It was the surveillance cameras trained on the dark corners of St Matthias church in the village of Castenray in the Netherlands that caught the creatures in the act.
The video footage is in black and white, the animals are entwined and upside down, and the events that unfold against a metal grill are more frantic than romantic.
But the recording may nonetheless prompt the rewriting of textbooks. Researchers believe the film of serotine bats is the first documented evidence of any mammal mating without intromission. In plain English, that’s having sex without penetration.
“It was a surprise,” said Dr Nicolas Fasel, an expert in bats at the University of Lausanne. “But in the evidence we’ve gathered, it’s quite clear there is no penetration.”
Scientists have long been stumped by sex in serotine bats, or Eptesicus serotinus. The reasons become obvious from a glance at their anatomy. The male’s erect penis is enormous and ends with a heart-shaped head that is seven times wider than the female’s vagina.
And so researchers suspected one of two scenarios. Perhaps the penis became engorged only once inside the vagina? This would create the kind of “copulatory tie” that makes mating dogs hard to separate. The second possibility was that bats mated through contact, similar to the “cloacal kiss” in birds. But such behaviour had never been seen in mammals before.
The latest work swung into action when a message arrived in Fasel’s email inbox in early 2020. Written in Dutch and containing the word “penis”, it was destined for the spam folder. But Fasel stopped when he saw the Latin name for the serotine bat. The message was from Jan Jeucken, who monitors bats at St Matthias Church, and contained the unusual footage.
Months later, another message arrived for Fasel, this time from Ukraine. Researchers at a bat rehabilitation centre in Kharkiv had their own footage that seemed to show bats mating without penetration. In total, the team now had video from 97 mating events – 93 from the Dutch church and four from the rehabilitation centre.
Analysis of the videos found that none of the males engaged in penetrative sex. Instead, they wielded their outsized penis like an extra arm, to push aside the female’s tail membrane and make contact with the vulva. The process involved some fumbling around, but continued for around an hour on average, and nearly 13 hours at best, according to their report in Current Biology.
The scientists followed up by examining the anatomy of serotine bats, including live animals which helpfully develop erections when anaesthetised. This confirmed the impressive proportions of the penis but also revealed hairs on the heart-shaped head that may provide sensory feedback when searching for the vulva.
While the footage doesn’t prove non-penetrative sex in mammals, some females were left with fluid on their abdomens, suggesting males had at least attempted to deposit their sperm. The scientists now hope to swab females after mating and are building a “bat porn box” to film couples from all angles.
Brock Fenton, emeritus professor at the University of Western Ontario, said the study took him back to the 1960s and his own field studies of little brown bats in an abandoned mine. Pulling mating pairs from the wall would often reveal a female damp with seminal fluid without any sign of penetration. “The paper nicely demonstrates that intromission doesn’t seem to be taking place,” he said.
Prof Paul Racey, a bat expert at the University of Exeter, has witnessed penetrative sex in serotine bats and was sceptical of the findings. “Why would they want to mate without intromission, which runs the risk of ‘wasting’ semen?” he said. But he conceded that given recent revelations about bat sex, he had to be open-minded.
Prof Gareth Jones at the University of Bristol, who won an Ig Nobel prize for documenting fellatio in fruit bats, said he found the evidence convincing, though confirming sperm in the vagina afterwards would prove successful copulation. “The sexual behaviour of bats never ceases to amaze: males urinating into wing sacs to attract females, males mating with torpid females, female relatives sharing sexual partners, fellatio, and now mating without intromission!” he said.
Dr. Thomas Hughes is a UK-based scientist and science communicator who makes complex topics accessible to readers. His articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.