Moult Timing as a Measure of Condition in a Northern Hemisphere Ungulate

Many vertebrates that live in seasonal climates moult once or twice a year, with the effect of refreshing worn coat or plumage, increasing insulation or improving camouflage. The timing of moult may be in response to a variety of cues, but because it is costly, it is also likely to reflect the health and wellbeing of individuals, here referred to as their condition. Using the individually-monitored red deer from the Isle of Rum, in which summer and winter coats are easily distinguished, we (a) test for traits that are predictive of moult timing, (b) explore traits that moult timing is predictive of and (c) analyse covariances between moult timings and calving dates at the individual, annual and residual level. We show that adult female moult timing in July and November is associated with age, reproductive status and parasite burden, and has been getting earlier over time in November. A female's moult timing also predicts the overwinter survival of herself and her current calf as well as her future fecundity. Moult timing varies among individuals and years. Covariances between calving dates and the timing of each moult shed light on the temporal patterns of condition within individuals and across years, for example, across years, winter moult covaries with the following calving date but not the summer moult that occurs shortly after. The timing of moult appears to be a non-invasive measure of condition. Monitoring moult timing across years at the population level may shed light on year-to-year variation in population health and variation within populations is likely indicative of the relative conditions among individuals. Increased knowledge of population and individual condition may be of benefit for ungulate population management and conservation.