Cryptic female choice: frogs reduce clutch size when amplexed by undesiredmales

In species with internal fertilization, females can 'cryptically' choose (e .g. through sperm selection) which individuals sire their offspring, even w hen their overt preferences for copulatory partners are overrun by male-mal e competition and sexual coercion. The experiment presented here reveals th at control of paternity after copulation has begun is also possible in spec ies with external fertilization. Females of the hybridogenetic Rana lessona e-Rana esculenta (LL-LR) waterfrog complex adjust their clutch size in resp onse to mate type: they release fewer eggs when amplexed by hybrid LR males -who jeopardize successful reproduction-than when amplexed by parental LL m ales. This reduction in the number of eggs laid can increase a female's res idual reproductive value through a second mating in the same breeding seaso n or a larger clutch size in the next year. We argue that cryptic female ch oice through clutch size adjustment (i) may have evolved more often than pr eviously assumed, and (ii) can arise even where females mate only once duri ng a reproductive period.