Eunuch Spiders Have The Edge

In the Nephilengys Malabarensis spider species, the male and female are vastly different in size as shown by his resting on the female’s abdomen after escaping being eaten by her during mating. Often, the male spiders will castrate themselves, just so their male equipment does not weigh them down when fighting off the competition.

Researchers led by National University of Singapore professor Daiqin Li found that eunuch males make better fighters, simply because they are significantly lighter and have increased amounts of endurance. Their aim always is to prevent other males from mating with the female, even though, in a number of spider species, females are much larger and more aggressive than males.

Li and his team studied the tropical species –  Nephilengys malabarensis - females almost always cannibalizing the male during mating, but what has, up to now really puzzled researchers is that, during copulation, males break off either one or both of their sperm-containing palps – akin to boxing gloves in shape – with tubes attached inserted into the female for sperm transfer, because, once broken off, those the palps remain attached and block the reproductive tract of the female.

Researchers now think that these deliberate spider castrations are purely intended to continuously transfers sperm to the female after the male is gone, via that plug, as well as preventing the sperm of competing males from entering, though other males can easily remove the mating plugs. Scientists found that the 25% males surviving the mating process are better fighters, perhaps because they become significantly lighter compared to the intact male and are able to run faster and longer.

The team tested this hypothesis by creating spiders having one or both palps taken away with forceps, before weighing both half-eunuch and full-eunuch spiders to compare weights to those of intact males, finding the full eunuchs to be 10% lighter, making them 80% more durable in terms of energy expenditure and thus better fighters than those males who were still completely sexually endowed.