Stereotypical resting behavior of the sperm whale

Though very little is known about sleep in wild cetaceans, toothed cetaceans in captivity sleep with one side of their brain at a time [1]. Such uni-hemispheric sleep is thought to enable swimming, voluntary breathing, predator avoidance and/or social contact during sleep at sea 2 and 3. Using suction cup tags, we discovered that sperm whales (Physeter macrocephalus) worldwide conduct passive shallow ‘drift-dives’ in stereotypical vertical postures just below the sea surface. Bouts of drift-dives accounted for 7.1% of recording time, or 36.7% of non-foraging time. Drift-dives were weakly diurnal, occurring least from 06:00–12:00 (3% of records), and most from 18:00–24:00 (30% of records). A group of vertically drifting whales were atypically non–responsive to a closely-passing vessel until it inadvertently touched them, suggesting that sperm whales might sleep during these stereotypical resting dives.