Order Cetacea
dolphins, porpoises, and whales

The order Cetacea comprises two extant sub-orders and one extinct sub-order. The extant sub-orders are Mysticeti (baleen whales) and Odontoceti (toothed whales). Both mysticetes and odontocetes are thought to be descendants of archaeocetes (Archaeoceti, ancient whales), an extinct sub-order. There are at least 83 living species of cetaceans, with 46 genera in 14 families. Of the two extant suborders, Odontoceti is larger and more diverse, with at least 70 species, 40 genera, and 10 families. Cetaceans, along with bats, are considered some of the most derived mammals on the planet. They evolved from terrestrial animals to an entirely aquatic life form that is completely separated from the land in all aspects of biology. Cetaceans live, breed, rest, and carry out all of their life functions in the water. (Gingerich et al., 2001; Reeves et al., 2002; Rice, 1984)

Cetaceans inhabit all of the world's oceans, as well as some freshwater lakes and rivers in South America, North America, and Asia. Some species, such as killer whales (Orcinus orca) are found in all of the world's oceans. Others are limited to one hemisphere (Antarctic minke whales) or ocean (Pacific white-sided dolphins). Still others have highly restricted ranges. For example, vaquitas (Phocoena sinus) only occur in the northern part of the Gulf of California. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans are exclusively aquatic. Most are species are marine, inhabiting coastal areas as well as open ocean. A few species inhabit freshwater rivers and lakes. Others live in the brackish waters of estuaries and coastal marshes. (Nowak, 1999; Rice, 1984)

Traditionally, cetaceans were thought to have arisen from mesonychians, an extinct order of hooved carnivorous mammals. There was debate over which modern order of hooved mammals is most closely related to mesonychians, and therefore, to cetaceans (O'Leary and Geisler (1999) summarize this debate). Certain shared morphological characteristics: paraxonic limbs, large lacrimals, presence of three bronchi, long incrudal crus breve, and smooth muscle-based penile erection, suggest a close relationship between cetaceans and artiodactyls (even-toed hoofed ungulates) (O'Leary and Geisler 1999). Molecular studies in the 1990s began to show that the order Cetacea is actually nested within the order Artiodactyla. This Cetacea + Artiodactyla clade has been given the name Cetartiodactyla. (O'Leary and Geisler, 1999; Reeves et al., 2002; Rice, 1984)

On the basis of various protein and DNA sequences, Graur and Higgins (1994) came to the conclusion that ruminants (i.e., cows) are the closest living relatives of whales, and that whales and ruminants form a clade exclusive of other artiodactyls (i.e., pigs and camels). However, Graur and Higgins (1994) did not include the family Hippopotamidae in their study, and later researchers determined that cetaceans and hippopotamids are each other's closest living relatives, followed by ruminants and then other artiodactyls. These researchers based their conclusions on various sets of molecular data, including IRBP and vWF gene sequences (Gatesy et al. 1999), cytochrome b and 12S rRNA sequences (Montgelard et al. 1997), y-Fibrinogen gene sequences (Gatesy 1997), SINES (Shimamura et al. 1997), and milk casein genes (Gatesy et al. 1996). (Gatesy et al., 1996; Gatesy et al., 1999; Gatesy, 1997; Graur and Higgins, 1994; Montgelard, Catzeflis, and Douzery, 1997; Shimamura et al., 1997)

In the face of such overwhelming molecular evidence, taxonomists have been forced to re-examine fossil and morphological data that seems to favor a cetacean-mesonychian clade. Mesonychians lacked specialized ankle bones that are characteristic of artiodactyls, and until recently, it was thought that cetaceans also lacked these bones. However, recently discovered fossils show that early cetaceans do possess ankle bones similar to those of artiodactyls (Gingerich et al. 2001; Thewissen et al. 2001). Furthermore, Boisserie et al. (2005) examined 80 morphological characteristics of extant and extinct cetartiodactyls and determined that hippos evolved from a group known as the anthracotheres, and that this hippos-anthracothere clade--not Mesonychia--appears to be sister to Cetacea. An anthracothere origin for Hippopotamidae would fill in the 30 million year gap in the fossil record between the origin of whales in the Eocene and the origin of hippos in the Miocene. (Boisserie, Lihoreau, and Brunet, 2005; Gingerich et al., 2001; Thewissen, Williams, and Hussain, 2001)

