For a male ornamental tree trunk spider (Herennia multipuncta), copulation can get a bit dangerous. His mate is several times larger and if she’s hungry, she’ll eat him up. But that’s not even the most disturbing thing that can happen after sex among these arachnids, also called coin spiders: The male chews off one or both of his palps (the organs used to deliver sperm), leaving him a survivor but a full or partial eunuch.
Scientists call this “self-emasculation,” and it’s known to happen in more than two dozen species of spiders. Some spiders remove their genitals before they’re fully mature. Others do it during mating. Still others are like the coin spiders and get rid of them after the act. The trait that surely has some of you squirming has evolved multiple times, so it should provide some sort of evolutionary advantage. Matjaž Kuntner of the Slovenian Academy of Sciences and Arts in Ljubljana and colleagues wanted to figure out what that advantage might be for coin spiders. Their study appears in the January Behavioral Ecology and Sociobiology.
The researchers collected coin spiders from several locations across their Southeast Asian range and brought them into the lab. First, Kuntner and his team set up 60 mating trials in which they introduced a well-fed virgin female to a virgin male. Only about half those pairings worked out (even in the spider world, not every couple is meant to be, it seems). In the 32 that did, 9 males became half eunuchs and 23 lost both palps. Mating itself didn’t result in emasculation; that left the palps merely disfigured. But within 24 hours of hooking up, the male always finished the job and chewed off what was left.
One hypothesis for why emasculation works is that the act of mating leaves behind bits of the palp that become lodged in the female genitals and prevent other males from mating with her. And when the researchers tried to get the mated females to mate with new virgin males, the guys gave it a go but weren’t able to insert their bits.
But why would males bother to chew off all their bits after sex if all they need to do is have sex to block her up? The researchers did a couple of other experiments that start to explain that. The team staged contests between eunuchs and virgin males, and the eunuchs were more aggressive and able to stick closer to the females they had mated with. The mated males also had better endurance, possibly because, having lost a palp or two, they weighed less. If virgin males were actually able to mate with already-mated females (despite the findings in this study), then being a better fighter and having better endurance would help a guy prevent that from happening.
Alternatively, the researchers say, these behaviors might simply be evolutionary leftovers from an ancestor species in which sex was not guaranteed to plug up a female and a male would have had to guard her if he wanted to prevent another male from getting lucky.