Scientists discover first mammal that mates without penetration

Nicolás Fasel, a biology professor at the University of Lausanne, noticed something odd while observing serotine bats in his lab: the males possessed penises seven times longer and wider than the females' vaginas. The anatomical mismatch posed an obvious question—how did these animals reproduce at all? Penetration seemed impossible, yet the species clearly survived. Fasel found himself stuck on the problem until an email arrived from Jan Jeuker, a Dutch bat enthusiast who had filmed serotine bats mating inside an abandoned church. Combined with additional recordings from a bat rehabilitation center in Ukraine, Fasel and his collaborators gathered footage of 97 separate mating encounters, which they analyzed and published this year in Current Biology.

What the videos revealed was unprecedented in mammalian biology. The males did not penetrate. Instead, during mating, a male would grip the female from behind, biting her at the nape of the neck. Between the female's hind legs and tail lies a membrane called the uropatagio—a natural barrier that would seem to prevent copulation entirely. But the male's extraordinarily long penis functioned almost like an arm, pushing past this membrane to make direct contact with the vulva. The penis itself bore sensory hairs at its tip, which the researchers believe act as a locating mechanism, helping the male find his target. Along the upper surface of the erect penis sat a hollow structure that may function as a suction cup, allowing sustained contact during what turned out to be remarkably prolonged encounters. Half of the recorded matings lasted less than 53 minutes. The longest stretched beyond 12 hours.

After mating, the females showed wet abdominal fur, suggesting ejaculation had occurred. Yet the researchers acknowledged a significant gap in their understanding: they have not yet documented actual sperm transfer or determined the precise mechanism by which it happens. Susanne Holtze, a co-author of the study and senior scientist at the Leibniz Institute for Zoo and Wildlife Research in Berlin, told CNN that the question remains open. Some form of suction may be involved, she suggested, but the exact process remains unknown—a puzzle for future investigation.

The mating style of serotine bats bears a striking resemblance to the reproductive behavior of birds, which engage in what is known as a cloacal kiss, where both sexes press their cloacas together to transfer sperm. Among mammals, however, non-penetrative copulation is virtually unheard of. This is the first time scientists have documented such a mating system in any mammal species. The researchers suspect the mechanism may occur in other bat species as well, though confirmation awaits further study.

The discovery carries practical implications beyond pure curiosity. Holtze, who specializes in assisted reproduction in animals, sees potential applications for conservation. More than a thousand bat species exist, and many face extinction. To date, no effective strategy for assisted reproduction in bats has been established. Understanding how serotine bats actually transfer genetic material could open pathways to help endangered populations breed in captivity. The team is already investigating the genital anatomy and mating behavior of other bat species, with Fasel joking to colleagues that they are developing a "porn cabin for bats"—essentially an aquarium equipped with cameras on all sides to capture the intimate details of reproduction.

Bats have long surprised scientists with their sexual behavior. In 2009, researchers documented that fruit bats engage in oral sex, with females regularly licking the male's penis during copulation. That finding was remarkable enough at the time; only bonobos had been observed performing fellatio among non-human animals. The researchers speculated that the behavior might extend copulation duration, which could improve sperm transport or increase the likelihood of fertilization. Now, with the serotine bat discovery, the field of bat reproduction—long understudied—is beginning to attract serious scientific attention. Each new finding suggests that the private lives of these creatures hold far more complexity than previously imagined.