Whales and dolphins

The cetaceans whales, porpoises, and dolphins have the most highly evolved brain of all aquatic animals and include some of the most intelligent animals on earth. Indeed, they are sometimes considered to be as intelligent as humans. Although cetaceans are descended from land-dwelling mammals, they have nevertheless become so well adapted for an aquatic life that they cannot leave the water. Except for the few species of dolphins of the family Platanistidae, which live in rivers and lakes, all cetaceans are marine mammals.

The order Cetacea divides into two main suborders, according to method of feeding. The suborder Odontoceti, which has seven families, is made up of the toothed whales. The suborder Mysticeti, the baleen whales, has three families.

The common names of these animals are confusing. Most large species are called whales and most of the smaller ones are called dolphins. The killer whale, however, is a member of the dolphin family (Delphinidae), yet there are several species of whales considerably smaller than it.
A further inconsistency is that in the United States, most dolphins are called porpoises, whereas in Europe, this name is reserved for six species of small, blunt-faced dolphins of the family Phocaenidae.

The bottle-nosed dolphin occurs in all the world’s oceans, but is most common along the Atlantic coast of the United States. It eats fish and, like other true dolphins, can often be seen swimming alongside or in front of ships.

Anatomy and physiology

Of all the mammals, whales and dolphins are the most highly adapted for life in water. The outline of their body has become streamlined, and their forelimbs have become broad flippers, which they use for steering and balance. Although these flippers still have all the same bones of the vertebrate forelimb, they have been modified. The hind limbs have disappeared, but cetaceans still have a reduced pelvis that in the male supports part of the reproductive apparatus. The dorsal fin and the tail are folds of skin that are not supported by any skeletal elements. Cetaceans’ smooth outline is partly due to the absence of fur-cetaceans have only a sparse covering of hair.

Cetaceans propel themselves through the water by up-and-down movements of the broad horizontal tail flukes. The body is protected by layers of fat, or blubber, which act as an efficient insulator to keep the body warm and also serve as an energy store. The presence of large amounts of fat in the surface tissues allows subtle variations in the animals’ shape, which reduce drag and make deep diving easier.

Cetaceans usually show only a small part of their back when they come up to breathe, although even the largest ones can leap clear of the water, a behavior known as breaching. The nostrils are on the top of the head, forming a single blowhole in toothed whales, and two blowholes in baleen whales. As a cetacean surfaces, it blows, sending up a distinctive spout of watery spray. This jet is a cloud of droplets caused by water vapor in the breath condensing as the pressure suddenly drops when the animal shoots up to the surface.

The circulatory and respiratory systems of cetaceans are well suited for deep diving. Cetaceans carry a relatively small proportion of their oxygen intake in their lungs. Evidence suggests that the lungs are emptied before a deep dive and that oxygen is stored elsewhere, most of it combined with hemoglobin and myoglobin in the blood and tissues. Because there is not enough oxygen to supply all body functions during a long dive, cetaceans compensate by directing oxygen-rich blood toward the brain and nerves, where it is most needed.

These animals also have evolved some muscles that can work for short periods anaerobically (without oxygen), although they must be replenished with oxygen soon afterward. The reduced amount of oxygen cetaceans take on deep dives, and the diminished rate of blood flow to the tissues prevents the problem of nitrogen coming out of solution as bubbles in the blood when these animals return to the surface a dangerous condition for human divers called the bends.
Cetaceans have a highly developed sense of touch. They have very little sense of smell, and a variable ability to taste. Whales and dolphins, in fact, have no sense of smell. Many species have good eyesight, but vision is not very useful because little light penetrates the depths of the sea.
Because sound travels well in water, cetaceans depend chiefly on their sense of hearing. Whales and dolphins are able to produce a range of different noises, with which they communicate and find their way. Schools of dolphins chatter continually to one another, using clicking and whistling sounds, and the “songs” of the humpback whale are known to carry hundreds of miles. Such calls inform the cetaceans of the identity and mood of their fellows. Distress calls, for instance, seem to summon other cetaceans to an injured individual. Some species of toothed whales, especially dolphins, are known to use echolocation. Like bats, these creatures emit streams of high-frequency clicks and ultrasonic squeaks and use the returning echoes to locate prey and to orient themselves. There is evidence that dolphins and killer whales have a language, and an ability to reason, learn, remember, and use information.

A baleen whale filters the tiny organisms in plankton from seawater using baleen plates that hang down from the upper jaws (right, A). The blue whale (farright, B), is a baleen whale. It is the largest mammal that has ever lived and is now in danger of extinction.

Teeth and feeding

The whales of the suborder Mysticeti have no teeth. Instead, they have a series of comblike baleen plates made of keratin (the same substance as fingernails) hanging down from the upper jaw, or palate. Their food consists of plankton, the mass of tiny organisms that drifts at or near the surface of oceans, and also sometimes small fish. The whale draws water containing the food into its mouth and then squirts it out through the baleen plates, which act as a strainer, keeping the food in the mouth.

Toothed whales include all nonbaleen whales. They have numerous conelike teeth. Some toothed whales, such as the sperm whales (family Physeteridae), have teeth in their lower jaw only, and the beaked and bottle-nosed whales (family Ziphiidae) usually have only one or two pairs of teeth showing, but even these may never break through the gums. The male narwhal has a single tooth that has become a long, twisted tusk on one side of its head.

The teeth of most toothed whales are adapted for holding prey but not for chewing. As a result, they swallow their food whole. Most toothed whales feed on fishes, squid, and cuttlefish, but certain groups of killer whales specialize in hunting seals.

Reproduction

Cetaceans breed seasonally, mating during spring and early summer. They produce single offspring, and birth takes place from 10 to 12 months after conception in most species; although sperm whales have a very long gestation period of some 16 months. One or more females may help a mother cetacean give birth. Calves are born under water and may be as much as a third of the mother’s size at birth. A calf emerges tail first and is pushed to the surface by its mother to take its first breath and to suckle from the teats, which lie in the folds of blubber on each side of the reproductive opening. The milk is 50 per cent fat and is very rich in protein and minerals.
Baleen whales seem to breed only every two years. As a result, the numbers already severely depleted by the commercial whaling industry will take a long time to recover. In addition, large whales start to reproduce only when they are about 12 years old, which obviously affects the replacement of individuals in nature.

A young humpback whale swims above its mother. It is fed on mother’s milk for five to ten months after birth, but does not grow to full size until it is 10 years old. These whales migrate annually from Arctic and Antarctic waters (map, (eft) to spend the winter in warm tropical seas.