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Cetaceans

Cetaceans

Mysticeti: Baleen whales

Odontoceti: Toothed whales

Anatomy and physiology

Sensory perception

Social behavior

Pregnancy and birth

Intelligence and communication

Commercial whaling and other threats

Resources

Human contact with cetaceanswhales, dolphins, and porpoiseshas a rich history, beginning with some of our earliest civilizations. Although ancient people believed cetaceans were fish, they are actually aquatic mammals, which means they bear live young, produce milk to feed their offspring, and have hair (albeit just a few sensory hairs). The Greek philosopher Aristotle (384322 BC) was the first to record this fact; in his Historia Animalium, Aristotle noted that whales and dolphins breathe air through a blowhole, and therefore have lungs; and that instead of laying eggs like fishes, they deliver their offspring fully developed.

Modern biologists believe that life first appeared in the sea; from these marine beginnings, land-dwelling organisms such as mammals gradually evolved. Cetaceans have returned to the marine environment after an ancestral period on land. As evidence of their terrestrial pedigree, consider that a whale fetus possesses four limb buds, a pelvis, tail, and forelimbs with five fingers like any land mammal. Adult whales and dolphins have the streamlined, fish-like appearance befitting their watery existence, but they have maintained and modified key terrestrial features (e.g., a much-reduced pelvic girdle in the tail, and forelimbs now used as flippers for swimming). A blowhole atop the head (one in dolphins, two in whales) replaces the nostrils, and thus the passageways for food and air are completely separate, as opposed to the usual terrestrial condition, in which food and air partly share a common tube. Other anatomical changes in cetaceans include a reduced neck, sensory modifications, and the addition of a thick layer of blubber to insulate against the cold of the ocean depths and to provide extra energy stores.

The order Cetacea is divided into three suborders. The Archaeoceti are a group of extinct cetaceans with elongated bodies, and are known only from fossils that are still being discovered and described. The living cetaceans are the Mysticeti or baleen whales, 10 species restricted to the ocean, and the Odontoceti or toothed whales, whose many species (including dolphins and porpoises) are found in diverse habitats ranging from deep oceans to freshwater rivers great distances from the sea. Cetologists (scientists who study cetaceans) still disagree about how many toothed whales may be distinguished, but at least 68 species are recognized.

Mysticeti: Baleen whales

Baleen whales include the great whales or rorquals, a word that comes from the Norse for grooved whale, owing to the conspicuous grooves or pleats on the throat and belly of these huge animals. There are seven rorquals, including the blue whale (Balaenoptera musculus ), the largest creature that has ever lived. The largest blue whale ever recorded, according to the Guiness Book of World Records, was a female caught in 1926 off the Shetland Islands, measuring 109 ft (33.3 m) long. The blue whale belongs to the family Balaenopteridae, a group that migrates from summer feeding grounds in cold polar waters to breed in warmer waters in the fall and winter. Additional species are the fin whale (B. physalus ), sei whale (B. borealis ), Brydes whale (B. edeni ), minke whale (B. acutorostrata ), humpback whale (Megaptera novaeangliae ) and gray whale (Eschrictius robustus ).

Rounding out the baleen whales are the Balaenidae, three genera that include the right whale (Balaena glacialis ), the bowhead whale (B. mysticetus ), and their elusive cousin, the pygmy right whale (Caperea marginata ). Many cetologists recognize three subspecific forms of the right whale, and some argue that the southernmost of these deserves recognition as a separate species, the southern right whale (B. australis ).

Although baleen whales are very large, they subsist on some of the smallest creatures: tiny oceanic plankton, or krill. Baleen whales are filter feeders, straining the water to collect their microscopic food, swallowing them in vast numbers; one mouthful of water may net its owner tens or hundreds of thousands of these tiny prey, which are trapped on the baleen as the seawater rushes back out. The baleen are rows of flexible, horny plates suspended from the upper jaw, each one fringed with a mat of hair-like projections to create an effective filtering device. Baleen was used for many years in ladies corsets and other fashions, and thus was given the misleading name whalebone. It is not bone at all, but rather the tough, flexible protein called keratin. As a group, baleen whales are larger and slower moving than toothed whales, perhaps in part because they do not need to pursue their prey.

