New Zealand Whales and Dolphins
The Biology Of Cetaceans
The Biology Of Cetaceans
Cetaceans have a very smooth body which slips through the ocean with little effort; their streamlining is essentially fish-like but with one big difference — the tail flukes of a cetacean are horizontal rather than vertical as in fishes. The similarity with fishes stops with body form, for cetaceans are modified mammals whose ancestors once lived on land.
Like all mammals, cetaceans have mammary glands and hair, although the latter is often present only as sparse stubble in newborn calves. The teats, along with the sex organs and ears, do not protrude from the body as in other mammals, but are either hidden within slits in the body wall or, as in the case of ears, are reduced to a pinhole opening or a membrane flush with the skin. Thus, apart from compact forelimbs (flippers), a dorsal fin, and tail flukes, which are all necessary for balance, steering, and propulsion in the water, cetaceans are almost perfectly streamlined (Fig. 2).
The great size of whales is no legend, for adult Blue Whales may grow to 100 ft. These creatures are the largest known animals, living or extinct. There is a skeleton of an 87ft specimen in the Canterbury Museum. The smallest cetacean in New Zealand waters is Hector's Dolphin, which barely exceeds 4ft in length.
The hind limbs of whales are only vestiges, hidden deep in the body muscles above the vent. In baleen whales there is a small bone representing part of the pelvis, and occasionally there may be tiny pieces of cartilage repesenting the actual limb. Toothed whales have only the small pelvic bone.
The tail flukes and dorsal fin of cetaceans have no skeletal support, and are not related to any typically mammalian structures. They are outgrowths of skin and connective tissue, and the flukes are connected to the body muscles by a complex of ligaments and tough fibres. The powerful up and down movement of the flukes produces forward propulsion.
The fins and flukes are well served by systems of subcutaneous veins and arteries, and act as heat exchangers during great activity or when a cetacean is swimming in warm waters.
The extremely smooth skin of whales and dolphins consists of several layers. The outermost epidermis is very thin, and peels off like rice-paper from sun-scorched stranded whales. Separated from the thin outer skin by a tough layer of fibres is the thick (seventeen inches in sperm whales), fatty blubber.
Figure 3: Skeleton of a Hector's Dolphin. The small outlying bone near the tail is vestigial pelvis.
Figure 4: The different kinds of whale skulls viewed from above. A: Fin Whale type; B: Large Sperm Whale; C: Pygmy Sperm Whale; D: Beaked Whale (Ziphius-Berardius type); E: Beaked Whale (Mesoplodon type); F: Dolphin — Pilot Whale type.
The lungs of cetaceans are long and light and are situated in a dorsal position in the body cavity — a position which enhances stability and horizontal floating. To cope with pressure changes underwater, the lungs are very elastic. The lungs of dolphins have about one and a half times the capacity of those of land animals, whereas other whales have about half the capacity. The diaphragm is large and muscular and lies more horizontally than in other mammals, so it is able to press against more area of lung, enabling almost complete evacuation of air.
The lungs are filled by inhalation through a blowhole (two in baleen whales, one in toothed whales) which is situated on top of the head. Powerful muscles close the blowhole underwater. Whales are twice to three times as efficient at utilising oxygen in the inhaled air as land mammals. Exhalation of air takes place through the blowhole at the surface, and is accompanied by a cloud of vapour — the ‘spout.’ It is thought that the vapour is formed either by the cooling effect and consequent condensation of a gas escaping from the lungs under great pressure, or by the expellation of a fine spray of mucus from the air sinuses. Also, Sperm Whales have been seen to start blowing just before emerging from the surface, so their spouts could be partly a spray of seawater. Many whales have a characteristic blow, and some of these are illustrated in Fig. 5.
Some cetacean senses are highly sophisticated, whereas others are greatly reduced. The ‘feeling’ sense of the skin is probably poorly developed, although some whales may be fairly sensitive around the mouth and snout. The sense of smell is completely absent in toothed whales but is developed to some extent in baleen whales. Taste buds have not been found in cetaceans.
Hearing is highly refined in cetaceans and is the most important sense connected with food finding, navigation, and communication. This acute hearing compensates for the poor sense of smell and the uncertainties of underwater visibility. According to various researchers, sounds enter the middle ear either by way of the tiny external ear opening, through the side tissues of the head, or through a fat body within the posterior portion of the lower jaws.
Baleen whales also produce a variety of sounds, mainly of lower frequency and lesser complexity than those of toothed whales. Humpbacks are said to emit an eerie undulating wail not unlike the page 12 sound of distant bagpipes! There is no evidence that baleen whales echolocate but it is thought that the regular low-pitched calls of the fin whale may have an orientation function.
There is some controversy about how good cetaceans' eyesight is underwater, and it is considered to vary with different species, depending on whether they are oceanic or coastal, and on how they feed — a toothed whale chasing squids and fishes would need better eyesight than a plankton feeder. Anyone who has seen performing dolphins leaping into the air to snatch small fishes from a trainer's hand will realise how good their eyesight must be out of water. The field of vision from each eye does not overlap in whales, but probably does slightly in dolphins and beaked whales with short, sharp foreheads.
Dolphins and porpoises may become sexually mature at only fourteen months old, whereas large whales such as the Sperm take ten years to mature. Cetaceans breed seasonally, and after gestation periods of from nine to sixteen months, usually bear only one young. The calf is born tail first and is immediately able to swim and keep up with its mother. A mother dolphin is often assisted in looking after a young calf by an ‘auntie’ dolphin. The young are suckled for about a year. The sex of a cetacean can be determined by examining the distance betwen the genital aperture and the anus; in the female they are close together, and in the male they are separated.
