So far, little is known about how bats mate – their nocturnal habits and well-hidden roosts make it difficult to catch them in the act. But now, for the first time, biologists have managed to film broad-winged bats (Eptesicus serotinus) mating. It turned out that these bats apparently mate without penetration – this is unique among mammals. Instead, the male bat uses his oversized penis to push aside the sliding membrane on the female’s tail. The actual mating then takes place, similar to birds, through mere contact between the genital regions.
For mammals, mating through penetration has so far been the rule. While birds mate through mere contact between the cloacas, male mammals have developed a penis with which they penetrate the female’s vaginal tract and secrete the ejaculate with the sperm there. Although there are countless variations of this principle, until now no mammals have been known that reproduce completely without penetration. “Copulatory behavior is one of the essential pillars of sexual selection, but at the same time it is still mysterious in many species,” write Nicolas Fasel from the University of Lausanne and his colleagues. This is particularly true for bats. Because mating often takes place in the darkness of their caves and even then you can hardly see more than the backs of the animals involved, it is unclear how exactly mating occurs in many bat species.
Male bat with giant penis
To find out more about this, Fasel and his team tricked the shy fluttering animals: They installed cameras under a grid that served as a holder for the animals. They installed this system in a church tower inhabited by broad-winged bats (Eptesicus serotinus) in the Netherlands and in a bat sanctuary in the Ukraine. This enabled them to film the mating of this bat species from a new perspective and from below for the first time. In total, Fasel and his team managed to observe 97 copulations up close. They first noticed that the male’s penis swelled to around 20 percent of the bat’s body size: “When erect, it was seven times longer than the vagina,” the biologists report. The tip of the penis also had a heart-shaped thickening, which meant that its width did not even begin to fit into the female’s vagina.
“We were wondering how this would work,” says Fasel. “But we thought that it might be like dogs, where the penis only swells to full size after penetration.” This subsequent swelling leads to the so-called copulation lock, which is intended to prevent it from slipping out before the deed is done. But this is apparently not the case with broad-winged bats, as the video recordings showed: the males’ genitals were already too large for penetration at the beginning of mating. The heart-shaped tip was seven times the diameter of the vaginal opening. Nevertheless, the bats apparently did not allow themselves to be dissuaded from mating: Typically, the male would bite the female lightly on the neck and make searching movements with his abdomen for a while before both remained still. On average, the animals stayed in the mating position for 53 minutes, and the longest mating lasted 12.7 hours, as the team reports.
Copulation through mere contact
Closer analyzes of the video material then revealed what exactly happens during the copulation of the broad-winged bats. This showed that copulation actually takes place without penetration. Instead, the male bat appears to use his oversized penis as a tool to expose the female’s genital opening, which is covered by the tail skin. “Bats normally need their tail skins to fly, but females can also use them to cover their abdomens to protect themselves from males,” explains Fasel. “But the males can then use their large genitals to push cock skin to the side.” Individual hairs on the tip of the penis could also be used to sense the position of the female genitals. For the actual mating, the male bat only presses itself against the vaginal opening from the outside and thus transfers the ejaculate, as the team observed.
This means that the broad-winged bats could be the first known mammal species for which mating without penetration has been documented. “Assuming that sperm were actually transferred during these matings, our study reveals a copulation pattern that is completely new to mammals,” state Fasel and his team. It remains to be investigated whether this form of non-penetrative mating also exists in other bat species.
Source: Nicolas Fasel (University of Lausanne, Switzerland) et al., Current Biology, doi: 10.1016/j.cub.2023.09.054