The Anatomy of Dolphins

ok, this was A LOT OF WORK

I hope you enjoy:

sorry-the photos might be a little shaky

Basic Anatomy:

Evolution and Taxonomy Behaviour
Anatomy Human and Dolphin
Table of Contents Previous Page

Basic Anatomy

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Head Body Shape Dorsal Fin Skin Tail Flukes
Dolphin Anatomy
Pectoral Fins Mammary and Genital Slits

Body Shape:

An examination of the profile of modern dolphins shows the extent to how mammals have actually evolved to become adapted to their aquatic environment.

All dolphins have a similar streamlined, torpedo-shaped body – their bodies are larger at the front than at the back. The dolphin has a short, stiff neck. As with all the ceteceans, all visible traces of hind limbs have disappeared. The forelimbs have developed into paddle-shaped flippers which helps in steering through water. The tail of a dolphin (called the fluke) is used for propulsion: it is therefore very muscular.

Many features of the former land-dwelling mammals have disappeared, for example, dolphins do not have significant body hair, an external ear lobe (pinna), a projecting nose, and externally projecting genitals or mammary glands. All their protruding parts are reduced or tucked away, so as to improve hydrodynamic efficiency.

Their body shape differs very little in comparison with other ceteceans (whales and porpoises). Sometimes, dolphins are even mistaken for sharks. It is easy to distinguish between a dolphin and a shark on the surface; the latter has a vertical tail fin which projects out of the water behind the dorsal fin. Comparatively speaking, dolphins have a smaller head and shorter body than whales. Dolphins have a beak and but porpoises do not and dolphins usually have dorsal fins.

Dorsal Fin:

There is generally a prominent dorsal fin on the back of the dolphin though it is absent in northern and southern rightwhale dolphins. Some of the river dolphins do not have a dorsal fin, but only a slight ridge or hump on the back.

The size, shape and position of the dorsal fin varies from one species to another. The fin is not supported by any bone, but by tough fibrous tissue inside it. Dolphins can have fins that are falcate, triangular, bluntly rounded or totally absent. The functional significance of these different styles of in is uncertain. Some people believe that dorsal fins help dolphins to maintain stability in the water while others believe that a well-developed fin is not essential for survival.

However, the blood vessels in the dorsal fin do help to control body temperature. It acts as a heat exchanger during intense activity or when swimming in particularly warm water.


A dolphin has extremely smooth, firm and velvety skin, which helps them to slip through water with apparent ease. Water resistance is greatly reduced and this effect, together with the body’s streamlining and the power of the flukes, allow the dolphin to swim at high speeds.

The dolphin’s smooth skin is constantly being sloughed off and replaced. It is also very sensitive to touch, and easily scarred. Almost all adult dolphins have numerous scars, nicks and notches on their skin and these help researchers to identify and study their interaction with companions, enemies and the environment.

Unlike most other land mammals, dolphins do not have thick coats of hair to keep their bodies warm although new-born calves sometimes have a small amount of stubble on their beaks. To maintain a stable body temperature of around 36-37 degree celsius is particularly difficult in water, because water can conduct heat away from the body twenty-five times faster in air. Therefore, dolphins have developed a thick layer of insulating fat, known as blubber. The thickness of blubber differs between species and according to the average temperature of the water.


The head of the dolphin has many interesting features. The face of a dolphin is rather unexpressive. Dolphins seem to wear a permanent smile, but this is deceptive since the head, like the rest of the body, carries significant blubber under the skin. Blubber prevents major muscles on the face to reach the surface. Therefore, dolphins are capable of only a limited range of facial expressions.


Like all mammals, dolphins have to go the surface to breathe. During the course of evolution, the nostrils have moved to the top of the head and is now known as the ‘blowhole’. The blowhole is situated behind the melon, in the center of the top of the head, for easy breathing at the surface. In most species of dolphins, the blowhole is crescent-shaped, with the curve towards the back. The hole is naturally closed and must be opened by muscular action. It opens briefly for a fast exhalation and inhalation. There are air sacs under the blowhole which help in the closing of the hole and are also used for sound-formation.


