Volume 4 of the Handbook of the Mammals of the World (HMW) covers 19 families in three orders. Although united here, their grouping as “Sea Mammals” is more practical than phylogenetic. Mammals that occupy oceanic habitats have traditionally been placed together, so we have grouped the three families containing the earless seals, the eared seals, and the Walrus (Odobenus rosmarus), commonly called pinnipeds meaning “fin-footed,” with the sirenians (manatees, Trichechus spp., and the Dugong, Dugong dugon) and cetaceans to form this volume. Phylogenetically, the pinnipeds belong with the order Carnivora, covered in Volume 1 of this series. The sirenians are evolutionarily distant from both pinnipeds and cetaceans, but ecological similarity argues for keeping these three groups together in one volume.

We begin the volume with a special chapter on sea mammals and conservation, in the context of marine conservation and climate change because these are topics of such growing concern. Another topic of concern and considerable controversy involves the classification of the order Cetacea. We follow the traditional arrangement exemplified by the Third Edition of Mammal Species of the World (MSW), published in 2005. Nevertheless, we have made several improvements and updates. The views of the authors, all of whom are leading authorities, have been incorporated into this volume. We have added descriptions of new species and ongoing systematic revisions, which continue to add to our knowledge of the phylogenetic relationships of the families covered by this volume. The Systematics section in each family account reviews the ongoing taxonomic work and recent research using new molecular techniques, which have revolutionized our ability to analyze evolutionary relationships.

Recently, there is strong molecular evidence for close evolutionary relationship between cetaceans and the family Hippopotamidae in the traditional order Artiodactyla. This has led to some well-reasoned arguments for combining the two groups into a single order, Cetartiodactyla. We think that most of our readers will find it easier to use the volume as it is arranged, but with the caveat that an extensively modified classification of higher categories of mammals may soon be accepted. For the latest versions of this ongoing study to properly outline the phylogenetic relationships of all sea mammals, we recommend visiting the website of The Society for Marine Mammalogy (http://www.marinemammalscience.org/).

Pinnipeds are found in coastal waters worldwide. The eared seals of the family Otariidae contain fur seals and sea lions. They live in all oceans except the North Atlantic and are better adapted to locomotion on land than their phocid relatives because they can rotate their hindflippers under their bodies. Like phocids, they have an amphibious lifestyle, breeding on land but spending most of their time in the water. Highly specialized carnivores, they feed on fish, crustaceans, and cephalopods, and have adapted to a wide variety of coastal marine habitats.

The Odobenidae is perhaps the most specialized, with only a single living species, the Walrus which inhabits the arctic and subarctic waters of the Northern Hemisphere. Walruses are huge, with bodies almost as large in circumference as in length and with small heads perched on top. Both sexes have elongated canine teeth forming distinctive tusks. When swimming, their hindflippers propel them as they search along the bottom for food. Ashore, Walruses can walk on all four limbs, using their tusks to help move their heavy bodies. Like phocids, Walruses lack external ear flaps. Unlike male otariids and phocids, adult male Walruses are almost entirely naked, with only scattered hairs over the body. Nevertheless, the skin is very thick to protect them when fighting.

The last of the pinniped families, the earless seals of the family Phocidae are more common in temperate and polar waters than in tropical seas. True seals, as they are also known, are more adapted to ocean life than their relatives the eared seals and fur seals. They can swim long distances, dive deeply for long periods of time, and are generally much more at home in the water than on land.

The bulk of the volume is devoted to the cetaceans, by far the largest group of mammals inhabiting the oceans of the world full time. The right whales belong to the family Balaenidae, which consists of four species in two genera living in all but tropical waters. One genus contains only the Bowhead (Balaena mysticetus). The three species of true right whales belong to the genus Eubalaena. Their curiously arched upper jaw contains exceptionally long baleen plates, which they use to filter krill and small crustaceans from the water. Their stocky appearance and large heads give them a distinctive look among the larger cetaceans. The name right whale comes from early whalers, who considered them the “right” whale to hunt.

The sole member of the family Neobalaenidae, the Pygmy Right Whale (Caperea marginata), is a “right whale” in name only, because it is related to a fossil group known as cetotheres and is probably more closely related to rorquals and the Gray Whale (Eschrichtius robustus) than to right whales. It is restricted to the Southern Ocean and comes in contact with the tips of the southern continents. It is easily the smallest of the baleen whales.

Another monotypic family, the Eschrichtiidae, contains only the Gray Whale. Now restricted to the northern Pacific Ocean where it is a familiar migrant along the west coast of North America, the distribution of the Gray Whale extends across the Arctic to an endangered population in the western Pacific Ocean off the coast of Asia. The population of Gray Whales in the eastern Pacific Ocean breeds off the coast of Mexico, where they are popular with whale watchers.

The largest of the cetaceans belong to the family Balaenopteridae, present in seas worlwide. There are seven species of rorquals (Balaenoptera spp.), or large baleen whales, plus the Humpback Whale (Megaptera novaeangliae). The family includes the largest living mammal, the Blue Whale (B. musculus), which can weigh up to 150 tons. This family suffered mightily from the 19^th century whaling industry, and several species are now listed as endangered on The IUCN Red List.

The magnificent Sperm Whale (Physeter macrocephalus) is the only species in the family Physeteridae. Sperm Whales were hunted extensively in the 19^th century for ambergris and spermaceti oil. The lower jaw contains large teeth that are also prized items in many cultures. Herman Melville’s Moby Dick was a Sperm Whale. They were indeed among the most dangerous whales to hunt. Despite their persecution, Sperm Whales persist throughout the world’s deep-water oceans.

