Phylogenetic analysis of the ecology and evolution of mammalian sleep.
The amount of time asleep varies greatly in mammals, from 3 h in the donkey to 20 h in the armadillo. Previous comparative studies have suggested several functional explanations for interspecific variation in both the total time spent asleep and in rapid-eye movement (REM) or "quiet" (non-REM) sleep. In support of specific functional benefits of sleep, these studies reported correlations between time in specific sleep states (NREM or REM) and brain size, metabolic rate, and developmental variables. Here we show that estimates of sleep duration are significantly influenced by the laboratory conditions under which data are collected and that, when analyses are limited to data collected under more standardized procedures, traditional functional explanations for interspecific variation in sleep durations are no longer supported. Specifically, we find that basal metabolic rate correlates negatively rather than positively with sleep quotas, and that neither adult nor neonatal brain mass correlates positively with REM or NREM sleep times. These results contradict hypotheses that invoke energy conservation, cognition, and development as drivers of sleep variation. Instead, the negative correlations of both sleep states with basal metabolic rate and diet are consistent with trade-offs between sleep and foraging time. In terms of predation risk, both REM and NREM sleep quotas are reduced when animals sleep in more exposed sites, whereas species that sleep socially sleep less. Together with the fact that REM and NREM sleep quotas correlate strongly with each other, these results suggest that variation in sleep primarily reflects ecological constraints acting on total sleep time, rather than the independent responses of each sleep state to specific selection pressures. We propose that, within this ecological framework, interspecific variation in sleep duration might be compensated by variation in the physiological intensity of sleep.
Capellini
Isabella
I
Evolutionary Anthropology Research Group, Department of Anthropology, Durham University, DH1 3HN Durham, United Kingdom.|E-mail: Isabella.Capellini@durham.ac.uk.
Barton
Robert A
RA
Evolutionary Anthropology Research Group, Department of Anthropology, Durham University, DH1 3HN Durham, United Kingdom.|E-mail: R.A.Barton@dur.ac.uk.
McNamara
Patrick
P
Department of Neurology, Boston University School of Medicine and VA New England Healthcare System, Boston, Massachusetts 02130.|E-mail: pmcnamar@verizon.net.
Preston
Brian T
BT
Max Planck Institute for Evolutionary Anthropology, Deutscher Platz No 6, D-04103 Leipzig, Germany.|E-mail: brian.preston@eva.mpg.de.
Nunn
Charles L
CL
Max Planck Institute for Evolutionary Anthropology, Deutscher Platz No 6, D-04103 Leipzig, Germany.|Department of Integrative Biology, University of California, Berkeley, California 94720.|E-mail: nunn@eva.mpg.de.
D000818:Animals; D016477:Artifacts; D001699:Biometry; D001921:Brain / Q000033:anatomy & histology* / Q000254:growth & development; D003071:Cognition / Q000502:physiology; D004032:Diet; D004734:Energy Metabolism; D020387:Food Chain; D008322:Mammals / Q000502:physiology*; D009929:Organ Size; D010802:Phylogeny; D011235:Predatory Behavior; D012895:Sleep, REM