Rest and activity states in the Commerson's dolphin (Cephalorhynchus commersonii)

The unihemispheric slow-wave sleep, the ability to sleep during swimming with one open eye and the absence of paradoxical sleep in the form of it is observed in all terrestrial mammals are unique features of sleep in cetaceans. Visual observations supplement electrophysiological studies and allow obtaining novel data about sleep of cetaceans. In the present study we examined behavior of 3 adult Commerson's dolphins Cephalorhynchus commersonii which were housed in the oceanarium Sea-World (San Diego, USA). The behavior of the dolphins can be subdivided into 5 swimming types: 1) active swimming marked by variable speed and irregular trajectory of movement (on average for 3 dolphins 35.1 +/- 2.7% of the 24-h period) was scored as active wakefulness; 2) circular swimming was divided into slow and fast swimming and occupied, on average, 44.4 +/- 3.8 and 9.7 +/- 0.8% of the 24-h period, respectively; while in circular swimming, dolphins swam from 1 to 6 circles on one respiration pause; 3) quiet chaotic swimming (3.9 +/- 1.2%) that occurred at the bottom and was not accompanied by signs of activity; 4) floating, and 5) slow swimming at the surface (4.1 +/- 0.5 and 2.8 +/- 0.4%), respectively; the latter two swimming types were accompanied by frequent respiration (hyperventilation). We suggest that sleep in Commerson's dolphins occurred predominantly during the circular and quiet swimming. From time to time the dolphins slowed down their speeds and even stopped for several seconds. Such episodes appeared to be the deepest sleep episodes. In all dolphins muscle jerks as well erections in the male were observed. Jerks and erections occurred during the circular and quiet chaotic swimming. Similar to other studied small cetaceans, Commerson's dolphins are in a state of almost uninterrupted swimming during 24 h per day and they sleep during swimming. Some muscle jerks that we observed in the dolphins in this study might have been episodes of paradoxical sleep.