Dolphins have a full sex life, with frequent dalliances that go beyond reproduction. One reason could be is that like female humans, female dolphins also have a fully functional clitoris.

A close examination of clitoral tissue from common bottlenose dolphins (Tursiops truncatus) reveals many similarities to the human clitoris. Researchers write in Current Biology on Jan. 10 that many sensory nerves and spongy tissues in the genitalia of our female flippered buddies suggest that the dolphin clitoris may be very responsive to physical contact.

"The dolphin clitoris has many features to suggest that it functions to provide pleasure to females," Patricia Brennan, an assistant biology professor at Mount Holyoke College in Massachusetts, said.

Brennan was motivated to do the investigation while studying dolphin clitoral evolution.

Brennan and her colleagues investigated the anatomy of 11 bottlenose dolphins discovered dead on US beaches to prove that dolphins' genital organs aren't exclusively for procreation.

They discovered that their vulvas were unexpectedly "similar" to the shape of a human's, which Brennan found remarkable given that "the entire pelvis of dolphins is so different."

The female bottlenose's sex organ, like those of their bipedal counterparts, is encased in a hood, according to New Scientist. As the animal ages, this cover wrinkles, potentially causing the vulva tip to become engorged with blood when aroused.

Teri Orr, a physiological ecologist at New Mexico State University in Las Cruces who was not involved in this work, described the new findings as "striking but not surprising." Brennan's previous postdoc, Orr, studies genitalia in many animals. Dolphin community dynamics, like those of bonobos and chimps, entail bonding and pleasure-seeking through sex, she claims.

Orr said there are at least two more possible explanations for the origin of a larger clitoris in female dolphins. The clitoris may have a thicket of nerves supplying it, indicating that it shares an embryological root with the penis in males. It's hardly unexpected that the clitoris is equally well-supplied with nerves, as both come from the same types of tissues during development.

Furthermore, whereas modern dolphins do not have ovulation driven by an external stimulus, Orr speculates that clitoral stimulation may have had a role in triggering ovulation in dolphin ancestors. Women's clitoral stimulation can lead to orgasm, and pleasure and fertility may have coevolved, according to theories from many species.

Because of a long-standing and persistent gender bias in reproductive biology research, we still don't know a lot about female sexuality in all species.

Dolphins "might have something to tell us about ourselves," Brennan said. "We have a lot to learn from nature."