The spider that eats its own penis

On the southwestern coast of India lives Nephilengys malabarensis, the Malabar Spider. It’s an orb weaving spider. The females can grow up to 3cm long, which is 3-6 times larger than the male. All pretty standard stuff for a spider. But when it comes to the act of courtship, things get very strange indeed. Rebecca Coffey is the author of Beyond Primates: Every Sperm is Sacred...

Rebecca - For the Malabar spider. Oddly enough, he has two penises. They're called palps and they dangle from either side of his face. Though the female has two receptacles or vaginas, they're actually called epigynum or epigyna and they are on the abdomen. So what he generally does is climb up on the web, jostle it a little so that she knows he's there, go over to her, stroke her, and then mount her from the underside and he takes one of his palps and puts it into one epigynum and starts ejaculating. And that's where the odd stuff begins. His palp is detachable, it breaks off. So he puts it into the epigynum and then he breaks it off and leaves it in the epigynum to enjoy the sex, right? And he comes back out on the web and stands ready to defend her from any other potential mate. This is an area of evolutionary theory called sperm competition. What sperm competition is, is the male trying to guarantee that if his female has many mates, his are the sperm that get to the egg. So the male Malabar leaves his ejaculating palp inside the epigynum and comes out to defend her from other males. And then if one approaches, things get really weird. You would think that he would stick his other palp into her other epigynum, break it off, let it ejaculate, and then he would have two plugs in her two epigyna and the whole battle would be over. But no, what he does is he eats his other palp, he eats his penis.

Will - You could've given me a hundred guesses and I wouldn't have been able to guess that that was what happened.

Rebecca - Right? You wouldn't go there at all. So no one knows why because no one has ever asked a male Malabar why he eats his own penis at that point. But a team of scientists from Asia and Europe built basically little boxing rings for male Malabar spiders and paired male eunuchs with fully bodied males. And they paired male partial eunuchs with fully bodied males. And the name of the paper says it all - eunuchs are better fighters.

Will - The mind boggles, but is there anything to be said that eating your own palp might give you that burst of energy needed to defeat whoever you are fighting?

Rebecca - Certainly that's one interpretation. Another is that it just pisses you off so much that you become a monster. But after he has left one in an epigynum and eaten the other, he has no more reproductive potential in his life. Those things aren't going to grow back. And Charles Darwin defined procreation as the whole point of life. It's why sex feels good, right? <laugh>, we just keep going out and doing it. And the result is that we send our genes into the next generation. Well, he can't do that anymore. And so his next act, to me, is almost as astonishing as eating his own palp. And it is that he offers himself as dinner to the female. Why? <Laugh> Because he's just inseminated maybe 200 eggs. She has a huge nutritional need and he is there to satisfy it.

Will - That is absolutely amazing and almost it's kind of a one shot deal. If nothing's growing back, then you've really got to hope that this is a successful mating attempt.

Rebecca - That's right. And you know, male spiders mate very rarely. They're not always eaten and they're not always threatened by another male. But you only get so many shots in life if you're a male Malabar spider and they're willing to give it their all.

Will - And nothing but respect to them. And people thought that male praying mantis had it bad, just having their head ripped off. Imagine having to rip off your own organ and then be eaten as well.

Rebecca - And you know, across species there are sperm competition strategies. They can be biological, they can be physiological, and they can be behavioural. With canines like coyotes and dogs, there's a bulb at the base of the penis that inflates, sticking the female onto the male for long enough for his sperm to get to know her eggs really well before another canine approaches. For humans mate guarding, jealousy on the part of the male, is possibly just a sperm competition strategy. It keeps other males away from her so that your sperm gets to her eggs.Across species, sperm competition is absolutely fascinating.