Odontoceti: Toothed whales

In contrast, the faster-moving, smaller-bodied toothed whalesincluding dolphins and porpoises pursue squid, fishes of many sizes, and in the case of killer whales (Orcinus orca ), sea birds and mammals, including other cetaceans. Many toothed whales travel in groups of from five to many dozens of animals, whose purpose seems to be in part to hunt cooperatively. Killer whales have been observed to gang up on

and kill larger whales such as gray whales. They also collectively hunt seals resting on ice floes. Once a seal is spotted, the whales will dive together, causing a great wave that upsets the ice, dumping the unfortunate seal into their midst. (Interestingly, there is no record of a killer whale having killed a human.) Cooperative behavior has also been suggested for several dolphin species; bottlenose dolphins (Tursiops truncatus ) have been observed to circle a school of fish, causing them to group more tightly together, and then take turns lunging through the school, grabbing mouthfuls of fish as they pass. Perhaps in these species, individuals hunting together can catch more food than each would hunting alone. This is probably not true of the plankton-eating baleen whales.

There are many solitary species of odontocetes, however, each with innovative solo feeding strategies. Odontocete teeth are generally conical in shape, except in porpoises; these animals have spade-shaped teeth, and they lack the dolphins protruding rostrum, or beak. However, the bouto (Inia geoffrensis ) of the Amazon and Orinoco rivers in South America has rear teeth that resemble molars, used for crushing the armored catfish that is among its favorite foods. The susu river dolphins (Platanista minor and P. gangetica ) of the Indian subcontinent, have long, pincer-like jaws for grabbing prey out in front of them. The largest odontocete, the sperm whale (Physeter catodon ), is also the deepest diver. These animals have been observed diving to 4, 000 ft (1, 220 m), and have been captured with squid inhabiting depths of 10, 000 ft (3, 050 m) within their stomachs; however, their average dives are probably around 1, 100 ft (335 m).

Anatomy and physiology

The sperm whales deep dives raise interesting questions about cetacean anatomy and physiology. Even the shallower dives performed regularly by many cetaceans would jeopardize the health of a human diver. One important issue for a diving animal is keeping warm in the cold depths. All cetaceans have a thick layer of blubber insulating them from the frigid water. In addition, a diving cetacean is aided by the increasing pressure of the greater depths, which reduces blood circulation automatically. Blood flow to peripheral body areas is further reduced by proximity to the cold water, keeping the warm blood circulating to the internal organs. It is still a mystery that the cetacean brain can function normally at great depths; the heartbeat drops during a long dive, but somehow the brain maintains its normal temperature.

Another important issue to a diving animal is oxygen deprivation. Many rorquals routinely stay under for 30 minutes before surfacing for a breath. Longer dives are always followed by a certain amount of panting at the surface, so the whale can restore its depleted blood oxygen levels. This is not the whole story, of course, since no land animal could hope to match this feat of breath-holding. Cetacean muscle tissue contains much greater amounts of myoglobin, the oxygen-binding protein found in the muscles of all mammals. This means that ounce for ounce, cetacean muscle is capable of storing more oxygen where it is needed most, even when new oxygen is not being provided via the lungs. In addition, cetaceans apparently have a high tolerance for the waste products (lactic acid and carbon dioxide) that accumulate in working muscle in the absence of sufficient oxygen.

Of special interest to people who dive for recreation is the dangerous phenomenon known as the bends. Human divers take to the depths with a tank of compressed air, whose pressure equals or exceeds that of the surrounding water. Otherwise, our relatively feeble chests would collapse under the pressure of the surrounding water. The bends occurs when nitrogen gas present in the compressed air dissolves into our blood and tissues, forming bubbles when the pressure is reduced too rapidly upon ascending. How do cetaceans avoid this deadly condition? Upon diving, their remarkably flexible chests and small, elastic lungs collapse; the tiny pouches that absorb oxygen within the lungs (the alveoli) are forced shut and gas exchange ceases. This means that little or no nitrogen is transferred into the bloodstream, and the dangerous bubbling-up of dissolved gas does not occur upon resurfacing.

