If a dolphin loses consciousness, he will not be able to keep his blow-hole clear of the water and will suffocate and die. Other dolphins will jostle a dolphin at risk in this way to keep him awake, and will hold the dolphin to the surface so that he can breathe.
Helping other dolphins seems to be so strongly ingrained that that may be why dolphins sometimes come to the assistance of humans in trouble in the water. Blowholes are conveniently located at the top of their heads so that they can easily inhale air when they come to the surface. Dolphins also use their blowholes to exhale, which they tend to do sharply when they come to the surface.
No, dolphins do not have gills. Gills are used to breathe by sharks, fish and rays that are able to absorb oxygen from water passing over their gills. Dolphins and other marine mammals do not have this ability and therefore depend on breathing air through their lungs to survive. Unlike most mammals, dolphins need to be conscious to breathe, meaning they need to be actively aware of their breathing in order to survive.
But as you can imagine, this could be pretty tricky to do while asleep! But dolphins are thought to essentially turn off half of their brain while they sleep, so that they can remain almost half-conscious enough to help regulate their breathing. Since dolphins are half conscious while sleeping, it allows them to control their breathing, which they need to do in order to continue breathing. Dolphins will sleep in short bursts near the surface so that they can quickly and easily pop to the surface for air.
That being said, dolphins do not need nearly as much sleep as humans do! While they need to breathe air to survive like all mammals, dolphins can not survive on land. Dolphins have evolved to survive in the ocean, and being on land for too long can be deadly. For one, dolphins are not able to walk or slither, and their streamline body is not built for land, which would make it impossible for them to survive.
Dolphins are fairly large-bodied and being on land would likely make it difficult for them to breathe. In the water, buoyancy is on their side, which prevents their large body size from impeding their breathing. Dolphins can also dry-out. These differences allow the dolphin to have a much more efficient exchange of gases from the lungs to the bloodstream.
Dolphins are aided by the fact that they can use a sort of selective circulation process; when diving, the blood flow to the skin, digestive system and outer extemities slows down or stops altogether.
This leaves the heart, brain and tail muscles able to still function. The atmospheric pressure of a deep dive forces air out of the lungs and into the nasal passages and forces blood from the heart into a complex network of capillaries.
The dolphin is able to squeeze every bit of oxygen from its lungs in this way to stay down. Humans that went down as far as dolphins do and then came up would develop the decompression sickness known as the bends, since they are breathing highly compressed air as they dive. But since dolphins are simply holding their breath they do not suffer the same consequences.
Dolphins do not drown when they sleep because they are able to float just beneath the surface of the water due to their bone structure and differences in their lungs from other mammals. This makes them more buoyant, and small movements of their tail flukes propel them to the surface so they can take a breath every now and then as they sleep.
John Lindell has written articles for "The Greyhound Review" and various other online publications. A Connecticut native, his work specializes in sports, fishing and nature.
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