Biased adult sex ratio can affect fecundity in primiparous moose Alces alces

Recruitment rate has declined in many Norwegian moose Alces alces populations during the last two decades and at the same time the population size has increased and the proportion of males to females has decreased. Although density-dependent food limitation may explain part of the decrease in productivity, we evaluated to what extent the decline could also be a result of an insufficient number of males in the populations to fertilise all females. To test this hypothesis, we compared, based on ovary analyses, the age-specific variation in fecundity rates in eight Norwegian moose populations with annual variation in the adult sex ratio. Differences in female age and body mass explained a significant proportion of the variance in the probability of pregnancy. In addition, there was a significant relationship between the population sex ratio and the probability of pregnancy among 2-year-old females, but not the probability of pregnancy of older females. Because biased sex ratio only affects first-time breeders, the effect on the population recruitment rate may be limited. However, we also call attention to other possible effects of biased sex ratio, e.g. delayed parturition date, which in the long term may decrease population productivity through effects on survival and fecundity.