In a swimming cetacean, the tail is the main source of forward propulsion, pushing the animal forward by an up-and-out movement against the water (rather than side-to-side, like a fish). The tail is a flexible extension of the last vertebra, which supports the muscular flukes. Rolling and changing position in the water is accomplished by the flippers; the flippers and dorsal fin (which is not present in all species) act together as stabilizers. Cetacean bones are heavier than water, but the body floats easily owing to buoyant blubber, oil in the bones, and air in the lungs. Some great whale species have been observed to sleep for hours, usually at night, their heads passively rising clear of the water to expose the blowhole. Both eyes are closed, and the body floats motionless with tail and flippers hanging limply, while the whale breathes once or twice per minute with a brief, snorting exhale. Amazingly, the smaller dolphins, who have more to fear from sharks and other predators, appear to sleep with only half of their brain at a time; one eye remains open, enabling the animal to rouse itself should danger arise.

Sensory perception

Cetologists have been able to infer a good deal about the sensory powers of whales and dolphins. Their vision is good, but is limited to about 45 ft(13.7 m) or so, even in the clearest water; the depths of the ocean are quite dark, and vision is of no use. Their sense of hearing is much more important, in part because water is such an excellent conductor of sound. In addition, many cetaceans navigate and find food using echolocation, or sonar.

An echolocating animal perceives objects in its path by listening for the reflected echoes of pulsed sounds that it produces. In keeping with this practice, the cetacean hearing range is much greater than ours; some species can hear sounds up to 180, 000 Hz. The sound is produced in a complex chamber in the airway atop the head, and is conducted out through the melon, a waxy, lens-shaped structure in the forehead. The melon functions to focus the beam of sound the way a magnifying glass focuses a beam of light. We know that around a dozen species of toothed whales and dolphins use sound to find food items; for instance, blindfolded bottlenose dolphins can find fish swimming in their tanks. Baleen whales lack the sophisticated structures for true echolocation, but may use echoes from lower-frequency sounds for a more rudimentary echonavigation.

Scientists once believed that cetaceans had no sense of taste or smell. More recently, bottlenose dolphins have been shown to distinguish the four basic taste stimuli (sour, sweet, salty, and bitter). In addition, beluga whales (Delphinapteras leucas ) have been observed to show alarm when swimming through areas where other belugas have been killed and quantities of blood are present in the water. Several cetacean species are sensitive to substances found in mammalian urine and feces, which could provide information on the identity or status of other individuals. Most modern cetologists agree that taste and smell are important to many cetaceans.

The cetacean sense of touch is very keen, as becomes obvious to anyone who watches two familiar animals interacting: whales and dolphins large and small rub up against each other, stroking and petting one another with flukes or flippers. Such touching clearly feels good to the recipient, and captive dolphins have been trained to do various tricks using touch alone as the positive reinforcement.

Scientists have suggested the existence of a cetacean magnetic sense; this would help to explain their remarkable navigational powers during long migrations in the otherwise featureless marine environment. In support of this possibility, the mineral magnetite has been found in the brains of some species (including common dolphins, Dalls porpoise, humpback whales, and beaked whales). A magnetic sense could help explain the bizarre phenomenon of live stranding; although rare, stranding is generally fatal to the whale, which is ultimately crushed by its own weight out of the supporting water. A magnetic sense might not be fool-proof, and could be upset by various disturbances from on shore, leading the animals to beach themselves. Others have proposed that mass strandings of cetaceans are the result of the intense social bonds that form among members of some species. Perhaps the urge to avoid the dangers of separation is stronger than that to avoid the fatal risk of stranding along with a sick or injured comrade seeking shallow water.

Social behavior

Social behavior in cetaceans runs the gamut from species that are largely solitary to those that are highly social. Baleen whales are rarely seen in groups of more than two or three; however, gray whales and other rorquals may form transient groups of five to 50 animals during migration. Among toothed whales, river dolphins and narwhals (Monodon menoceros ) are examples of species that appear to have mainly solitary habits.

