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Weight of genitals reduces physical endurance in male orb web spiders, researchers find

eunuch N. malabarensis spider

This scanning electron microscope image of a eunuch N. malabarensis shows its head with eyes and mouthparts, and the two broken stumps of its pedipalps. (Photo courtesy Matjaž Kuntner)

Without their genitals to weigh them down, male spiders of the species Nephilengys malabarensis become lean, mean fighting machines. Carried on both sides of the head, the spider’s two sexual organs, or palps, equal about 9 percent of their total body weight. During mating with the much larger female one or both of the male’s palps usually snap off inside the female.

If he’s not eaten as an after-sex snack by the female, the newly castrated male will hang around and aggressively fend off approaching suitors who must battle the eunuch with their heavy and cumbersome palps still intact. It’s not a fair fight and the eunuchs normally win.

In a series of new experiments 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 proved just how much the weight of the palps cost the male spiders in physical endurance. Using forceps, a razor blade and a microscope the scientists carefully removed the genitals from a number of male N. malabarensis spiders, weighing the spiders both before and after their amputations. Both palps were removed from one spider group, only one palp was removed from a second group, and a third group was left un-castrated.

Next, the scientists made the spiders exercise by irritating them with a small paint brush and causing  them to move around until they were exhausted. Spiders from the group with palps removed were able to travel 300 percent further than spiders with their palps intact, the researchers learned. Spiders with only one palp also performed significantly longer than males with both palps intact.

“Palp removal produced significant reduction in male body weight and enhanced the physical stamina of the spiders,” the scientists conclude in a new paper published in the journal Biology Letters. Large palps are favorable for reproduction but hinder locomotion, the scientists write. Simply, eunuch spiders are superior fighters because the heavy genitals that intact males carry around makes them grow tired faster.


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