SLEEP IN CAPTIVE BOTTLENOSE DOLPHINS

Student Report- University of Groningen 

Sleep can be identified using electrophysiological and/or with behavioral criteria. Both methods have been proven valid throughout numerous mammalian sleep studies. If the animal exhibits all mammalian sleep criteria (provided by numerous mammal sleep research studies over the past years), then sleep is present. The behavioral sleep criteria, however, have been founded on bi-hemispheric sleeping mammals. Studies on Cetaceans have proven they rely on uni-hemispheric sleep instead. The bottlenose dolphin (Tursiops truncatus), for instance, shows constant movement throughout the night. This constant seemingly alert state is argued to be wakefulness. But what if we were to apply existing mammalian behavioral sleep criteria on their resting behavior? How well would these criteria apply to them, if at all? 

The captive bottlenose dolphins exhibit two types of rest during low activity hours (18:00 – 06:00). Slow Circular Swimming (SCS) and logging behavior, recognized by v.d. Klij (unpubl.) during a previous study, has also been acknowledged by numerous other behavioral studies on bottlenose dolphins. 

Mukhametov 1985 drugged the dolphins to sleep, measured EEG, and recorded similar behavioral patterns of a logging dolphin. This observation allows us to combine both EEG, which proposed bilateral desynchronization, with a behavioral state which had been repetitively described as passive hanging of the bottlenose dolphin (logging behavior). He also concluded longer periods of unilateral desynchronization, lasting up to 2.5 hours, with similar behavioral description of a dolphin in SCS. We therefore assume that these 2 behaviors, observed only during low activity time, are in fact rest and perhaps even, sleep. 

With the predetermined mammalian sleep criteria we wish to conclude whether this established rest is in fact sleep in our captive bottlenose dolphins. With 24-hour behavior analysis we attempted to establish whether they exhibit a resting pattern. This combined with arousal threshold analysis and anticipatory feeding behavior , which is driven by an internal clock, enabled us to conclude if they exhibit two very important behavioral criteria: Namely, increased arousal threshold and a circadian rhythm. 

The bottlenose dolphin fits extremely well in the given mammalian behavioral criteria. They exhibit two types of rest during the night, Slow Circular Swimming (SCS) and Logging behavior. Both states exhibit an increase in arousal threshold, a circadian rhythm, a quiescent state with a certain posture and location. Their logging behavior fits most perfectly in the mammalian sleep model, their SCS behavior fits 6 of the 8 criteria. Because of this, we conclude sleep to be present in our captive bottlenose dolphins. 

Keywords: Sleep, Uni-hemispheric sleep, Captive bottlenose dolphins, Tursiops truncatus, Mammalian behavioral sleep criteria, Arousal threshold, Anticipatory feeding behavior, Circadian rhythm, Slow Circular Swimming, SCS, Logging, Surface rest.