Before I talk about the impossibility of dolphins drowning, even when they are in a deep slumber, let me clarify a query some of you might have regarding the title of this article. Why am I only talking about dolphins and not ‘fish’ in general? Well, dolphins are technically not ‘fish’. In fact, they are more like us – humans – because they are also mammals. To learn more about human-dolphin interaction, click here.
Being a mammal, they need to periodically come back to the surface to breathe. Some breeds of dolphins, such as the bottlenose dolphin, can only hold their breath underwater for 7-8 minutes! With that in mind, how’s a poor dolphin supposed to doze off to rest their body after a tiresome day of swimming and playing in the sea?
How Do Dolphins Sleep?
Upon careful observation by aquatic experts and research scholars, it was discovered that dolphins (and even some species of whales), have two basic methods of sleeping:
- They quietly rest in the water, horizontally or vertically.
- They simply sleep while swimming slowly next to another animal.
Individual dolphins also enter a deeper form of restfulness, often at night, a state called sleep logging. That’s because, in this state, a dolphin looks much like a log floating at the water’s surface.
When marine mammals like dolphins simultaneously sleep and swim, they are in a state that is similar to napping. Young dolphin calves actually eat, rest and sleep while their mother swims, towing them along in her slipstream. This placement is technically called echelon swimming. During these times, the mother dolphin will also sleep on the move with her calves. This might surprise many of you, but mother dolphins cannot stop swimming for the first few weeks after a new dolphin is born. If she does try to stop at certain intervals of time, there is a risk that the calf may even sink, as the newborn dolphin is not born with enough body fat to keep it afloat.
Dolphins Sleep in Groups
As dolphins are aquatic, but still mammals, lots of swimming will tire out a newborn dolphin much more easily. This makes them weak and susceptible to infection or attack from other aquatic predators. Now, adult male dolphins generally travel in pairs. Oftentimes, they swim slowly, side by side and turn by turn, as they sleep. Similarly, female dolphins and their young also travel together, albeit in much larger pods. They may rest in the same general area, or compatible dolphins may even pair together for sleeping while swimming in a group, in order to stay protected in unison as they sleep.
Another very interesting characteristic about dolphin slumber is that they only shut down half of their brain, so the opposite eye is closed and resting. The other half of the dolphin’s brain stays awake, albeit with a mitigated level of alertness. This attentive side of the brain helps the dolphin keep an eye out for hungry predators and unwarranted obstacles. That waking part of the brain even signals when to rise to the surface for a breath of fresh air, even if the dolphin is in sleep mode. After approximately two hours, this process will be reversed, i.e., the active side of the brain will rest and the rested half will take over. This pattern is called cat-napping.
Why Don’t Dolphins Drown While Sleeping?
Time for the million-dollar question. Why don’t dolphins drown in their sleep? To avoid sinking under the sea, it is mandatory that dolphins retain control of their blowhole. Now, you may ask, what is a blowhole? Well, the blowhole is nothing but the flap of skin that opens and closes under the voluntary control of the dolphins. Although this “control” is fiercely debated, most researchers opine that in order to breathe, a dolphin must be conscious/alert to recognize that its blowhole has reached the surface.
Humans are lucky creatures—we breathe despite the conscious mind being in a deep sleep. However, our subconscious mind takes charge at that point and ensures that subconscious mechanisms have control over this involuntary system. Now, the dolphin respiratory system is voluntary—meaning that dolphins must keep a part of the brain awake to trigger each breath.
Unlike humans, marine mammals like dolphins can take in more air with each breath. This is because their lungs are proportionately larger compared to the lungs of humans. This large lung size helps dolphins exchange more air in each inhalation and exhalation cycle.
Their red blood cells also carry more oxygen when compared with humans. Moreover, dolphin physiology is such that, when they dive, their blood only travels to the essential organs of the body that need oxygen—the brain and the heart. Digestion and many other processes must wait until the marine animal stabilizes underwater after a dive.
Dolphins have a higher tolerance for carbon dioxide (CO2) than humans. Their brains only trigger when a level of CO2 becomes very high, signaling them to return for another breath.
All of these processes, part of the marine mammal diving response, are adaptations to living in an aquatic context. The processes help dolphins stay safe during the process of dozing off. Dolphins are highly adaptable when it comes to breathing in slumber, and they can reduce the number of breaths they take during rest periods. A dolphin that averages 8 to 12 breaths a minute when active can reduce their breathing rate to only 3 to 7 per minute while resting.
Factually speaking, it is very rare for a dolphin to drown, but they might suffocate from a lack of air. For a newborn dolphin, being underwater can cause some trouble. It is the touch of air on the dolphin’s skin that activates the first, crucial breath. However, if the dolphin is unable to reach the surface, or if it finds itself in a state of panic, the dolphin may dive deeper, where it will be unable to breathe, but this is a very rare occurrence. Clearly, there are problems associated with sleeping safely under the sea, but Mother Nature has designed the marine mammal system such that it can adjust to the ‘half-brain sleeping’ trait and manage to catch some Z’s without drowning!