Individual and geographical variation in age at maturity in female moose (Alces alces)

Variation in age at maturity among 2764 female moose (Alces alces) in 14 populations in Sweden was investigated and related to patterns of body growth and characteristics of the environment. Data were collected from animals shot during the hunting season in 1989-1992, and all females were aged and examined for previous pregnancy by inspecting the uterus. Individual variation was large; 10 (1.3%) out of 776 yearlings were classified as previously pregnant, and must have become fertile during their first year of life. Among 2-year-old females 31.2% were mature, while the majority (80.5%) of females became mature at 3 years of age. Age at maturity, calculated as the age at which 50% of the females were mature, was also highly variable among populations and ranged between 2.06 and 3.17 years. Population-specific age at maturity was significantly related to mean yearling carcass mass among populations but not to juvenile, 2-year-old, or adult carcass mass. Approximately 50% of the variation in age at maturity among populations could be explained by the mean rate of body growth between juvenile and yearling stages. High initial rates of body growth and early maturity in a population were strongly associated with low rates of body growth after the yearling stage. No clear relationship was found between age at maturity and population density, amount of browse, latitude, or climatic conditions among populations. This suggests that multiple factors are involved in shaping the geographical pattern of age at maturity and that additional factors such as nutritional quality and age-specific survival must be considered when explaining the mechanisms behind the apparent variation in age at maturity among populations of female moose.