of nerves located in skin under the clitoral hood, which would increase sensitivity.

"The nerve bundles were very dense, very large and very abundant, to the point they were almost 10 times larger in the dolphin clitoris than found in other animals known to experience sexual pleasure."

CT scan of the clitoris of the bottlenose dolphin. The erectile tissue in the centre is surrounded by dense muscle and connective tissue. (Dara Orbach, Mount Holyoke College)

Position of the dolphin clitoris

Although the similarities to the human clitoris are remarkable, Orbach found a couple of significant differences. But these differences also lend support to the idea that female dolphins are equipped to experience pleasure in sex.

"Dolphins have a ventral opening to their vagina and the clitoris is right at the top of it essentially, but it's located on their belly, on their ventrum," said Orbach. "And that means that on any angle that the male can possibly mate with the female, the penis will be in contact with the clitoris."

Another difference between the human and dolphin clitoris is the lack of a vestibular bulb in the dolphin. In humans, this area of erectile tissue surrounds the vaginal opening and is known to contribute to orgasm.

All of this strongly suggests that the clitoris could be functional in dolphins, but Orbach says it doesn't necessarily prove that they experience sexual pleasure. To build the case further, they'd need to take physiological measurements while the dolphins were engaging in intercourse, which presents practical difficulties.

"In other animals, you could look at things like changes in heart rate, changes in blood pressure, changes in breathing rate, vocalizations, things like toes curling," said Orbach. "These are really hard things to look at in a dolphin, which is submerged beneath the water. Plus they don't have toes."