Superorder Cetartiodactyla
cetaceans and artiodactyls

Cetartiodactyla is a group comprised of two orders of mammals that are superficially quite different and that, until recently, were recognized as two separate monophyletic clades. These orders are Artiodactyla, the even-toed ungulates, including animals such as cows (Bovidae), camels (Camelidae), and deer (Cervidae), and Cetacea, a group of mammals that are highly specialized for an aquatic lifestyle, including whales, dolphins (Delphinidae), and porpoises (Phocoenidae). Recent molecular evidence suggests that Cetacea evolved from artiodactyl ancestors. Making Artiodactyla non-monophyletic unless Cetacea is included. Experts suggest the monophyletic clade representing artiodactyls and cetaceans be called Cetartiodactyla. (Boisserie, Lihoreau, and Brunet, 2005; Gatesy et al., 1996; Gatesy et al., 1999; Gatesy, 1997; Graur and Higgins, 1994; Milinkovitch and Thewissan, 1997; Montgelard, Catzeflis, and Douzery, 1997; Naylor and Adams, 2001; O'Leary and Geisler, 1999; Shimamura et al., 1997; Thewissen, Williams, and Hussain, 2001)

Cetartiodactyls are found the world over, from north of the Arctic Circle to the waters surrounding Antarctica. Artiodactyls are native to all continents except Antarctica and Australia, and some artiodactyls are domesticated and have been introduced around the world by humans. Cetaceans inhabit all of the world's oceans and some freshwater lakes and rivers in South America, North America, and Asia. (Nowak, 1999; Rice, 1984; Simpson, 1984)

Most artiodactyls live entirely on land and reside in a range of terrestrial habitats, such as savannah, forest, mountains, desert, and farmland. One artiodactyl family, Hippopotamidae, is semi-aquatic and can be found in freshwater lakes, ponds, streams, and rivers. Cetaceans, on the other hand, are exclusively aquatic and inhabit the world's oceans, as well as some freshwater rivers and streams. (Nowak, 1999; Rice, 1984; Simpson, 1984)

Traditionally, Cetacea and Artiodactyla were classified as two separate monophyletic orders. Most morphological and fossil data supported this classification; after all, artiodactyls have unique characteristics that were not known to be shared with cetaceans, such as a specialized ankle bone (astragalus). 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. (O'Leary and Geisler, 1999)

Molecular studies in the 1990s began to show that not only are cetaceans most closely related to artiodactyls, but that Cetacea is nested within Artiodactyla. On the basis of various protein and DNA sequences, Graur and Higgins (1994) came to the conclusion that whales and ruminants (i.e., cows) were each other's closest living relatives and formed 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). (Boisserie, Lihoreau, and Brunet, 2005; 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 exclusive of artiodactyls. If whales belong nested within Artiodactyla, then cetaceans--thought to lack astragali--are not after all closely related to mesonychians, which also lack astragali. However, recently discovered fossils show that early cetaceans do possess ankle bones similar to those of artiodactyls (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 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; Thewissen, Williams, and Hussain, 2001)

Because cetaceans are so highly specialized for their aquatic lifestyle, they bear little resemblance to their artiodactyl ancestors. They have nearly hairless, fusiform bodies. They lack hind limbs except for tiny internal pelvic vestiges, and the forelimbs are modified into streamlined flippers. The tail bears a flattened fluke. In addition, cetacean skulls are highly modified so that the nares are located on the top of the head. On the other hand, most artiodactyls are specialized for cursorial locomotion, with long, hoofed limbs, and they lack the extreme aquatic specializations found in cetaceans. Most cetartiodactyls are relatively large animals, but there is an enormous range of body sizes in this group. Blue whales, Balaenoptera musculus, are the largest animals on earth, growing over 27 meters in length and weighing over 190,000 kg, whereas the smallest artiodactyl, the lesser mouse deer (Tragulus javanicus), is just 45 cm long and weighs 2 kg. Many species of cetartiodactyls exhibit sexual dimorphism, with males larger than females or vice versa. Also, male artiodactyls often bear antlers or large horns, and some male cetartiodactyls (narwhals (Monodon monoceros), tragulids, and suids) bear large tusks. (Nowak, 1999; Rice, 1984; Simpson, 1984)

Cetartiodactyls are relatively long-lived mammals. Most species live for at least a decade in the wild, and captivity usually prolongs life expectancy by several years. Cetaceans are especially long-lived; 116-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)

