Serotine bats
11-20-2023

Peculiar anatomy: Serotine bats have oversized penises that are not used for penetration 

Scientists have discovered a unique mating behavior in serotine bats (Eptesicus serotinus). Unlike typical mammalian mating practices that involve penetration, serotine bats engage in a rare form of non-penetrative sex. This study marks the first instance where such behavior has been documented in mammals.

Strange anatomy 

The peculiar anatomy of serotine bats plays a crucial role in their mating process. The researchers observed that the bats’ penises are about seven times longer than the females’ vaginas and possess a “heart-shaped” head that is seven times wider than the vaginal opening. 

Such dimensions make penetration post-erection unfeasible. Instead, the bats utilize their oversized penises, not for penetration but as an extra limb to maneuver the female’s tail sheath, facilitating contact mating. This behavior closely resembles “cloacal kissing” seen in birds.

Oversized penises 

Nicolas Fasel, the first author of the study from the University of Lausanne, shared their initial curiosity about the bats’ disproportionately long penises. “By chance, we had observed that these bats have disproportionately long penises, and we were always wondering ‘how does that work?’”

“We thought maybe it’s like in the dog where the penis engorges after penetration so that they are locked together, or alternatively maybe they just couldn’t put it inside, but that type of copulation hasn’t been reported in mammals until now.”

Clear evidence

The researchers collaborated with a Ukrainian bat rehabilitation center and Dutch citizen scientist Jan Jeucken. The team analyzed 97 mating events captured through video recordings. 

The recordings provided clear evidence of the non-penetrative nature of the bats’ mating, with no penetration observed during any of the mating events.

Remarkably, the male bats were seen grasping their partners by the nape and positioning their pelvises and fully erect penises in a probing manner until contact was made with the female’s vulva. 

These interactions, sometimes lasting up to 12.7 hours, ended with the female bats’ abdomens appearing wet. This hints at the possibility of semen transfer, but further research is needed for confirmation.

Genital morphology 

Additionally, the researchers conducted detailed studies of the bats’ genital morphology. They measured the erect penises of live serotine bats and performed necropsies on deceased bats, confirming the significant size disparity between the male and female genitalia. 

The experts also noted the unusual length of the female bats’ cervixes, which could play a role in sperm selection and storage.

Mating strategies 

The study suggests that the evolution of the serotine bats’ oversized penises might be a mechanism to bypass the females’ tail membranes, which are typically used for protection against males. This finding sheds light on the complex and diverse mating strategies in the animal kingdom

“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, but the males can then use these big penises to overcome the tail membrane and reach the vulva,” explained Fasel.

Looking ahead, the researchers aim to explore bat mating behaviors in more natural settings and examine penis morphology and mating habits in other bat species. “We are trying to develop a bat porn box, which will be like an aquarium with cameras everywhere,” said Fasel.

Image Credit: Alona Shulenko

The research is published in the journal Current Biology.

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