Dolphins: characteristics, evolution, habitat, reproduction - science - 2023



The dolphins (Delphinidae) or oceanic dolphinsThey are placental mammals of the order Cetacea that inhabit oceans and seas, unlike river dolphins (Platanistoidea) that are found in rivers.

Regardless of habitat, dolphins must rise to the surface of the water in order to breathe. These breathing cycles, where they emerge and then submerge, are carried out at intervals of different times, according to the characteristics of the species.

The anatomy of this animal has undergone several adaptations that allow it to live in water. Although all the species of this family share some morphological and anatomical aspects, they differ between them by their color, shape and size.

In their natural habitat, dolphins could live between 10 and 50 years. Its size is varied, being the orca (Orcinus orca) the largest and heaviest specimen of this genus.

Porpoises can often be mistaken for dolphins. This is because their appearance is quite similar. However, porpoises are smaller and have a more rounded snout than dolphins.

Danger of extinction

Dolphins that do wildlife face natural hazards that put their lives at risk. However, the main threat is that which comes from humans.

Many are the species that are in danger of extinction. One of these is the common dolphin (Delphinus delphis), which is at risk of disappearing from the Mediterranean Sea. This is due, among other things, to the loss of the optimal environmental conditions of the place where it lives, as a result of contamination.

Some activities of man affect the population of the family Delphinidae in various ways. Among these factors, the following could be highlighted:


The process of capturing dolphins to transfer them to scientific institutes, in order to be part of research, is contributing to the death of these animals.

When removed from their natural environment, dolphins are exposed to many threats. These could be the procedures in the capture, the methods of the transport, and the exhibition to the own diseases of the captivity

Chemical contamination

This type of contamination, caused by spills into the water of oil, chemical compounds and heavy metals, notably affects the dolphin's habitat. The effects it produces on the animal are diseases and the high mortality rate in young dolphins.

The polluted waters also affect other fish, which are the basis of the dolphin diet. In this way, the risk of extinction increases for this group of animals.

Noise pollution

This type of contamination represents a danger for dolphins. Noises from oil extraction activities and from ship engines create underwater noise currents, which could frighten or disorient dolphins.

This could force them to move away from their natural feeding and breeding habitats, causing alterations in their life cycles.

Bodily trauma

Another cause of death is the injuries these animals suffer when they become entangled in fishing nets. Dolphins collide with boats are also very frequent.

Adaptations to the aquatic habitat

Body morphology

Its torpedo-shaped body and the lack of hairs facilitate its movement in the water, reducing its resistance. The front fins assist in steering and the dorsal fins use it for balance when swimming. Their tail is oriented horizontally, which helps them propel and move their heavy bodies more quickly.

Instead of nostrils, as in other mammals, dolphins breathe through a hole in the top of their head.


Although many species may have poor eyesight, dolphins can be efficient hunters. This is thanks to echolocation.

This sophisticated system is based on the emission of high-frequency waves by dolphins. When these collide with solid objects, the waves are returned and captured by the animal. These waves are transformed into nerve impulses that reach the brain.

The interpretation of these impulses tells the dolphin where the prey, any other object or predator is located. The information is so detailed, you could know the dimensions and how far away the object or other animal is.

Fast swimmers

Dolphins swim with great speed and agility. This favors their ability to hunt and to avoid their predators. The bottlenose dolphin species can reach speeds greater than 18 mph. In general, members of this family could jump up to 6 meters out of the water.

General characteristics

-Body size and shape

Dolphins vary markedly in weight and size. The Maui dolphin is a species that measures on average about 1.7 meters long, weighing around 50 kg. The killer whale is the heaviest representative of the Delphinidae family, it can weigh 10 tons and be almost 10 meters long.

The body is aerodynamic, designed to reach high speeds while swimming, even for long distances. In adult males there is a post-anal hump, located in the lower part of the body.

In addition, the body is fusiform and hydrodynamic, which allows them to live in various aquatic habitats.


The bone structure is lighter than that of those mammals that live on land. These are because the dolphin must support a lower weight, since it lives in the water. His neck is short, his 7 cervical vertebrae are fused.


The skin of specimens of the Delphinidae family is very sensitive, it can easily be injured if it rubs against rough surfaces. However, the dolphin has a very rapid healing process, even in the case of very deep wounds.

These animals can be born with a few hairs, which they lose at a very early stage. In this way, in its young state, the skin is free of any type of hair.

