Cetacean
Cetaceans are marine mammals belonging to the infraorder Cetacea, a secondarily aquatic clade under the order Artiodactyla that include whales, dolphins, porpoises and extinct groups such as Basilosaurus. Most cetaceans live in marine environments, particularly the pelagic zone, but some reside solely in brackish or fresh water. Having a cosmopolitan distribution, they can be found in some rivers and all of Earth's oceans. Many species migrate seasonally over vast ranges for food advantages.
Key characteristics of cetaceans are their fully aquatic life cycle, streamlined, fish-like body shape, the need to periodically surface and breath air, and exclusively carnivorous diet. All extant cetaceans are capable of echolocation.
As nektonic animals, cetaceans propel themselves through the water with powerful up-and-down movements of their tails, which have evolved into in a horizontal paddle-like fluke. Their hindlimbs have disappeared with only some vestigial skeleton of the pelvis and femurs, and their forelimbs have evolved into flippers which they use to paddle and steer. Some fast-swimming groups, most notably the smaller dolphins and porpoises, have a dorsal fin to facilitate directional stability. Cetaceans also have large brains and have high intelligence, complex social behaviour, and song-like communication. Some cetaceans have large bodies, such as the blue whale, which reaches a maximum confirmed length of and a weight of 173 tonnes, making it the largest animal known to have existed.
There are approximately 90 living cetacean species split into two parvorders: Odontoceti or toothed whales, which contains 75 species including porpoises, dolphins, the beaked whales and other predatory whales like the beluga and sperm whale, who prey upon fish, cephalopods and other marine mammals such as pinnipeds; and Mysticeti or baleen whales, which contains 15 species of large whales including the blue whale, humpback whale and bowhead whale among others, who are mostly filter-feeding planktivores using oral bristle plates known as baleen to sieve out and feed on large swarms of small invertebrates, usually crustaceans such as krill. Despite their highly modified bodies and carnivorous lifestyle, genetic and fossil evidence places cetaceans within the terrestrial even-toed ungulates, most closely related to the hippopotamids.
Since the Industrial Revolution, cetaceans have been extensively hunted by humans for their meat, blubber and oil by commercial whaling operations. Although the International Whaling Commission has agreed on putting a halt to commercial whaling, whale hunting is still ongoing, either under IWC quotas to assist the subsistence of Arctic native peoples, or in the name of "scientific research" despite a large spectrum of non-lethal methods are now available to study marine mammals in the wild. Cetaceans also face severe environmental hazards from underwater noise pollution, entanglement in ropes and nets, ship strikes, build-up of heavy metals and plastic pollutions, food scarcity due to overfishing down the food web, and anthropogenic climate change, but how much they are affected varies widely from species to species, from minimally in the case of the southern bottlenose whale to the functional extinction of baiji due to impacts of human shipping activity.
Etymology
The scientific name Cetacea comes, meaning "whale"; which in turn comes, meaning "huge fish" or "sea monster"Baleen whales and toothed whales
s and toothed whales are thought to have diverged from one another around thirty-four million years ago. Ninety extant species of whale are currently accepted, with 75 being toothed whales and the remaining 15 baleen whales.Baleen whales have bristles made of keratin instead of teeth. Gray whales feed on bottom-dwelling mollusks. Baleen whales belonging to the rorqual family use throat pleats to expand their mouths to take in small invertebrates like krill and sieve out the water. 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 around the poles 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 toothed whales include sperm whales, beaked whales, dolphins and porpoises. These whales have teeth evolved to catch fish, squid or other marine invertebrates and swallow their prey whole instead of chewing it. Tooth shape can vary between groups, with cone-shaped teeth in dolphins and sperm whales, spade-shaped teeth in porpoises, peg-like teeth in belugas, tusks in narwhals and many different shapes in the ornamental teeth of male beaked whales. The teeth of female beaked whales are hidden in the gums and are not visible, and most male beaked whales have only two short tusks. Narwhals have vestigial teeth alongside their tusk, which is present on males and 15% of females. A few toothed whales, such as some orcas, feed on marine mammals such as pinnipeds and other whales.
