Well-Endowed Bat Becomes First Mammal Known To Reproduce Without Penetration

With a heart-shaped tip, it could've been so romantic were it not seven times too big to function.


Rachael Funnell


Rachael Funnell

Digital Content Producer

Rachael is a writer and digital content producer at IFLScience with a Zoology degree from the University of Southampton, UK, and a nose for novelty animal stories.

Digital Content Producer

a serotine bat

"It's the size that counts, not how you use it, right?" – Misled serotine bat, 2023.

Image credit: Alona Shulenko /

With a penis that’s seven times too large to fit inside the vaginal opening of its species, the serotine bat, Eptesicus serotinus, has had to get creative when it comes to sexual reproduction. It’s physically impossible for the massive appendage to achieve penetration post-erection, but video footage has revealed how they’ve evolved to achieve contact mating by wielding their enormous penises like an arm.

Looking at a serotine bat, you might not realize that it’s concealing some intimidating genitals. These fluffy mammals are fairly large Eurasian bats, about the size of a lemon, and under that fur, the males are packing a penis that’s too big to function.


“By chance, we had observed that these bats have disproportionately long penises, and we were always wondering ‘how does that work?’,” said first author Nicolas Fasel of the University of Lausanne in a statement. “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.”

Typically considered a penetrative organ in mammals, it would seem the species was doomed given that the males’ penises are seven times longer and wider than the average vaginal opening and canal of the females. With a heart-shaped tip, it could’ve all been so romantic were it not so disproportionate. 

a serotine bat on stone
The bats made the researchers' lives easier by *ahem* standing to attention while under anesthesia.
Image credit: Olivier Glaizot

If you’re wondering how you come about such precise measurements of a bat’s penis, the species comes with the convenient quirk of getting an erection while under anesthesia, a trait shared by several vesper bats. Awkward for the anesthetist, but convenient for research.

Despite the ill-fitting puzzle pieces of their genitalia, serotine bats persist, so how are they making baby bats without the kind of penetration that typically characterizes mammalian sexual reproduction? Mating is a behavior we know little about in a lot of bat species, but researchers were able to obtain footage of mating serotine bats with a few choicely placed cameras.


The resulting footage captured 97 mating events that took place in a bat rehabilitation in Ukraine and a church attic in the Netherlands. Watching them back revealed that the pats don’t engage in penetrative sex, luckily for the females, and that instead, the males were wielding their erect penises a bit like an arm to clear the way so they could make better contact with the vulva.

Female serotine bats have tail membranes they can use as a form of protection against unwanted male attention. It’s possible that the enormous penis size may have been an adaptation for the males to overcome this barrier, by forcibly pushing it out of the way with their huge erections.

a serotine bat on wood
They may look cute, but serotine bats have some pretty pushy mating tactics.
Image credit: Alona Shulenko

Once in position, the mating pairs stayed still in an embrace that could last from 53 minutes to 12.7 hours. You’ve got to admit, the serotine bat doesn’t do things by halves.

The researchers noticed that the females’ abdomens appeared to be wet after the extensive hugging episodes, indicating that semen may have been exchanged. However, further research is needed to confirm if this is indeed the source of the wetness.


Fortunately, the researchers are already creating the perfect tool for a serotine bat peep show.

“We are trying to develop a bat porn box, which will be like an aquarium with cameras everywhere,” said Fasel.

The study is published in Current Biology.


  • tag
  • evolution,

  • sex,

  • bats,

  • animals,

  • mating,

  • animal behavior,

  • reproduction,

  • penis