Spiders Try to Ensure Paternity

Male dwarf spiders deploy sperm-blocking plugs to thwart the reproductive chances of their mate’s subsequent suitors.

Bob Grant
Bob Grant
Jun 11, 2014

Ventral view of an Oedothorax retusus female. Red square indicates the position of the genital region (epigyne) that is plugged by the male.MELANIE WITTHUHNThe plugs that male dwarf spiders (Oedothorax retusus) insert into the copulatory ducts of females with whom they’ve just mated act to block fertilizations by other males, according to researchers studying the arthropods. “The mating plug in the dwarf spider clearly functions as a mechanical obstacle to rival males,” lead author Katrin Kunz of the Zoological Institute and Museum in Greifswald, Germany, said in a statement. “Mating plugs are a powerful mechanical safeguard whose efficacy varies with plug size and age.”

Kunz and her coauthors, who’s study of the spider’s mating habits was published yesterday (June 10) in Behavioral Ecology and Sociobiology, determined that the mating plugs are composed of a liquid material that hardens with time to block access to females’ sperm storage organs shortly after they’ve copulated. When the researchers mated females bearing such plugs to subsequent males only about 32 percent resulted in successful copulation. And the chances of a rival male’s sperm reaching the sperm storage organ of the female decreased the longer the plug had to dry and harden in the genital openings. The team also discovered that larger plugs were more effective at blocking subsequent fertilizations than were smaller ones, which males had an easier time punching through or removing.