The most highly social species are found among the toothed whales. In bottlenose dolphins and killer whales, for instance, individuals typically have social bonds with many others, which may last for life. In these species, females form the core of the society: long-term bonds between females and their adult daughters are important in many aspects of life, including foraging, fending off predators and aggressive dolphins, and delivering and raising the young. For example, sperm whale mothers often leave their infants in the company of a babysitter, while they make the deep dives for squid where a baby could not follow. In bottlenose dolphins, social alliances between adult males are a prominent feature of society. Trios of adult male bottlenose dolphins perform together in synchronous aggressive displays, herding a sexually receptive female, or attempting to dominate rival males from similar alliances.

Courtship and mating remains largely undescribed for many species, owing to the difficulty of observing events under water; systematic study of most species is simply lacking. The mating systems of many that have been studied are classified as promiscuous, meaning that individuals select a new mating partner each year, and both males and females may mate more than once in a given season.

Some cetacean speciesfor example, narwhals, killer whales, and sperm whalesexhibit rather pronounced sexual dimorphism in size, meaning that males are noticeably larger than females. Such a size difference is believed to indicate a relatively high degree of competition among males for a chance to mate. These species are assumed to be polygynous, meaning that a few males mate with most of the available females, excluding the rest of the males for mating opportunities. Male-male fights in the presence of a receptive female may become fierce, and adult males often bear the physical scars of such contests. Adult male narwhals have a long, spiral tusk made of ivory growing out from their head, which they use in jousting and sparring with their competitors.

Pregnancy and birth

Most baleen whales first mate when they are four to 10 years of age. Many toothed whales take longer to mature, and in sexually dimorphic species, males take longer still. Sperm whales require seven to 12 years, killer whales eight to 10, and false killer whales (Pseudorca crassidens ) need up to 14 years to mature.

Gestation in the baleen whales lasts 1013 months on average. Many toothed whales have similar gestation times but some are longer: pilot, sperm, and killer whale pregnancies last up to 16 months. The birth of a baby has rarely been witnessed by humanstypically, a formerly pregnant female simply appears one day with her new infant at her side. On rare occasions, however, a human observer has been lucky enough to see a birth. One laboring gray whale spent the last 10 minutes of her labor hanging vertical in the sea, head down, with her flukes held 6 ft (1.8 m) out of the water. As she lowered her flukes to a horizontal position, the calfs snout was seen to be emerging from her belly. The mother shifted to a belly-up position, just at the surface, as the calf continued emerging; then the mother submerged, and the calf popped up to the surface, separate from its mother. Bottlenose dolphins are thought to usually deliver their calves tail-first, but head-first deliveries have been observed in captivity. In a captive situation, the dolphin mother typically uses her tail and rostrum (beak) to guide the baby to the surface for its first breath of air.

In all species, the baby soon takes up the infant position below and to her side, near her mammary slits. The nipple of a nursing mother protrudes from this slit on her belly, and the milk is ejected into the babys mouth by her mammary muscles with no effort on the part of the calf. Nursing bouts are relatively brief, since the calf must surface to breathe. Even so, the baby gains weight steadily; blue whale infants gain 200 lb (90.8 kg) per day! Blue and gray whales nurse for a period of about seven months; bottlenose dolphins nurse for three to four years or more. The record must be for pilot and sperm whales, who are occasionally observed to be nursing at 1015 years of age. Of course, these youngsters have been eating other foods as well.

Intelligence and communication

Lengthy juvenile periods are typical for animals of greater intelligence. In Greek myths and other ancient sources, whales and dolphins have been accorded the attributes of higher intelligence, congeniality, and kindness to humans. Observations of wild and captive dolphins supporting a dead baby dolphin at the surface for hours and daysprobably an instinctive act by a naturally protective motherare no doubt the source of long-standing anecdotes about dolphins saving injured human divers from drowning.

It is generally agreed that cetacean intelligence surpasses other non-human mammals, such as dogs, seals, and even many primates. Curiosity, affection, jealousy, self-control, sympathy, spite, and trick-playing are all common observations by human handlers of captive cetaceans. Their brains feature a sizeable and deeply convoluted cortex, suggesting considerable higher learning ability. Dolphins in particular are known for their powers of innovation. An especially large supralimbic area of the brain explains their excellent powers of memory and social intelligence.