The skin is soft to the touch, giving the feeling of looking like rubber. The outer layer, known as the epidermis, is up to 20 times thicker than that of other mammals. It is lined by cornified cells and no sweat glands are present.

Under the skin, dolphins have a thick layer of fatty tissue. This fat helps in body temperature control, insulating your body from the low ocean temperatures. It also helps the animal float in the water.


Dolphin skin coloration is mostly gray-blue on the dorsal area and white or light gray on the belly. However, there are also species that can have it in black, gray, white or bluish tones.

The orcaOrcinus orca) has completely different shades from the rest of the Delphinidae family. The dorsal area is black on the sides and its skin is white on the belly. Behind the eyes, the killer whale has a characteristic white spot.

The common dolphin is easily recognized because its dorsal region is dark, with a cream colored V on the sides.

These colors are useful to the animal because, seen from above, its skin blends in with the darkness of the ocean. Whereas if it is seen from below, the white of its belly blends in with the luminosity of the water's surface.


The dolphin has two curved fins on each side of its body, called pectoral fins, which it uses to direct its body while swimming. The dorsal fin is on your back and provides you with balance.

The caudal fin or tail is made up of two fins. These work as propellants when swimming, since it moves from top to bottom, contrary to the fish that do it from side to side.



This group of cetaceans have large brains. Research shows that its structure is complex, much more than other mammals.


These are located on both sides of the head, which gives the dolphin a fairly wide field of vision. Each eye can move independently, but they hardly see directly up or down.


These animals do not have external ears. However, they have very small openings located behind the eyes that lead to an ear canal.


This is a hole that is located at the top of the head. Its function is to participate in the breathing process and in the emission of sounds. To prevent water from entering the dolphin's body, when it is submerged, the blowhole has a muscular membrane.

Through the blowhole, this cetacean inhales and exhales oxygen. They also expel carbon dioxide and mucus. This organ connects to the dolphin's lungs through the trachea.


This organ is spherical in shape, due to the adipose tissue that forms it. It is located in the front part of the skull, giving it the characteristic shape that this species presents.


The dolphin's snout is long and conical in shape. In it are the teeth, which it uses to grab its prey. In addition, some species use this structure to explore the bottom of the sea or the river.


The mouth has several teeth, the number of which varies according to the species. However, they generally range from 80 to 100 teeth. The jaws are elongated in shape, playing a very important role in the sensory system of the animal.


Dolphins are mammals that live in water, and they use their lungs to breathe.Members of the Delphinidae family are aware of respiration, deciding when they need to go up to seek oxygen.

Evolutionary history

Scientists are of the opinion that the ancestors of dolphins were not animals that lived in water. According to studies, they lived on land and migrated to the sea.

Dolphins were long thought to be descendants of the Mesonychians, an extinct order of land mammals, ungulates, and carnivores. However, recent genetic studies show that cetaceans, including dolphins, are related to artiodactyls.

The study of the fossils found of the Indohyus indicates the close relationship of this species with cetaceans. Indohyus is a member of the family Raoellidae, which belonged to the primitive artiodactyls. It lived in the lower and middle Eocene, between 55 and 45 million years ago.

One of the characteristics that supports this position is the shape of some of the bones that make up the ear. The walls of the middle ear are made up of a bone called the ectotympanum. In artiodactyls the thickness of this wall is invariable, while in cetaceans the internal part is thicker than the external one.

Ectotympane in Indohyus has a very thickened inner lip. This is an important foundation that supports its close relationship with cetaceans.



The Pakicetus, belonging to the Artiodactyls, is considered the forerunner of the cetaceans. This species lived around 50 million years ago.

It was distributed in what is now known as the Middle East. This region, at that time, was a marshy area, bordering a shallow sea.

The Pakicetus were perfecting their fishing skills, which was possibly inherited by later generations. In addition to this, the ability to swim was inherited, as well as the adaptation that their eyes and ears suffered to function under water.


Ambulocytids were semi-aquatic mammalian animals that formed a family, around 48 million years ago. They were better swimmers than the Pakicetus, due to their webbed feet and short legs.

In addition, its dorsal vertebrae were adapted to be able to make an upward and downward undulating movement, synchronized with its hind legs. Their swim is comparable to today's otters.

The lower jaw, linked to the reception of the waves in echolocation, and the ear, underwent significant changes.


The earliest known fossils of Pakicetus and ambulocetus are from India and Pakistan. With the emergence of protoketids, cetaceans spread across Asia, Africa, Europe, and North America. This species lived in the middle Eocene, between 49 and 40 million years ago.

