To become both a lover and a fighter, the male spider Nephilengys malabarensis snaps off his penis inside his partner while they have sex. He becomes better at fending off other males who try to mate with her, because his now-lightened body can fight for longer without tiring. And while he’s playing the guardian, his detached genitals can continue pumping sperm into the female. Through self-castration, he gets more stamina, and he gets more stamina.
I first wrote about the self-castrating spider a few months ago. Then, Daiqin Li from the National University of Singapore confirmed that the severed penis continues to pump sperm into the female. That allows the newly minted eunuch to fertilise her remotely, while also blocking the way to other males.
Li suggested that this extreme strategy evolved to counter the female’s penchant for cannibalism. She almost always turns on her partner during sex, and has a 75 per cent chance of eating him. She usually does this well before he has dispensed all his sperm, so by detaching his penis, he gets to continue having sex even while he’s being devoured.
But the male gains other advantages too. Last year, Li showed that the eunuchs make better fighters. If they survive their dangerous liaisons, they guard the female aggressively and can even fend off larger opponents. Now, Li has discovered the secret to the eunuch’s success.
Male spiders deliver sperm though a large pair of structures on either side of their heads, known as palps. If both of these break off, Li’s colleague Qi Qi Lee found that the spiders become 9 per cent lighter. He then measured their endurance by gently nudging them with a paintbrush until they were too exhausted to scurry away. With both palps, they lasted just 16 minutes. With one gone, they lasted 21, and with both gone, they endured for around 29.
Without his bulky sperm pumps, a male’s stamina leaps up by 80 per cent. This probably explains his ability to trounce other suitors, and Li calls this the “gloves-off hypothesis”. There could be other explanations too. They male might also become more motivated to defend his shot at fatherhood (especially since he won’t get any more). For the moment, there’s no evidence for this idea, but maybe that will come in future studies.
Reference: Lee, Oh, Kralj-Fiser, Kuntner & Li. 2012. Emasculation: gloves-off strategy enhances eunuch spider endurance. Biology Letters http://dx.doi.org/10.1098/rsbl.2012.0285
Image by Joelyn Oh