These bats are the only mammals known to mate more like birds

As the only mammals that can fly, bats are the oddballs of the mammalian world. But serotine bats stand out for another, glaringly obvious reason — when erect, a male’s penis can swell to almost a quarter of its body length. How these bats use their humongous genitals to mate — without penetration — is a method never seen in a mammal before, researchers report in the Nov. 20 Current Biology.

At more than 16 millimeters when erect, the penis of male serotine bats (Eptesicus serotinus) has no chance of fitting inside the female’s approximately 2-millimeter-long vagina, a discrepancy that prompted biologist Nicolas Fasel to wonder how these bats go about getting it on. Videos collected at the Ukrainian Bat Rehabilitation Center in Kharkiv from 2018 to 2021 and the attic of St. Matthias Church in Castenray, Netherlands, from 2016 to 2022 revealed the answer. With footage, taken from underneath the bats, “we could see actually what was happening,” says Fasel of the University of Lausanne in Switzerland.

Instead of inserting its penis into the vagina, a male uses it to move aside a membrane covering the female’s genitals and then presses the penis against the female’s vulva and holds it there, often for just under an hour. In one observation, the behavior lasted more than 12 hours.

Once the deed was done, the fur around the female’s vulva appeared wet, which Fasel and colleagues suspect is due to semen. Other scientists told Fasel the wet spots look similar to those in which they’ve found semen after mating in other bat species, he says. These observations suggest the serotine bats are mating without penetration, a common practice in birds but one never observed in a mammal before (SN: 1/16/09).

“I think it’s super interesting because it sort of brings up a different function for a penis” aside from penetration, says Patricia Brennan, an evolutionary biologist at Mount Holyoke College in South Hadley, Mass. “It doesn’t surprise me that this is a bat because bats have some of the craziest reproductive strategies in mammals,” such as females being able to store sperm for six months or males having spines on their penises.

Fasel agrees. Among mammals, bats “just have to be the weirdos of the group.”