Behavioral Ecology Blog

In a paper publisher in the January 2006 edition of Behavioral Ecology, a group of German scientists have published thier work that looks at pre-copulatory sexual conflict in Orb-weaver spiders. Apparently, these spiders have evolved the capability of detaching and depositing part of their body inside the female reproductive tract to serve as a copulatory plug.

ABSTRACT:

The morphology of male genitalia often suggests functions besides sperm transfer that may have evolved under natural or sexual selection. In several species of sexually cannibalistic spiders, males damage their paired genitalia during mating, limiting them to one copulation per pedipalp. Using a triple-mating experiment, we tested if genital damage in the orb-web spider Argiope bruennichi increases male fitness either through facilitating his escape from an aggressive female or by obstructing the female’s insemination ducts against future copulation attempts from other males. We found no survival advantage for males damaging their pedipalps; however, copulations into a previously used insemination duct were significantly shorter when the previous male had left parts of his genitalia inside the insemination duct. Because copulation duration determines paternity in this species, our result suggests that male genital damage in A. bruennichi is sexually selected. By breaking off parts of their intromittent organs inside a virgin female, males can reduce sperm competition and thereby increase their paternity success.

Believe it or not, this is not terribly uncommon in the animal world- copulatory plugs that is. While the spiders leave behing parts of their body inside the female reproductive tract, other animals have evolved even more bizzare adaptations. Same argue that human semen, as it coagulates, may serve the same function…

Other animals whose males are known to deposit a copultory plug:

1. Guinea Pig
2. Chimpanzee’s
3. Lemur
4. Iberian rock lizards
5. masked palm civets
6. Garter snake
7. Ground Squirrels
8. Australian Hopping Mouse

A quick shout out for some alternative hypotheses:

• Copulatory plugs may serve as a sprem storage reservior.
• C. Plugs may serve to reduce sperm leakage after insemination.