NEW YORK, NY.- A few years ago, Nicolas Fasel, a biologist at the University of Lausanne in Switzerland, and his colleagues developed a fascination with the penises of serotine bats, a species found in woodlands and the attics of old buildings across Europe and Asia.

Serotine bats sport abnormally long penises with wide, heart-shaped heads. When erect, the members are around seven times longer than the female’s vagina, and their bulbous heads are seven times wider than the female’s vaginal opening.

“We wondered: How does that work? How can they use that for copulation?” Fasel recalled.

What they discovered has overturned an assumption about mammalian reproduction, namely that procreation must always involve penetration.

In a study, published Monday in the journal Current Biology, Fasel and his colleagues presented evidence that serotine bats mate without penetration, making them the first mammals known to do so. Instead of using their penises to penetrate their partners, the scientists found, the male bats use them to push their partner’s tail membrane out of the way so they can align their openings and engage in contact mating, a behavior similar to one found in birds and known as “cloacal kissing.”

To learn how these bats overcome their substantial genital size difference, Fasel and his colleagues analyzed nearly 100 videos of serotine bats mating. The videos were provided by a bat rehabilitation center in Ukraine and a citizen scientist filming bats in the attic of a church in the Netherlands. The footage revealed a mating strategy unlike any other used by mammals.

While the two bats hang upside down, the male climbs on the female’s back and grasps the nape of her neck. Once he has a firm hold, the male will use his erect penis to push the female’s tail membrane to the side and probe between her legs until he has located her vulva. The male then presses the heart-shaped head of his penis to the female’s vulva and holds it there until the deed is done. While this process took less than an hour for most of the couples the researchers observed, one pair went at it for nearly 13 hours.

“It’s a really weird reproductive strategy, but bats are weird and have a lot of weird reproductive strategies,” said Patty Brennan, a biologist at Mount Holyoke College in Massachusetts who studies the evolution of genital morphology but was not involved in the study. She notes that bats have evolved penises outfitted with everything from hair to spines, so she was not surprised that one species ended up with a penis too big to penetrate its partner.

While the serotine bat’s spade-shaped penis may not be well suited for penetrative sex, its size and shape make it the perfect tool for moving the tail membrane of females. This flap of skin, which connects the bat’s tail to its legs, not only aids in flight but also offers females a means of covering up their vulva, making it difficult for potential mates to access.

Fasel and his colleagues suspect that male serotine bats evolved their disproportionately large penises to bypass the female’s tail membrane as part of “a really interesting evolutionary arms race based on socio-sexual conflict,” Fasel said.

Like the males of their species, female serotine bats also have unique genitalia. Their cervices are unusually long, which most likely helps them select and store sperm.

Fasel and his colleagues plan to study the mating habits of serotine bats in greater detail, as well as those of other bat species. He wouldn’t be surprised, he said, to find contact mating in other bat species, or even in other mammals. “There is some doubt, but let’s keep our minds open,” he said.

Brennan said, “I think that there are probably quite a lot of weird morphologies and behaviors that we just don’t know anything about.” For too long, she said, such research “hasn’t been considered a serious academic pursuit.” The findings of this paper, she argues, are proof that even the most scandalizing fields of science are worthwhile.

This article originally appeared in The New York Times.