Cetacea

Cetaceans (/sɪˈteɪʃəns/) (from Latin: cetus, lit. 'whale', from Ancient Greek: κῆτος, romanized: kētos, lit. 'huge fish') are aquatic mammals constituting the infraorder Cetacea. There are around 89 living species, which are divided into two parvorders. The first is the Odontoceti, the toothed whales, which consist of around 70 species, including the dolphin (which includes killer whales), porpoise, beluga whale, narwhal, sperm whale, and beaked whale. The second is the Mysticeti, the baleen (from Latin: balæna, lit. 'whale') whales, which have a filter-feeder system, and consist of 15 species divided into 3 families, and include the right whale, bowhead whale, rorqual, pygmy right whale, and gray whale. The ancient and extinct ancestors of modern whales (Archaeoceti) lived 53 to 45 million years ago. They diverged from even-toed ungulates; their closest living relatives are hippopotamuses and others such as cows and pigs. They were semiaquatic and evolved in the shallow waters that separated India from Asia. Around 30 species adapted to a fully oceanic life. Baleen whales split from toothed whales around 34 million years ago. The smallest cetacean is Maui's dolphin, at 1 m (3 ft 3 in) and 50 kg (110 lb); the largest is the blue whale, at 29.9 m (98 ft) and 173 t (381,000 lb). Baleen whales have a tactile system in the short hairs (vibrissae) around their mouth; toothed whales do not have vibrissae. Cetaceans have well-developed senses—their eyesight and hearing are adapted for both air and water. They have a layer of fat, or blubber, under the skin to maintain body heat in cold water. Several species exhibit sexual dimorphism. Two external forelimbs are modified into flippers; two internal hindlimbs are vestigial. Cetaceans have streamlined bodies: they can swim very quickly, with the killer whale able to travel at 56 kilometres per hour (35 mph) in short bursts, the fin whale able to cruise at 48 kilometres per hour (30 mph), dolphins able to make very tight turns at high speeds, and some species diving to great depths. Although cetaceans are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend their lives in the water of seas and rivers; having to mate, give birth, molt or escape from predators, like killer whales, underwater. This has drastically affected their anatomy to be able to do so. They feed largely on fish and marine invertebrates; but a few, like the killer whale, feed on large mammals and birds, such as penguins and seals. Some baleen whales (mainly gray whales and right whales) are specialised for feeding on benthic creatures. Male cetaceans typically mate with more than one female (polygyny), although the degree of polygyny varies with the species. Cetaceans are not known to have pair bonds. Male cetacean strategies for reproductive success vary between herding females, defending potential mates from other males, or whale song which attracts mates. Calves are typically born in the fall and winter months, and females bear almost all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively short period of time, which is more typical of baleen whales as their main food source (invertebrates) aren't found in their breeding and calving grounds (tropics). Cetaceans produce a number of vocalizations, notably the clicks and whistles of dolphins and the moaning songs of the humpback whale. The meat, blubber and oil of cetaceans have traditionally been used by indigenous peoples of the Arctic. Cetaceans have been depicted in various cultures worldwide. Dolphins are commonly kept in captivity and are even sometimes trained to perform tricks and tasks, other cetaceans aren't as often kept in captivity (with usually unsuccessful attempts). Cetaceans have been extensively hunted by commercial industries for their products, although hunting the largest whales is now forbidden by international law. The baiji (Chinese river dolphin) has become 'Possibly Extinct' in the past century, while the vaquita and Yangtze finless porpoise are ranked Critically Endangered by the International Union for Conservation of Nature. Besides hunting, cetaceans also face threats from accidental trapping and environmental hazards such as marine pollution, noise pollution and ongoing climate change. The two parvorders, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have diverged around thirty-four million years ago. Baleen whales have bristles made of keratin instead of teeth. The bristles filter krill and other small invertebrates from seawater. Grey whales feed on bottom-dwelling mollusks. Rorqual family (balaenopterids) use throat pleats to expand their mouths to take in food and sieve out the water. Balaenids (right whales and bowhead whales) have massive heads that can make up 40% of their body mass. Most mysticetes prefer the food-rich colder waters of the Northern and Southern Hemispheres, migrating to the Equator to give birth. During this process, they are capable of fasting for several months, relying on their fat reserves. The parvorder of Odontocetes – the toothed whales – include sperm whales, beaked whales, killer whales, dolphins and porpoises. Generally the teeth are designed for catching fish, squid or other marine invertebrates, not for chewing them, so prey is swallowed whole. Teeth are shaped like cones (dolphins and sperm whales), spades (porpoises), pegs (belugas), tusks (narwhals) or variable (beaked whale males). Female beaked whales' teeth are hidden in the gums and are not visible, and most male beaked whales have only two short tusks. Narwhals have vestigial teeth other than their tusk, which is present on males and 15% of females and has millions of nerves to sense water temperature, pressure and salinity. A few toothed whales, such as some killer whales, feed on mammals, such as pinnipeds and other whales.