Dolphins are some of the most intelligent animals in our oceans. They’re capable of problem-solving, self-awareness, and play.
They are some of the only animals that can send a shiver up the spine of some of the biggest ocean predators, the Great White shark.
Typically dolphins live in pods of up to 2 – 30 individuals, giving them the advantage of numbers when it comes to hunting prey.
Today, we’re going to take a closer look at dolphins and answer a question that often comes up when discussing these animals. Are dolphins fish?
No, dolphins are categorized as mammals. Unlike fish that breathe through gills, dolphins have lungs and must surface to breathe air. They are also warm-blooded, give birth to live young, and secrete milk for their young.
Are Dolphins A Type Of Fish?
Despite dolphins living in the ocean alongside many species of fish, they are not fish and are instead a class of mammal.
Dolphins differ from fish in a number of ways, but mainly because they evolved from land mammals that had legs underneath their bodies.
For this reason, dolphins’ tails move up and down when they swim instead of side-to-side. They are also warm-blooded mammals and are unable to breathe underwater like fish.
They are able to hold their breath for between 8 – 10 minutes, but once their bodies have used up the oxygen in their lungs they must return to the surface.
What Are Dolphins Classified As?
Dolphins are categorized as mammals as they have all of the characteristics of such. Although they appear ‘fish-like” they are categorized as cetaceans (marine mammals).
Cetaceans include dolphins, whales, and porpoises – the ancient Greeks recognized that cetaceans breathe air, give birth to live young, produce milk and also have hair.
These are all distinct features of mammals, and it’s what makes cetaceans different from fish.
Below is some brief detail on why dolphins are mammals and the reasons for this:
Warm-blooded animals are able to maintain body temperatures higher than their surrounding environment through metabolic processes that are different from cold-blooded animals.
The ability to maintain body temperature regardless of the environment is a characteristic of homeothermic animals.
This allows dolphins to be able to swim in incredibly cold waters without actually feeling the cold and suffering from the cold temperatures.
Whilst dolphins typically prefer warmer, coastal waters, there are some species that are found in the coldest regions on earth in Antarctica.
Lungs Not Gills
Unlike fish, dolphins do not have gills, but instead, they have lungs. If you’ve ever been dolphin watching or witnessed them whilst out on a boat, you’ll notice that they regularly return to the surface for air.
When dolphins surface for air, they exhale to blow away water covering their blowhole which is why you’ll often see water spray away from the top of their heads.
This then allows dolphins to inhale oxygen through their blowhole and into their lungs, providing them with enough oxygen to stay submerged in order to hunt their prey.
Dolphins only ever breathe through their blowholes, just like whales. Their mouth is used strictly for catching prey and not to breathe.
They also have collapsable lungs to avoid decompression sickness when they dive to the depths.
These collapsable lungs allow dolphins to breathe much more quickly than humans, which is why they only need to surface for around 2 seconds to fill their lungs with air.
Give Birth To Live Young
Like all mammals, dolphins give birth to live young. Whereas fish, insects, and reptiles lay eggs that then hatch at some point in the future.
The sexual maturity of dolphins varies depending on the species, but its also determined by the size of the dolphin and not the age.
Reproductive maturity in dolphins will typically occur at 85 – 95% of their mean adult body length, in most cases, this is between the ages of 5 – 12 years old.
A dolphin pregnancy lasts around 12-months and female dolphins typically bear calves every 2 to 4 years but the average interval between dolphin birth is three years.
This one may come as a surprise to many, but dolphins actually have hair on their snout when they are born which falls out soon after they are born.
That said, hair follicles are still visible throughout their adult lives. Typically the hair is used to help maintain body temperature, but dolphins do not need this as they have a thick layer of blubber that acts as insulation.
Evolutionary biologists believe that the presence of dolphin body hair at birth indicates that the ancestors of dolphins (land mammals) had hair to help them maintain body temperature in colder environments.
Marine mammals that live on both land and water use body hair as a primary means of insulation, but being that dolphins live strictly in the water they do not need body hair.
Secrete Milk For Calves
One of the main defining factors for an animal to be classed as a mammal is the possession of a mammary gland that allows females to feed their young calves.
Dolphins do have mammary glands, and dolphin calves typically nurse for two to four years and some will even remain with their mothers for four or five years.
The milk produced by dolphin mothers is incredibly rich in fat, helping their calves grow into adulthood as soon as possible so that they can fend for themselves.
Calves are able to drink milk from their mother’s teet whilst underwater because they are able to manipulate their tongues in a “straw-like” formation to form a seal over the mother’s nipple.
So, are dolphins fish? No, they are cetaceans which are marine mammals. Dolphins do not have gills and are therefore unable to breathe underwater as fish can.
They have all of the characteristics of mammals, including being warm-blooded, having lungs, giving birth to live young, having hair when they are born, and also females having a mammary gland.
These incredibly intelligent marine mammals differ from fish in a number of ways, and despite living in the same oceans they are not the same.
Hopefully, this post has been helpful and you’ve learned why dolphins are not fish but instead are marine mammals.
Thanks for taking the time to read this post and feel free to share it with others who may find it of value.
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Hi, I’m George – the founder of MarinePatch. I created this blog as marine wildlife has been my passion for many years and I’ve spent decades learning and dedicating myself to documenting all I can about the topic.