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How Male Spiders Use Genital Plugs and Self-Castration to Ensure Paternity

Just when you thought spiders couldn't get any kinkier in the sack.
Leucauge argyra, which is known to self-castrate for genital plugs. Image: Marc AuMarc/Flickr

Spiders are all about pushing the limits of sadomasochistic love-making. A few weeks back, Motherboard reported on the ins-and-outs of sexual cannibalism in wolf spiders. You'd think that would be enough kinky news from these arachnid weirdos for one year, but now it's time to talk about genital plugs.

According to a new study published in Behavioral Ecology and Sociobiology, male dwarf spiders create a plug made of "amorphous material"—basically a glue that dries into a hard plug—during copulation in order to block off the reproductive duct of the female.

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"The mating plug in the dwarf spider clearly functions as a mechanical obstacle to rival males," says lead author Katrin Kunz in a statement. "Mating plugs are a powerful mechanical safeguard whose efficacy varies with plug size and age."

A small dwarf spider plug (A) and a larger, more effective one. IMage: Kunz et. al

The approach makes sense when you consider that the dwarf spider mating scene is highly competitive—and plugs make even more sense in species in which the male gets eaten soon after mating, such as the case with the orb weaver spider Leucauge argyra. But the study's close-up look at this odd ritual reveals that there is more than one way to plug a spider's epigyne (so the saying should go).

Kunz and her colleagues at the Zoological Institute in Greifswald, Germany, selected hundreds of juvenile dwarf spiders from the wild and isolated them in Plexiglas cages until they were sexually mature.

Once they were ready to mate, the males were released into webs with the females, and were observed blocking the female epigynum (genital duct) by depositing liquid material into it, which then hardened into an effective plug. The males were then rescued by the researchers before the females could enjoy their post-coital meal.

The second phase of the experiment examined whether a group of males could circumvent this traffic jam inside the female spiders. Males always use their right pedipalp to penetrate the females' right epigynum (and same with the left reproductive organs), so the researchers artificially removed pedipalps from the males before releasing them for round two.

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A male dwarf spider. Image: Michael Hohner

Some males were paired up with females whose reproductive ducts would be open to them, and some with females whose tubes had been plugged in round one. The results confirmed that the blocking strategy is incredibly effective. The males paired off with plugged females still inserted their pedipalps into the epigynum, but they only had a 32 percent chance of fertilization due to the blocked duct. The bigger the implanted barrier, the more difficult it was to circumvent.

Though dwarf spiders were the focus of this study, they are far from the only spiders that have learned to obstruct the female epigynum. Indeed, some species are even more hardcore about barricading the entrance. For example, male wasp spiders will self-castrate while mating in order to ensure paternity. By depositing their full pedipalp inside the female, they literally cock-block any other male that attempts to mate after them.

The Tidarren genus of spiders are also known to leave the full organ inside the females, but not of the male's own volition. Instead, the female Tidarren severs the male's body from his sexual organ like a twist-off bottle cap. She then feeds on the body while the dead male's phallus continues to pump sperm into her.

So while spider sex can get a bit extreme, at least it's always there to remind us that while human courtship can be frustrating, at least it is not a waking nightmare.