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, 22 (5), 847-853

Energetic Constraints, Not Predation, Influence the Evolution of Sleep Patterning in Mammals

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Energetic Constraints, Not Predation, Influence the Evolution of Sleep Patterning in Mammals

I Capellini et al. Funct Ecol.

Abstract

Mammalian sleep is composed of two distinct states - rapid-eye-movement (REM) and non-REM (NREM) sleep - that alternate in cycles over a sleep bout. The duration of these cycles varies extensively across mammalian species. Because the end of a sleep cycle is often followed by brief arousals to waking, a shorter sleep cycle has been proposed to function as an anti-predator strategy. Similarly, higher predation risk could explain why many species exhibit a polyphasic sleep pattern (division of sleep into several bouts per day), as having multiple sleep bouts avoids long periods of unconsciousness, potentially reducing vulnerability.Using phylogenetic comparative methods, we tested these predictions in mammals, and also investigated the relationships among sleep phasing, sleep-cycle length, sleep durations and body mass.Neither sleep-cycle length nor phasing of sleep was significantly associated with three different measures of predation risk, undermining the idea that they represent anti-predator adaptations.Polyphasic sleep was associated with small body size, shorter sleep cycles and longer sleep durations. The correlation with size may reflect energetic constraints: small animals need to feed more frequently, preventing them from consolidating sleep into a single bout. The reduced daily sleep quotas in monophasic species suggests that the consolidation of sleep into one bout per day may deliver the benefits of sleep more efficiently and, since early mammals were small-bodied and polyphasic, a more efficient monophasic sleep pattern could be a hitherto unrecognized advantage of larger size.

Figures

Fig. 1
Fig. 1
Phylogenetically independent contrasts of sleep-cycle length with (a) body mass and (b) sleep site exposure. The index of exposure quantifies vulnerability of sleeping sites from the least exposed site (lowest values) to the most exposed site (highest values). Sleep-cycle length and body mass were log-transformed.
Fig. 2
Fig. 2
Evolutionary history of phasing of sleep (monophasic sleep in white, polyphasic sleep in black) reconstructed with maximum likelihood (see text). Areas of pies indicate the relative support for each of the two possible character states at each given node. Because all reconstructions at each node along the phylogeny strongly supported only one state (probabilities were 99% in favour of one state), circles appear to be filled. Support for polyphasic sleep as ancestral character state is 99% (actual calculations from Discrete; Pagel 1994, ; see text). Species with missing values for phasing of sleep are not shown. Phylogenetic tree assembled using published phylogenies (sources in Appendix S1).
Fig. 3
Fig. 3
Phylogenetically independent contrasts of phasing of sleep with (a) body mass and (b) sleep site exposure. Phasing of sleep was treated as a dummy variable and analysed with independent contrasts; lower values indicate monophasic sleep and higher values indicate polyphasic sleep (see text). The lowest values of sleep site exposure indicate most protected sites, the highest values the most exposed sites (see text).
Fig. 4
Fig. 4
Phylogenetically independent contrasts of phasing of sleep with NREM sleep (a), REM sleep (b) and sleep-cycle length (c). Phasing of sleep was analysed as a dummy variable; lower values indicate monophasic sleep and higher values polyphasic sleep (see text).
Fig. 5
Fig. 5
Phylogenetically independent contrasts of sleep-cycle length with contrasts of REM (a) and NREM sleep duration (b).

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