Male Traits as Life-History Variables: Annual Variation in Body Mass and Antler Size in Moose (Alces alces)

Abstract

Annual variation in two sexually selected male traits, carcass mass and number of antler points, was studied in 2,862 moose ( Alces alces ) collected during the autumn hunting season in northern Norway during 23 years. Yearling males grew larger during cool and dry summers, but a decrease in mean carcass mass of adult males was best predicted from the decline in sex ratio. However, because population density was significantly correlated with sex ratio, the two effects were difficult to separate. Males were smaller when population density was high and the male: female ratio was low. Carcass mass in female moose was unaffected by density or sex ratio, except in the case of yearlings. The cohort effect of large size of yearling only persisted for 1 year among males. Antler size did not change during the study despite the decline in body mass of males; this implies that the costs of bearing antlers of a given size increased. Variation in sexually selected characters in males may be understood by considering them to be life-history traits.