Ageing and reproductive effort in male moose under variable levels of intrasexual competition

1. Capital breeding is a resource use tactic common among polygynous mammals, such as many ungulates. Even though large, prime-aged males stop eating during the rutting season, younger individuals often do not and may adopt alternative mating tactics that are less strenuous. The pattern of reproductive effort is therefore probably very variable among age classes, but has rarely been quantified in male mammals. 2. Based on data of body weight of 9949 male moose (Alces alces L.) aged up to 21 years from seven populations in Norway, we tested hypotheses regarding the pattern of reproductive effort (weight loss during rut) in this capital breeder, and how this may be affected by factors such as age, population sex ratio and density. 3. Reproductive effort increased with age, even after prime-age was reached around the age of 6 years. This provides the first evidence consistent with the terminal investment hypothesis in male mammals. For the very oldest males (> 12 years) data were limited, but the tendency was that effort stabilized or even decreased slightly. Effort did not depend on the population sex ratio or density for adult males. 4. Yearling males also lost some weight during the rutting season, but this was not related to population sex ratio. Decreasing trends in yearling body weight as seen in many strongly harvested moose populations, therefore are due probably to other causes than earlier onset of rutting. Further, effort in yearlings decreased with increasing density. 5. Despite the lack of a correlation between effort and population sex ratio, a significantly faster ageing after prime-age was observed with an increasingly female-biased population sex ratio. This is consistent with the hypothesis that strongly skewed sex ratios may affect the ageing process. 6. Most ungulate populations in Europe and North America are heavily harvested or otherwise managed extensively, harvest being typically male-biased to varying degrees. The resulting skews in sex ratio provide manipulations that give unique opportunities to study life-history variation, in particular for large mammals that are otherwise difficult to manipulate. These opportunities are currently not fully utilized.