These primitive cetaceans were adapted to life in the water. They probably only came to land to breed and raise offspring.

Another change that occurred was the loss of coat and the deposit of fat under the skin. The senses were developed to hear and see underwater. The nostrils evolved, appearing in them some plug-like structures that prevented the passage of water into his lungs.


At the end of the Middle Eocene, approximately 41 million years ago, a new cetacean species emerged, much more similar to current cetaceans: the Basilosauridae. This was characterized by having a nasal opening displaced towards the eyes, thus forming a nostril.

The front limbs have fins and the hind limbs are too small to support its weight on the ground.


Animal Kingdom.

Subkingdom Bilateria.

Chordate Phylum.

Vertebrate Subfilum.

Tetrapoda superclass.

Mammal class.

Subclass Theria.

Infraclass Eutheria.

Order Cetacea.

Suborder Odontoceti.

Family Delphinidae

Genera: Cephalorhynchus, Globicephala, Grampus, Sotalia, Lagenodelphis, Pseudorca, Lagenorhynchus, Lissodelphis, Orcaella, Orcinus, Peponocephal, Sousa, Stenella, Steno, Tursiops and Delphinus.


The Delphinidae family is divided into the following genera:


The oceanic common dolphin (Delphinus delphis) is slim, with a short snout. In the dorsal area it has dark gray tones and the ventral area is white. On the side, from head to tail, the color is light gray.


A representative of this genus is the bottlenose dolphin (Tursiops truncatus). These inhabit the warm seas around the world, being found in all oceans except the Arctic and Antarctic. They can swim between 5 and 11 km / h.


The striped dolphin (Stenella coeruleoalba) has the underside of its body white or pink. Dark blue bands emerge from both eyes to the tail. The back, the dorsal fin, the snout and the melon are also dark blue.


The Hong Kong pink dolphin (Sousa chinensis). This animal has a fat hump under its dorsal fin. It is about 2 meters long. When it is born, its skin is black, but as it matures this color changes, reaching a pink hue.


One of the representatives is the melon-headed dolphin (Peponocephala electra). Its body is torpedo-shaped, having a light gray color, except for the head which is dark gray.


The orcaOrcinus orca) has a robust complexion, being the largest species of the Delphinidae. Its dorsal region is black; the chest, the ventral area and the flanks are white. It also has a white patch behind each eye.The killer whale has a large triangle-shaped dorsal fin.


The southern smooth dolphin (Lissodelphis peronii) has a slender and elongated body. Its main characteristic is the lack of a dorsal fin. The dorsal area is black and the ventral is white.


One of the members of this genus is the Irawadi river dolphin (Orcaella brevirostris). Its head is rounded. The dorsal fin is triangular in shape.


The dark dolphinLagenorhynchus obscurus) largest is found in Peru, at 210 cm long, weighing 100 kg. The dorsal area is dark gray, almost black. It has long patches on both sides, in a light gray shade. Its throat and belly area are white.


To this genus belongs the black killer whale (Pseudorca crassidens), whose length oscillates around 3.7 and 5.5 meters. Its weight could be between 1 and 2 tons. Its dorsal fin could reach 30 cm high. The false killer whale, as they are also known, has a uniform coloration from dark gray to black.


Fraser's Dolphin (Lagenodelphis hosei) reaches 2.75 meters, weighing about 200 kilograms. The dorsal part can be blue-gray. From muzzle to tail they have a cream colored band. The belly is white.


The tucuxi (Sotalia fluviatilis) is bluish gray in the lateral and dorsal area. The belly is gray. The dorsal fin is hook-shaped.


This group includes the tonina overa (Cephalorhynchus commersonii), whose size is not greater than 1.45 meters. Males weigh around 42 kg, while females reach 50 kg.


The pilot whale (Globicephala melas) has dark gray, brown or black skin. It has some light areas, like a pale spot behind each eye.


The gray dolphinGrampus griseus) is representative of this genus. Their skin is gray, with numerous markings. It has a robust body, mainly at the base of its dorsal fin.


The family Delphinidae or oceanic dolphins are widely distributed worldwide. They can be found in all the oceans and seas of the world, except the ecosystems of the Arctic and Antarctic oceans, and the Caspian and Aral Seas, in central Asia.

The killer whale is the only animal belonging to the Delphinidae family that can be found living in Arctic areas. However, it prefers warm or slightly cold waters.

They can be distributed from the zone of the equator to the subpolar areas. However, the vast majority of species are concentrated in areas with temperate or tropical climates.

