Why Are Dolphins Mammals?


Dolphins have the scientific class of Mammalia which specifies that they are mammals. Dolphins are the marine mammal family with the largest array of species. 

Dolphins are warm-blooded, females give birth to live young and provide their young with milk produced by their bodies, which are all characteristics of mammals. Dolphins even have hair when they are born, another characteristic found in mammals.

Dolphins are part of the Cetacea order in scientific classification, along with whales and porpoises.

Cetaceans are mammals which are entirely aquatic, to learn more about cetaceans check out my other article Why are Whales Mammals, which explains more about cetaceans. 

What Is A Mammal? 

When we traditionally think of mammals we think of domestic pets like cats and dogs, humans and even elephants. 

Rarely do we think first about dolphins, perhaps because they live in the sea and many people think they are fish.

A mammal by definition is a “warm-blooded vertebrate animal of a class that is distinguished by the possession of hair or fur, females that secrete milk for the nourishment of the young, and (typically) the birth of live young”.

Dolphins and other marine mammals are classed as such because they possess all of these traits.  

Below I have explained more about why dolphins are mammals, from dolphins being warm-blooded to breathing air. 

Striped-dolphin
Striped-dolphin

Are Dolphins Warm-Blooded?

Dolphins are warm-blooded; their internal temperature is roughly 98˚F (36.6˚C), they need to conserve heat while submerged in cool waters.

Cetaceans can have between blubber (a layer of fat), that ranges between 2 inches and 1 foot (30cm) thick. 

As dolphins are smaller than many other species of cetacean such as whales, the layer of blubber they possess is thinner. 

Due to this thinner layer of blubber, dolphins are more commonly found in warmer climates, as well as some cooler regions of the world. 

This is compared to large whales that can be found in freezing (or near freezing) climates near the northern and southern poles. 

A dolphin’s blubber contains a higher concentration of lipids with a low concentration of water than other cetaceans. This keeps them warmer than other cetaceans that have a lower concentration of lipids combined with a higher concentration of water. 

Risso’s Dolphin

The concentration of lipid versus water and blubber thickness differs between dolphin species.

Those dolphins that have a lower lipid-high water concentration in their blubber and those who have thinner blubber may be limited to the world’s warmer regions (such as on the equator).

Dolphins are also able to maintain their body temperature by increasing their metabolic rate. This means they burn calories faster and transfers heat to their vital organs to help them when they are cold.

Dolphins, like many mammals, can also shiver to help alleviate being cold. 

Some species of dolphins also are born with hair but lose it shortly after. This is to help keep their body temperature constant in cooler waters. 

Dolphin Reproduction

Reproduction for dolphins varies from seasonal to year-round. In various species, seasonal reproductivity is related to geographical distribution. 

Dolphins have a similar reproductive process to that of other mammals including humans. A female will begin to ovulate when sexual maturity is reached. After this time, she can become pregnant. 

Dolphins give birth in the same way that other mammals do; to live young. 

Spinner-dolphin

Dolphins reach sexual maturity at different ages depending on gender and geographical location. A male dolphin can reach sexual maturity anywhere from 10-15 years of age whereas a female dolphin can reach sexual maturity anywhere from 3 to 13 years of age. 

Sexual maturity in dolphins is more of a function of size rather than age. 

Dolphins males have various mating behaviours, for example, some males form ‘alliances’ to consort with females. This is often only a pair of males; however, trios have also been formed. 

Gestation for Dolphins varies between species however, it is roughly a year. Mid-to-late pregnancies can be noticed due to the increased girth of the female Dolphin. 

Female Dolphins have mammary glands, which is what produces milk to feed their calves. Female Dolphins can have distended (swollen) mammaries, which is a sign of lactation when they are pregnant or after the birth of their calves. Mammary glands are a key feature of mammals. 

A Dolphin’s mammary gland can be seen near its genital region. Female dolphins have been observed lactating and nursing from the birth of one calf to the birth of the next, which can be up to 5 years.

Dolphin milk is very thick in consistency due to its high fat concentration. To learn more about how dolphins breastfeed check out my other article, How Do Dolphins Breastfeed Underwater?.

Calving intervals which include, gestation, lactation and sometimes a resting period between having calves, varies between species. Although the most common length of time for a calving interval is between 3 and 5 years. Shorter calving intervals can be because of the loss of a calf. 

Reproductive senescence (the age-related decline of reproductive output and success) is present in many mammals and dolphins are no exception. This is demonstrated in a study on bottlenose dolphins which shows that calf survival decreases with the age of the mother. Lactation and the interbirth interval in female Dolphins increased with age, demonstrating reproductive senescence. 

Do Dolphins Breathe Air?

Dolphins breathe air like any other mammal, above the surface of the water. They lack the gills that fish have which are necessary to extract oxygen directly from the water. 

Dolphins can hold their breath for roughly between 8 and 10 minutes before they surface to breathe. 

Dolphin blowhole

Although, this varies between species. Some can hold their breath for up to 15 minutes.

Dolphins breathe through a blowhole, which is covered by a muscular flap when they go underwater, preventing water from reaching their lungs. 

Dolphins remain conscious even when sleeping because their breathing is not automatic the way human’s is, so if they slept in an unconscious state, they would be unable to breathe. 

Dolphins allow one half of their brain to sleep at a time, this means that the other half is conscious and can allow the dolphin to breathe and look out for dangers at the time. This is also why dolphins only sleep with one eye closed. 

Dolphins sleep either motionless at the surface of the water or swimming very slowly near the surface of the water. 

Do Dolphins have similarities to Land Mammals? 

Despite being marine mammals and being unable to walk, Dolphins have similarities to land mammals.

Cetaceans, (dolphins, porpoises and whales) closest relative is a hippopotamus, a semi-aquatic mammal.  

Dolphins have similarities to land mammals in the basic sense that they all share the characteristics of mammals, breathing air, being warm-blooded and so on. 

However, it is more than this; despite having evolved to be aquatic dolphins have similar skeletons to that of mammals.

Dolphins have three middle ear bones, the same as all mammals, but even more impressively inside their pectoral fins, dolphins have a similar bone structure to that of a human arm and hand!

Dolphins have a humerus complete with a ball and socket joint as well as a radius and ulna and phalanges inside their pectoral fins. 

These mimic a human arm and their five phalanges are very similar to a humans five fingers.

Bryan Harding

Bryan has spent his whole life around animals. While loving all animals, Bryan is especially fond of mammals and has studied and worked with them around the world. Not only does Bryan share his knowledge and experience with our readers, but he also serves as owner, editor, and publisher of North American Mammals.

Recent Content