Relationships within Cetacea have also been a source of controversy over the years. Based on morphological evidence, some authors came to the conclusion that Cetacea is in fact paraphyletic, and proposed independent origins for mysticetes and odontocetes (i.e., Yablokov 1964). Molecular and morphological studies published since then have rejected this hypothesis, but controversy has continued over the position of sperm whales (Physeteridae) within Cetacea. Based on mitochondrial gene sequences, Milinkovitch et al. (1993) formulated the hypothesis that the family Physeteridae is more closely related to mysticete whales than to odontocete whales, rendering the suborder Odontoceti paraphyletic. Since then, a large number of papers on cetacean phylogeny have been published, and the physeterid-mysticete clade has been rejected on the basis of both molecular (Nikaido et al. 2001, Cassens et al. 2000; Gatesy et al. 1999; Messenger and McGuire 1998) and morphological (Geisler and Sanders 2003; Messenger and McGuire 1998) evidence. The current consensus is that Cetacea is a monophyletic order consisting of two monophyletic suborders, Odontoceti and Mysticeti (Price et al. 2005). Sasaki et al. (2005) estimated that these two suborders diverged in the late Eocene, about 35 million years ago, perhaps before they had adapted to an exclusively aquatic lifestyle. (Cassens et al., 2000; Gatesy et al., 1999; Geisler and Sanders, 2003; Messenger and McGuire, 1998; Nikaido et al., 2001; Price, Bininda-Emonds, and Gittleman, 2005; Sasaki et al., 2005)

All cetaceans share a number of similarities: they have a fusiform, or streamlined body shape; paddle-shaped front limbs; vestigial hind limbs (which are within the body wall); no external digits or claws; tail flattened laterally and bearing horizontal flukes at the tip; vestigial ear pinnae; basically hairless body (some young have hair on their snouts); thick subcutaneous blubber layer filled with fat and oil; telescoped skull bones; external nares (blowhole) on the top of the head; addition of compressed vertebrae; shortening of the neck; lack of sweat glands; internal reproductive organs; 3-chambered stomach; and an airway reinforced with cartilage to the alveoli. Many of these characteristics are adaptations to reduce drag for fast swimming in an aquatic environment. Protuberances such as external ears or genitals would create turbulence and would be inefficient for an animal in the water. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans are white, black, gray, bluish-gray, or pink in color, and many are spotted, mottled, streaked, or boldly patterned. Most exhibit some countershading, tending to be lighter ventrally than dorsally. They are large animals, ranging in size from 20 to 180,000 kilograms and from 1.2 to 30 meters. Blue whales (Balaenoptera musculus) are the largest animals that have ever existed. Some species are sexually dimorphic in size. For example, female blue whales are larger than males and male bottlenose dolphins (Tursiops truncatus) are larger than female bottlenose dolphins. In a few species, such as narwhals (Monodon monoceros) and beaked whales (Ziphiidae), males have enlarged, protruding teeth that may be used in aggressive male-male encounters. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Some cetaceans are thought to be the most intelligent non-primates and many have proportionately large brains. They also have remarkably efficient lungs and circulatory systems, allowing them to dive for extended periods of time. Cetaceans use about 12% of the oxygen that they inhale, compared to 4% used by terrestrial mammals. They also have at least twice as many erythrocytes and myoglobin molecules in their blood, for efficient capture and transport of oxygen. When cetaceans dive, their heart rates slow by as much as 80 beats per minute, so their bodies use less oxygen than they would otherwise. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans are found in all climates, including regions where sea water is near freezing. Small cetaceans can cope with cold temperatures because they have high metabolic rates. Also, their flippers and flukes have a countercurrent heat exchange system, wherein heat from arterial blood warms venous blood as it returns to the heart. Large cetaceans have a small surface area to volume ratio, so they lose little heat to the surrounding environment. Both small and large cetaceans are insulated by their thick blubber layer.