Mating is usually preceded by various forms of courtship behaviour. For example, dolphins have been seen to slap their tails madly on the surface of the water and rush at a partner to make a gliding contact, followed by intense rhythmical flipper stroking and genital contact. Large whales dive and approach each other rapidly to emerge vertically from the water belly to belly; copulation occurs very rapidly before the animals crash back into the water and separate. In dolphins, copulation takes place with the male swimming upside down beneath the female (Fig. 6).
Dolphins sometimes copulate with individuals of different species or even genera, but although there are several strangely anomalous skeletons in museums, and an offspring from a Tursiops x Stenella cross has been reported in a Hawaiian dolphinarium, naturally occurring hybrids have not been recorded.
The degree of healing of external scars on the body has been used, without great success, to gauge the age of whales, but the counting of white tissue or scars in the ovaries, which are indications of periodic ovulation, has been more promising. Other ageing methods involve measuring the changing colour of the eye lens, which turns golden with increasing age, and the counting of growth ridges on baleen plates, or layers in teeth dentine. Sperm whales are thought to reach 60 years of age.
A recent method of ageing baleen whales by counting rings formed in the wax ear plugs has been more successful than the others, and an age of forty years at physical maturity has been estimated for Blue Whales in this way.
With a body so beautifully proportioned it is not surprising that cetaceans are the fastest swimmers in the sea. Dolphins can swim at 20-22 knots for sustained periods, and even the huge Sperm Whale has been recorded doing 20 knots in a short burst. Although the Sei Whale can apparently speed along at 35 knots on occasions, the larger baleen whales probably cruise at between 5 and 12 knots.
The skin of dolphins has a remarkable property of being able to dampen vibrations and turbulence at its surface and thereby decrease resistance. This effect, together with the body streamlining and powerful tail oscillations, enables high speeds to be achieved with comparatively little effort.
Many sea-going New Zealanders will have enjoyed the company of dolphins around their boat at some stage. Bow-wave riding is a favourite pastime for dolphins, and it is a profitable one too, for they are carried along at a good pace for virtually no effort. A dolphin will position itself in the bow-wave of a ship and hold its flukes either slightly up or down depending on the type of wave page 14 (blunt bows force water down, sharp ones force it up). When the flukes are held at a slightly negative angle to the water deflecting from the bow, they act as a hydrofoil producing a downward and forward lift force. This lift pushes the dolphin along at the same speed as the ship. Although the tail flukes seldom move during wave riding, the front of the body does nod noticeably as the dolphin tries to maintain its position and keep its snout down.
In New Zealand the Common, Bottlenose, Dusky, and Hector's Dolphins all ride on bow-waves, and some manage to ride on waves in the wake of a ship. The Bottlenose is very entertaining under the bow, and may ride upside-down for several minutes.
Cetaceans dive to considerable depths, mainly in search of food. Sperm Whales, which feed on deepwater squids, are known to reach 3,000ft, and stay under for one and a half hours. After such a deep dive the whales spend about ten minutes at the surface breathing about six times a minute before diving again. Dolphins are generally not deep divers and usually stay under for only up to five minutes. A Bottlenose Dolphin has been known, however, to dive to 900ft and back in four minutes. The plankton-feeding baleen whales dive for shorter periods (10-15 mins) to depths of about 300ft.
Deep-diving cetaceans do not suffer from decompression sickness (the bends) which afflicts human divers. This happens when nitrogen from air in the lungs, which has dissolved into the bloodstream under pressure, bubbles out of solution as the diver reaches lower pressures near the surface and causes blockages in blood capillaries which sometimes results in death. As cetaceans dive, their lungs are compressed and air is forced into the windpipe and nasal passages, thus preventing the absorption of nitrogen through the lung walls. A foamy mucus in the air sinuses may also assist by absorbing excess nitrogen. Good oxygen storage in the blood, a slowing of the heart beat, and the constriction of all but vital blood vessels, compensate for the fact that oxygen as well as nitrogen is squeezed from the lungs during deep dives.
Cetaceans sleep in the sea either floating horizontally or obliquely with the blowhole just at the surface. Their breathing rhythm is reduced, and they may lie motionless or give a few slow strokes with the tail, and even open their eyes periodically.
Although the Sperm Whale apparently sleeps so soundly as to be a hazard to shipping, most cetaceans probably only doze in short ‘naps.’ Whales and dolphins extend conscious control over breathing, and for this reason captive animals cannot be totally anaesthetised during operations without fear of a complete breathing stoppage.
Almost 2,000 cetaceans have stranded on the New Zealand coast since records were begun about 100 years ago. In the past much useful biological information has been obtained from stranded animals, and today museums are still very interested in examining specimens and skeletons, for much remains to be learnt from them.
Common stranding sites in New Zealand are the northern coasts of Cook Strait, the Hawke Bay-Mahia area, the Hauraki Gulf, and the Chatham Islands. Baleen whales are rare stranders but always do so singly, and probably when they are sick or dying. Some toothed whales, on the other hand, strand relatively often, and en masse. The commonest mass stranders are Pilot Whales, False Killer Whales, and Sperm Whales. Dolphins strand rarely.
Species which strand in mass are those which normally travel in social groups under leaders, and a ‘herd instinct’ could play a major part in these strandings. It is generally believed that once a group of whales have stranded, they will refuse to swim off when returned to the water. This could happen when only a few individuals of a large herd are set free, for the instinct to remain with the rest of the herd may be overpowering. When most of the herd is returned to the water however, refloating may be successful — as in the case of thirty-six out of forty Pilot Whales which stranded in Hawke Bay in 1967. In most instances, the problem of moving a large number of heavy animals prevents attention being given to more than one or two individuals.