How big is too big? Bat’s enormous penis makes penetration impossible

If you go out walking at dusk nearly anywhere in Europe or Asia, you’re likely to catch a glimpse of a serotine bat ( Eptesicus serotinus ) in flight. These furry creatures are a familiar sight, known to roost in chimneys and in the gables of old churches. But although serotine bats are quite common, scientists have yet to unravel all their mysteries. In fact, up until recently, the male of this species was hiding a truly astonishing secret: a penis so large that it makes “traditional” sex impossible.

“People have always wondered why bats have such large penises,” says Brock Fenton, a biologist at the University of Western Ontario who’s been studying the animals since the 1970s. Even then, he and his fellow researchers pondered whether such oversize members might pose a problem for penetration. “The penis is too big. The vagina is too small. How does it work?”

According to a study published today in Current Biology , at least in the case of serotine bats, it doesn’t . Instead of mating via penetrative sex, the authors report, these bats engage in contact mating, in which sperm is transferred simply by rubbing the genitalia together, a behavior never before documented in mammals. Fenton, who wasn’t involved in the research, says the finding represents an important contribution to the “fascinating topic” of bat reproduction.

When it comes to sex, bats have always been full of surprises. Some mate hanging upside down from branches, whereas others get down and dirty on cave walls and in rocky crevices. Few are monogamous, and many different species are known to engage in same-sex behavior. There’s some evidence that female short-nosed fruit bats ( Cynopterus sphinx ) fellate their mates to prolong copulation , and male Indian flying foxes ( Pteropus giganteus ) have been observed performing cunnilingus on their partners with the same goal in mind.

Nicolas Fasel, an evolutionary ecologist at the University of Lausanne and lead author of the new study, has long been fascinated by what he describes as “the quite incredible reproductive system of bats.” Although bat sex might seem salacious to some, Fasel believes it can help advance our understanding of how and why animals’ wide variety of sexual behaviors and preferences have evolved.

As part of his research, Fasel often handles live, anesthetized bats—a routine activity that several years ago led to a notable discovery. When a male bat is sedated, he gets an erection, and the most impressive erection of all belongs to the unassuming serotine. “The erect penis of this species is just amazing,” Fasel says. “It’s superlong.”

As it turns out, it’s a bit too long. When engorged, the penis reaches a length of about 1.6 centimeters. That might not sound like much, but the serotine bat is only about 7 centimeters from head to tail, so the penis spans a whopping 22% of his entire body length. This enormous organ is seven times longer than the female bat’s vagina and ends in a heart-shaped swelling seven times wider than the vaginal opening.

Instead of engaging in penetrative sex, the male serotine bat uses his penis like an extra “arm” to push aside the female’s protective tail membrane. Jan Jeucken/Stichting De Laatvlieger

Based on these observations, Fasel and his colleagues suspected these bats didn’t engage in penetration, although they couldn’t say for sure. As Fenton notes, although nonpenetrative sex is common in birds, which transfer sperm by touching their genitals together in an act known as a cloacal kiss, many scientists consider penetrative sex to be a “sacred” aspect of mammalian reproduction. “We assume that, because we mate in a certain way, everybody else does the same thing.” Bats’ nocturnal and elusive lifestyle also means scientists rarely have the chance to observe them in flagrante delicto.

That all changed when Fasel received an email from a bat enthusiast in the Netherlands, which included video footage depicting serotine bats mating in a church attic. Fasel also collaborated with employees at a bat rehabilitation center in Ukraine, who, after overhearing amorous rustling coming from their indoor roosts, were able to film the animals getting up close and personal.

In most instances, the male bat mounted the female from behind, bit her nape, and moved his penis in a probing fashion until the head was firmly pressed against her vulva. There he remained, shifting occasionally, for close to an hour. One particularly energetic male waited almost 13 hours before letting go; the female was left with a patch of wet fur on her abdomen—likely her partner’s seminal fluid. No penetration was observed at any point.

The researchers think these male bats may have evolved their supersize shafts to use as an “extra arm.” Bats possess a tail membrane, which they use when flying and catching insects, but the female can also use it to shield her genitalia from unwanted advances. “For the male, it would be advantageous to have this huge penis to actually pass by the membrane and reach the vulva,” Fasel explains. Although in this case, he notes, evolution appears to have gotten a tad overzealous.

The female bat also possesses an unusually long cervix, which suggests she’s not as passive a participant as she seems. Females of some bat species tend not to be very choosy when it comes to selecting mates, but they have a remarkable capacity for storing sperm. As a result, a single female can mate with multiple males. Then, once the mating season is over, she can take her time deciding which sperm will end up fertilizing her eggs. The female serotine bat’s long cervix indicates that, although she might not be able to exercise much agency during the actual act of copulation, she’s adept at storing and selecting sperm afterward.

In the future, Fasel hopes to investigate penis morphology and mating behavior across different bat species. One of his goals is to develop a so-called bat porn box, which will allow researchers to observe captive bats mating in real time from several different camera angles to suss out their copulatory quirks. Fenton, for his part, says he’s pleased to see sexual behavior in these animals getting the attention it deserves. “Bats keep interesting questions alive.”