20.02.2019 – Dreaming dolphins
The Atlantic had us beaming with joy today as our team managed to encounter a total of four dephinid species. In the morning we encountered the interactive Short-beaked common dolphins (Delphinus delphis) and the more evasive Striped dolphins (Stenella coeruleoalba). On both tours we encountered a small herd of Bottlenose dolphins (Tursiops truncatus) as well as some relaxed Short-finned pilot whales (Globicephala macrorhynchus) during our afternoon tour.
A frequent question asked by our guests, particularly during sightings with extrmely active and dynamic species such as the bottlenose dolphins, is how do they sleep? This is a very good question, particularly since all cetaceans are voluntary breathers that consciously open their blowhole to respire at the surface before closing it again as they dive.
Sleep generally varies in the animal kingdom. For many land mammals, humans included, sleep involves partial or total unconsciousness during which voluntary muscle control as well as the activity of the senses are entirely shut down. Marine mammals, however, need to pay attention to their breathing and their surroundings as they rest and therefore do things a little differently. Like our brain, dolphin brains are divided into two hemispheres that „communicate“ with one another through a bridge known as the corpus callosum. During sleep one hemisphere goes into „standby-mode“, while the other hemisphere remains almost fully active. While one half of the brain is „asleep“, the opposite eye is closed and the other hemisphere will monitor the surrounding environment and control breathing functions. The length of such sleeping periods depends entirely on the species and usually involves the animal resting motionless at the surface or swimming very slowly for some minutes.
During this so-called „slow-wave unihemispheric sleep“ dolphins store and centralize new memories in their brains and rest their bodies from their daily activities. So now yet another question pops up; do dolphins actually dream? In humans dreaming is described is related to a physiological event known as REM (rapid eye movement). REM was infact recorded for six minutes in a single nights recording of a Short-finned pilot whale as well as in Bottlenose dolphins.
Despite the fact that they are still aware of their environment as they rest, dreaming dolphins are nonetheless vulnerable as they sleep. Here their social bonds come in handy; close peers within the group ensure the animals safety and that it regularly comes up for air. These so-called „guardian animals“ give us yet another reason to cherish these sophisticated, remarkable creatures.
By Paula Thake
Sightings of the day
10:00 Bottlenose dolphins, Short-beaked common dolphins, Striped dolphins
15:00 Bottlenose dolphins, Short-finned pilot whales