Fine-scale spatial variation in fitness, inbreeding, and inbreeding depression in a wild ungulate.

Environmental stress can exacerbate inbreeding depression by amplifying differences between inbred and outbred individuals. In wild populations, where the environment is often unpredictable and stress can be highly detrimental, the interplay between inbreeding depression and environmental variation is potentially important. Here, we investigate variation in inbreeding level, fitness and strength of inbreeding depression across a fine-scale geographic area (~12 km²) in an individually monitored population of red deer (Cervus elaphus). We show that northern regions of the study area have lower birth weights, lower juvenile survival rates, and higher inbreeding coefficients. Such fine-scale differences in inbreeding coefficients could be caused by the mating system of red deer combined with female density variation. We then tested for an inbreeding depression-by-environment interaction (ID × E) in birth weight and juvenile survival, by fitting an interaction term between the inbreeding coefficient and geographic location. We find that inbreeding depression in juvenile survival is stronger in the harsher northern regions, indicating the presence of ID × E. We also highlight that the ability to infer ID × E is affected by the variation in inbreeding within each geographic region. Therefore, for future studies on ID × E in wild populations, we recommend first assessing whether inbreeding and traits vary spatially or temporally. Overall, this is one of only a handful of studies to find evidence for ID × E in a wild population-despite its prevalence in experimental systems-likely due to intense data demands or insufficient variation in environmental stress or inbreeding coefficients.