Whales, Dolphins, and Porpoises: Cetacea
WHALES, DOLPHINS, AND PORPOISES: Cetacea
Cetaceans (sih-TAY-shunz) are whales, dolphins, and porpoises. These mammals live, eat, reproduce, and rest in the water. They range in size from that of a small human—5 feet (1.5 meters) and 110 pounds (50 kilograms)—to huge, building-sized animals of 110 feet (33 meters) and 400,000 pounds (180,000 kilograms). Their ancestors were land mammals. More than fifty million years ago, these ancestors evolved physical characteristics that allowed them to live successfully in the water. Today scientists believe that the closest living land-based relative of whales, dolphins, and porpoises is the hippopotamus.
All cetaceans share certain physical characteristics that allow them to live their entire life in the water. Most notably, they all have streamlined, smooth, bodies to cut down on friction and turbulence as they move through the water. This streamlining has come about because the bones in their front legs are shortened and compressed to form paddles called flippers that have no fingers or claws. In addition, their back legs are so reduced that all that remains are a few internal pelvic bones. Likewise, they have no external reproductive organs. Male cetaceans have a retractable penis, which means that they can draw it up inside their body. The nipples of the female are also hidden in a slit within their belly.
The need to be streamlined has affected the shape of the skull and the sense organs found it in. The bones of the skull and the jawbones have become elongated, stretched out. The nostrils, usually on the front of the face in land mammals, have moved to the top of the head and are called blowholes. There can be one or two blowholes, depending on species, or a single slit on the top of the head. Blowholes are connected to the lungs and can be closed to keep out water when the animal dives.
Cetaceans have no external, outside, ears, although they have very good hearing. Sound is transmitted to the internal ear through bones. Most members of this order have good eyesight, although some species that live in cloudy water have lost most of the ability to see. Cetaceans use a complex system of communication and are thought to be highly intelligent. They have large brains in proportion to their body size.
All members of this order are hairless, they may have a few hairs at birth, but have a thick layer of oil and fat called blubber under the skin. They are warm-blooded; their core body temperature stays about the same as that of a human, even in cold Arctic waters. Cetaceans have no sweat glands. They regulate their temperature by controlling the amount of blood flowing through their flippers and fins, which are not covered with blubber.
Members of this order are known for their ability to make deep dives and remain underwater for long periods. Sperm whales have been known to dive more than 6,080 feet (1,853 meters). They have an efficient circulatory system that allows them to store and retrieve large amounts of oxygen in their blood and muscle tissue. In addition, when they dive, they reduce blood flow to their skeletal muscles, decreasing oxygen use in the muscles while keeping blood flow to the brain. Finally, when they dive, they expel, push out, the air in their lungs. Reducing the amount of air in the lungs helps them withstand the high pressure that occurs when they dive deeply.
WHY DON'T WHALE CALVES DROWN WHEN THEY NURSE?
Whale calves must nurse from their mothers while in the water. How can the baby suckle and not suck huge amounts of water into its lungs when it breathes? The answer lies in an adaptation to aquatic life. Unlike land mammals where air and food share a single passage into the body, the digestive system and the breathing passage of the whale are separate. The whale's blowhole leads directly to the lungs, while the mouth and esophagus, throat, lead only to the stomach. This allows the whale calf to eat and breathe at the same time.
Although all cetaceans have common characteristics that suit them to life in the water, different species have evolved physical and behavioral features that allow them to eat certain foods or inhabit specific zones. There are two suborders of whales, each with identifying physically characteristics. Mysticeti are the baleen (buh-LEEN or BAY-leen) whales. These whales have no teeth. To feed, they filter large amounts of water through flexible plates in their mouth called baleen. The baleen strains out krill, small shrimp and plankton, which they collect with their tongue and swallow. This suborder includes the largest whales on Earth.
Odontoceti, the other suborder of whales, all have teeth that they use to catch fish, squid, octopus, and marine mammals such as seals, dolphins, and other whales. They are often referred to as toothed whales to distinguish them from baleen whales. These whales use echolocation (eck-oh-loh-KAY-shun) to navigate and find prey. Echolocation involves making sounds that bounce off objects. Sense organs pick up the echo or reflected sound and use the timing, direction, and strength of the echo in order to locate objects. In some species, echolocation is so sensitive that it can locate an object less than 0.5 inches across (1.25 centimeters) at a distance of 50 feet (15 meters). Unlike toothed whales, baleen whales do not have a highly developed sense of echolocation.
This order also contains porpoises and dolphins. These animals are smaller than most whales, and some dolphins and porpoises live in fresh water rivers rather than in salt water. Strictly speaking, porpoises belong to only one family and are distinguished by their spade-shaped teeth. However, casual language makes little distinction between the terms porpoise and dolphin.
