Rest and Sleep in the Hippopotamus (Hippopotamus amphibious)

Rest and sleep states of an adult pair of hippopotamuses (Hippopotamus amphibius) and their two-month-old calf are described in a zoo (Tampa, United States). During the day, the animals spent an equal amount of time on land and in water; at night, they spent on average 76% of their time in water. Rest occupied 48-53% of the 24-hour period (11.4-12.8 h). On land, the animals rested in a lying position (18% of rest in the calf); in water-lying, sitting, or standing on the bottom with nostrils, eyes, and ears above the surface (80% in the female), or submerged and lying on the bottom (29% in the calf). All displayed eye jerks and twitches featured characteristics of REM sleep. On land, 98% of breathing pauses lasted <30 s. In water, they ranged from 4 to 145 s, with most lasting <1 min. The male's eyes were closed more often (up to 72%), the female's partially open (84%). Slow wave sleep in the hippopotamus is likely bilaterally symmetrical, as in terrestrial mammals. The female's sleep was more fragmented, less deep and less vigilant due to calf care.