Evolutionary (psychology) theory of sexual selection suggests that organisms have a biological imperative to produce as many genetically-related and viable offspring as possible. One strategy is to mate as much as possible. Another strategy is to limit a partners's access to other mating partners. Destroying your partner's genitals after mating take this approach to an extreme.
From National Geographic:
This Spider Destroys Female Genitalia to Prevent Future Mating
Some orb-weaving spiders secure their fatherhood by mutilating their partners’ genitalia—the first such discovery in nature, a new study says.
By Michael Greshko
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.
Published November 5 in Current Biology, the discovery also adds further nuance to the theory of sexual selection, which holds that males and females within a species compete for opportunities to mate—even if it kills them.
“All the time, we’re discovering [such] new, astonishing adaptations,” says Jutta Schneider, a biologist at the University of Hamburg who wasn’t involved in the study but has collaborated with some of its authors. “This competition has enormous power.”
Read the rest here.