Muscle jerks during behavioral sleep in a beluga whale (Delphinapterus leucas L.).

We conducted video recording of the behavior of one captive adult male beluga (or white) whale over eight nights aiming to quantify muscle jerks and to evaluate their relationship to the sleep-waking cycle. Presumably, the whale was asleep during a significant portion of the time it spent lying on the bottom of the pool. Individual sleep episodes lasted between 20 and 492 s and on average occupied 66.7+/-2.6% of the nighttime (n=8). Muscle jerks were quantified in the last three nights, during which an average of 144+/-24 jerks were documented per night. Forty-six percent of all jerks occurred within 10 s of each other. Series of jerks lasted 2-21 s (on average 4.8+/-0.5 s, n=97) and in total occupied 0.3-0.7% of the rest time (0.2-0.5% of total nighttime). Jerks occurred more frequently at the end of rest episodes. A significant portion of rest episodes with jerks (62%) followed each other. These series of episodes with jerks alternated with periods when jerks were not recorded over 8-37 min. We conclude that some jerks meet the behavioral criteria of paradoxical [or rapid eye movement (REM)] sleep (PS). On the other hand, definitive conclusions about the presence and duration of this sleep stage in cetaceans cannot be reached without further combined electropolygraphic studies and visual observations.