Behavioral sleep in the walrus

In this study we examined behavioral sleep in the walrus, the only living species of the family Odobenidae. The behavior of four 1.5-2-year-old captive walruses was videotaped continuously for 7-17 days and scored in 1-min epochs. When walruses had access to water and land, behavioral sleep, the combined amount of quiet and rapid eye movement (REM) sleep, occupied on average 17+/-4% of 24 h (n=4) with the majority of sleep occurring on land. All walruses alternated periods of almost continuous swimming lasting for 40-84 h with periods of rest on land lasting for 2-19 h. When in water they were predominantly awake (88-99% of the time). On land walruses were asleep on average 40-74% of the time. The total sleep time varied between 0 and 60% of 24h with the daily amount of REM sleep ranging from 0 to 5% of 24 h. In water, walruses slept while floating at the surface, lying on the bottom or standing and leaning against the pool wall. REM sleep in water occurred in all positions. On land the breathing pattern was regular during quiet sleep (most pauses were <30s) and arrhythmic in REM sleep (apneas lasted up to 160 s). While in water the irregularity of breathing further increased (apneas were >4 min) and all REM sleep episodes occurred during a single apnea. Data indicate that the pattern of sleep and breathing in walruses is similar to the Otariidae seals while on land and the Phocidae seals while in water.