The ear of the dolphin is hardly visible as there is no flaps or lobes, with only a pinhole opening on either side of the head and just behind the eye. Instead of using the ear, a dolphin makes use of a highly developed acoustic faculty to hear sounds with high frequencies between 75 Hz and 150 kHz. The optimum sensitivity of the dolphins for sounds is between 40 and 70 kHz.

The well developed acoustic ability is attributed to the auditory system found in the brain of a dolphin. The auditory system of dolphins is more strongly developed than the auditory system of humans, as seen in the fact that the dolphin auditory nerve has double the amount of nerve fibres compared to the human auditory nerve.

Sound is transmitted to the dolphin’s middle ear via the blubber and the lower jaw. Blubber is mainly fats and the lower jaw is filled with fatty tissues. Since fats can conduct sound well, both the blubber and the lower jaw are good conductors of sound.

The dolphins have middle ear cavities that are independently suspended and surrounded by air-filled spaces, reducing the contact with the bones. This probably helps in directional hearing. The middle ear functions to stiffen the sound transmission system, optimising it for high frequencies and also helps to balance the pressure between the inner ear and the external environment.


Dolphins have two eyes – one on each side of the head. Their eyes are built with a jelly-like substance that looks like tears. This mucus like substance protects the eyes from seawater and prevents irritation of the eyes. Their eyes function independently of each other and there are no eyebrows or lashes.

Light penetration is poor in water and the importance of sight varies between species and their particular habitats. Generally, dolphins can see well both in and out of the water. However, the eyes of the dolphins are optimized for underwater vision.

Unlike the human eye that loses its refractive power under the water, the dolphin eye has very great refractive power because the lens is located further up and is completely spherical, facilitating underwater vision. Because of this, the dolphin eye looks a lot like a fish eye.

Instead of having an iris, the dolphin pupil uses a kind of “lid” called operulum that slides down, covering the centre of the pupil under bright light conditions. This action will leave narrow slits on the pupil’s edges and these slits can give the dolphin better vision.

The dolphin cornea is somewhat different from the usual mammal eyes. The dolphin cornea have two yellow spots (the area on the retina where light-sensitivity is the highest). One of the yellow spots is believed to be used for forward vision and the other for lateral vision


On the back of the upper jaw, and on the dolphin’s top lip, is the bulbous ‘melon’, a fatty substance used to focus echolocation clicks. The melon serves as an acoustical lens and focuses the clicks into a narrow beam.


Some dolphins have a prominent bird-like beak, known as the rostrum, in front of the eyes, and others do not. The rostrum, which is very hard, is the dolphin’s nose or snout. It is not used for smelling but for digging and attacking enemies.


As in all cetaceans, the forelimbs have evolved into pectoral fins. Dolphins have well-developed pectoral fins (flippers) placed behind the head and below the midline.

The flippers vary widely in shape, size and colour from one species of dolphins to another. Flippers are important in steering and stopping. They provide excellent paddles for steering and stability. Shaped like paddles, these modified front limbs are used primarily for twisting, turning and for manoeuvring. The flippers also appear to be important as organs of touch in social and sexual contexts.

Pectoral fins are made of cartilage and bone. They are similar to the skeletal structure of land mammals with fingers and ball and socket joints. In some cases, the flippers are held rather rigidly out from the body as dolphins have a stiff foreflipper without the movable elbow joint in most mammals.

Mammary and Genital Slits:

The difference between male and female dolphins can be found out by examining the genital area which is near to the tail of the dolphin.

In females, there is a single urogenital slit, which contains the genital opening, the urinary tract opening and the opening. The mammary slis of the female can be seen beside the urogenital slits. However, some males may also have mammary slits.

In males, there are two openings within the urogenital slit. One of which is a small opening and the other is a genital slit.