Also, this group of aquatic mammals are found in seas with shallow waters, such as the Mediterranean and the Black Sea. They also live in ports, estuaries, bays, gulfs and estuaries.

The habitat of the Delphinidae can be affected by environmental factors and by the availability of food. This causes that, sometimes, these animals are seen in the need to migrate from their natural habitat.

Such is the case of the killer whale, which can travel thousands of kilometers in search of a suitable place to live and reproduce.

Extensive habitats

Some specimens are found in fairly extensive habitats, while others may be regional or even characteristic of a small geographic location.

It is currently claimed that these animals can swim up to 300 meters deep in the ocean. This is due to the fact that remains of fish, such as benthic ones, have been found that live at great depths in the stomach of dolphins.

Dolphin species that thrive in open waters tend to tolerate low water temperatures better than coastal dolphins. The latter prefer warm and tropical waters.

An example of this is Hector's dolphin (Cephalorhynchus hectori), which can only live in shallow waters, near the coast. While the acrobat dolphin (Stenella longirostris) lives almost exclusively in the middle of the ocean.

Specific habitats

Some dolphins are endemic to an area, found exclusively in the saline waters of one country, while others barely separate them for kilometers from another sister species. They can even be found in the same habitat, separated by natural barriers.

Such is the case of the bottlenose dolphin, which is found in three naturally divided regions of the Mediterranean Sea and the Black Sea.

This barrier does not prevent them from moving, but rather defines the characteristics of each area. In relation to the bottlenose dolphin, the three populations are genetically different from the species that lives in the northeast of the Atlantic Ocean.

Geographical distribution

Members of the family Delphinidae live in all the oceans of the planet, except the Arctic and Antarctica. They usually inhabit the tropical Atlantic, between the tropics of Cancer and Capricorn.

This is due to the constant temperatures throughout the year, the calm tides and the abundant variety of foods.

Dolphins can migrate stationary. The reasons that lead them to this could be the significant variations in the temperature of the water and the movement towards other habitats of the fish that are part of their diet.

In addition, variations in the physical-chemical characteristics of the water, such as pH, salinity and density, cause these animals to leave their natural habitat and look for others where they can develop.

These migrations are more common in some high-latitude shoreline dolphins, which often travel south in winter. Those that live in temperate waters rarely migrate due to the changes of the seasons.

Various species inhabit the Pacific Ocean, such as the bottlenose dolphin. This extends from Japan to Australia and from North America to Chile. This species is also found in the Atlantic from the United States to Argentina and from Norway to South Africa.

Distribution of the common dolphin

The Common Dolphin (Delphinus delphis ) is the species with the widest distribution worldwide. It is found around temperate, subtropical and tropical seas.

Atlantic Ocean

In the western Atlantic Ocean it is located all along the South American coasts, from Venezuela to Uruguay. It also lives in the Caribbean and the Lesser Antilles.

It can also be found from Nova Scotia, one of Canada's maritime provinces, to the coasts of Florida, in the United States.

The eastern Atlantic includes the areas of the North Sea and in the waters of the United Kingdom, the Bay of Biscay and the Azores islands.

On the African coasts the common dolphin can be found from Morocco to the Gulf of Guinea. There are some populations in the Black and Mediterranean Seas.

Pacific Ocean

The geographic range of this species in the western Pacific Ocean is made up of the saline waters of Japan, Indonesia, and the Philippines. In addition to the coasts of New Zealand and the Coral Sea, close to Australia.

The west coast of North America, Central America and the coasts of South America to the south of the Chilean republic are examples of the distribution in the eastern Pacific.

Indian Ocean

In this ocean, the common dolphin is found in Sri Lanka and in India. In addition to the Arabians, the Gulf of Aden, and the coast of Natal and Madagascar.


Sexual maturity in dolphins will depend on the characteristics of each genus and species. On average, males can begin to reproduce at around eleven years of age, while females do so at nine years of age.

Occasionally they may begin to have some sexual contact before they are able to reproduce. These animals are very sexually active species, which means that a male can copulate repeatedly with a female, or with several of them, within the same reproductive time.

In addition, throughout their lives they could have different sexual partners, within their own group or with females from other family groups.

Males have two openings. In the longest are the genital organs, while in the shortest is the anus. During erection, the penis extends forward from the cleft where it is located.

Females have a cleft in which the external genitalia and the anal opening meet. On both sides of this there are two slits, where the mammary glands are located.


External factors, such as some threatening situations that can cause stress in the animal, could affect the mating of dolphins. This is because at that time the priority of the animal is its own survival.

