694 Mammals, Biodiversity of Cetacea The discovery of Eocene fossil whales confirms an early branching of the Cetacea from the primitive ungulates, closely related to modern artiodactyls The 11 families of whales, dolphins, and porpoises show a wide diversity of body size from the finless porpoise, Neophocoena phocoenoides, at 30 kg, to the world’s largest mammal, the blue whale, B musculus, at 150 metric tons There is an equally large diversity of diet from tiny shrimp or krill to seals, fish, and squid – practically anything that swims Social structure is variable and ranges from solitary individuals to the communal pods of the Orca, Orcinus orca, or killer whale, whose social system more closely resembles that of wolves than other whales Cetaceans share many adaptations critical to true aquatic living The skulls are ‘telescoped,’ with the premaxillary and maxillary bones forming the roof of the skulls and the occipital bones forming the back of the skull In all whales the tail fin is horizontal Whales are divided into two suborders, the Mysticeti, or baleen whales, with 13 recent species in four families, and the Odontoceti, or toothed whales, which includes 71 species in seven families Baleen whales, which include the aforementioned blue whale, are filter feeders Incongruously, these leviathans live on tiny prey, from krill and small planktonic copepods, often less than cm in length, to small, schooling fish Migratory patterns across entire oceans from summer to winter feeding grounds are not uncommon, with many baleen whales being truly cosmopolitan species Baleen whale populations, severely decreased by aggressive hunting during the nineteenth and twentieth centuries, are still in the process of recovering The toothed whales in the family Odontoceti are found in all oceans and all seas Some of the smaller members of the family are also found in rivers and lakes on all continents except Australia Like bats, the toothed whales all use echolocation, allowing them to hunt and live in turbid waters where visibility is low Although echolocation ‘pings’ or clicks are used for locating prey, longer, more continuous tones are also important for communication among conspecifics The toothed whales include three species of sperm whale, including the giant sperm whale (Physter catodon, family Phsteridae) made famous by Herman Melville in Moby Dick; six species of Phocoenidae, or porpoises, found throughout the Northern Hemisphere and in near-shore environments of South America; 21 species of the poorly known beaked whales (Ziphiidae), some of whose species have never been seen alive; and two species of highly gregarious Monodontidae, limited to the Arctic oceans, the beluga whale (Delphinapterus leucas), or white whale, and the narwhal (Monodon monoceros) The narwhal has a long, spirally grooved single upper tooth, used by males in intrasexual competition, which may provide the origin of the unicorn myth Sirenia The five species of the families Dugongidae (with two species: the Dugong, Dugong dugon, and Steller’s Sea Cow, Hydrodamalis gigas) and Trichechidae (with three species of manatee) are the sole modern representatives of the Sirenia An anomalous, nearly hairless group of mammals weighing up to 1600 kg, the common name for these animals,‘sea cows,’ is remarkably accurate The only large, fully aquatic mammal that lives on grasses, the sirenids have extremely long intestines Like perissodactyls, cellulose fermentation takes place in the hindgut, enabling the sea cows to process large volumes of relatively coarse seagrass vegetation Reproduction is both delayed and slow, with long interbirth intervals and the production of a single offspring that remains with its mother for up to years Sirenids have a near-global distribution in tropical and subtropical waters Manatees are found up the Amazon and in major river systems of West Africa They suffer little competition for their main resource, aquatic grasses Yet, despite this, all species are threatened with extinction These large mammals are docile, represent a large package of meat to human hunters, and are supposed to taste good This, combined with slow reproduction, has led to overharvesting throughout their range Pollution, dam building, and injury from the propellers of motorboats all amplify the problems of overhunting Proboscidea Elephants were widely spread across North America, Asia, and Africa until recent times Throughout the Tertiary, proboscideans showed a relatively wide range of adaptations and a diversity of species Today, only three species remain: the African bush and forest elephants (Loxodonta africana and Loxodonta cyclotis, respectively) and the Asian elephant (Elephas maximus) These hulking creatures are the largest terrestrial mammals, weighing up to six metric tons Although the Asian elephant is now limited to the forest of Asia, this probably reflects a historical shift from open grassland and woodland habitat, which has now almost universally been converted to human agricultural use In Africa, elephants are found throughout the forests, woodlands, and savannas, but at highest densities in savanna woodlands Some ecologists believe the split of the two African elephants, once defined as subspecies, reflects a more recent fragmentation of populations, with a former, clinal structure of genetic variation now lost In both Africa and Asia, elephants are ecological keystone species, one of the few species, other than humans, having a direct impact on the form and structure of the environment in which it lives Elephants keep savanna ecosystems from reverting to woodland when they knock down trees ‘Catastrophic’ destruction of trees may occur in droughts but is clearly part of a long-term natural cycle In Asia and Africa, the fruits and seeds eaten by elephants are transported and then defecated, seeding the plains and the forests and depositing the seeds in their own fertilizer Elephants, fossil and modern, have a highly specialized dentition with six cheek teeth on either side of the jaw At any one time, one to two teeth are exposed in the jaw As these teeth wear down, they are replaced by a tooth erupting from behind the current tooth This pattern of sequential replacement of grinding teeth is unique in mammals Tusks are formed from elongated upper incisors Highly social, female elephants live in extended, matriarchal groups Adolescence is prolonged, with males remaining in the herd until they are 10–15 years old and females not breeding for the first time until they are 15 years old Males are solitary, and breeding priority is determined by an