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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 . 6 and Gnone . 7 challenge our conclusions.

METAMORPHOSIS of holometabolous insects involves extensive reorganization of their nervous systems1,2. In spite of such changes, behavioural modifications can be demonstrated at the adult level after manipulations in the larval stage3–5. For example, the adult reactions of Nemeritis and Drosophila to chemical cues have been altered by exposure to these cues during the larval stage3,4. One limitation of these experiments, as Thorpe points out, is that the behavioural modification cannot be assessed directly in the larval stage. Using a more formal conditioning procedure, Borrell du Vernay5 found that Tenebrio trained to discriminate between surfaces and thus avoid shock in the larval stage showed subsequent savings in relearning the discrimination as adults. Unfortunately, this study did not include controls for such variables as sensitization to shock and familiarization with the test situation.

EARLIER work1 with fresh canine brains has shown that there is a linear increase in the dry matter content from birth to 6 weeks of age (10 per cent to 19 per cent) and then a more gradual maturation to the adult level of 20 per cent. A rapid increase in brain volume up to 6 weeks of age was also found.