Greater communication skills often accompany greater powers of intelligence. An underwater listener in the vicinity of a group of cetaceans may be surprised by the great variety of sounds they produce, from the repetitive tonal pulses of fin whales; to the moans and knocks of gray whales; the whoops, purrs, and groans of bowhead whales; and the elaborate, eerie songs of humpbacks. Cetaceans do not have vocal cords; odontocete vocalizations are produced in a group of air sacs in the region below the blowhole atop the head, and are projected out through the melon. Mysticete sounds seem to come from an area off the lower side of the larynx, but the exact origin is unclear. Many of the social odontocetes emit whistles, and intense whistling seems to accompany excitement surrounding feeding, sex, and the joy of riding a wave before a speeding boat. Individuals may recognize one other from their unique whistles and other sounds.

Male humpback whales, who sing primarily during the breeding season, are the greatest balladeers of all cetaceans. In addition to their melodic, haunting songs, humpback whales produce a harsh gurgling noise, something like the sound a drowning person might make. Their vocalizations, which may go on for hours, were audible through the wooden hulls of old whaling vessels. Superstitious sailors, hearing these voices out of the deep below their ship, believed they were the ghosts of drowned comrades, coming back to haunt their old vessel.

Commercial whaling and other threats

Cetaceans have been harvested on an individual basis by native peoples, including Inuit, in many parts of the world since before recorded time. Commercial whaling was underway in earnest by the twelfth century, when the Basque people of the French and Spanish coasts harvested whales harpooned from small boats, called shallops, in struggles lasting many hours. Such struggles were worth the risks, because a single whale yields enormous amounts of valuable commodities. The victorious whalers returned with huge quantities of meat and blubber, which was rendered down into valuable oil for fuel and lubricants. By the eighteenth century, commercial whaling was a burgeoning industry, notably for baleen, to be used in ladies corsets.

Modern whaling methods are viewed by many outside the industry as grossly inhumane. Whales today are killed by a harpoon whose head has four claws and one or more grenades attached; when a whale is within range, the harpoon is fired into its body, where the head explodes, lacerating muscle and organs. The whale dives to escape, but is hauled to the surface with ropes and shot again. Death may not be swift, taking 15 minutes or more.

Commercial whaling inflicted a devastating blow on the worlds baleen whale populations until 1986; most populations have yet to recover, and many populations and some species of whales are endangered. Public outcry resulted in reduced catch quotas during the 1970s, and finally in 1982 the International Whaling Commission set a moratorium on commercial whaling (to which all nations complied by 1989). Since then, however, Iceland, Norway, and Japan have demanded resumption of whaling; when the IWC refused, Norway announced plans to resume anyway, and Iceland left the IWC. Commercial whaling continues, although at a much reduced scale compared to former times.

Further dangers to cetacean populations include marine pollution and loss of food resources due to human activity. Drift nets hanging invisible in the water cause the death of cetaceans along with seals, seabirds, and fish; nets that are torn free during storms may drift at sea for many years. Thus, although the use of drift nets was halted by 1992, their effects persist. Meanwhile, purse-seine fishing methods for tuna have killed an estimated seven million dolphins since 1959.

Resources

BOOKS

Evans, P.G.H., and J.A. Raga, eds. Marine Mammals: Biology and Conservation. New York: Kluwer Academic/Plenum, 2001.

Hoelzel, A.R., ed. Marine Mammal Biology: An Evolutionary Approach. Oxford, U.K.: Blackwell Scientific, 2002.

Mann, Janet, et al., eds. Cetacean Societies: Field Studies of Dolphins and Whales. Chicago: University of Chicago Press, 2000.

Perrin, W.F., B. Wrsig, and J.G.M. Thewissen, eds. Encyclopedia of Marine Mammals. San Diego, CA: Academic Press, 2002.

Reeves, R.R., et al. National Audubon Society Guide to Marine Mammals of the World. New York: Knopf, 2002.

Twiss, J.R., Jr., and R.R. Reeves, eds. Conservation and Management of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Susan Andrew

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