Most cetartiodactyls are highly social animals. Some live in large herds or pods numbering hundreds or even thousands of individuals. Group living commonly results in the formation of dominance hierarchies among both male and female cetartiodactyls. A few species, such as greater mouse-deer (Tragulus napu) and river dolphins (Platanista) are solitary. Many cetartiodactyl species migrate seasonally or are nomadic, and some bovid species (Bovidae) are territorial. Different species vary in their timing of daily activities: some are diurnal, some are nocturnal, some are crepuscular, and some are active at any time of the day or night. (Nowak, 1999; Rice, 1984; Simpson, 1984)

Cetartiodactyls perceive the world through visual, tactile, auditory, and chemical means. Some cetaceans, the Odontoceti, navigate and hunt using echolocation. Terrestrial cetartiodactyls often communicate with scent; many species, such as those in the family Cervidae, have specialized glands for doing so. Communication in cetaceans is accomplished largely by sound, as sound waves travel well in water. Low frequency sounds produced by baleen whales (Mysticeti) may travel for hundreds of kilometers, allowing individuals to communicate with one another over great distances. (Nowak, 1999; Rice, 1984; Simpson, 1984)

As a group, cetartiodactyls consume a wide array of terrestrial and aquatic food items. Most artiodactyls are herbivores, consuming grass, leaves, bark, and other plant parts. Those in the family Suidae are omnivorous, and eat small mammals, reptiles, amphibians, insects and other invertebrates, fruit, bulbs, rhizomes, fungi, carrion, and bird eggs. Cetaceans consume plankton, fish, squid, crustaceans, and aquatic birds and mammals (including other cetaceans). (Nowak, 1999; Rice, 1984; Simpson, 1984)

Artiodactyls are an important food source for many large mammalian carnivores, notably felids, canids, and ursids. Cetaceans, on the other hand, have few natural predators, save other cetaceans (killer whales, Orcinus orca), sharks, and occasionally walruses (Odobenus rosmarus). Group living ("safety in numbers") and camouflage are two defenses often employed by cetartiodactyls against predation. (Nowak, 1999; Rice, 1984; Simpson, 1984)

Cetartiodactyls are primary, secondary, and higher-level consumers, filling roles of predator (most cetaceans) and prey (most artiodactyls). Terrestrial cetartiodactyls are plagued by ectoparasites such as fleas, lice, and bot flies. Cetaceans, though aquatic, are not free from external parasites either, and are host to barnacles, copepods, and whale lice. Both terrestrial and aquatic species host internal parasites as well, such as tapeworms, flukes, and nematodes. Interestingly, birds have evolved commensal relationships with both aquatic and terrestrial cetartiodactyls. Seagulls follow schools of dolphins and consume small fish stirred up by the cetaceans, and cowbirds follow herds of cattle and consume insects stirred up by the hooves of the artiodactyls. Also, some cetartiodactyl species are mutualists with animals that feed on their ectoparasites: topsmelt (Atherinops affinis) consume whale lice that live on the skin of gray whales (Eschrichtius robustus), while oxpeckers (Buphagus) remove fleas and other parasites from the skin of various African artiodactyls. (Nowak, 1999; Rice, 1984; Roberts, Janovy, and Jr., 2000)

Artiodactyls, especially domesticated species, sometimes carry diseases that are transmissible to humans or other domestic animals. Wild artiodactyls sometimes interfere with farming operations by eating crops. (Nowak, 1999)

Cetartiodactyls are of immense economic importance to humans. They have been hunted for thousands of years, for food, for sport, and for various body parts. They are important for ecotourism, be it a whale-watching boat off the coast of Maine or a safari on the African savannah. Several artiodactyl species have been domesticated for thousands of years and are used to produce meat, milk, leather and wool, and their dung is used as a fertilizer. Cetaceans are sometimes kept in captivity and taught to perform tricks. Many cetartiodactyls are the focus of research programs that help us to better understand many aspects of evolution, physiology, and behavior. (Nowak, 1999)

Currently, the IUCN classifies 54 cetartiodactyl species as data deficient, 146 as lower risk, 40 as vulnerable, 32 as endangered, 14 as critically endangered, 7 as extinct, and 2 as extinct in the wild. The biggest threats to many terrestrial species are habitat loss and fragmentation and overhunting. Several large cetacean species were hunted nearly to extinction until an international treaty banned commercial whaling in the 1980s. Populations of many large, commercially-valuable whales remain severely depleted. Today, cetaceans face threats associated with global climate change, which could have widespread impacts on their food supply in the near future. (IUCN, 2004)