Are dolphins fish or mammals?

In this post, we will understand if dolphins are fish or mammals. Also, we will learn dolphins’ biological and behavioural characteristics.

Are dolphins fish or mammals?

Dolphins are not fish, they are mammals. They are closely related to whales, together they form the group of Cetaceans. Whales and dolphins belong to the infraorder of Cetaceans, mammals that are fully adapted to the aquatic environment. In total, the infraorder has approximately 86 species, which are divided into two groups: the odontocetes (Odontoceti superfamily) and the mysticetes (Mysticeti superfamily).

General characteristics of cetaceans

Diving and its relationship with the respiratory and circulatory system

Their nostrils are located on top of the head, allowing the animal to breathe without sticking its entire head out of the water. In Odontoceti, this breathing orifice is composed of only one opening and, in Mysticeti. there are two openings.

Cetaceans experienced a process called telescopy, the nostril migration to the top of the head. As these animals hunt for their food under the water, they have developed strategies to hold their breathing for some time, once their breathing strategy is pulmonary. The nostril orifice closes, preventing the entry of water, the vessels constrict and blood is taken only to vital organs such as the brain and heart. The heart rate decreases. In addition to red blood cells, cetaceans have a large amount of myoglobin. This molecule binds more easily to oxygen and retains it for longer periods as well.

Temperature control

As warm-blooded animals, cetaceans produces body heat. To maintain the warm temperature, cetaceans have a thick layer of fat, as they do not have any body hair. In addition, they also have fins as thermal windows. The fins are warmer than the rest of the body using the blood heat in that area to both, cool and warm the body.

Nervous system

Cetaceans have a very well developed vision. They have a structure called tapetum lucidum that helps to increase vision in low-light environments. The sense of smell is not very well developed. They do not have an outer ear, but the inner ear is pretty dense. Cetaceans have a well-developed acoustic communication system. And in general, are very intelligent and can develop abilities through cognitive training. 

Excretory system

Their kidneys have several excretory units similar to many small kidneys that work independently. Some ducts join the kidney to the urethra. The many excretory structures help with fluid retention as they can dehydrate due to the environment salinity. The sea or estuarine water has more salts than the body of animals.

Reproductive system

The fertilization of cetaceans is internal and there is an interval of one to many years between copulations and pregnancies. In some species, the females stop feeding during the nursing season, in others, the calf accompanies the mother while she feeds.

Digestive system and foraging habit

Cetaceans have a complete digestive system. The digestive tract is long and the stomach has no glands attached. Some of the species have migration habits. There are different feeding and breeding periods. During feeding periods, the animals move to the poles, where there is a higher level of food production. However, there is also the species that do not migrate and do not have any seasonality for foraging or breeding.

Odontoceti and Mysticeti


Mysticetes are cetaceans with no teeth. They have plates that filter food from the water instead. They have two nasal openings at the top of their head. 

Females are generally larger than males. They form seasonal groups, especially during the mating season. There are 4 families: Eschrichtidae (Grey whale), Balaenidae (Right whale), Neobalaenidae (Dwarf Right whale), Balaenopteridae (Humpback and Blue whale).


Odontocetes are cetaceans that have teeth. Their body is smaller than the mysticetes. There are 9 extant families: Delphinidae (general dolphins and killer whales), Phocoenidae (porpoises), Kogiidae (Dwarf sperm whale), Ziphiidae (Beaked whale), Physeteridae (Sperm whale), Monodontidae (Beluga), Iniidae (Amazon boto), Platanistidae (Ganges’ river porpoise), Pontoporiidae (Franciscana). 

Odontocetes echolocate, they have a structure in the skull called melon and they hunt in groups using this acoustic ability. During echolocation events, air enters through the nostril at the top of the head and vibrates structures called bursas that propagate vibration to the melon. Then the melon transmits sound waves to the water. Sound reflex is received by acoustic channels in the lower jaw and transmitted to the inner ear.

Delphinids, or dolphins

Echolocation system

Dolphins have a complex echolocation system used for navigation, food capture, and socialization. Echolocation allows detecting the position and distance of objects or animals by emitting ultrasonic waves and the time taken for these emitted waves to reflect on the target and return to the animal in the form of an echo. They emit the sound through the nasal structures, propagate using the melon, and receive the echoes through their jaw, passing to the inner ear. The inner ear sends the message to the brain where the recognition of the signal occurs, all this in milliseconds. In addition, the melon directs the sound waves, alters their length and frequency, that is, it controls the intensity of the sound emitted by the animal.

With this complete echolocation system, dolphins have developed a “linguistic wealth”. Some studies argue that dolphins’ communication goes further and that they have specific sounds to warn of danger and food presentation, but with the usage of vocabulary and dialects.