However, if conditions are favorable, dolphins may mate throughout the year, with a preference for the warmer months.

Males often have to fight each other in order to mate with a female. This fight can be by colliding their bodies, thus measuring their forces. Also one of the males may emit vocalizations, warning the other to move away.

Courtship is part of a kind of mating ritual. The male begins by doing stunts, swimming and brushing the female with his snout. The female responds by emitting some whistles. When both are ready, they put their bellies together, initiating copulation.


The gestation period of the Delphinidae family can vary, depending on each species. However, it is estimated that the gestation time could be between 10 and 12 months. In the killer whale, this period can reach up to 17 months.

Once the male and the female have copulated, producing the fertilization of the female gamete, the development of the embryo begins. This occurs in the uterus, in a transitory organ called the placenta.

During gestation, the female usually emigrates to regions with a temperate climate, with warm waters. The female's appetite increases, due to the strong demand for energy she needs in this new stage of her life.


Once the newborn has been expelled from the womb, the umbilical cord breaks. The newborn's tail comes out first, and the head comes out last. Then the mother propels her young to the surface, to breathe for the first time.

The female usually gives birth to a single young for each birth. In some species, due to their small size, they could gestate up to two young.


In the first months, the young dolphin feeds on the mother's milk. Then when he is able to fend for himself, he starts eating some small fish.

Dolphins are carnivorous animals. They can adapt their eating habits to the characteristics of the environment where they are.

Their diet is based on fish, squid, crustaceans and cephalopods. Dolphins chase their prey to hunt them, turning them into active predators

They have several teeth, all the same size. However, they do not use their teeth to chew food, they use them to hold their prey. Once they catch it, they swallow it whole.If the animal is very big, they shake it or squish it until it breaks into pieces.

The dolphin's stomach has three sections. The first cavity is an adaptation that the distal part of the esophagus has undergone. In this the food that has been consumed is stored. In the second and third cavity the food is digested.

Members of the Delphinidae family eat about 6% of their body weight daily. If it is a female in a pregnant state, they could ingest up to 8% of her weight.

Hunting techniques

Dolphins usually hunt in groups, making a total of 6 or 10. This is done to benefit from this stalking technique. To carry it out, the dolphins surround a school of fish and take turns one by one to eat the animals they have enclosed.

Another technique is to take the prey to a shallow area, where it is more difficult for them to escape from the dolphin. They also tend to hit the animal they are going to consume with their tails, stunning it so they can catch it more easily.

The Delphinidae use echolocation to detect the location of prey. In addition to this, they emit sounds to stun the other animal, making it easier to hunt.

Killer whales can create large waves with their powerful tail to knock down seals or penguins found on ice floes. They also go to the beach to catch the sea lions.

These animals try to turn the sharks over before killing them, thus inducing the so-called "tonic immobility." This is a temporary paralysis that sharks experience when they feel they are upside down.



During gestation, the specimens that make up the herd, especially the male, protect the pregnant female until the moment of delivery. They even do it for a long time after this. In this way they prevent predators, attracted by the blood of childbirth, from approaching the mother or the young.

In groups of dolphins there are usually some females that fulfill the role of "midwives". These are in charge of helping the female during delivery.


Much research maintains that these animals have empathy and are in solidarity with other animals, including humans.

Dolphins establish strong bonds with others of their kind. If a dolphin is injured, the others in the group help it to surface and breathe.

Dolphin-assisted therapy is a therapeutic method that helps people who have disabilities in their mental, physical or emotional development. Thanks to these techniques, it is possible to relieve pain and increase the motor skills of these patients. However, it is a practice that is criticized because dolphins are not found in their natural habitat.

The success of this therapy is based on the unconditional love that the dolphin offers to the people who participate in it, helping them to strengthen their confidence and self-esteem.

Some think that this is simply a method of modifying behavior, rewarding the individual with the possibility of swimming with the dolphins. However, some scientists believe that interaction with dolphins increases endorphin levels.


These animals form social groups, of up to 12 members, being able to freely leave the group to which they belong and join another. They chase each other and throw seaweed, an activity that could be preparing them for hunting.

Living in groups allows them to hunt cooperatively, as well as to monitor and defend their members. They do this by communicating with each other, using shrieks, whistles, among other sounds.

Bottlenose dolphins often join groups of other species, such as the rough-toothed dolphin, Risso's dolphin, and the spotted dolphin. From this relationship, the animal obtains greater protection and greater efficiency in hunting fish.

