This Bat Uses Its Extra Long Penis Like an Arm While Mating

Bats are unusual creatures. They’re the only mammals that can truly fly, they sleep while hanging upside down and they use echolocation to hunt and navigate their surroundings.

Now, scientists have discovered yet another unique trait to add to this list. Serotine bats (Eptesicus serotinus), a large species in the southern United Kingdom, are the first mammals known to mate without penetration, researchers reported Monday in the journal Current Biology.

Male serotine bats have long penises with bulbous heads. When erect, their penises measure roughly 0.6 inches (1.6 centimeters) long, which represents more than one-fifth of their 2.7-inch (7-centimeter) body length.

But perhaps more importantly, their penises are too big for penetrative mating. Their erect members are more than seven times longer than the length of a female serotine bat’s vagina. The wide, heart-shaped heads, meanwhile, are also seven times wider than a female’s vaginal opening. “We were thinking it would be really difficult for [the penis] to enter anything,” study co-author Nicolas Fasel, a biologist at the University of Lausanne in Switzerland, says to Nature News’ Gayathri Vaidyanathan.

So, how do they copulate? This was the question scientists set out to answer when they analyzed footage of 97 serotine bat mating events. Most of the videos came from a citizen scientist named Jan Jeucken who filmed bats mating in a church attic in the Netherlands, while a few also came from a bat rehabilitation facility in Ukraine. (Jeucken is also a co-author of the new paper.)

What the scientists saw when they watched the footage surprised them. Male bats used their abnormally large penises like an extra arm to move the females’ tail membranes out of the way. Then, the male pressed its penis head against the female’s vulva. For half of the observed bat pairings, this lasted for less than 53 minutes, but one duo persisted for nearly 13 hours. Afterward, the females’ abdomens appeared to be wet, which the scientists suspect indicates the presence of semen—but they don’t know for sure.

This all occurred while both bats were hanging upside down, with the male biting the nape of the female’s neck. The researchers did not observe any penetration. The bats’ non-penetrative reproductive method is similar to how birds mate, a behavior known as “cloacal kissing.”

The findings are novel, but they’re not necessarily surprising to researchers who study bats.

“It’s a really weird reproductive strategy, but bats are weird and have a lot of weird reproductive strategies,” says Patty Brennan, a biologist at Mount Holyoke College who was not involved in the new research, to the New York Times’ Annie Roth.

Scientists don’t know for sure why serotine bats evolved to have such large penises. However, they suspect it’s an adaptation designed to help them get around the female’s tail membrane, which they can use to shield themselves from unwanted mating attempts.

“Bats use their tail membranes for flying and to capture the insects [they eat], and female bats also use them to cover their lower parts and protect themselves from males,” Fasel tells Live Science’s Sascha Pare. “But the males can use these big penises to overcome the tail membrane and reach the vulva.”

In the future, the scientists hope to study bats in captivity so they can observe real-time mating from different angles, as well as verify if sperm is actually being transferred from males to females. These initial insights into serotine bats also raise questions about other bat species—have they also evolved unique genitalia and mating methods?

The new serotine bat findings overturn the idea that mammals need to have penetration to reproduce, opening the door to the possibility that other mammals engage in contact mating. “We assume that, because we mate in a certain way, everybody else does the same thing,” says Brock Fenton, a biologist at the University of Western Ontario in Canada who was not involved in the new research, to Science’s Phie Jacobs.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.