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Sleep behaviour

Sleep in continuously active dolphins; Activity and sleep in dolphins (Reply)


All terrestrial mammals studied so far do maximal amounts of sleeping at birth, with sleep time gradually decreasing to adult levels. This has led to the concept that sleep, with its characteristic immobility and unresponsiveness, is necessary for brain and body development. We reported1 that dolphins and killer whales have a very unusual developmental pattern: neonates are maximally and continuously active at birth, and this activity diminishes over a period of months to the adult level; in the postpartum period, mothers also abruptly cease the characteristic ‘hanging’ that constitutes the typical sleep behaviour in bottlenose dolphins and killer whales2,3,4,5. We did not claim that all sleep was abolished in the postpartum period. Rather, we reported that immobility was absent and that typical sleep posture increases with age, a pattern opposite to that seen in all land mammals studied so far. Sekiguchi et al.6 and Gnone et al.7 challenge our conclusions.

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  1. 1

    Lyamin, O., Pryaslova, J., Lance, V. & Siegel, J. Nature 435, 1177 (2005).

  2. 2

    Flanigan, W. F. Sleep Res. 3, 84 (1974).

  3. 3

    Flanigan, W. F. Sleep Res. 4, 139 (1975).

  4. 4

    Lyamin, O. I. et al. Behav. Brain Res. 129, 125–129 (2002).

  5. 5

    Lyamin, O. I., Mukhametov, L. M. & Siegel, J. M. Arch. Ital. Biol. 142, 557–568 (2004).

  6. 6

    Sekiguchi, Y., Arai, K. & Kohshima, S. Nature 441, doi:10.1038/nature04898 (2006).

  7. 7

    Gnone, G., Moriconi, T. & Gambini, G. Nature 441, doi:10.1038/nature04899 (2006).

  8. 8

    Siegel, J. M. Nature 437, 1264–1271 (2005).

  9. 9

    Siegel, J. M. & McGinty, D. J. Science 196, 678–680 (1977).

  10. 10

    Siegel, J. M. Brain Res. Rev. 1, 69–105 (1979).

  11. 11

    Siegel, J. M. & Tomaszewski, K. S. J. Neurophysiol. 50, 696–716 (1983).

  12. 12

    Siegel, J. M. in Principles and Practice of Sleep Medicine (eds Kryger, M. H., Roth, T. & Dement, W. C.) 120–135 (Elsevier Saunders, Philadelphia, 2005).

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Correspondence to J. M. Siegel.

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