Disentangling seasonal effects of environmental variability and population density on life‐history traits in a capital breeder

High-latitude environments are characterised by strong seasonality, with resident animals experiencing a very short plant-growing season for reproduction and acquisition of resources before the long and demanding winter. These seasonal fluctuations in food availability are likely to affect density-dependent processes, with density-dependent food limitation expected to be stronger outside the plant-growing season. Density dependence and environmental variability can have both direct and indirect effects on reproductive success through their influence on important indicators of individual quality, such as body mass. Untangling the direct and indirect effects of environmental factors and population density on life-history tactics is fundamental for understanding population and food-web dynamics. We developed a mechanistic path model to quantify the relative importance of direct and indirect effects of density dependence and environmental variability on body mass and reproductive success of female reindeer. Long-term individual measurements before and after lactation allowed quantifying simultaneously the direct negative effect of reproductive success on autumn body mass and the strong positive seasonal covariation among the individuals' body mass measurements. This emphasises the co-occurrence of a cost of reproduction and individual heterogeneity. We show that the estimated impact of density dependence on female body mass at the end of the summer is only 25% of the impact of density dependence during winter, as failed reproduction induced by low spring body mass leads to a large compensatory growth over summer. While such a pattern is expected, our estimates provide a unique and robust quantification of its importance. This result challenges the use of body mass data of adult individuals harvested in autumn when studying temperate large herbivores, as density dependence during winter and compensatory adjustments of reproductive effort may not be revealed without considering body mass both before and after lactation.