The Pygmy Sperm Whale (Kogia breviceps) and the Dwarf Sperm Whale (K. simus) were once placed in the family Physeteridae, but they now are considered a separate family, the Kogiidae. They are much smaller and not really all that similar morphologically to Sperm Whales. The family is cosmopolitan, occurring in both temperate and tropical oceans. One characteristic they do share with their larger relative is the presence of a spermaceti organ.

With 22 species in six genera, the wide-ranging pelagic family Ziphiidae, or Beaked Whales, are among the most diverse of cetacean families. Even though they make up about one-quarter of known cetacean species, they remain among the most poorly understood groups because of their pelagic habits. Their conservation status is especially difficult to assess because most of our knowledge comes from the occasional stranded individual that washes up on a beach. An example of recently recognized species is Mesoplodon hotaula, a name resurrected from the synonymy of Mesoplodon ginkgodens for seven specimens from scattered islands in the Indian and Pacific oceans. Described as a new species in 1963 only because the describer was unaware of the earlier description of M. ginkgodens, the description turned out to be curiously prescient when recent molecular studies confirmed the existence of both species.

Interestingly, several species of dolphins have adapted to freshwater habitats in widely disparate regions of the world and find their closest relatives among the wide-ranging pelagic beaked whales. They were once thought to belong to two families, Platanistidae and Iniidae, but we now recognize four families that are known collectively as freshwater dolphins.

Platanistidae contains the single species the South Asian River Dolphin (Platanista gangetica), from the Ganges and Indus rivers. These dolphins are nearly blind and probably rely largely on echolocation to navigate and find food.

The family Iniidae offers a striking illustration of how even presumably very well known species may turn out to be composite. Amazon River dolphins had been thought to belong to a single species, Inia geoffrensis (the MSW arrangement), or more recently, to two species, by also recognizing the Bolivian Boto (I. boliviensis). Nevertheless, in 2014, a third species, the Araguaian Boto (I. araguaiaensis), was described from an isolated river in central Brazil. Because river dolphins are among the most rare and endangered mammals in the world, the discovery of a new species is exciting news indeed.

The family Lipotidae contains only a single species, the Baiji (Lipotes vexillifer), found in the middle and lower reaches of the Yangtze River in China. The Baiji is endangered because of industrial development in China, pollution of the rivers, and commercial fishing. A recent survey of the species resulted in no sightings, and it may be extinct.

Rounding out the river dolphin families, the family Pontoporiidae contains only the Franciscana (Pontoporia blainvillei). This species is not a true freshwater dolphin, although it does penetrate into the La Plata River and estuary. It is primarily a coastal marine species, ranging as far as 30 miles offshore along eastern South America.

Two of the more curious cetaceans, the Narwhal (Monodon monoceros) and the Beluga (Delphinapterus leucas), compose the family Monodontidae. Both species are restricted to arctic and subarctic waters in the Northern Hemisphere. Narwhals are the unicorns of the sea, with the males bearing a single long tusk that extends straight out in a long spiral from the blunt snout. Belugas are the only completely white whales and travel in pods of up to several hundred individuals.

With 36 species in 17 genera, Delphinidae clearly is the most diverse family of cetaceans. It also is arguably the most complex taxonomically. This oceanic dolphin family includes some of the most familiar cetaceans, the bottlenose dolphins (Tursiops spp.) and the Killer Whale (Orcinus orca). Delphinids occupy almost all marine waters, from coastal zones to deep-sea habitats. Although we present the current consensus arrangement of species here, this family is among the most likely to undergo changes in classification in the near future.

The family Phocoenidae, the porpoises, includes seven species in four genera. Porpoises are primarily coastal animals and occur throughout the oceans of the Northern Hemisphere, along the coast of South America, and in circum-antarctic waters. They extend into South-east Asia, where some penetrate into a few river systems. Although the most familiar species, such as the Harbor Porpoise (Phocoena phocoena), are found primarily in bays and estuaries, some species range into deep water.

Lastly the sirenians comprise perhaps the most unusual of marine mammals. As the name suggests, they bring to mind ancient tales of mythical sea creatures. The family Trichechidae, the manatees, contains three species, widely separated into Caribbean, Amazonian, and West African populations. The other family, Dugongidae, contains only a single living species, the Dugong, restricted mostly to Australian and Indo-Pacific waters. A recently extinct species, Steller’s Sea Cow (Hydrodamalis gigas), was restricted to North Pacific waters. All living sirenians are slow-moving, gentle creatures that feed on sea grasses in shallow coastal waters and rivers.

Finally, we offer a word or two about different conventions in this volume because marine mammals have unique characteristics compared with all other mammals. In preceding volumes, we routinely used head-body length, but for cetaceans, the most common length measurement is total length, measured from the tip of the upper jaw to the notch in the flukes. Similarly, a word of caution about the distribution maps is in order. For terrestrial mammals, we have more confidence in species distributions because they normally adhere to given habitats across relatively small areas. For ocean-going species, we present distributions as shape maps covering vast areas of ocean waters, even though we assume that the animals are not uniformly distributed across such areas. Many distributions of marine mammals are known primarily from observations along coastlines, or from individuals stranded on beaches. Pelagic sightings suggest broad distributions for many species, but the details remain sketchy for many of them.

Don E. Wilson