By John Barrat
Image right: A scanning electron microscope image of a eunuch N. malabarensis showing its head with eyes and chelicerae (mouthparts), and the two broken stumps of its pedipalps. (Photo courtesy Matjaž Kuntner)
If he’s lucky a male orb web spider of the species Nephilengys malabarensis,can count on having both his genital appendages, known as pedipalps, break off during sex. (Male spiders have two pedipalps, one located on each side of the head.) The detached appendages plug up the female spider and prevent her from mating with other males. If unlucky, he’ll also be eaten by her after copulation, which lasts about 10 seconds.
In a recent study of the mating behavior of these Asian spiders, researchers found that 87.5 percent of males had both palps amputated during copulation. “The two palps connect to the female epigynum, a sclerotized plate with double copulatory openings positioned ventrally on her abdomen,” says study co-author Matjaž Kuntner, spider expert at the Smithsonian’s National Museum of Natural History and the Slovenian Academy of Sciences and Arts.
This photo shows a female N. malabarensis spider after copulation with the palp of a male spider protruding from her abdomen.
Palps that remain intact are chewed off after sex by the males, making them voluntary eunuchs. Rendering yourself sterile may seem counterproductive to the laws of evolution, but in spiders, the researchers found, there is a distinct benefit to being a eunuch. Simply put, eunuchs are more agile, better fighters and better at defending a mate.
After copulating with a female, the castrated males remain close-by to guard her and prevent other males from approaching. In a series of staged contests between male spiders whose sexual equipment was intact and eunuchs, the eunuchs won hands down, the researchers found.
“Eunuchs guarded females, where highly aggressive and active and initiated and won contests more often,” the scientists write in their paper in the journal Animal Behaviour. “Intact males and half-eunuchs (those with only one palp) showed significantly lower levels of guarding behavior, aggression and general activity.”
Basically, the scientists write, “a sterile male has no reproductive future and nothing to lose in a fight.” Intact males with a reproductive future are more likely to avoid fights and risk injury. “Genital amputation enhances the eunuch’s mate-guarding activity and male to male aggressiveness, which help the eunuch win contests against rivals and increase his paternal success.”
The paper “Eunuchs are Better Fighters” is co-authored by Simona Kralj-Fišer and Matjaz Gregorič, Slovenian Academy of Sciences and Arts; Shichang Zhang, National University of Singapore; Daiqin Li, Hubei University, China; and Matjaž Kuntner, National Museum of Natural History, Smithsonian and the Slovenian Academy of Sciences and Arts.
Related posts:
Male spider’s sexual organs work fastest only when a female breaks them off
Weight of genitals reduces physical endurance in male orb web spiders, researchers find
Suitor’s gentle massage soothes aggressive, cannibalistic female spiders, researchers find
These newly discovered pelican spiders will make you want to visit Madagascar
