The bat with an enormous penis

The serotine bat is a fairly common species of chiroptera, found everywhere in Eurasia from England to China. And if you saw one at night, you might think ‘that’s pretty big.’ But you’d have no idea how right you were. In fact, the male's organ is so big that it led researchers to wonder just how on earth it could possibly function in copulation. One of those people was the University of Lausanne’s, Nicolas Fasel…

Nicolas - When it's flaccid, already it looks big. But actually we had the chance to see it in erection, and it was really surprising for us. The size of it, it's really large comparatively to the body size. It's also quite wide. And at the top of the penis, you have this heart shape, which is very strange because it doesn't look like something that will penetrate in anything. It doesn't look right.

Will - So if that's the case, then if it doesn't look like it's going to fit anywhere useful, what is the bat using that for?

Nicolas - For us, it was a bit of a mystery because we saw this big penis and we were thinking, 'okay, but there is no way you can use it as an intermittent organ.' So we were thinking, okay, either you can use it having an erection after the intermission and be locked inside a bit like you will find in dogs. Or maybe it doesn't have penetration. But this was a bit unsure because it was never reported in any other mammal species. But actually when we could have access to some videos provided, then we could see that actually the bat is not penetrating the female, but he used this huge penis to actually move the tail membrane. So the female can use the tail membrane to protect her belly and her genitalia. And the male can then use this big membrane to pass by this tail membrane and reach the vulva.

Will - So just so I've got this right, as you say, it's copulation without penetration. The bat's, penis moves the membrane out of the way of the female's genitalia, deposits the sperm. Then what happens so that the female can be fertilised?

Nicolas - I would like to be able to answer you. The thing is that when you just see the videos, you don't see what actually happened inside. And this will be super interesting to see, but sadly, it'll involve killing the bats or using some special rays that may not exist now in order to see what is really going inside. So we don't have this possibility, but to some extent I am not super surprised that the sperm can find a way and that the female is also making the sperm move through the genital tract. So I think we think too much that the female is very passive and you know, she's just waiting for the motile sperm to go through and reach eggs on its way. But I think we quite forget that the females may have a big mechanism to deport or to decrease the capacity of the sperm to access the fertilisation site. So you don't actually need to put the sperm super far in order for the sperm to reach eggs. And I think the best example is, for example, the birds. The birds, they don't have information. They don't have penetration. Most of the birds, you still have ducks and ostrich, but most of the birds don't have penetration and they succeed quite well.

Will - So for the moment, these bats and their enormous penises get to keep their secrets,

Nicolas - Like in many aspects. You can speak about longevity. We can speak about immunity. So bats are quite incredible because they remain quite mysterious in many, many aspects.