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Cetacea (Whales, dolphins and porpoises)

Cetacea > General of Cetacea

Linnaeus originally assigned the name Cete to the order of mammals consisting of whales, dolphins, and porpoises. The term is derived from the classical noun cetos, meaning a large sea creature. Linnaeus conceived Cete to be the sole member of the group Mutica, one of his three primary subdivisions of placental mammals. The term Cetacea is the plural of cetos and was coined by Brisson in 1762. The study of cetaceans has come to be known as cetology, those who practice it as cetologists.

Cetacea-A spinner dolphin

The lines of demarcation between the living cetaceans and other orders of mammals are firmly drawn, and there is no ambiguity. Similarly, the two living suborders of Cetacea are unequivocally distinct from each other, but also monophyletic; that is, derived from a common ancestor. The Mysticeti, or baleen whales, and Odontoceti, or toothed whales, differ fundamentally in the ways that the bones of their skulls have become “telescoped.” The mysticete skull features a large, bony, broad, and flat upper jaw, which thrusts back under the eye region. In contrast, the main bones of the odontocete upper jaw thrust back and upward over the eye sockets, extending across the front of the braincase. Mysticetes have baleen and no teeth as adults, and they have paired blowholes (nostrils).

Odontocetes, in contrast, have teeth and no baleen (in some species, many or most of the teeth are unerupted and non-functional, however), and a single blowhole. A major additional factor in the anatomical divergence of the two groups is the development in odontocetes of a sophisticated echolocation system, which has required various unique anatomical specializations for producing, receiving, and processing sound. Mysticetes generally lack the enlarged facial muscles and nasal sacs that characterize odontocetes. Below the level of suborder, many different approaches to classification have been proposed, involving varying numbers and combinations of infraorders, superfamilies, families, and subfamilies. For simplicity here and in what follows, only families, genera, and species are considered. The present-day consensus among cetologists is that there are four extant families, six genera, and at least 14 species of mysticetes, and ten families, 34 genera, and about 72 species of odontocetes.

These numbers will inevitably change as larger samples become available and as more sophisticated analytical methods are applied. It is instructive that no less than five “new” species of cetaceans have been described over the past 15 years, including two mysticetes (Antarctic minke whale, Balaenoptera bonaerensis, and pygmy Bryde’s whale, Balaenoptera edeni) and three odontocetes (pygmy beaked whale, Mesoplodon peruvianus, spade-toothed whale, Mesoplodon traversii, and Perrin’s beaked whale, Mesoplodon perrini). Some of these represent the formal recognition and description of species long known to exist, but others are genuine discoveries. More of both types of developments are to be expected.

Vernacular uses of the terms whale, dolphin, and porpoise have always been complicated and, occasionally, confusing. All baleen-bearing cetaceans are considered whales, but any of the three terms can be applied to toothed cetaceans, depending upon a number of factors. Body size is a useful, but not definitive, basis for distinguishing whales from dolphins and porpoises. In general, cetaceans with adult lengths greater than about 9 ft (2.8 m) are called whales, but some “whales” (e.g., dwarf sperm and melon-headed; Kogia sima and Peponocephala electra, respectively) do not grow that large and some dolphins (e.g., Risso’s and common bottlenosed; Grampus griseus and Tursiops truncatus, respectively) can grow larger. There is considerable overlap in body size between dolphins and porpoises as well. Strictly speaking, the term porpoise should be reserved for members of the family Phocoenidae, all of which are relatively small (maximum length less than 8 ft [2.5 m]) and have numerous small, spatulate (spade-shaped) teeth.

The proclivity of seafarers and fishers to apply the term “porpoise” (singular and plural) to any small cetacean that they encounter has led to its rather loose application to marine dolphins by scientists as well. It is occasionally suggested that porpoises can be distinguished from dolphins by their lack of a pronounced beak (the elongated anterior portion of the skull that includes both the upper and lower jaw), but a number of dolphins are at least as blunt-headed as any porpoise. In fact, there is no strict definition of “dolphin,” as the term is equally valid for species as diverse as the very longbeaked, bizarre-looking river dolphins (superfamily Platanistoidea), the round-headed “blackfish” (pilot, false killer, and pygmy killer whales; Globicephala spp., Pseudorca crassidens, and Feresa attenuata, respectively), and the archetypal bottlenosed and common dolphins (Tursiops spp. and Delphinus spp., respectively). One other variant that often finds its way into the popular lexicon is “great whales.” In most contexts, those who use this term mean it to refer to all of the baleen whales plus the sperm whale (Physeter macrocephalus). In essence, the great whales are those that had great commercial value and therefore were seriously depleted by the whaling industry.

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