A female burrowing wolf spider (Lycosa tarantula), the Mediterranean tarantula, is photographed while feeding on a male.

Cape Town - Some spiders take desperate measures to ensure that only their sperm will make spider babies: after mating they sever their own genitals and use them to plug the female to prevent any other male mating with her.

Scientists say this phenomenon of “mate plugging” through genital mutilation occurs in many animal groups, including spiders. It is a way for male spiders to reduce sperm competition and exercise “paternity protection”.

But in the giant wood spider, Nephila pilipes, the male and female genitals have evolved to become simpler, with the result that male genital plugging has become ineffective.

While researchers can see parts of the males’ genitals lodged in the females, these plugs did not prevent further copulation.

What scientists were aware of, was that the female giant wood spider had another bigger, reddish, solid plug that covered her entire genital area.

What they did not know, was where it came from.

To find this out, Matjaz Kuntner of the Smithsonian National Museum of Natural History and other researchers set up mating trials of the giant wood spiders. The females are the “giants” and the males are small.

The mating trials sought to establish which sex produced this bigger plug, how it was formed and whether it did prevent copulation.

They speculated that it could be formed by either the male or female, or by both spider sexes together.

Their findings, published in the journal PLos ONE last month, were that these genital plugs are made by the female herself – but only after this highly polygamous spider had mated many times and had laid her eggs.

They also found that it was in the interests of the female spiders to mate many times. None of the females that mated only once ever laid eggs, while 30 percent and 60percent of those that had mated with three and five males respectively, produced viable egg-sacs.

The females that mated with more males, for longer and with a higher number of insertions, were more likely to lay eggs.

The scientists say they “lean toward the interpretation” that polyandry (a female having several male partners) is in fact necessary for successful egg fertilisation. They found that when the plug hardened, it did prevent subsequent copulation. They concluded that the newly discovered self-plugging mechanism represented a “female adaptation to sexual conflict through the prevention of unwanted and excessive copulations”.

Or in more colloquial terms: “No more sex thanks; I’ve done it enough and have had my kids.” - Cape Times