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Skeletal Structure
Respiratory System Dolphin Physiology Thermoregulation
Excretory System Reproductive System


The dolphin is a fast and active swimmer, capable of astonishing feats of speed and agility. Dolphins can travel on its belly, on its back or even on its side. It can make sharp turns easily and can jump to great heights. Swimming at about 35 km (22 miles) per hour, the dolphin seems to be indefatigable. The dolphin can even swim up to 50 km (30 miles) per hour. However, when physiologists studied its size and shape, they did not expect the dolphins to be so fast and agile in water. Why is it that dolphins are able to be so fast and agile?


To maintain the swimming speeds in dolphins, the tail would have to develop about ten times as much propulsive power as the muscles of other mammals. Therefore, it is seen that the dolphin’s tail is extremely muscular and powerful.

Dolphins have a disproportionately large horizontal tail, known as a fluke. The flukes contain no bones and are mostly made up of connective and muscular tissues, forming a tough assemblage of muscles and fiber. Tendons of the muscles in the flukes are so developed that even the most violent muscular activity will not break the spinal column.

Not only is the tail stock a powerful propeller, it is also used in stabilization and steering. It allows the dolphin to vary its movement, to change direction or position. This explains why the dolphin is so agile, having the ability to turn easily, to swim on all its sides and to spin about freely.


Despite the powerful propulsive force produced by the tail, physiologists concluded that dolphins can never reach a speed of 50 km (30 miles) per hour. In 1963, P.E Purves introduced the assumption that dolphins swim as well as they do because their bodies generate almost complete laminar flow. In other words, the dolphins are able to eliminate frictional drag and turbulence. Significant reduction of frictional drag over the body of the dolphin results in a smaller propulsive force required for high speed swimming.

How the dolphin managed to reduce laminar flow is partly because of its streamlined body, allowing it to move through water easily. However, the main reason lies in the dolphin’s silky smooth skin.

The water nearest to the skin creates the most frictional force. To minimize this, the skin secretes a high polymer of ethylene oxide that acts as lubricant, sloughing off skin cells. The skin cells shedds and renews every two hours, compared with every eighteen hours in human skin.

The dolphin’s skin is formed of small folds, called dermal ridges. These are tiny ridges running parallel to the length of the dolphin. that are constantly moving. Usually, when a body passes through water, little swirls of water, known as eddies, are formed along the body’s surface and these eddies create frictional drag. These tiny movements of the skins help to prevent the slowing effects of the water, by stopping the formation of eddies. The water around the body thus becomes calm, allowing the body to glide easily.


Dolphins often make use of the positive pressure field created by a object moving through the water, for example a ship, to be propelled forwards. It can be done so because there is a difference in the pressure on both sides of the flukes. In this way, the dolphins are enjoying a free ride in the pressure field and also saving some of the energy of locomotion.

However, dolphins do swim on their own most of the time. The main organ of propulsion is the flukes and the hind part of the body, which are moved up and down in a vertical plane, developing the thrust that propels the body along.

The flukes, attached to the vertebrae of the tail, are enveloped by bundles of ligaments that resist the bending of up or down. The fibres in the core of the flukes are pleated, with the lower surface having more pleats than the upper surface. If the flukes are raised upwards against the resistance of the water, the pleats on the upper side will cause the fluke to bend slightly. But if the flukes are moved downwards, the pleats on the lower side will open up and cause the fluke to bend considerably.

The beating of the tail is brought about by two pairs of muscles: the apaxial muscles and the hypaxial muscles. The epaxial muscles that lie above the vertebral transverse process, are much larger than the hypaxial muscles situated below the vertebral transverse process. This shows that it is the epaxial muscles, which are responsible for the upward movement of the tail that is the main driving force in swimming.

After the epaxial muscles have effected the power stroke, the tail then rises upwards, and the body moves forwards and downwards. The hypaxial muscles now pull the tail downwards, to get ready for the next power stroke. The buoyancy of the head and thorax, enhanced by the large amount of oil and fat contained in them, causes the front of the body to rise as the tail descends. The cycle then repeats itself, resulting in a upward and downward propulsive body movement.


One response to “The Anatomy of Dolphins

  1. Pingback: Dolphin Times: Issue 2 | Stop Cetacean Captivity

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