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-hours) 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 4 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.