Vegetation and local density are associated with red deer calving dates but don’t explain the long-term temporal trend

Kirsty H. Macphie, Shane Butt, Sean J. Morris, Alison Morris, Fiona E. Guinness, Tim Clutton-Brock, Loeske E. B. Kruuk, Hannah Froy, Josephine M. Pemberton

Long-lived mammal species can show advancing reproductive phenology even though prolonged gestation means adaptive phenotypic plasticity to warmer springs, as observed for reproductive phenology across other taxonomic groups, is unlikely. A constraint model in which oestrus date depends on achieving a certain level of body condition seems more likely, so the question is which variables are promoting changes in condition? Here we investigated the role of food availability as a predictor of subsequent calving date in the individually-monitored red deer (Cervus elaphus) on the Isle of Rum, Scotland, where calving date has got 12 days earlier over 45 years. Food availability and competition were represented by three individual-level environmental variables: satellite-derived NDVI (a measure of greenness), local deer density and proportion of grassland, estimated within individual home ranges. Higher NDVI, lower density and higher access to grassland were all associated with earlier calving dates, supporting the hypothesis that higher resource availability increases individual condition, allowing earlier reproduction. Whilst the connection between resource availability and calving dates was clear, temporal trends in NDVI and density were weak and the associations explained a maximum of one day of the shift observed in calving dates. We discuss explanations for this contrast, including the potential that by eating NDVI, the deer generate error in its measurement. Regardless, we evidence that spatiotemporal heterogeneity in resources among individual home ranges contribute to the high variance observed in reproductive phenology. As a trait with consequences for fitness, this highlights the importance of incorporating spatiotemporal variation when considering broad-scale responses in a rapidly changing world.