Survival of the fittest, physically? or mentally?

'Chastity belts' block rival sperm in female spiders
12:12 06 March 2007
NewScientist.com news service
Roxanne Khamsi

Some male spiders up-and-leave right after sex for good reason – they risk being eaten by their female partners if they linger too long. In the process of making a swift exit, many leave part of their genitalia inside their mates.

Now a new study reveals that detaching part of the genital organ is not a means to help the male escape a murderous attack. Instead, the abandoned genitals act as "chastity belts" and block the entry of sperm from competitors into the female.

Gabriele Uhl at the University of Bonn, Germany, and colleagues watched wasp spiders (Argiope bruennichi) mate. During the act, a male must insert one of its two sperm-carrying organs, known as pedipalps, into the female’s genital openings. After delivering the sperm, the tip of the pedipalp becomes stuck inside the female, forming a plug in her reproductive tract.
To find out if leaving behind part of the pedipalp helped the males escape death, researchers compared the damage to this organ during first-time sexual encounters with damage sustained in subsequent encounters.

Evolutionary benefit

The experienced males monitored in the experiment had mated once before and only had a single remaining pedipalp intact. These males would enjoy no evolutionary benefit from surviving after mating a second time because they have generally lost both of their pedipalps at this point, and can therefore no longer inseminate females.

According to Uhl, if detaching the pedipalp did offer a survival benefit, one would expect to see it happen more among virgin males, which could mate again, than experienced males, which could not. But the researchers found an equal amount of pedipalp damage among these two groups.
They therefore concluded that detachment of the pedipalp tip cannot significantly enhance a male spider’s chance of escaping attack by his mate.

Uhl’s team also found that a pedipalp tip left inside a female affected how long she copulated for in subsequent encounters. Normally virgin females mate for about 16 seconds, but those with a pedipalp plug mate for only half as long.

This is important because female spiders are more likely to deliver the offspring of those males with which they copulate longest, Uhl says.

She concludes that the pedipalp plug acts as a chastity belt to prevent sperm from competing spiders from entering the females. Uhl's group has also found other types of wasp spiders with a similar "plugging mechanism".

Journal reference: Behavioral Ecology (DOI: 10.1093/beheco/ar1074)

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