Sexual selection is a mode of natural selection where typically members of one biological sex choose mates of the other sex with whom to mate (intersexual selection) and competition between members of the same sex to sexually reproduce with members of the opposite sex (intrasexual selection). These two forms of selection mean that some individuals have better reproductive success than others within a population either from being more attractive or preferring more attractive partners to produce offspring. For instance in the breeding season sexual selection in frogs occurs with the males first gathering at the water's edge and making their mating calls: croaking. The females then arrive and choose the males with the deepest croaks and best territories. Generalizing, males benefit from frequent mating and monopolizing access to a group of fertile females. Females have a limited number of offspring they can have and they maximize the return on the energy they invest in reproduction.