Tough love

Male Larinia jeskovi spiders such as this one prevent their sexual partners from future hookups. Photograph by Gabriele Uhl

Michael Greshko writes at National Geographic,

Some orb-weaving spiders secure their fatherhood by mutilating their partners’ genitalia — the first such discovery in nature, a new study says.

...Talk about tough love — some male spiders lop off parts of females' genitalia to prevent her from mating again, a new study says.

The behavior, which guarantees that the male will father all of her offspring, is the first to suggest that males evolve behaviors to maim external parts of the female genitalia.

The team observed that after mating, many females were missing their scapus, a bicycle seat-shaped knob that sits above the genitalia. But why?

To find out, the researchers caught wild L. jeskovi and allowed them to mate under careful observation in the lab. When a male successfully mounted a virgin female, researchers froze the pair with blasts of liquid nitrogen, allowing the team to microscopically scan the spiders’ interlocked genitalia.

Freezing spiders in the act proved challenging; their hookups last mere seconds. “We had to be super quick and super lucky,” Uhl says. (Read about how some male spiders seduce using back rubs.)

A male spider delivers its sperm via pedipalps, a pair of leg-like appendages near its mouth that latch onto the female's scapus from above and below.

The scans revealed that the L. jeskovi pedipalp grasps and twists the scapus as the male dismounts, snipping it off as if with scissors. Without this crucial handle, other males can’t grasp the female at all, preventing her from having another sexual partner.

It’s a twist on the typical arachnid battle of the sexes. Many female spiders have sex with multiple males but fertilize their eggs with only one suitor’s sperm. This competition has prompted some species’ males to take drastic action, such as castrating themselves to plug the females’ reproductive tract.

In this case, however, “males have found a very clever means to prevent females from remating without mutilating themselves,” says Uhl.

Read more here.