Bird-like Mating Habits: Bats Shine as the Sole Mammal Species

Bats, known as the only mammals capable of flight, are considered peculiar in the mammalian world. Yet among them, serotine bats are distinctively peculiar due to a unique characteristic in males – when erect, their penis can grow to nearly a quarter of their body length. Researchers have reported in the Nov. 20 Current Biology that these bats exhibit a never-before-seen method of mating without penetration which involves their unusually sizable genitals.

The penis of male serotine bats, scientifically known as Eptesicus serotinus, can grow to more than 16 millimeters when erect. This large size makes it incapable of fitting inside the female's roughly 2-millimeter-long vagina. This intrigued biologist Nicolas Fasel to investigate the mating behaviors of these bats. Evidence was compiled from video footage gathered at the Ukrainian Bat Rehabilitation Center in Kharkiv from 2018 to 2021 and at the attic of St. Matthias Church in Castenray, Netherlands, from 2016 to 2022. The videos, taken from below the bats, provided clear insight into their mating practices, according to Fasel from the University of Lausanne in Switzerland.

Instead of the male bat penetrating the female with its penis, it uses its penis to shift a membrane covering the female’s genitals aside. The male then holds its penis against the female’s vulva, often for just under an hour although there was one incident where this action lasted for more than 12 hours.

Upon completion of mating, the fur surrounding the female’s vulva appeared moist, which Fasel and his colleagues hypothesize may be semen. Other researchers confirmed to Fasel that the dampness resembled the wetness they observed in other bat species post-mating, which they attributed to semen. These observations indicate that the serotine bats could be mating without penetration, an act prevalent in birds but never previously observed in mammals.

Patricia Brennan, an evolutionary biologist at Mount Holyoke College in South Hadley, Mass., describes this as "super interesting" as it presents an alternative role for the penis beyond penetration. She expressed little surprise at the bat species exhibiting this unusual mating behavior given their known array of unusual reproductive strategies like females' ability to store sperm for six months or males having spines on their penises.

Agreeing with Brennan, Fasel commented that among mammals, bats are undeniably the "weirdos of the group".