A serotine bat bares its fangs against a black background.

While serotine bats are widespread across Europe and Asia, this behaviour has not been studied before. Image © Alona Shulenko, licensed under CC BY-SA 4.0 via EurekAlert!

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Serotine bats have sex unlike any other mammal

A bat which lives in the UK has developed a novel way to reproduce.

With the penis of male serotine bats being too big for penetration, instead the animals touch their genitals together to mate - a style of sex more similar to birds than any known mammal.

When it comes to the birds and the bees, all mammals were thought to mate by penetration. But the unusual genitals of one bat species have thrown this into doubt.

When researchers discovered that the penis of male serotine bats could be as much as seven times wider and longer than the female’s vagina, they wondered how the species could possibly reproduce.

It turns out that the bats do it in a way never seen in mammals before. Males and females touch their genitals together to mate in a way similar to many birds, which typically touch multi-purpose organs called cloacas together to reproduce.

Dr Nicolas Fasel, the lead author of a new paper on the issue, says, ‘We were quite surprised to see this, as mammals have only ever been observed copulating by intromission before.’

‘It’s possible that it’s not just limited to serotine bats either, as I’ve observed similarly strange genitals in other bats. We now aim to get some funding to describe copulatory behaviours in other species, and explore the more secretive aspects of bat reproduction.’

The findings of the study were published in the journal Current Biology.

A group of bats clustered together in the rafters of a building.

As bats often live together, and hang from the ceiling, their mating habits are hard to research. Image © COULANGES/Shutterstock. 

How do bats reproduce?

Despite being one of the largest groups of mammals, making up around 20% of all species, surprisingly little is known about how bats reproduce. Being nocturnal species that often live in difficult to reach places, it’s been hard for researchers to study their sexual antics.

What is known, however, suggests that bat reproduction can be quite unusual when compared to other species.

The females of some species, for example, are known to store sperm that allows them to mate before hibernation but only become pregnant after they’ve woken up. Other species fertilise their eggs immediately, but can then delay the development of the embryo until conditions are better to raise offspring.

Until this point, however, it was still assumed that the actual act of mating was the same in all mammals, in which one individual penetrates the other with their penis.

The research team only began to realise this might not be the case when studying serotine bats, a species that has a wide distribution across much of Europe and Asia.

‘By chance, we had previously observed that the males of these bats have disproportionately long penises when compared to females,’ Nicolas says. ‘We thought it could have been similar to dogs, where the penis engorges after penetration to lock the male and female together.’

‘The other possibility was that males were simply unable to copulate inside females, but this hadn’t been reported in mammals before.’

Using hours of footage from a Dutch church that the bats were known to frequent, as well as captive animals in a Ukrainian bat rehabilitation centre, the team were able to observe the process of mating in the most intimate of details.

They saw that, when fully erect, male serotine bats have a ‘heart-shaped’ structure on the end of their penis covered in hairs. The team describe this as acting like a ‘copulatory arm’, moving the females’ tail membrane out of the way so that males can feel their way to the correct position for mating.

The male then touches their penis to the female’s vulva for copulation. These bouts of mating take about 53 minutes on average, but in some instances lasted more than half a day.

In future research, the researchers want to find out exactly why this is, as well as to confirm that this unusual behaviour is definitely mating. As they only studied recordings, the scientists were unable to confirm if the males’ behaviour actually led to the fertilisation of eggs.

If it is mating, then the team believe that this could be an example of an evolutionary process known as sexual conflict. 

Two red deer clash antlers while rutting.

Male deer rut with each other to gain mating rights over females. Image © Giedriius/Shutterstock. 

Battle of the sexes

Some of the most striking characteristics in the animal kingdom come about as a result of sex. From the tridents of extinct trilobites to the antlers of deer, a variety of features help to give animals a better chance of reproducing and passing on their genes.

However, the priorities between males and females can differ. For instance, while a male animal tends to have a better chance of reproducing by mating with more females, females often try to reduce mating attempts as it can impact their survival.

Over time, this can mean males and females of the same species evolve in response to each other as they try to get the upper hand. This is sexual conflict, and can involve changes in body size, characteristics and behaviour.

The unusual reproduction of the serotine bats appears to be an example of this conflict. Like many insect-eating bats, the species has a tail membrane which they can use to help capture prey in flight.

But female serotine bats have adapted this structure to act as a defence against unruly males, using it to cover their lower body and protect them from unwanted suitors.

In response, males seem to have adapted their genitals to push the membrane aside so that mating can take place, with the result being a larger than usual penis.

The team now hope to find out more about the role of female bats in choosing their partner, and broader questions about sex in the evolution of these flying mammals.