A new study published in Current Biology has shed light on the reproductive practices of serotine bats, revealing a unique mating behavior that has never been documented in mammals before. The penises of male bats are about seven times longer than the vaginas of their partners and have a head-shaped seven times wider than the vaginal opening, making penetration impossible. Instead, researchers found that male bats use their oversized penises to move the female’s tail sheath away and maintain contact mating.

Nicolas Fasel from the University of Lausanne in Switzerland, who led the study, stated that “We think perhaps it is like in the dog, where the penis becomes engorged so that it gets stuck or they simply could not insert it. But that type of copulation had not been described in mammals until now.”

The researchers observed genitalia during copulation using images from cameras placed behind a grate that they could climb onto. They analyzed a total of 97 pairings from two different sites: Dutch church and Ukrainian center. They also observed that the female’s abdomen appeared moist after copulation, suggesting the presence of semen. However, more studies are needed to confirm if sperm was transferred.

Furthermore, the researchers characterized the morphology of serotine bat’s genitalia by measuring erect penises of live specimens and performing necropsies on those that died. When erect, these penises are about seven times longer and seven times wider than females’ vaginas. Researchers plan to study mating behavior of these animals in more natural contexts and also study penis morphology and bonding behavior in other bat species in future studies.

Overall, this study highlights a new chapter in our understanding of mammalian reproductive practices and raises many questions about other bat species’ mating behaviors.