Skeletal Structure

1. Backbone
2. Dorsal Fins
3. Fore Limbs
4. Flukes
5. Hind Limbs
6. Ribs
7. Skull
8. Soft Tissues
9. Teeth

The basic skeleton of a dolphin has undergone a number of changes and 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 cervical vertebrae are shortened, the seven neck vertebrae have become greatly compressed, and some or all of them fused, making the neck short and rigid. This would mean that most dolphins can only move their 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 front limbs have evolved into flippers so as 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 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 mammalian 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 often delicate in build, and not strongly attached to the spine or to the breastbone. The ribs are said to be "floating". The ribs that are attached are often joined, enabling the rib-cage to collapse under the pressure of a deep dive without being damaged.

The skull of the dolphin has departed from the normal mammalian structure by being 'telescoped', which allows the face bones to elongate greatly, therefore meaning that both the upper and lower jaws are unusually long. This can be done by compressing from front to back of the skull so that certain parts overlap each other - like the sections of a folded up telescope). The skull has become tilted upwards in line with the spinal column and 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.

Other aspects of the skulls of dolphins also seem to be 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, the earbone that carries the organs of hearing and balance, is not fixed to the skull in dolphins. The external ear canal is vestigal. Sound is perhaps transmitted from the water to the internally placed ear boens via a thin 'pan-bone' in the lower jaw and a fatty channel from the pan-bone.

The soft tissues of dolphins are very much different from land mammals as do other parts of their anatomy. Sensory organs associated with the nervous system show many remarkable features. The olfactory (smelling) apparatus associated with the blowholes is apparently 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 probably transmitted to the ears through soft tissues.

The brain of a dolphin is comparably large to its body size. The lungs of a dolphin are supplied by a short and wide trachea. 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 like them. All species of dolphins do not have a gall bladder and appendix. Their live is not lobed either. The kidneys are large and possess many separate lobes.

The teeth of the dolphins particularly interesting because they are different from other mammals in a number of ways. 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.

Dolphin teeth are not differentiated into incisors, canines, pre-molars and molars, as in the case of most mammals. Instead, they are all conical, an ideal shape for grasping rapidly. Dolphin teeth starts 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.

Bibliography

Carwardine, Mark. The Book of Dolphins. Dragon's World Ltd, 1996

Harrison, Sir Richard, et. al. Whales, Dolphins and Porpoises. New York: Facts on File, Inc., 1994.

Tursi. "Tursi's Dolphin Page" Available http://www.neteng.bc.ca/~tursi/dolphins/index.htm, 4 September 1997, Accessed 4 April 1998