Orb web spiders become better fighters without their massive genitals

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It appears that Varys "the Spider" from Game of Thrones may be onto something. In what appears to be a very mixed blessing, castrated male orb spiders are far more agile and energetic than their fully endowed rivals.

During mating, males typically have their junk snipped off and are often killed and eaten by females. But for those who survive, the dramatic decrease in weight turns them into formidable fighters.

Top image: Ian Waldie/Getty Images.

And yes, the reduction in excess weight is significant. The genitals of an orb spider accounts for 9% of its total mass. Just for reference, if a 185lbs human male had the same proportions, his genitals would weigh nearly 17lbs.


The discovery was made by scientists from the Smithsonian's National Museum of Natural History, National University of Singapore, China's Hubei University and the Slovenian Academy of Sciences and Arts. By using razor blades and a microscope (of course), they were able to compare the physical endurance of male orb spiders before and after genital amputations (both for half-eunuchs with one palp removed, and full-eunuchs with both palps removed).


The scientists then irritated the spiders with small paint brushes until they were exhausted.

What they discovered was, when compared to non-castrated spiders, the full-eunuchs exhibited an 80% increase in physical endurance, while the half-eunuchs experienced a 32% increase — a change of affairs that likely accounted for their success in fending off belligerent rivals.


So, while large palps are favorable for reproduction, they are clearly a hindrance for endurance and mobility. Clearly, half-eunuchs and full-eunuchs (who together account for 25% of males who survive mating) have been given physical advantage. But why?

The researchers believe there may be a very good evolutionary reason for this. Surviving males are better able to protect their females from the advances of non-castrated spiders. In other words, female spiders may castrate males in order to make them better protectors.


You can read the entire study at Biology Letters.

Top image via tumblr. Inset image via Smithsonian Science/Matjaž Kuntner.