When animals compete for copulations, large body size often implies high mating success. Small males may, however, compensate for their competitive disadvantage by special adaptive traits. This is the case in marine iguanas, iguanid reptiles of the Galapagos archipelago. In marine iguanas, females copulate only once per season. During copulations, males needed 2.8-3.1 min to initiate ejaculation. Large territorial males copulated long enough (more than 3.1 min) to achieve ejaculations in 95% of copulations, even when disturbed. Small males were separated from females by rival males in 29% of copulations before the start of ejaculation. Small males nevertheless fertilized females using prepared viable ejaculate, which was kept in readiness in hemipenis pouches and transferred into the female cloaca at intromission. This tactic increased the mating success of small males by 41%. This tactic demonstrates the adaptive significance of a trait which is functionally equivalent to non-ejaculatory `masturbation', and appears to be unique for vertebrates. However, similar phenomena have occasionally been observed, but mostly lacked functional explanation. An `ejaculation-in-advance' could occur in many taxa in which there is size-dominance hierarchy and sexual interference, but this remains to be determined.