The argument that an optimal age at first reproduction evolves where the benefits of reproduction outweigh the costs implies that where conditions change, age at first reproduction should also change. We studied six captive populations of fallow deer (Dama dama) that differed in the age structure of males, male density, and sex ratio. We examined responses of males to changes in competition by simulating the presence of additional males by providing visual, olfactory, and auditory stimuli. Our results suggest that dominance rank was the most important factor in determining the level of reproductive behaviors exhibited. The participation in reproduction increased with dominance status, and this association held for males that are usually socially immature (3-5 years old). Hence, age apart from dominance was relatively unimportant in affecting the reproductive behavior. Male density was positively correlated with the time spent walking and standing The female-to-male ratio was positively related to male-female interactions, but negatively related to male-male interactions. In response to the simulations, males older than 3 years increased the rate of reproductive behaviors, but younger males did not change or decreased the rate. We conclude that only males up to 3 years of age refrained from reproduction under competition by mature males. However, past 3 years of age, investment in reproduction is a function of dominance rather than a function bf age. This implies that 4- and 5-year-old males do not seems to defer reproduction until socially mature but that their participation is a correlate of the achievable dominance rank.