Reproductive effort in female ungulates originates from gestation and lactation and has been studied extensively; however, no comparable studies of reproductive effort in males (due to fighting for access to mates) have, to our knowledge, previously been reported. Here, we report on weight loss of male red deer during the annual mating season—a direct measure of male reproductive effort (or somatic reproductive costs). The ‘terminal investmen’ hypothesis predicts that reproductive effort should increase with age, given that costs remain stable. We also propose the ‘mating strategy–effort’ hypothesis, which predicts that reproductive effort peaks in prime–aged males, since they are most often the harem holders. Consistent with the mating strategy–effort hypothesis, relative weight loss during the rutting season peaked at prime age and was lower in younger and senescent males. Weight loss during the rut was relatively smaller as density increased and more so for older males. This is probably primarily due to males (particularly senescent males) starting their rut in poorer condition at high density. The pattern of reproductive effort in males with regard to age and density therefore differs markedly from the pattern reported for females.