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Cetacean: the story on HearLore | HearLore
Cetacean
The direct ancestors of today's cetaceans are found within the Dorudontidae, a group that lived at the same time as the massive Basilosaurus. These ancient creatures had already developed the fixed bulla, a sound-conducting element for submerged directional hearing, and their wrists were stiffened to contribute to the typical build of flippers. While they still possessed hind legs, these were significantly reduced in size and connected to a vestigial pelvis. The transition from land to sea began approximately 49 million years ago with the Ambulocetus, or running whale, which was up to 15 meters long. This animal had leg-like limbs but was already fully aquatic, indicating that a switch to a lifestyle independent from land happened extraordinarily quickly. The snout was elongated with overhead nostrils and eyes, and the tail was strong and supported movement through water. Ambulocetus probably lived in mangroves in brackish water and fed in the riparian zone as a predator of fish and other vertebrates. By the late Eocene, about 40 million years ago, cetaceans populated the subtropical oceans and no longer emerged on land. The transition from land to water was completed in about 10 million years, leaving behind the Wadi Al-Hitan, or Whale Valley, in Egypt, which contains numerous skeletons of Basilosaurus and other marine vertebrates.
The Evolutionary Paradox of Hippos
Molecular biology, immunology, and fossils show that cetaceans are phylogenetically closely related with the even-toed ungulates, or Artiodactyla. Whales' direct lineage began in the early Eocene, around 55.8 million years ago, with early artiodactyls. Most molecular biological evidence suggests that hippos are the closest living relatives. Common anatomical features include similarities in the morphology of the posterior molars, and the bony ring on the temporal bone, known as the bulla, and the involucre, a skull feature that was previously associated only with cetaceans. Since the fossil record suggests that the morphologically distinct hippo lineage dates back only about 15 million years, Cetacea and hippos apparently diverged from a common ancestor that was morphologically distinct from either. The most striking common feature is the talus, a bone in the upper ankle. Early cetaceans, archaeocetes, show double castors, which occur only in even-toed ungulates. Corresponding findings are from Tethys Sea deposits in northern India and Pakistan. The Tethys Sea was a shallow sea between the Asian continent and northward-bound Indian plate. Cetaceans are deeply nested within the artiodactyls, and the two groups together form a clade, a natural group with a common ancestor, for which the name Cetartiodactyla is sometimes used. Modern nomenclature divides Artiodactyla into four subordinate taxa: camelids, pigs and peccaries, ruminants, and hippos plus whales.
When did the transition from land to sea begin for Cetacean?
The transition from land to sea began approximately 49 million years ago with the Ambulocetus. This animal had leg-like limbs but was already fully aquatic, indicating that a switch to a lifestyle independent from land happened extraordinarily quickly. The transition from land to water was completed in about 10 million years.
What is the closest living relative of Cetacean?
Molecular biological evidence suggests that hippos are the closest living relatives of Cetacean. Cetaceans are deeply nested within the artiodactyls, and the two groups together form a clade known as Cetartiodactyla. Common anatomical features include similarities in the morphology of the posterior molars and the bony ring on the temporal bone known as the bulla.
How large can the largest Cetacean species grow?
The blue whale reaches a maximum confirmed length of 29.9 meters and a weight of 173 tonnes, making it the largest animal known to have existed. This species is part of the Mysticeti or baleen whales, which contains 15 species of large whales. Most mysticetes prefer the food-rich colder waters around the poles of the Northern and Southern Hemispheres.
When did commercial whaling operations begin to extensively hunt Cetacean?
Since the Industrial Revolution, Cetacean have been extensively hunted by humans for their meat, blubber and oil by commercial whaling operations. In the 18th and 19th centuries, baleen whales especially were hunted for their baleen, which was used as a replacement for wood. In the 1930s, 30,000 whales were killed, and this increased to over 40,000 animals per year up to the 1960s.
How long do wild Cetacean live compared to those in captivity?
In the wild, females who survive infancy live 46 years on average and up to 70 to 80 years, while wild males who survive infancy live 31 years on average and can reach 50 to 60 years. Captive whales have reduced life expectancy, on average only living into their 20s, although some live longer, including several over 30 years old and two, Corky II and Lolita, in their mid-40s.
