In polygynous species, male reproductive effort has been measured both directly in the form of somatic costs and indirectly using behavioral data. We used 12 years of data collected from a semi-domesticated reindeer population in northern Finland to investigate age- and time-specific patterns of dominant males' reproductive effort. Overall, we found that activity levels differed both between young and old dominant reindeer males, and among the early, peak, and late rut, the pattern being age-specific. Reproductive effort was generally higher for old than young dominant males; however, old males reduced their effort in the late-rut period, while young males maintained the same level of activity. There was a positive relationship between somatic costs and activity level only during the early rut for young dominant males, and only during the peak rut for old dominant males. Thus, old males incur the highest energetic costs from rut-related activities when most of the females in the herd are in oestrus. Conversely, young males appear to time their rut-related energetic cost to coincide with the early rutting period, before most females have reached oestrus. Old males are more efficient in timing their reproductive effort so as to maximize their reproductive success. This can be attributed to young males being less experienced than old males or to using an alternative mating tactic by young males who try to avoid competition with old males during the peak rut.