It looks like flying isn’t the only unusual thing that bats can do compared to other mammals. New research has found a bat species whose males use their large penises like an arm during sex. Rather than have penetrative sex, these bats and their genitals seem to engage in close-contact “kissing”—a method widely seen in birds but never before in mammals.
Bats are one of the most plentiful mammal groups around, accounting for 20% of known species (second only to rodents). But there’s still so much we don’t know about them, thanks in part to their nocturnal and secluded lifestyles. There have been hints that bat reproduction has its own quirks relative to other mammals, though.
In some species, for instance, female bats are capable of storing sperm collected before they undergo hibernation, allowing them to get pregnant after they emerge from their slumber in the spring. Other species can delay the development of their fertilized embryos until the conditions for a successful pregnancy are better, such as having more food available.
The authors of a new study, published Monday in the journal Current Biology, say their curiosity was initially sparked by a distinctive attribute found among male serotine bats (Eptesicus serotinus): their huge junk.
“By chance, we had observed that these bats have disproportionately long penises, and we were always wondering: ‘How does that work?’” said lead author Nicolas Fasel, a researcher at the University of Lausanne in Switzerland, in a statement from Cell Press. “We thought maybe it’s like in the dog where the penis engorges after penetration so that they are locked together, or alternatively maybe they just couldn’t put it inside, but that type of copulation hasn’t been reported in mammals until now.”
To find out, Fasel and his colleagues enlisted the help of a bat rehabilitation center in Ukraine as well as Dutch citizen scientist Jan Jeucken—both of which had serendipitously collected hours of footage on these bats going about their day. And nestled within the footage was plenty of bat-on-bat action. The team documented 97 such mating events, finding no evidence that these bats have penetrative sex.
Instead, the bat’s penis enlarges before it reaches a female bat’s vagina. During the mating process, which has the male grab onto the female by the nape of their neck, the penis seems to brush away the protective tail membrane surrounding the vulva, then stays pressed up against it for an extended time. The average length of these sexual encounters was around 53 minutes, but the longest recorded bout lasted over 12 hours.
The team also studied live and dead serotine bats in the lab, confirming a few other genital-related traits. For instance, the penises are about seven times longer than the vaginas of female bats, while the heads of these penises are heart-shaped and about seven times wider than the vaginal opening. The cervix in females is also unusually long, which might help their bodies store or be more selective about the sperm that inseminates them.
All in all, the evidence points to a long-standing arms race between male and female serotine bats in how they mate.
“Bats use their tail membranes for flying and to capture the insects, and female bats also use them to cover their lower parts and protect themselves from males,” said Fasel, “but the males can then use these big penises to overcome the tail membrane and reach the vulva.”
The researchers note that their findings right now only indirectly suggest that serotine bats use this method to mate. Female bats’ abdomens appeared wet after these episodes, indicating the presence of semen, for instance, but the team wasn’t able to prove that sperm was transferred over during the act, and that this transfer then led to a successful pregnancy.
The team does plan to follow up with more definitive research that will study serotine bat mating up close, in a naturalistic setting. They also hope to investigate whether this mating strategy occurs in other bat species.
“We are trying to develop a bat porn box, which will be like an aquarium with cameras everywhere,” said Fasel.