Cetaceans have diploid chromosome numbers of 42 to 44. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Because of the difficulties involved with tracking and studying cetaceans, lifespans are difficult to estimate. Available estimates suggest that most species live at least two decades, and some live much longer than that. One-hundred and sixteen-year-old fin whales (Balaenoptera physalus) have been reported from the wild and bowhead whales (Balaena mysticetus) may live up to 200 years. (Carey and Judge, 2002; George et al., 1999)

All cetaceans are completely natatorial. Swimming is accomplished by pumping the tail and flukes up and down and using the flippers for stability. Mysticetes can reach speeds of up to 26 km/hr when swimming; odontocetes can swim more than 30 km/hr. Many small odontocete species gain extra speed by riding waves created by wind or by boats. Cetaceans inhale before diving; they stay underwater anywhere from a few seconds to over an hour at a time. Some of the odontocetes, such as sperm whales (Physeter catodon), regularly reach depths of over 1,500 meters. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans are found in groups that range from single individuals (Inia geoffrensis), to small, unstable associations (many mysticetes) to herds of hundreds or even thousands of individuals (some odontocetes). Odontocetes, such as bottlenose dolphins (Tursiops truncatus) are known to form lasting social bonds with individuals of both sexes. Some groups of odontocetes, like killer whale pods, form stable dominance hierarchies. Some cetacean species frequently travel in mixed-species groups. For example, Fraser's dolphins (Lagenodelphis hosei) are often observed in association with melon-headed whales (Peponocephala electra). (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Some species of cetaceans (e.g., humpback whales, Megaptera novaeangliae) are known for their seasonal, long-distance migrations between temperate feeding grounds and tropical breeding grounds. Not all cetaceans make such extensive movements; some migrate on a smaller scale and others stay within one general area for their entire lives. Daily activity patterns of cetaceans are poorly known. Most activity is observed in the daytime, of course, this is when observations are easiest to make. Spinner dolphins (Stenella longirostris) are known to feed at night. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans produce a variety of sounds. Baleen whales moan, grunt, chirp, whistle, and click to communicate; these sounds are made by the larynx. Male humpback whales (Megaptera novaeangliae) "sing" for up to 40 minutes at a time, presumably to attract females. Low-pitched moans produced by some baleen whales may be the loudest sounds produced by any animal; they carry underwater for hundreds of kilometers. Odontocetes communicate with whistles; these sounds are most likely produced by opening and closing nasal plugs. Odonotocetes also use clicks for echolocation, to navigate and to find food. They have a fatty organ called a melon on the forehead that focuses acoustic signals as they are emitted, and they receive sounds in the middle ear via the mandible. Odonocetes have a hearing range that greatly exceeds that of humans; they can perceive ultrasounds up to 120 kHz.

Though they rely mainly on sound to communicate, most cetaceans are able to see fairly well in both water and air. River dolphins (Inia, Lipotes, Platanista, Pontoporia) are the exception; there is little use for vision in the turbid waters where they reside, so their eyes are greatly reduced and some are nearly blind. Social odontocetes use touch extensively with pod-mates, which may be an important form of communication. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Mysticetes are filter feeders, using their baleen to strain plankton and other tiny organisms from the water. Odontocetes primarily feed on fish, squid, and crustaceans, though the larger species also eat aquatic birds and mammals (including other cetaceans). (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans have few natural predators, save other cetaceans (killer whales, Orcinus orca), sharks, and occasionally walruses (Odobenus rosmarus) and polar bears (Ursus maritimus), which feed on belugas trapped in ice. Small odontocetes rely on their speed and agility to escape predators. Humans prey on cetaceans throughout the world. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans have vital ecosystem roles as consumers of plankton, fish, crustaceans, cephalopods, and other aquatic animals. They are host to a range of internal parasites, including cestodes in their intestines (Tetrabothrium and Diplogonoporus), plerocercoids in their blubber and peritonea (Phyllobothrium and Monorygma), trematodes in their stomachs, livers, intestines, and sinuses, acanthocephalans in their intestines (Bolbosoma and Corynosoma), and nematodes in their stomachs and urinogenital tracts (Anisakis, Crassicauda, and Placentonema). In addition, odontocetes are parasitized by cetacean lungworms in the family Pseudaliidae.

