Drivers of individual differences in the sleep behaviour of fallow deer neonates

Inter-individual differences are necessary for selection to act, while plasticity (intra-individual variation) may buffer against selection. Sleep is a critical self-maintenance behaviour but, unlike most behaviours, the causes and consequences of its inter- and intra-individual variation in wild animals is poorly understood, particularly in neonates where sleep plays a key role in development. We have shown previously that free-ranging neonate fallow deer (Dama dama) differ in sleep during the first few weeks of life. Here, we test whether individual variability in sleep is organised systematically across the population, and whether these individual differences are associated with chronic stress measured using hair cortisol, or the timing of birth. Four dimensions of sleep behaviour (total sleep time, sleep fragmentation, sleep quality, and sleep distribution over 24-h) were quantified using state-of-the-art triaxial accelerometers. We then used a multivariate mixed-effects model in a Bayesian framework to evaluate covariation between multiple dimensions of sleep behaviour, and quantify the relative importance of chronic stress and the timing of birth, while accounting for the confounding effects of environmental conditions and age. We found that the timing of birth and chronic stress were not associated with changes in sleep between individuals. While both total sleep time and the number of bouts per day declined with age, their rate of development covaried, but no other sleep dimensions covaried. Our results represent an in-depth analysis of natural variation in sleep, and show that individual differences in four aspects of sleep architecture in free-living fallow deer fawns are strong but independent of one another and unrelated to chronic stress or the timing of birth. We suggest that covariation between sleep dimensions might emerge later in life and effects of cortisol and birth timing might be very short and transient.