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, USA). During the day, the animals spent an equal 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 resembling REM sleep. On land, 98% of breathing pauses lasted <30 sec. In water, they ranged from 4 to 145 sec, 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 vigilant due to calf care.

rest, sleep, slow wave sleep, REM sleep, vigilance, common hippopotamus, Hippopotamus amphibius, ungulates, semiaquatic mammals, cetaceans