Promiscuity is wide spread in nature.

Females can have sexual contacts with many males, something their male partners cannot prevent.

That's why they have developed certain mechanisms that turn these actions sterile and make them father the offspring: this is called sperm competition.

In the wasp spiders, the males simply place a chastity belt on their partner while copulating: the tip of their genital breaks off during sexual contact, plugging the sexual orifice of the female, as German researchers discovered.

The male wasp spider is much smaller than the female. During sexual intercourse, he inserts his "penis" (the tip of a transformed leg filled with sperm) into the female's sexual orifice. After a few seconds, the female attacks her partner, and if he does not manage a quick escape, he will be eaten. "When the male detaches himself from the female, in more than 80 % of cases the tip of his genital breaks off," said the Bonn lecturer Dr. Gabriele Uhl. "The tip then remains in the sexual orifice like a cork, blocking it."

The team looked for the reason why spiders lose their penises. "There are basically two hypotheses. On the one hand detaching part of the genital organ could help the male to escape from the female's murderous attack. On the other hand it might be a mechanism ensuring that paternity is maintained, preventing or impeding further copulation by the female", said Uhl.

"The detached tip might prevent subsequent intercourse, like a chastity belt. The first male would thereby ensure that all the egg cells were fertilized by him rather than his rival." said Professor Jutta Schneider from the University of Hamburg.

When the female genitalia is plugged, the male copulates only for eight seconds; if not, the copulation lasts twice as long. "The results show that the blockage at least impedes copulation. Initial morphological studies show that the detached tip plugs the orifice so securely that the transfer of semen is probably largely excluded", said behavioral biologist Stefan Nessler from the University of Hamburg.

Other species of wasp spiders also displayed the "plugging mechanism" and in all of them, the females kill the males during intercourse. "We presume that genital mutilation only makes sense if there is hardly any chance of further copulation anyway. The males show maximum investment", said Uhl.

The team is now studying a different strategy in sperm competition: the dwarf spider secrets a viscous stuff in its genital which it ejects after ejaculation, forming a cork that impedes other copulations.

When these two mechanisms of impeding sperm competition are present, "the females have a separate aperture (through which deposit the eggs). In spider species with only one aperture for copulation and oviposition no such contraceptive strategies exist", said Uhl.