Dwarf spiders are one of several creatures that go to great lengths to ensure the fidelity of their mates. The males deposit a 'mating plug' inside the female to block out rival sperm and make sure that any offspring is theirs. Surprisingly, the effectiveness of these arachnid chastity belts depends not only on the plug’s size, but also on its age, according to research published this week in Behavioral Ecology and Sociobiology1.

The plug starts as a liquid secreted by a specialized gland, and then hardens to become an obstacle to any subsequent males seeking to mate with the female. The common European dwarf spider (Oedothorax retusus), which measures less than three millimetres long, is known to use a plug to secure paternity2, but its mechanism and effectiveness has until now been poorly understood.

In the latest study, a team led by biologist Gabriele Uhl of the University of Greifswald in Germany analysed how well the plugs of this species actually work. The team observed mating sessions between virgin females and males, removing the males after different lengths of copulation. The longer the copulation lasted, they found, the larger the plug left behind.

The researchers then introduced other males to the females with different plug sizes and the females were later analysed using a scanning electron microscope.

Size matters

The team found that the time between the first and second mating varied, depending on the plug’s age. The researchers also found that although four out of five males tried to copulate with a plugged female, only a few succeeded, either by breaking or removing the plug. And the smaller and fresher (and thus softer) the plug, the higher the second male’s chances were. Moreover, if plugs were more than a day old, the chances of a second male copulating with the female were much lower, says Katrin Kunz, an evolutionary biologist at University of Greifswald and co-author of the research.

But for a large plug, age does not matter, adds Kunz — subsequent males are not likely to remove it. “The plug-size effect has been seen in a few other spider species,” says William Eberhard, an evolutionary biologist at the Smithsonian Tropical Research Institute in Panama City. But the effect of age — that plugs are more effective the older they are — was unexpected, he says.

Uhl’s team also noticed that even if a second male was able to mate with a plugged female, it wasted much of its sperm, because secretions of the substance were found on the plugged female's genital area. That implies that “plugs at least reduce successful sperm transfer”, says Uhl. The team is now planning to perform paternity tests on spiderlings to determine whether second males successfully transferred sperm and fathered offspring.

Furthermore, finding the chemical composition of the material that the plug is made of could have useful implications in materials science, because the substance attaches itself firmly to a rough surface. If the material can be synthesized, says Uhl, it could “be used as a bionic superglue”.