In the family groups of bottlenose dolphins there are hierarchies. Males maintain their leadership by showing aggressiveness, maintaining dominance by hitting their tails against the water, chasing and impacting the body of other males, and emitting clouds of bubbles through the blowhole.

Despite being a docile animal, the bottlenose dolphin can be very hostile, even biting members of its own species with its teeth. This behavior is most fierce when he fights against sharks, to defend his life.


In the ocean all dolphins are vulnerable. Bottlenose dolphins are rarely prey to other animals. This is due to their size, the speed of their swim, their echolocation and their intelligence. In addition to this, their social organization allows them to stay in a group and intimidate the aggressor.

However, there are predatory animals of the Delphinidae. Two of these are in their natural habitat; killer whales and sharks. The other fierce predator is man.

Killer whales

Killer whales feed on a variety of fish, crustaceans, and mollusks. If they see a dolphin, despite belonging to the same family, they will not hesitate to catch it to eat it.

These animals are expert hunters, being more efficient when they are organized in groups. The killer whale, a genus of the family Delphinidae, may attack young, sick dolphins or their young, which they separate from their mother to prevent her from defending them.

A herd of killer whales could approach the dolphins, hitting them and launching them into the air to stun them.


Dolphins are preyed upon by some species of sharks, including tiger sharks, gold shark, sand shark, Sardinian shark, and great white shark.

When a member of the dolphin family group is threatened by a shark, the rest of the members come to their defense. These will surround the shark, swimming around it in all directions and hitting it with their tails. In this way they confuse the shark, which could flee.

If the dolphin is alone, it can use its great speed to swim and uses its long snout. The dolphin swims under the shark and hits it with this bone structure. This onslaught stuns the marauder, though it is sometimes strong enough to kill him.

The man

The human being is also a predator of dolphins. In its diet it has incorporated the meat of this animal, which has commercial value, although it is very high in mercury, a toxic element. This has resulted in a great problem worldwide, since human beings carry out atrocious kills of dolphins.

Every year, between the months of September and April, fishermen from Taiji, Japan, corner more than 20,000 dolphins in a cove, where they capture them. The objective is to obtain their meat and sell the live specimens for captivity. In this process, many dolphins are seriously injured, causing their blood to turn the sea red.

Some local groups are in favor of this activity, considering it as part of the culture. However, many global organizations such as One Voice, Elsa Nature Conservancy, and Earth Island Institute have documented this great slaughter, making it world-dominating.

In the same way, the animalistic and environmental protest does not stop. These organizations carry out various activities worldwide, in order to condemn and prevent these massacres.


  1. Suzanna Hulmes (2018). What Are the Characteristics of Dolphins? Sciencing. Recovered from
  2. Arlen Hershey (2017). What Are the Dolphin’s Body Parts ?. Sciencing. Recovered from
  3. Dan Fielder (2018). Three Adaptations for a Dolphin. Sciencing. Recovered from
  4. Ethan Shaw (2018). How Do Dolphins Survive in Their Natural Habitat ?. Sciencing. Recovered from
  5. ITIS (2018). Delphinidae. Recovered from
  6. Wikipedia (2018). Dolphins. Recovered from
  7. Delfinpedia (2018). Dolphin assisted therapy. Recovered from
  8. Dolphins-World (2017). Dolphin intelligence. Recovered from
  9. Animanaturalis (2018). The annual slaughter of dolphins in Japan. Recovered from
  10. Dolphins-World (2017). Dolphin evolution. Recovered from
  11. Peter J. Morganeab, Myron S. Jacobsab, Willard L. McFarlandab (1979). The anatomy of the brain of the bottlenose dolphin (Tursiops truncatus). Surface configurations of the telencephalon of the bottlenose dolphin with comparative anatomical observations in four other cetacean species. Sience direct. Recovered from
  12. Sernapesca (2018). Common dolphin. Recovered from
  13. Encyclopedia britannica (2018). Dolphins. Recovered from
  14. Dolphins-World (2017). Dolphin habitat and distribution. Recovered from
  15. Chris Deziel (2018). How Do Dolphins Fight Sharks? Sciencing. Recovered from
  16. G. M. Thewissen, Lisa Noelle Cooper, John C. George and Sunil Bajpai (2009). From Land to Water: the Origin of Whales, Dolphins, and Porpoises. Evolution: Education and Outreach. Recovered from
  17. Cetacea Association (2018). Bottlenose dolphin. Recovered from associaciocetacea.or.