Dolphins have family habits, usually spend long periods being raised by their mother. This raising period can last for up to 8 years. They have habits of being helpers and collaborative, especially when someone in the group gets hurt or sick. They always help those who need help. 

These animals normally hunt as a team, feeding on octopus, squid and small fish. When they identify their prey, they swim around them sometimes forming several bubbles. These bubbles serve to distract them and block their escape route, and thus, the prey becomes easier to capture. They have habits of communicating by echolocation, through the emission of different sounds.

They usually feed at night, when visibility in the water is poor. Dolphins use the sound of echoes to find their prey. On the other hand, they are easily preyed upon by sharks, sperm whales and humans, through predatory hunting.

What does cover the dolphins’ bodies?

Dolphins are hairless, their skin is smooth and soft, with elastic characteristics. This elasticity allows the animal to suffer a deformity when moving through the water mass. When the animal is swimming at a low speed, the skin is smooth, unaltered. When it travels at high speed, the skin deforms to absorb turbulence, thus, increasing its speed.

The dolphin’s skin constantly renews itself and is very sensitive to touch. This sensitivity to touch makes the animal hurt quite easily, however, they have high healing power.

Under their skin, the dolphins’ bodies are covered with a thick layer of fat that acts as a thermal insulator, keeping them warm even under the water and in cold conditions. This layer of fat also helps these animals during swimming behaviour.

Dolphin occurrence area

Most dolphins live in temperate and tropical ocean waters. However, few species can be found in freshwater rivers. Some still live in seas like the Mediterranean, the Red Sea, and the Black Sea. Also, some species of odontocetes occupies very cold waters, such as the narwhals, belugas, and orcas.

Dolphins curiosities

The sound emitted by dolphins is produced in the respiratory channels. When air passes under strong pressure, they emit sounds at different frequencies. Like pops, clicks and whistles. These sounds are used in both, communication and echolocation. These sounds are used similarly to sonars.

Researchers believe that dolphins engage in cultural activities. Behaviours are commonly transmitted and learned by different members of the same group. The use of objects such as algae and sponges to play between group members, throwing from one to another, and surfing are some of these activities that do not have a biological function related to avoiding predation, feeding or reproducing, but rather to play, socialize and exercise.


In this post, we learned that dolphins are not fish, but mammals. We also understand more about biological and behavioural characteristics.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Are dolphins fish or mammals?

Why is a dolphin not a fish?

Although many people think they are fish, dolphins are mammals and one of the smartest ones, as they have a very evolved brain. They live in water but breathe through their lungs. In addition, they have mammary glands, exclusive characteristics of mammals.

Where do dolphins live?

Dolphins are found in all marine environments around the world. There are species of dolphins that can be found in rivers, estuarine areas, along the coast, in open waters. In some cases, species have a very wide distribution, being classified as cosmopolitan. Some species live in great depths and under very cold conditions.

What do dolphins eat?

Dolphins in general feed on fish (teleosts), squids (cephalopods), and shrimp (crustaceans). 

What are the types of odontocetes?

There are 9 extant families: Delphinidae (dolphins and killer whales), Phocoenidae (porpoises), Kogiidae (Dwarf sperm whale), Ziphiidae (Beaked whale), Physeteridae (Sperm whale), Monodontidae (Beluga), Iniidae (Amazon dolphin), Platanistidae (Ganges’ dolphin), Pontoporiidae (Franciscana dolphin). The different species are classified according to their genetic and morphological characteristics.

What are the characteristics of a dolphin?

Dolphins are not aggressive, on the contrary, they are very affectionate and sociable. Dolphins can grow from 1.5 meters to 10 meters in length. About the weight, they can reach up to 7,000 kilos, depending on the species. They are hairless and have true teeth. 

What species is the killer whale?

The killer whale is the Orca (Orcinus orca). The orca is the largest member of the delphinids and is a versatile super predator. They feed on fish, molluscs, birds, turtles, seals, sharks, and larger animals when hunting in groups.

Why do they say dolphins are the smartest animal in the world?

Their brain is heavier and more complex than ours and several studies have shown that these cetaceans are very smart regarding cognitive abilities and learning.

Why are dolphins so smart?

Some dolphins have a very complex social structure between the group members, In addition, they have abilities of creativity, tool usage, collaborative groups, and other behaviours that classified them as very intelligent beings.


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Tyack, P. L., & Clark, C. W. (2000). Communication and acoustic behavior of dolphins and whales. In Hearing by whales and dolphins (pp. 156-224). Springer, New York, NY.

Tyack, P. (1986). Population biology, social behavior and communication in whales and dolphins. Trends in ecology & evolution, 1(6), 144-150.