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Why Are Dolphins Mammals?

Dolphins are in the class Mammalia, which specifies that they are mammals. However, many people still think they are fish.

Dolphins are classified as mammals as they have the five typical characteristics of mammals. They are warm-blooded animals and are born with small amounts of hair. Females give birth to live young and provide them with milk. Dolphins breathe oxygen from the air and cannot breathe underwater.

Dolphins, along with whales and porpoises, are part of the Cetacea order. Cetaceans are entirely aquatic mammals.  

Why Are Dolphins Mammals?

  • Dolphins are warm-blooded
  • Dolphins give birth to live young
  • Dolphins breathe oxygen from the air
  • Dolphins breastfeed their young with milk
  • Dolphins have a small amount of fur when born

Why Are Dolphins Not Fish?

Dolphins are not fish, as they don’t share the same characteristics. Dolphins use their lungs and breathe in oxygen from the air, while fish use girls for respiration and take oxygen from the water. Dolphins also differ from fish as they are warm-blooded (endothermic), while fish are cold-blooded (ectothermic) and cannot regulate their body temperature. Dolphins do not spawn eggs, unlike fish. Dolphins produce live young.

Why Are Dolphins Not Sharks?

All cetaceans, including dolphins and whales, are mammals, while sharks are classed as fish. Sharks use gills to extract oxygen from the water. In contrast, whales use their lungs to take in oxygen from the air.

Sharks are ectothermic (cold-blooded), while whales can control their body temperature (warm-blooded.) The main similarities between whales and sharks are that they are both large marine animals and some sharks give birth to live young.

What Is A Mammal? 

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.

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.

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

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

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 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. 

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

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 with 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 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. 

Do Dolphins Produce Live Young?

Dolphins give birth to live young in the same way that other mammals do. They feed the young milk to help them grow.

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 swim differently to fish and whales.  Find out more here

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. 

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. 

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 Produce Milk?

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.

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?

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

Do Dolphins Breathe Oxygen?

Dolphins breathe oxygen 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. 

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 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 any 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. 

Why are dolphins mammals and not fish?  Find out here

Do Dolphins Have Fur?

Although it may not be noticeable at first glance, dolphins are born with a small amount of fur.

Although they may not be visible hairy, dolphins have a small amount of hair on top of the snout. Hair can be seen on most newborn dolphins, which is shed within the first few weeks.

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 resemble a human’s five fingers.