Toothed whales have well-developed senses. Their eyesight and hearing are adapted for both air and water, and they can echolocate by producing sounds and using their melon. Some species—such as sperm whales and beaked whales—are well adapted for diving to great depths. Several species of toothed whales show sexual dimorphism, in which the males differ from the females, usually for purposes of sexual display or aggression.
Anatomy
Cetacean bodies are generally similar to those of fish, which can be attributed to their lifestyle and similar habitat conditions. Their body is well-adapted to their habitat, although they share essential characteristics with other higher mammals. Most notably, whales have a streamlined shape, and their forelimbs are flippers. Almost all have a dorsal fin on their backs, but this can take on many forms, depending on the species. A few species, such as the beluga whale, lack them. Both the flipper and the fin are for stabilization and steering in the water. The body is wrapped in a thick layer of fat, known as blubber. This provides thermal insulation and gives cetaceans their smooth, streamlined body shape. In larger species, it can reach a thickness up to. Hind legs are not present in cetaceans, nor are any other external body attachments such as a pinna.The reproductive organs of both sexes and the mammary glands of females are sunken into the body. The male genitals are attached to a vestigial pelvis.
Sexual dimorphism is prominent in many species of toothed whale, including sperm whales, narwhals, many members of the beaked whale family and several species of the porpoise and dolphin families. In some of these species, males have developed external features absent in females that are advantageous in combat or display. For example, many male beaked whales possess tusks which are used in competition and absent in females.
Head
Whales have an elongated head, especially baleen whales, due to the wide overhanging jaw. The jaws contain either teeth or baleen in species of the respective groups. Baleen is made up of long, fibrous strands of keratin. Located in place of the teeth of the upper jaw, it has the appearance of a huge fringe and is used to sieve the water for prey items. When present, the teeth or baleen in the upper jaw sit exclusively on the maxilla.Their nostril make up the blowhole, which has one opening in toothed whales and two in baleen whales. The nostrils are located on top of the head above the eyes so that the rest of the body can remain submerged while surfacing for air. By shifting the nostrils to the top of the head, the nasal passages extend perpendicularly through the skull. The back of the skull is significantly shortened and deformed, and the braincase is concentrated forwards through the nasal passage and is heightened, with individual cranial bones overlapping. Most cetaceans have fused neck vertebrae and are unable to turn their head at all, but river dolphins retain this ability.
Many toothed whales show a depression in the front of their skull, which houses a large mass of fat known as a melon and multiple asymmetric air bags. These soft tissues aid in biosonar and buoyancy, respectively. The sperm whale has a particularly pronounced melon and the shape of their head is further changed by the presence of the spermaceti organ, which contains the eponymous spermaceti, hence the name "sperm whale".
Skeleton
The cetacean skeleton is largely made up of dense cortical bone, which stabilizes the animal in the water. For this reason, the usual terrestrial compact bones, which are finely woven cancellous bone, are replaced with lighter and more elastic material. In many places, bone elements are replaced by cartilage and even fat, thereby improving their hydrostatic qualities. The ear and parts of the snout contain a high-density bone structure that is exclusive to cetaceans and resembles porcelain. This conducts sound better than other bones, thus aiding biosonar.The number of vertebrae that make up the spine varies by species, ranging from 40 to 93. The neck consists of seven vertebrae which are reduced or fused, providing stability during swimming at the expense of mobility. The fins are carried by the thoracic vertebrae, ranging from nine to seventeen individual vertebrae. The sternum is cartilaginous. The last two to three pairs of ribs are not connected and hang freely in the body. The stable lumbar and tail include the other vertebrae. The caudal vertebrae can be identified by the chevron bone, which hangs underneath them.
The front limbs are paddle-shaped with shortened arms and elongated finger bones, to support movement. They are connected by cartilage. The second and third fingers display a proliferation of the finger members, a so-called hyperphalangy. The shoulder joint is the only functional joint in all cetaceans except for the Amazon river dolphin. The collarbone is completely absent.