The oversized penis of the orchid bat (Eptesicus serotine) is not what it seems. While it may appear disproportionate and impossible for penetration, this species has actually evolved to use it as an extra arm for mating contact. A new study in the journal ‘Current Biology’ reveals that bats do not engage in penetrative sex, but instead use their large penises to push against the female’s tail membrane during mating. This surprising behavior, known as non-penetrative sex, resembles the cloacal kiss of birds and has never been observed in a mammal before.

Nicolas Fasel from the University of Lausanne and first author of the work explained how they discovered this behavior by chance while observing bats with disproportionately long penises. They thought maybe it was like a dog whose penis swells after penetration or perhaps they simply couldn’t get it inside, but no such copulation had been reported in mammals until now. The researchers analyzed 97 mating acts using images from cameras placed behind a grate that bats could climb onto, including videos from a bat rehabilitation center in Ukraine and a citizen scientist who filmed a garden bat for hours in the attic of a church in the Netherlands.

The recordings revealed that bats do not practice penetrative sex and that their erectile tissues enlarged before making contact with the vulva during mating. Male bats grabbed their mates by the back of the neck and moved their pelvises (and fully erect penises) in a probing fashion until they made contact with the female’s vulva. Then they stayed still in a long embrace on average, but some interactions lasted up to 12.7 hours. After copulation, females’ abdomens appeared moist, suggesting sperm transfer during these supposed mating events, although more research is needed to confirm this theory.

Furthermore, measurements showed that when erect, bats’ penises are about seven times longer and seven times wider than female vaginas, reaching about one-fifth their head-to-body length (about 7 centimeters). Females also have unusually long cervixes which could help them select and store sperm during mating contact. The team speculates that bats may have evolved their oversized penises to push away female tail webbing which they can use to avoid sexual intercourse if necessary. They plan to study bat mating behavior in more natural contexts and investigate penis morphology and mating behavior in other bat species as well as investigate further into this fascinating discovery.

This discovery adds another intriguing chapter to our understanding of different species’ sexual lives and shows us that nature always finds ways to adapt even when things seem impossible at first glance.