- Males who shed their sperm-containing sacs are lighter and more agile.
- Scientists believe this may make males better able to fight off competition.
Male spiders castrate themselves so their manhood doesn't weigh them down when they're fighting off competitors, say researchers.
Associate Professor Daiqin Li from the National University of Singapore, and colleagues, report their findings today in Biology Letters.
"The eunuch males are better fighters because they are lighter and they have an increased endurance," says Li, an expert in animal behavior.
"They try to prevent the other males from mating with the female."
In a number of spider species, the females are much larger than the males and more aggressive.
Li and colleagues studied one tropical species called Nephilengys malabarensis in which the female cannibalizes the male during copulation.
Not only that, but what has really puzzled researchers until now is that during mating the males break off one or both of their sperm-containing palps.
The palps, which look a bit like boxing gloves, have a tube attached which is inserted into the female to enable sperm to be transferred.
When the palps break off they remain attached to the tube and sit on the outside of the female's body blocking her reproductive tract.
This had led to some scientists to speculate that spiders castrate themselves to create a "mating plug" that continuously transfers sperm to the female after the male is gone - and at the same time stops competing males from inserting their sperm.
But other males can easily remove the mating plug, says Li, which called for other explanations for the emasculating behavior.
Li says scientists had noticed that the 25 percent of males that survive the terrifying ordeal of mating are better fighters.
One hypothesis is that castrated males are better fighters because they are not weighed down by their manhood and thus have more energy to fight off competing males.
"Relatively the palps are heavy compared to the whole body mass," says Li. "If they remove one or two palps, they become significantly lighter compared to the intact male and are able to run faster and longer.
Li and team tested this "gloves off" hypothesis by creating spiders with one or both palps removed.
He says the design of the palps mean they break off easily and his team used forceps to emulate this natural process.
They then weighed the half-eunuch and full-eunuch spiders and compared their weight to intact males.
Finally, they tested the endurance of the three types of spiders by prodding them to run around and measuring the time it took the spiders to get exhausted.
Li and colleagues found full-eunuchs weighed 9 percent lighter than intact males, and half-eunuchs weighed 4 percent lighter.
They also found that full-eunuchs had 80 percent more endurance than intact males and half-eunuchs had 32 percent more endurance, The findings show intact males are more easily exhausted than eunuchs and provides a mechanism to support the "gloves off" hypothesis, says Li.