Dolphin Anatomy

All dolphins are adapted to their aquatic environment. They have a similar streamlined, torpedo-shaped body - their bodies are larger at the front than at the back and they have a short, stiff neck. As with all the cetaceans, all visible traces of hind limbs have disappeared and they have lost most of their body hair. The forelimbs have developed into flippers, which helps in steering through water. The tail of a dolphin (called the fluke) is used for propulsion and is therefore very muscular.

There are also many other, less obvious adaptations. For example, they have excellent hearing, which compensates for a poor (or entirely lacking) sense of smell. For the uncertainties of visibility underwater; they have a high tolerance to carbon dioxide to help with lengthy dives, and are two or three times more efficient than land mammals at using the oxygen in inhaled air. Their rib cages are collapsible for deep diving; and they have layers of insulating fat to keep them warm

Dolphin skeletons are weaker than those of land mammals. Largely because of the buoyancy of the water, dolphins do not require strong limbs for support.

The backbone is very flexible, due to the reduced interlocking of individual vertebrae and the development of large fibrous discs between them, to allow powerful undulations of the tail for swimming.

The seven neck vertebrae have become greatly compressed, and some or all of them fused, making the dolphins neck short and rigid. This means that most dolphins can only move their head from side to side, and not nod up and down.

The dorsal fins of dolphins are not related to any other typically mammalian structure. The dorsal fin contains no bones but is made of a tough, fibrous fatty material.

The flukes are boneless and like the dorsal fin, they are made of a tough, fibrous fatty material. Attached to the end of the spine, they are always horizontal and form a perfect hydrofoil.

The hind limbs have all but disappeared from the dolphin skeleton structure, however there are still traces of the pelvic girdle, and in some cases, the femur is buried deep inside the abdominal blubber and muscle.

The ribs of dolphins are not strongly attached to the spine or to the breastbone. These "floating ribs" are often joined, enabling the rib cage to collapse under the pressure of a deep dive without being damaged.

The front limbs have evolved into flippers to minimise resistance in the water. The flippers are generally more rigid than the mammalian hand because the only mobile joint is the shoulder. The typical structure of the mammalian hand is still present in the skeleton, despite its hydrodynamic function. However, the fingers have become lengthened, thus increasing the surface area. The individual bones of fingers (known as the phalanges) are generally increased beyond the normal mammalian number but the number of fingers are reduced to four.

The upper and lower jaws of the Dolphin are unusually long or 'elongated'. The skull is tilted upwards in line with the spinal column and the bones of the cervical vertebrae (the neck) have become fused together in all species. The main bones of the upper jaw have been thrust backwards and upwards over the eye sockets to extend across the front of the brain case. This type of telescoping of the upper jaw may be associated with the well-known ability of dolphins to echolocate or use sonar.

The Dolphin's skull is adapted for producing and receiving high-frequency sounds. The expanded and backward-shifted upper jaw bones hold a large volume of facial muscles, and these muscles focus upwards and in towards the blowhole. At the blowhole, the muscles are attached to a series of sacs in the soft tissues of the nasal passages between the external blowhole and the bony nasal openings on the skull. In each ear, the middle ear cavity is expanded into a complex sinus on the skull base. These sinuses help to isolate the right and left ears from each other, making it easier for the animal to tell the direction of a sound source. The periotic, or ear-bone, that carries the organs of hearing and balance, is not fixed to the skull in dolphins. Sound is perhaps transmitted from the water to the internally placed ear bones via a thin 'pan-bone' in the lower jaw and a fatty channel from the pan-bone.

The olfactory (smelling) apparatus associated with the blowholes is absent in dolphins. Dolphin's eyes are relatively small. Their ears are well developed, the ear canal from the side of the head is usually closed, and sound is transmitted to the ears through soft tissues.

The brain of a dolphin is comparably large to its body size. A short and wide trachea supplies the lungs of a dolphin. The trachea is supported by cartilage rings and the bronchioles. A dolphin's stomach may be chambered, like those of some hoofed mammals. However, dolphins do not chew their cud. Dolphins do not have a gall bladder and appendix. The kidneys are large and possess many separate lobes.

All species have teeth, although the number of teeth varies between species. Some species of dolphins can have as many as 252 teeth while at the other extreme, some species can have as few as 4 to 14 teeth. The teeth are all conical in shape, which is ideal for grasping rapidly. Dolphin teeth start to form before birth and erupt when the calf is a few weeks old. Dolphins have only one set of teeth. They retain their first teeth (otherwise known as their 'milk' teeth) for the rest of their lives.