Cetacean anatomy is specificially designed to facilitate living entirely underwater, never emerging onto the land. There are several special adaptations which allow cetaceans to survive as they do in the oceans and rivers of the world.
The general and overall body shape of most cetaceans is a streamlined and sleek form, with few distruptions which are hindering to passage through the water. The unique anatomy of a cetacean typically consists of the flukes, peduncle, pectoral flippers, dorsal fin, rostrum, blowhole, melon, and genital region. All these anatomical features are designed to facilitate a life in an entirely aquatic environment. Additionally, cetaceans have special adaptations for their eyes, mouth, and ears that terrestrial mammals are generally lacking. Another note on general anatomy is that most cetaceans possess a form of camouflage known as countershading. This indicates that the animal is colored darkly along the dorsal aspect and lightly along the ventral side. This assists in disguising the animal under water, where the surface is bright when seen from below and the floor is dark when seen from above.
Flukes and Peduncle
The flukes of cetaceans consist of two lobes; each lobe is individually called a fluke. The 'tail stock' is called the peduncle, which is the large muscular extension of the body which the flukes are attached to. The flukes possess no bones whatsoever, being constructed entirely of dense and fiberous connective tissue which house a network of blood vessels which aid in body temperature regulation. The size and specific shape of the flukes is dependant on the species of cetacean in question. The flukes are raised and lowered by lengthy longitudinal muscles found in the cetacean's back and along the caudal peduncle. Thrust is provided by the upstroke of the flukes and peduncle, not the downstroke as some would believe. The following image demonstrates the motion of the flukes in the upstroke and downstroke.
The pectoral flippers of cetaceans are responsible for both steering and stopping while swimming. Flippers are the equivalent of forelimbs in terrestrial mammals and possess the same basic bone structure. Supporting the bones and holding them rigidly in place is thick and dense connective tissue and cartiledge. As with the flukes, a network of blood vessels serves to regulate body temperature.
The dorsal fin is a cartiledgeous structure which rests atop the back of most, but not all, cetaceans. There are no bones supporting this structure, thus it occassionally weakens and leans to one side or the other. The size and shape of the dorsal fin differs between species and, in some, is absent all together. It acts as a keel, assisting in the stabilzation of the cetacean during swimming, however it is not necessary, as evident from the many species which lack the fin. Like the flukes and pectoral flippers, the dorsal fin is a site of thermoregulation. The following diagram illustrates how the thermoregulation via blood vessels occurs. The picture below that is of a killer whale dorsal fin, most likely of a female animal.
Image from U.S. Government
The snoutlike projection on the end of the cetacean's head is known as the rostrum. In some species, the rostrum is very pronounced, such as the bottlenosed dolphin or spinner dolphin. In others, such as the killer whale or Pacific white-sided dolphin, it is subtle and hardly noticable except on close examination. The rostrum is often very hard and can be used as an offensive weapon against predators. For example, bottlenose dolphins are capable of ramming sharks with their rostrum with enough force to severely injure or kill them. In the rostrum is housed the animal's teeth. In odontocetes, teeth can be of many different forms. Dolphins typically have conical teeth, while porpoises have spade shaped teeth. Killer whales have extremely large, curved teeth. All odontoceti teeth are meant for holding prey, not ripping, tearing, or chewing. Food is swallowed whole, head first in the case of fish. The teeth of cetaceans are often an indicator of age, having rings like that of a tree.
The blowhole of a dolphin is formed by a muscular flap which is opened or closed voluntarily by the cetacean and provides a water tight seal. The flap is closed in its relaxed position, thus the cetacean must consciously think about breathing, unlike humans and most other terrestrial mammals. The blowhole's appearance and fuctions differs between some species. Most notably, one difference between those in the suborder Odontoceti and Mysticeti. Most species in the Odontoceti group have only one blowhole opening. Comparitively, cetaceans belonging to the suborder Mysticeti have two openings.
Odontocetes possess a unique feature called the melon. The melon is a large fatty area in the forehead region of the cetacean's head. This buldging fatty tissue assists in the focusing of ecolocation clicks and related sounds. In some species, it has been noted that the melon can be voluntarily reshaped to also assist in sound production and focusing. However, cetacean echolocation is not completely understood and research continues.
The genital region of cetaceans is far different from that of terrestrial mammals. To remain streamlined and reduce drag in the water, all external genitlia has evolved to be tucked within the abdominal cavity. This presents new problems which cetaceans have solved in their own unique way. In the male cetacean, the testes and penis are within the body cavity and hidden from view. This presents two issues which terrestrial mammals do not face. First of all, the internal temperature of the cetacean must be low enough that sperm production can take place and the sperm produced can survive. This is achieved apparently, as the average internal body temperature of a cetacean is approximately 35.5° Celsius. Also important to note is that the male penis can be retracted or extended at will by the animal, a factor influenced by its placement within the abdominal cavity. The female cetacean is less different from that of a terrestrial mammal than the male. All the female reproductive organs are located within the body, as with terrestrial mammals. One item of note is that cetaceans generally have one offspring, called a calf. It is exceedingly rare to find cetaceans with mutliple births. Another interesting point of note is that most cetacean's have the ovum implanted in the left side of the uterus. There has been no explaination for this, but it seems relatively constant... even when the female ovulates on the right side.