Nutritional and reproductive dynamics of moose near their southern range limit

Understanding the relationships between nutrition and life-history characteristics can provide insight into the factors limiting populations. Moose (Alces alces) at their southern range periphery are subject to nutritional constraints because of marginal climatic and habitat conditions, but how these limitations influence vital rates remains a point of uncertainty. We examined the nutritional and reproductive dynamics of moose at the fringes of their range in southeast Wyoming, USA. We used an individual-based approach to evaluate the nutritional condition (i.e., body fat) of female moose by recapturing the same individuals each autumn and spring to measure body fat, pregnancy, recruitment of offspring, and survival. Like other temperate ungulates, we anticipated that moose would accumulate body fat over summer and use those reserves to support survival and reproduction over winter. Throughout the annual cycle, we expected that body fat, an integrated measure of energetic gains and losses, would influence pregnancy, recruitment, and adult survival. Contrary to expectations, body fat varied by less than 1.5 percentage points between autumn and spring. Despite relatively low seasonal fluctuations, body fat was positively associated with probability of pregnancy and adult survival but did not influence recruitment of young. Moreover, age influenced probability of pregnancy and overwinter survival. We demonstrate clear signs of nutritional limitation and unexpectedly absent relationships between body fat and recruitment that may be unique to moose living at the fringes of their range, highlighting environmental constraints on reproduction at distributional limits. By investigating the connections between nutrition and reproduction in southern moose, we provide insight into how vital rates can be used to monitor demographic performance for at-risk populations.