Female genital mutilation and monandry in a spider, Cyclosa argenteoalba (#431)
The evolution and the ecological consequences of mating systems of animals, including human, has been studied extensively. Monandry, in which a female has one mating partner, could be established and maintained in two ways, i.e., 1) when a female spontaneously refrains mating with multiple males, and 2) when a partner male interferes re-matings of a female that otherwise tries to mate with other males. In the latter case, the interference that is realized by several mechanisms, such as guarding, mating plug or injecting chemical substances, is often incomplete, and the female can escape from the partner males to mate with other males. This may explain the reason why polyandry is ubiquitous in animals.
Genital coupling is necessary for successful copulation, and damaging female genitalia is expected another possible way to limit the number of mating partner of females. Spider is a candidate group that may use female genital damage to realize monandry, since in some species, female genitalia is found partly mutilated.
I examined this hypothesis using an orb-web spider, Cyclosa argenteoalba, by staged matings and artificial mutilation experiments. Females of C. argenteoalba accepted two palpal insertions in one mating bout in most cases. They were deprived of a small genital appendage (a scape) after the second palpal insertion of their first matings. This mutilation completely took copulation ability from the female that was still eager to re-mate. The mutilation rate was high and more than 90 % of females were forced to lifetime monandry. This mechanism is considered to provide great advantages to males in sexual conflicts over mating number of females. The occurrence in female genital mutilation may be widespread in spiders.