Cetaceans are found in all oceans of the world. In the Arctic and Antarctic they avoid ice-covered water, since they must rise to the surface to breathe. Dolphins live in the ocean, but are also found in several freshwater rivers in Asia and South America.
The ocean is divided into different zones or regions based on depth, closeness to land, and underwater features. Cetaceans inhabit virtually all ocean zones, including zones in semi-enclosed water such as the Red and Black Seas. Cetaceans that live in freshwater rivers inhabit clear, rapidly flowing water and dark muddy water.
Members of this order are primarily carnivores, meat eaters. Baleen whales have evolved special filter-like structures to gather small shrimp, small fish, squid, and plankton. Other cetaceans actively hunt prey, either alone or in cooperative groups. Typically they eat whatever fish are found in the oceanic zone that they inhabit. Many also eat squid, octopus, shrimp, and crabs. A few species, especially the killer whale, hunt other whales, seals, sea lions, sea turtles, and sea birds.
BEHAVIOR AND REPRODUCTION
Cetaceans generally have pregnancies that last ten to sixteen months. Like all mammals, they nurse their young. The young tend to stay with their mothers for at least a year and often much longer. Many cetaceans give birth only every two to five years. These animals do not become capable of reproducing for about three to ten years. Large whales may live for close to 100 years and are slow to mature.
Cetaceans have evolved a wide spectrum of behaviors. Some species such as the spinner dolphin are known for the way they leap out of the water, while other species, like almost all porpoises, rarely jump when they come to the surface. Some members of this order live in groups of up to one thousand, while others live in groups of ten or fewer animals. Some groups show great social stability and communication. Killer whales, for example, are known to hunt in packs. Other social groups are simply casual associations, with members coming and leaving at will. Communication seems to involves several different types of sounds combined with echolocation.
CETACEANS AND PEOPLE
People have been fascinated with cetaceans from the earliest times. These animals have figured in stories and mythology in many countries. Perhaps the best known example is the biblical story of Jonah being swallowed by a whale.
Whales have been hunted for their oil, meat, baleen, and bones for hundreds of years. As sailing and hunting technologies improved, increasing pressure was put on some whale species. Whaling, whale hunting, reached its peak in 1847 when about 700 American ships, along with ships from many other nations, took part in whale hunts. In 1935, the United States and several European countries entered into the first international agreement to protect certain species of whales. Since then, there have been other international agreements, all of which have loopholes that allow at least some whale hunting to continue. In 1972, the United States passed the Marine Mammal Protection Act. This legislation extended protection to all cetaceans as well as other marine mammals such as seals, sea lions, and sea otters. Today, whale hunting, along with regulation of other types of fishing, continues to be a source of international tension.
Other pressures on cetaceans include being trapped and exhibited, put on display, for entertainment. Many tourist destinations offer visitors the opportunity to swim with dolphins in confined areas, and businesses trap wild dolphins for this purpose. In addition, the United States Navy trains dolphins to retrieve potentially dangerous materials from under water.
Interest in protecting cetaceans is high, and several organizations such as the American Cetacean Society and the Whale and Dolphin Conservation Society in Great Britain work hard at promoting conservation awareness among the public. Public pressure has lead to the development of "dolphin-safe" fishing nets and "dolphin-free" tuna, but many cetaceans are still drowned when they accidentally become trapped in fishing gear. Estimates of populations of different species are difficult to make, but the population of many species appears to be declining. Some, such as the baiji, a Chinese river dolphin, are Critically Endangered, facing an extremely high risk of extinction.
FOR MORE INFORMATION
Carwadine, Mark, and Martin Camm. Smithsonian Handbooks: Whales Dolphins and Porpoises. New York: DK Publishing, 2002.
Gowell, Elizabeth T. Whales and Dolphins: What They Have in Common. New York: Franklin Watts, 2000.
Mead, James G., and Joy P. Gold. Whales and Dolphins in Question: The Smithsonian Answer Book. Washington, DC: Smithsonian Institution Press, 2002.
Nowak, Ronald. M. "Order Cetacea." In Walker's Mammals of the World Online 5.1. Baltimore: Johns Hopkins University Press, 1997. http://www.press.jhu.edu/books/walkers_mammals_of_the_world/cet acea/cetacea.html (accessed on July 8, 2004)
Whale and Dolphin Conservation Society. P.O. Box 232, Melksham, Wiltshire SN12 7SB United Kingdom. Phone: (44) (0) 1225 354333. Fax: (44) (0) 1225 791577. Web site: http://www.wdcs.org.