The simple answer is no, dolphins don’t have gills.
In fact gills are actually a common characteristic of fish, sharks (a variation of fish) and a variety of amphibious creatures that enables them to pull oxygen directly out of the water so that they can breathe in an aquatic environment without needing to rise to the surface of the water.
Depending on the species some amphibians can breathe both underwater and on the surface, however most fish and sharks can only breathe underwater and unlike mammals fish would actually suffocate above the surface of the water.
In contrast to fish and sharks dolphins are marine mammals and must come to the surface in order to breathe.
Dolphins are equipped with lungs (like humans and land animals) not gills and would drown if they tried to breathe underwater just as a human would drown if they tried to inhale while submerged underwater.
In order to breathe dolphins swim to the surface and inhale oxygen through their blowhole, which can be found on the top of their head.
One interesting characteristic that separates dolphins from land based mammals is that unlike humans and land animals dolphins are incapable of breathing through their mouth because their trachea and air passage are not connected.
This serves several purposes for dolphins.
First of all having a separate trachea and esophagus allows dolphins to surface partially above the water while only exposing their blowhole, which makes it easier to breathe and allows them to rest without having to constantly monitor whether or not their head is above the surface.
If a human or land animal were to obtain oxygen they would have to lift their head completely out of the water which would take additional energy and require the animal to orient their entire body into a position where they can successfully breathe above the water.
A dolphin on the other hand can simply log around at the surface using little or no energy to acquire its oxygen.
Secondly, by having a separate breathing system dolphins are able to hunt for and consume fish and various other prey while under the water without the fear of drowning.
They don’t have to be concerned with opening up their lungs and accidentally filling them with water every time they hunt for fish and other prey.
Third, having a blowhole on the top of their head allows them to maintain good visual awareness of objects, prey and predators around them as they do not have to be distracted with lifting their head out of the water.
When it comes to breathing some dolphin species such as the killer whale may be seen spouting water from its blowhole when it rises to the surface of the water.
When this occurs the dolphin is simply blowing the water that has collected around its blowhole away.
It isn’t water that has been ingested through the mouth, lungs or blowhole as most people assume, its just water surrounding the outside of the blowhole that has been powerfully forced away when the dolphin exhales.
Before the dolphin dives back underwater it takes in a fast deep breath and closes the sphincter muscles around its blowhole to prevent water from getting in so that it can hunt and travel underwater without drowning.
The amount of time a dolphin can hold its breath for varies from one species to the next with estimates of 3 – 15 minutes being considered average for certain species.
In order for dolphins to stay submerged for extended periods of time certain unnecessary biological functions shut down to conserve energy and as much as 90% of the air a dolphin inhales is considered usable oxygen which is several times higher than that of a human and some land mammals.
Living in the ocean for millions of years has allowed dolphins to become extremely efficient at minimizing energy usage and maximizing the amount of quality air they inhale.
Will dolphins ever develop gills?
It is extremely unlikely that dolphins will ever evolve gills; at least not for several million years.
In fact it took the ancestors of dolphins millions of years to evolve from land dwelling animals into the marine mammals we now know today.
All species of dolphin belong to the toothed whale suborder as they possess teeth and are equipped with echolocation.
In regards to why it is unlikely that dolphins will ever develop gills is because evolution is an extremely slow process that occurs over millions of years and while dolphins have evolved from land animals to marine mammals they still share many of the same characteristics as other mammals such as the need to breathe air, being warm-blooded animals, giving birth to children, having hair (some species have hair at birth which falls off shortly after they are born) and producing milk.
In order for a dolphin to develop gills its entire respiratory and lung systems would have to evolve.
It would also have to make evolutionary sense for dolphins to develop gills.
Fish can be found thousands of miles below the surface of the water and some species never come to the surface of the water throughout their entire lives, and while dolphin are able to dive to fairly deep they always rely on their ability to come back to the surface and thus are unlikely to dive deep enough to the point where they would require gills for survival.
Aside from living at deeper depths fish are also cold-blooded which means they’re able to survive in extremely deep oceans that possess very low temperatures; even in areas that do not possess any light.
Because fish are small in size and do not possess body fat their cold-blooded physiology allows them to survive at these depths despite not being able to develop their own internal heat and they can survive off of fewer calories than a warm-blooded animal that needs to keep itself warm.
If a dolphin were to develop gills it would also have to develop a biological system that would allow them to maintain their oxygen supply and body heat at extreme low depths for an indefinite amount of time.
In other words dolphins would likely have to become cold-blooded animals to survive at these depths or become really efficient at burning the calories they consume for body heat and to support other biological systems.
Unless evolution requires dolphins to survive at these extreme depths for huge amounts of time in order to survive the likelihood of the dolphins ever developing gills will remain extremely low.
Dolphins are marine mammals not fish and like humans these marine mammals would drown if water entered into their lungs.
Unlike fish they cannot breathe underwater so they must come to the surface to get air.
In addition to helping them get air their blowhole also helps them survive in the ocean by minimizing the energy it takes to breathe near the surface, allowing them to maintain awareness of their surroundings and minimizing the chances of taking water into their lungs when consuming food underwater.
Dolphins have also perfected their ability to inhale quality oxygen and use it effectively in order to stay submerged underwater for extended periods of time, search for food, navigate the ocean or escape an attack from a predator.