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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 most species. 

Dolphins have all the typical characteristics of mammals. They are warm-blooded animals, and females give birth to live young and provide them with milk. Dolphins also have hair when they are born, another feature of mammals.

Dolphins, along with whales and porpoises, are part of the Cetacea order. Cetaceans are entirely aquatic mammals. To learn more about cetaceans, check out my other article, Why are Whales Mammals, which explains more about cetaceans. 

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What Is A Mammal? 

When we traditionally think of mammals, we think of domestic pets like cats, 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 is a warm-blooded vertebrate animal of a class that is distinguished by the possession of hair or fur. Females secrete milk to nourish 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. 

Dolphins use echolocation. Find out more in this article I wrote


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 cetaceans species, such as whales, the layer of fat they possess is thinner. Due to this thinner layer of blubber, dolphins are more commonly found in warmer climates and some cooler regions of the world. 

This is compared to large whales 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 water concentration than other cetaceans. This keeps them warmer than other cetaceans with a lower concentration of lipids combined with a higher concentration of water. 

Did you know there are 16 species of dolphins in North America? Find out what they are here.

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

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

Dolphins can also 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 colder 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 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. 


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. 

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

Gestation for dolphins varies between species but is usually a year. Mid-to-late pregnancies can be spotted due to the increased girth of the female dolphin. 

Female dolphins have mammary glands, which produce milk to feed their calves. Female dolphins can have distended (swollen) mammaries, a sign of lactation when pregnant or after their calves’ birth. Mammary glands are a crucial 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.

Dolphins swim differently to fish and whales.  Find out more here

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 vary between species, including gestation, lactation, and sometimes a resting period between having calves. However, the most common length 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 mother’s age. 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 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 humans are, 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 enable 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 water’s surface or swim very slowly near the water’s surface. 

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Do Dolphins have similarities to Land Mammals? 

Despite being marine mammals and unable to walk, dolphins are similar to land mammals.

The closest relative to dolphins, porpoises, and whales is the 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 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 and a radius and ulna and phalanges inside their pectoral fins. 

These mimic a human arm, and their five phalanges are similar to a human’s five fingers.

Ever wondered if all mammals are warm-blooded? Find out here.