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Sex Is Scary and Castrating for Spider

Sex is fast and scary for some male spiders. And the outcome isn't the most comfortable for the females, either.
/ Source: Discovery Channel

Sex is fast and scary for some male spiders. And the outcome isn't the most comfortable for the females, either.

Copulation is so treacherous for the males, in fact, that some castrate themselves during the act, leaving behind their sexual organ that in turn, plugs up the cannibalistic female as they run for their lives.

New research shows that the males may win out in the end, however, since their severed part actually increases the amount of transferred sperm, heightening the now-sterile male's chances of paternity.

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The study, published in the Royal Society journal Biology Letters, is the first to demonstrate that the sexual phenomenon known as "remote copulation," when the male's sexual organ works without being attached to the male, has not evolved to benefit the female. In fact, the painful-sounding process turns out to be a win-win for males.

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"He achieves continuous sperm transfer after having been removed by the aggressive female, or has moved away himself," co-author Matjaz Kuntner told Discovery News. "At the same time, his palp (sexual organ) plugs the female, thereby monopolizing her."

Kuntner, an evolutionary biologist at the Slovenian Academy of Sciences, and his colleagues studied the highly sexually cannibalistic orb-web spider Nephilengys malabarensis. The findings, however, likely apply to other spiders, such as those in the genus Herennia and Tidarren, and could even apply to other non-spider species with similar mating habits.

For the study, the scientists collected numerous N. malabarensis spiders in the field. They then chose virgins for their experiments.

The researchers began by introducing a virgin male onto a virgin female web. They recorded what happened next, and then counted the amount of sperm under a compound microscope for each spider pairing.

"The copulation is very short, 3-35 seconds," lead author Daiqin Li of the National University of Singapore told Discovery News. "Copulation duration (mean: 7 seconds) resulting from a female-initiated break off is even shorter than that caused by a male-initiated break off (mean: 12 seconds). Males try to escape from females very fast, and then will guard the female if they can manage to escape."

If they don't escape, she eats them. If they do get away, chances are that they severed the joint attaching their sexual organ before running. Close to 90 percent of all spiders studied used cut-and-run tactic.

The male is left sterile, but seems to gain agility and testiness in his new eunuch state.

"We were conducting a study on eunuch adaptiveness...I observed that males escape very quickly from females during copulation while a whole, big plug remained in the female. Due to jet leg, I could not sleep during the night and spoke about this with my husband. He told me about bee stings, where the poison still leaks in despite the fact that the bee is away. It came in this moment...that's the mechanism."

-- Co-author Simona Kralj-Fiser of the Slovenian Academy of Sciences and Arts

"He lives for at least weeks longer," Kuntner said, explaining that males of this species don’t live all that long anyway. "The male benefits from being more aggressive in order to secure his paternity, that is, he defends the female from subsequent rivals."

The discovery helps to explain how mating succeeds for some amazingly different males and females. For many cannibalistic spiders, such as black widows and those of N. malabarensis, the females are enormous and deadly when compared to the smaller, less toxic males. But remote copulation and other evolved tactics keep their sexual conflicts in check.

For example, females of the Australian redback spider, one of the world's most poisonous spiders and a close relative to the black widow, demand 100 minutes of courting or else they usually cannibalize their male suitors. But scrawny males of this species can win at love without exerting much effort.

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Proving that bigger isn't always better in the mating game, the tiniest of males sometimes approach female redbacks after the critical 100-minute mark and successfully mate without being eaten.

"Based upon our data of the timing of premature lethal cannibalism, it appears as though females are not tuned to select male size, but rather the duration of courtship," co-author Jeff Stoltz of the University of Toronto told Discovery News, explaining that females don’t even discriminate much once the 100 minutes are up, so other males can come in at that point and win her favor.

In terms of spider self-castration, Kuntner suspects there are other benefits associated with the seemingly maladaptive behavior that additional research might unveil.