Cetaceans are a diverse group of marine mammals that include whales, dolphins, and porpoises. They are the only mammals that have evolved to live their entire lives in water. Cetaceans range in size from the small vaquita (Phocoena sinus) which is less than 2m long and weighs 60kg, to the blue whale (Balaenoptera musculus) which can grow to over 30m long and weigh 150 metric tonnes. Cetaceans are found in all oceans of the world, from cold polar waters to warm tropical seas.
The cetacean body plan is adapted for an aquatic lifestyle, with features such as a stream-lined shape for efficient swimming, skin without hair for reduced drag in water, and forelimbs that have been modified into flippers. Cetaceans breathe air through blowholes located on the top of their heads – a feature that helps them to remain buoyant in water.
Most cetaceans feed on fish, squid, and other marine invertebrates. Baleen whales strain huge volumes of seawater through their baleen plates to trap their prey, while toothed whales capture prey using their sharp teeth. Toothed whales also use echolocation – emitting clicks and listening for the echoes bouncing back off objects – to help them navigate and find food in dark or murky waters.
Cetaceans are highly social animals that live in groups called pods. These pods can vary greatly in size, from just a few individuals up to several hundred individuals. Pod members often stay together for life, forming strong bonds with one another. Communication within pods is vital for coordinating activities such as hunting and caring for young calves. Cetaceans use a variety of vocalisations (including clicks, whistles, grunts) to communicate with each other underwater. Some species also use touch and body language as part of communication within pods .
Cetus is a taxonomic order comprising all extant and extinct carnivorous ungulates—that is land mammals with hooves but who do not fit comfortably into any other category owing to having lost some typical herbivore characteristics after evolving away from an ancestral diet containing significant amounts of meat—and consequently including many fossils traditionally placed within Artiodactyla because they share some superficial similarities due largely to convergent evolution brought about by similar lifestyles based around running at speed on land after escaping predators or chasing prey over long distances; fossil evidence suggests most early artiodactyls were at least opportunistic scavengers if not outright carnivores anyway so this shift did not require much change anatomically speaking since many primitive artiodactyls were already well equipped with large canine teeth used primarily for display purposes rather than tearing flesh like those seen on later representatives of this clade such as pigs or hippopotamuses. The earliest known member of this group was Arctocyon, an arboreal forest dweller from Late Paleocene North America 55 million years ago, thoughmost members had emerged by the Early Miocene 20 million years ago.