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Interesting Facts About the Sleeping Habits of Dolphins

Leena Palande
If dolphins sleep like human beings, they might suffocate. If they fail to maintain vigilance, they would fall prey to predators. Then, how do dolphins sleep? AnimalSake provides an answer to this question, and also presents some amazing facts about the sleeping habits of these incredibly intelligent aquatic mammals.

Did You Know?

A dolphin needs 8 to 12 breaths a minute when fairly active. The breathing rate can drop to 3 to 7 breaths per minute while resting. It can catch a catnap between trips to the ocean surface.
Dolphins, the aquatic mammals, are known for the acrobatic leaps that they make out of the water and also for their stunning performances in various shows. They have been very friendly to humans since time immemorial. They are helpful and playful by nature, and are quite energetic too. Trained dolphins can be quite entertaining and so are commonly seen in aquariums, TV shows, and movies. You must have observed that they need to come to the surface for breathing.
Most fish breathe through their gills, but dolphins breathe air using their lungs. They breathe through a blowhole on top of their head. It is covered by a muscular flap when they are underwater, and it opens to exhale as they reach the surface. They surface at regular intervals to exhale and inhale air. A group of dolphins is called a 'school' or a 'pod'. Male dolphins are called 'bulls', females 'cows', and young dolphins are called 'calves'.
Humans breathe involuntarily, but dolphins have to decide when to breathe. They have to be conscious to breathe. This means that they cannot enjoy a sound, deep sleep, because then they would run out of breath. Mother Nature has solved this problem by offering them an exclusive skill with the help of which they can let one half of their brain sleep at a time. Various studies on dolphins have proved this fact.
Usually, fish reduce their activity and metabolism, and take some rest. But they remain alert to prospective predators. Most fish do not have eyelids, so their eyes remain open during such periods of 'suspended animation.' This type of rest performs the same restorative function as 'sleep' does in other animals and humans. But if dolphins sleep like this and if they fail to come to the surface for breathing, then they would suffocate and die. So, how do they manage to sleep without drowning?

How Do Dolphins Sleep?

Like all other fish, dolphins get plenty of exercise as they're constantly swimming around. They also need rest. With only one of the cerebral hemispheres resting at a time, dolphins can sleep with one eye open. The cerebral hemisphere that is working (not shut down) monitors the activities in the surrounding environment and controls breathing functions.

Dolphins in Captivity

In captive dolphins, scientists have found that both hemispheres of brain can go into a 'slow-wave sleep' mode simultaneously. They can sleep with both eyes closed. They do not respond to mild external stimuli when they are fast asleep. The process of respiration continues and does not get affected. A tail kick reflex helps keep the blowhole above the water, whenever required. More research is needed to know whether dolphins in the wild reach this state.

When Do Dolphins Sleep

Dolphins don't sleep at a specific time, like we humans do, and they usually sleep as a group. Most of the pod goes into the half-sleep mode at the time. They just 'switch off' when safe. So, they are often found 'daydreaming', saving energy.

Where Do Dolphins Sleep?

While going into the sleep mode, dolphins prefer to stay close to the surface of the ocean so that they can come up for air easily. They can also rest at the surface with their blowhole exposed. This is the reason why they are sometimes seen 'logging', swimming slowly along the surface, with very little movement.

Pattern of Swimming During Sleep

A study was conducted by the University of California at Santa Cruz on Pacific white-sided dolphins. Researchers found that the dolphins swim in a circle with the open eye facing other dolphins in the circle. Predators are less likely to attack a group of large animals. The tight circle minimizes the chances of being attacked by predators. Once the dolphins get sufficient rest, the school breaks up, and they continue with their normal activities. Adult male dolphins usually travel in pairs, and often swim slowly side by side as they sleep.

How Can Dolphins Hold Their Breath Longer?

  • They have proportionately larger lungs.
  • They can exchange more air at each breath.
  • Their red blood cells can carry more oxygen.
  • As they dive, only the brain, heart, and the swimming muscles receive blood (oxygen). Other processes like digestion are halted for the time being.
  • They can tolerate relatively higher levels of carbon dioxide.

What About a Cow and a Calf?

Unihemispheric sleep is beneficial for both the mother dolphin and her calf. Like in all other species, dolphin calves are especially vulnerable to predators like sharks. So the mother swims, towing the calf along in her slipstream. This type of positioning (placement) is called echelon swimming. The mothers also want their calves near them for nursing.
Human breathing is a constant, involuntary process. On the contrary, dolphins are conscious breathers. The sleeping habits of dolphins are suitable for schooling behavior in a marine environment. They are well-equipped with a voluntary respiratory system to handle underwater sleeping. It's good that human beings have not yet mastered the art of putting half of their brains to sleep, otherwise most people would have been found half-asleep, daydreaming at their desks!