Animal behaviour: continuous activity in cetaceans after birth

Abstract

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.

Figure 1 |. Duration of rest behaviour in whales and dolphins.

a, b, Time spent by killer whales floating at the surface and lying on the bottom of the pool (a) and by bottlenose dolphins floating at the surface (b). The data represent percentages of the night, the main sleep period in captive cetaceans. Data for one non-pregnant orca female and one adult male are the averages (with s.e.m.) of 12 nights. All other killer-whale, dolphin and calf data are averages over two consecutive nights. Red, mothers; yellow, calves; blues, adult females without calves; green, adult males.