Phenotypic plasticity in a maternal trait in red deer

Summary Phenotypic plasticity and microevolution represent the two processes by which phenotypic traits in a population can track environmental change. While there is a growing literature documenting microevolution in reproductive traits in naturally occurring animal populations, few studies to date have examined either between‐individual variation in levels of plasticity or how selection acts on plasticity. We present here mixed‐effect linear models analysing changes in calving date in relation to autumn rainfall observed over a 30‐year study of 2147 red deer on the Isle of Rum, Scotland. The study period is characterized by a phase of low and rising population density (up to and including 1980), followed by a phase of high and fluctuating population density (1981 to present). Variation within individual females explained a population‐level trend of delayed calving dates following years of high autumn rainfall. There was significant variation between females both in their average calving dates and in their individual plastic responses of calving date to autumn rainfall. Females born in the low population density phase were, on average, phenotypically plastic for the calving date–autumn rainfall relationship, and showed significant variation in plasticity. Selection favoured individuals with early average calving dates among these females. Among females born at high population density, there was on average no significant plasticity for calving date, but variation in plastic responses was still present. Selection favoured females with increasingly positive plastic responses of calving date to autumn rainfall. We argue that early experience of high population density affects the physiological condition of females, making an environmental response (calving early following dry autumns) in later life physiologically untenable for all but a few high quality individuals. These same few individuals also tend to be fitter and have higher reproductive success.