Serotine Bats Mate Without Penetration

While studying Eptesicus serotinus – the serotine bat – researchers at the University of Lausanne made a surprise discovery: it had a peculiarly large penis.

Specifically, the bat's penis is seven times longer than the female vagina, and has a heart-shaped head that is seven times wider than the vaginal opening.

“We were always wondering, ‘how does that work?’,” says Nicolas Fasel, professor at the Faculty of Biology and Medicine at the University of Lausanne. “We thought maybe it's like in the dog where the penis engorges after penetration so that they are locked together.”

Another hypothesis was that perhaps the penis didn’t enter the vagina during penetration. But this type of copulation wasn’t known to exist in mammals – the research literature is somewhat scarce when it comes to bats’ sex lives.

Fasel and colleagues collaborated with researchers at a bat rehabilitation center in Ukraine to film 97 bat mating events. They were joined by citizen scientist and bat enthusiast Jan Jeucken, who helped to film hours of serotine bat mating in a church attic.

The recordings revealed a first-of-its-kind discovery: the serotine bat mates without penetration.

No intromission observed in serotine bats

Male serotine bats grasp their partner, moving their pelvis around until they make contact with the female’s vulva. “The male bit the skin on the nape, and the lateral movements of the male hindquarters were then accompanied by rapid probing movements of the fully erect penis,” Fasel and colleagues write.

At this point, they become still, almost as though they are embracing the female. The longest interaction of this kind was 12.7 hours, but on average lasted 53 minutes.

“No intromission was observed at any point. In addition, the erectile tissues of the penis were enlarged before the contact with the vulva and formed a shape unsuitable for intromission,” the researchers write. “Following copulation, the fur on the female’s abdomen appeared wet, indicating the presence of semen.” 

Fasel and colleagues did not, however, test for the presence of ejaculation, and suggest that a vaginal swap would be required to prove their observation.

A novel copulatory pattern in mammals

The researchers suggest that female serotine bats could theoretically use their tail membranes to avoid copulation, but the male’s long penis might serve as a “copulatory arm”, used to bypass the membrane. “Bats use their tail membranes for flying and to capture the insects, and female bats also use them to cover their lower parts and protect themselves from males,” says Fasel. “But the males can then use these big penises to overcome the tail membrane and reach the vulva.”

Studying bat mating behavior in more natural contexts is the researchers’ next step. They are also eager to explore penis morphology and mating activities in other species.

“This study reveals a novel copulatory pattern in mammals. Further investigation should focus on the role played by pre- and post-copulatory female choice as well as male competition in the evolution of this prolonged and particular mating behavior,” Fasel and colleagues conclude.

Reference: Fasel NJ, Jeucken J, Kravchenko K, et al. Mating without intromission in a bat. Curr. Biol. 2023;33(22):R1182-R1183. doi:10.1016/j.cub.2023.09.054

This article is a rework of a press release issued by the University of Lausanne. Material has been edited for length and content.