All mammals previously studied take maximal rest or sleep after birth, with the amount gradually decreasing as they grow to adulthood, and adult fruitflies and rats die if they are forcibly deprived of sleep. It has therefore been assumed that sleep is necessary for development and serves a vital function in adults. But we show here that, unlike terrestrial mammals, killer-whale and bottlenose-dolphin neonates and their mothers show little or no typical sleep behaviour for the first postpartum month, avoiding obstacles and remaining mobile for 24 hours a day. We find that neonates and their mothers gradually increase the amount of time they spend resting to normal adult levels over a period of several months, but never exceed these levels. Our findings indicate either that sleep behaviour may not have the developmental and life-sustaining functions attributed to it, or that alternative mechanisms may have evolved in cetaceans.

Adult dolphins are capable of sleeping with one eye open and exhibiting slow wave activity in the electroencephalogram (EEG) of one hemisphere at a time. The aim of this study was to examine the postpartum sleep behavior of bottlenose dolphin calves and their mothers. The behavior of three dolphin mother-calf pairs was monitored from birth to 13 months postpartum. Dolphin mothers and their calves exhibited a complete disappearance of rest at the surface for a minimum of 2 months postpartum, swimming in echelon formation on average in 97-100% of the observation time. Calves surfaced to breathe more often than their mothers between the postpartum age of 2 and 8 weeks. During the first postpartum month two dolphin mothers surfaced with both eyes open on average in 93 and 98% of the time while in their calves both eyes were open in 90 and 60% of the cases. In calves, the eye directed toward the mother was open more often (on average in 95% of all observations in calf 1 and 99% in calf 2) than the eye directed to the opposite side (82% in calf 1 and 60% in calf 2). Our data indicate that dolphin mothers and calves are highly active and vigilant during the initial period of the calf's life, continuously monitoring their position relative to each other by sight during wakefulness and sleep. We hypothesize that episodes of EEG slow wave activity at this time are likely to be brief, fragmenting EEG defined sleep into short episodes.

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