Cetacea (Whales, Porpoises and Dolphins)

Abstract

Abstract Cetaceans (whales and dolphins) are streamlined aquatic mammals that spend all their lives in water. They are all carnivorous, taking either many small prey by bulk filter‐feeding (Mysticeti, baleen whales), or larger prey by echolocation‐assisted hunting (Odontoceti, dolphins and toothed whales). The main living groups, Mysticeti and Odontoceti, arose from archaic whales – Archaeoceti – some 35 Mya. Cetaceans have been distinct for more than 50 My. Their closest relatives are the hoofed mammals, artiodactyls, such as hippos and cows. Cetaceans include the largest living animals, and range through all oceans and into some rivers. Their active aquatic lifestyle makes them difficult to study. Developments in electronic data‐gathering, tissue analyses, genetic sequencing and phylogenetic analyses, and discoveries of new fossils, have hugely expanded recent understanding. Most of the diversity of living cetaceans (currently 87 species) is concentrated in the oceanic dolphins – Delphinidae (36 species), Ziphiidae (beaked whales, 21 species) and Balaenopteridae (rorquals, 8 species). Key Concepts: Cetaceans form a monophyletic group of marine mammals, with three divisions: the living mysticetes (filter‐feeders), odontocetes (echolocators) and the extinct archaeocetes. Currently, 87 species are recognised: 14 mysticetes (baleen whales) and 73 odontocetes (dolphins, porpoises and toothed whales). Cetaceans are difficult to study because of their habits, size and rarity, so that, in terms of conservation status, many species are data‐deficient and of uncertain population trend. As a group, cetaceans range through all oceans, and into some estuaries and rivers; individual species tend to be regionally or hemispherically restricted. Molecular (genetic) studies of phylogeny will continue to change concepts of relationships, leading to revised classifications at species and genus level. The earliest known fossil cetaceans lived more than 50 Mya, in what is now India and Pakistan. Cetaceans are related closely to hoofed mammals (artiodactyls), particularly the hippopotamus.