Early life sleep in free-living fallow deer, Dama dama: the role of ontogeny, environment and individual differences

An individual's future behaviour and fitness are strongly influenced by early life experience. Within the suite of factors that underpin juvenile development, sleep plays a particularly important role, fulfilling vital physiological and cognitive functions. Sleep ontogeny is the process by which sleep time becomes shorter and more consolidated into fewer bouts from in utero development to adulthood; however, how sleep quantity, fragmentation and quality develop in neonates in the wild is unknown. We investigated this question in 19 free-ranging fallow deer fawns, Dama dama, during the first 5 weeks of life. Specifically, we examined how sleep developed, how it differed between and within individuals, and how it was affected by environmental conditions, using accelerometer-derived estimates of sleep and a Bayesian hierarchical modelling approach. We showed that sleep duration rapidly decreased and became more consolidated, quickly approaching an adult-like condition. Moreover, fawns exhibited consistent individual differences in sleep quantity, fragmentation and quality, as well as in the rate at which sleep developed. Finally, environmental conditions affecting thermoregulation mediated sleep behaviour; sleep time was reduced and was of lower quality on warmer days, and sleep quality was further compromised in more humid conditions but was higher with greater rainfall. While sleep ontogeny in free-ranging fawns is partially shaped by the environment, our study reveals previously unknown individual differences in sleep behaviour present from birth, and in the rate of sleep development. We suggest that such individual differences may represent pace-of-life syndromes and may have important consequences for individual fitness later in life.