Cetaceans are not free from external parasites, either. Cookie-cutter sharks (Isistius brasiliensis) attack them and bite off chunks of blubber, and lampreys (Petromyzon lampetra) have also been known to attack them. Cetaceans are host to small parasites such as barnacles, which live on or in their skin, and copepods, such as Balaenophilus unisetus and Pennella, which live on their skin or in their blubber. Diatoms (Cocconeis ceticola) coat the skin of mysticetes with a greenish slime. Nematodes (Odontobius ceti), protozoans, and whale lice (Cyamidae) also inhabit the skin. Some cetacean species are mutualists with animals that feed on these ectoparasites; for example, topsmelt (Atherinops affinis) consume whale lice that live on the skin of gray whales (Eschrichtius robustus). Some birds have commensal relationships with cetaceans. Seagulls often follow schools of dolphins and consume small fish stirred up by the feeding cetaceans. Remoras (Remora australis) sometimes attach themselves to the skin of cetaceans and hang on for the ride, and pilotfish (Naucrates ductor) sometimes accompany killer whales and eat scraps from their kills. (Nowak, 1999; Reeves et al., 2002; Rice, 1984)

Cetaceans may negatively impact commercial fisheries, because they compete for fish and become entangled in fishing nets. (Nowak, 1999; Reeves et al., 2002)

For centuries, cetaceans have been hunted for their meat, blubber, and oil. The late 19th and 20th centuries saw a boom in commercial whaling to provide oil for lighting and heating and facilitated by newer and deadlier technologies for hunting whales. However, in 1986 a moratorium on commercial whaling was enacted by the International Whaling Commission, and today only Norway continues a commercial harvest. Harvesting for subsistence and scientific purposes continues, however, and several countries, including Iceland and Japan, continue to hunt whales under the name of "scientific" whaling. Currently cetaceans are important for entertainment and tourist industries: captive odontocetes are trained to perform tricks for large crowds of spectators, and whale-watching boats are popular attractions for tourists wishing to catch a glimpse of cetaceans in the wild. Whales are also important cultural and mythological icons for peoples around the world. (Nowak, 1999; Price, Bininda-Emonds, and Gittleman, 2005; Reeves et al., 2002)

The IUCN lists 28 cetacean species as lower risk, 5 as vulnerable, 7 as endangered, 2 as critically endangered, and 39 as data deficient. All cetaceans are listed in CITES Appendix II, except for Lipotes vexillifer, Platanista spp., Berardius spp., Hyperoodon spp., Physeter catodon, Orcaella brevirostris, Sotalia fluviatilis, Sousa spp., Neophocaena phocaenoides, Phocoena sinus, Eschrichtius robustus, Balaenoptera acutorostrata, Balaenoptera bonaerensis, Balaenoptera borealis, Balaenoptera edeni, Balaenoptera musculus, Balaenoptera physalus, Megaptera novaeangliae, Balaena mysticetus, Eubalaena spp., and Caperea marginata, which are listed in Appendix I. Commercial whaling in the 18th, 19th, and 20th centuries took a huge toll on many populations of mysticetes, which have low reproductive rates and cannot recover quickly from overexploitation. Since whales were given legal protections in the latter half of the 20th century and commercial whaling was banned in 1986, some populations seem to be recovering, albeit slowly. Continued whaling by some nations as "scientific whaling" may still threaten some species. Many species of small odontocetes are threatened by commercial fishing operations. They become entangled in nets and drown or they are killed by explosives. Sometimes they are killed on purpose by fishermen who see them as competitors. They also risk being struck by vessels, and some wild populutions have been depleted by people capturing live animals for oceanariums. Captive breeding programs may be the only hope for such critically endangered odontocetes as Yangtze river dolphins (Lipotes vexillifer). The use of military sonar in the oceans, as well as the increase of ocean noise, is becoming more widely recognized as a serious and growing threat to cetaceans. Finally, all cetaceans face the threats of pollution and global climate change. (baiji.org Foundation, 2005; IUCN, 2004; Nowak, 1999; Reeves et al., 2002)