Cetaceans are the mammals of the order Cetacea; a diverse group that includes whales, dolphins, and porpoises. The name “cetacean” comes from the Greek word for whale, κέτος (kétopos). Cetaceans are aquatic mammals; they live in the water their entire lives.
Cetaceans evolved from land-dwelling mammals about 50 million years ago. They share a common ancestor with artiodactyls, a group that includes pigs, deer, and cattle. Cetaceans are thought to have evolved from a group of Artiodactyla known as mesonychids. Mesonychids were relatively small animals, similar in size to modern coyotes or foxes. They had hooves instead of nails, and were well-adapted to life on land. However, they also spent time in water, and may have been semi-aquatic. Over time, mesonychids became more and more specialized for an aquatic lifestyle. They began to resemble modern cetaceans in their skeletal structure and physiology.
The first cetaceans were small animals; the largest was about the size of a modern dolphin. Over time, they grew larger and larger until some reached sizes exceeding 30 meters (100 feet). The blue whale is the largest animal on Earth today; it can reach lengths of over 30 meters (100 feet) and weigh over 150 metric tons (330,000 pounds).
Cetaceans are air-breathing mammals; they must surface periodically to breathe through blowholes located on the tops of their heads. While most cetaceans are obligate carnivores, meaning that they require animal protein to survive, some do eat significant amounts of plants material. Baleen whales strain huge quantities of seawater through their baleen plates to capture tiny prey such as krill and copepods. Toothed whales use their conical teeth to seize fish or squid.
Most cetaceans live in saltwater oceans but several species inhabit freshwater rivers and lakes including: beluga whales in the Arctic Ocean; bottlenose dolphins in Shark Bay Australia; Amazon river dolphins throughout South America; Irrawaddy dolphins off Myanmar (Burma); pink river dolphins throughout Central America & South America plus China & Tibet; Yangtze finless porpoise only within China’s Yangtze River basin – sadly this species is now critically endangered with perhaps just 1% remaining compared with historical populations due largely to pollution & heavy fishing within its very limited range but also because it gets caught incidentally by fishermen targeting other species such as Chinese sturgeon & paddlefish elsewhere along China’s rivers.. In addition two species – tucuxi & boto – both freshwater river dolphins inhabit much of Amazonia while another – Orcaella brevirostris – inhabits brackish coastal waters around parts Indonesia & Philippines plus estuaries down into freshwaters of Vietnam’s Mekong delta region.. Freshwater cetaceans tend be smaller than those inhabiting marine environments although there are exceptions e