November 30, 2004
How Do Dolphins Sleep?
This is Genevieve Johnson speaking to you from the Odyssey in the Mediterranean Sea.
When giving education presentations in schools, one of the most common questions asked of the Odyssey crew is - "How do dolphins sleep?"
Dolphins do not sleep in the sense that we understand it, as a prolonged, unconscious period of rest. The main reason they do not sleep soundly is quite simple. For a dolphin, breathing is a conscious activity, meaning that if it losses consciousness, it does not breath. Humans and other land mammals have a 'breathing reflex', so if we sleep or are become unconscious, we continue to breath automatically.
In 1964, John Lilly first observed that an important feature of sleeping behavior in bottlenose dolphins (Tursiops truncates) was that they keep their eyelids closed alternately and he proposed the concept of unilateral sleep in dolphins.
In 1985, a Russian scientist, L.M. Mukhametov, described unihemispheric slow wave sleep in the bottlenose dolphin. Investigations on brain electrical activity showed that in sleeping bottlenose dolphins, slow wave electrical patterns are recorded in one hemisphere (alternately in the right or left hemisphere) while activity typical of being awake is recorded in the other hemisphere. His work demonstrated that dolphins never sleep with both cerebral hemispheres simultaneously and never exhibit paradoxical 'rapid eye movement' (REM) sleep observed in land mammals.
Dr Guido Gnone with Iain Kerr and Genevieve Johnson at the Genoa Aquarium.
Photo - Chris Johnson
Earlier this year while the Odyssey crew worked out of Genoa, we met a scientist studying sleeping patterns in dolphins. Italian scientist Dr. Guido Gnone has studied sleeping and resting behavior in captive bottlenose dolphins at the Genoa Aquarium for several years. Dr. Gnone and his colleagues have identified two patterns of resting behavior based on visual observations and acoustic recordings.
The researchers termed the first behavior 'rest swimming' during which animals tend to pair off, follow a regular trajectory and slowly swimming side by side in strong synchrony. In such a state, they typically close one eye, coordinating with their partner which eye is open. If they change position, they also change which eye is open.
Interestingly, there is no strict correlation between which eye is open or closed and which brain hemisphere is awake or asleep. It appears the open eye performs a purely sentinel function and is most often directed at the partner.
During the second behavior termed 'rest at the surface', dolphins stop swimming and rest at the surface with the blowhole emerged. From the experience of Dr. Gnone and his team, the dolphins choose between these two resting options depending on water movement and the availability of a cooperative partner. This behavior was most often observed in a lone animal. In both patterns the animals show no interest in their immediate environment and produce no sounds.
The only resting behavior observed in young dolphins is 'rest swimming' in association with the mother with regular body contact observed. A calf has never been observed resting at the surface.
Because it is difficult to observe dolphins over long periods of time in the wild, little research has been carried out on sleeping patterns. Most of what has been learned and is currently understood about small cetacean resting and sleeping patterns is learned from captive animals. Indeed 'rest at the surface' behavior has never been observed in wild bottlenose dolphins and may in fact be a partial by-product of captivity.
FIGURE 1 - The side-by-side position of two adult bottlenose dolphins while 'rest swimming' as a pair.
Each individual keeps the open eye directed to the partner.
FIGURE 2 - The 'ventro-caudal' position of a bottlenose dolphin calf close to the mother's body.
Both dolphins keep an eye open and check each other's position with gentle lateral movements. Body contacts were often observed.
FIGURE 3 - An adult bottlenose resting at the surface.
Illustrations courtesy of Dr. Guido Gnone
A behavior called 'logging' is often observed in the wild and is a regularly recorded behavior of cetaceans sighted by the Odyssey crew. However, it is most often observed in association with calm sea conditions that allows a stationary resting position without continued wave disturbance. Perhaps dolphins in the wild partake in a similar 'rest swimming' pattern to that observed in captive animals in rougher weather in an effort to save energy and maintain a good state of vigilance against predators while resting.
The work of Dr. Guido Gnone and his colleagues is helping us understand more about some of the basic ecology of small cetaceans - especially in bottlenose dolphins. However, there is still much research to be done on the many other species of dolphins in their natural environment in order to fully understand and properly answer the question - "how do dolphins sleep?"
- Gnone, G. Benoldi, C. Bonsignori , B. & Fognani, P.
Observations of rest behaviours in captive bottlenose dolphins (Tursopis truncates)
Aquatic Mammals 2001, 27.1, 29-33.
Log written by Genevieve Johnson.