Why do Cetacean strandings occur due to human activity?
Underwater noise from military sonar and vessel traffic can disrupt Cetacean behavior such as their use of biosonar for orientation and communication. Severe instances can panic them, driving them to the surface, which leads to bubbles in blood gases and can cause decompression sickness. Since 2000, whale strandings frequently occurred following military sonar testing, with the US Navy admitting partial responsibility for the beaching and deaths of several marine mammals in March 2000.
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 of 50 centimeters. 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. 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.
The Symphony of the Deep
Cetaceans have excellent hearing, and the external ear has lost the pinna, but still retains a narrow ear canal. The three small bones or ossicles that transmit sound within each ear are dense and compact, and differently shaped from those of land mammals. The semicircular canals are much smaller relative to body size than in other mammals. A bony structure of the middle and inner ear, the auditory bulla, is composed of two compact and dense bones, the periotic and tympanic. It is housed in a cavity in the middle ear; in all toothed whales, with the exception of sperm whales, this cavity is filled with dense foam and completely surrounds the bulla, which is connected to the skull only by ligaments. This may isolate the ear from sounds transmitted through the bones of the skull, something that also happens in bats. Baleen whales have exceptionally thin, wide basilar membranes in their cochleae without stiffening agents, making their ears adapted for processing low to infrasonic frequencies. Cetaceans use sound to communicate, using groans, moans, whistles, clicks or the singing of the humpback whale. Toothed whales are generally capable of echolocation. They can discern the size, shape, surface characteristics, distance and movement of an object. Because of this they can search for, chase and catch fast-swimming prey in total darkness. Echolocation clicks also contain characteristic details unique to each animal, which may suggest that toothed whales can discern between their own clicks from those of others. While differences in ear structure associated with echolocating abilities are found amongst Cetacea, cranial asymmetry has also been found to be a factor in the ability to produce sounds used in echolocation. Mysticeti, who don't have the ability to echolocate, possess general symmetry of the skull and facial region, while Odontoceti display a nasofacial asymmetry that is linked to their echolocating abilities. Differences in the level of asymmetry also seem to correlate with differences in the types of sounds produced. The brain to body mass ratio in some odontocetes, such as belugas and narwhals, is second only to that seen in humans. In some whales, however, it is less than half that of humans. Sperm whales have the largest brain mass of any animal on Earth, averaging 6.9 kilograms and 9.2 kilograms in mature males. The neocortex of many cetaceans is home to elongated spindle neurons that, prior to 2019, were known only in hominids. In humans, these cells are thought to be involved in social conduct, emotions, judgment and theory of mind. Cetacean spindle neurons are found in the same areas of the brain where they are found in humans, suggesting they perform a similar function.
The Giants of the Ocean
Many species migrate seasonally over vast ranges for food advantages. The blue whale reaches a maximum confirmed length of 29.9 meters 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. 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.
The Shadow of Human Industry
Since the Industrial Revolution, cetaceans have been extensively hunted by humans for their meat, blubber and oil by commercial whaling operations. In the 18th and 19th centuries, baleen whales especially were hunted for their baleen, which was used as a replacement for wood, or in products requiring strength and flexibility such as corsets and crinoline skirts. In addition, the spermaceti found in the sperm whale was used as a machine lubricant and the ambergris as a material for pharmaceutical and perfume industries. In the second half of the 19th century, the explosive harpoon was invented, leading to a massive increase in the catch size. Large ships were used as mother ships for the whale handlers. In the first half of the 20th century, whales were of great importance as a supplier of raw materials. Whales were intensively hunted during this time; in the 1930s, 30,000 whales were killed. This increased to over 40,000 animals per year up to the 1960s, when stocks of large baleen whales collapsed. Most hunted whales are now threatened, with some great whale populations exploited to the brink of extinction. Atlantic and Korean gray whale populations were completely eradicated and the North Atlantic right whale population fell to some 300 to 600. The blue whale population is estimated to be around 14,000. The first efforts to protect whales came in 1931. Some particularly endangered species, such as the humpback whale, which then numbered about 100 animals, were placed under international protection and the first protected areas were established. In 1946, the International Whaling Commission was established, to monitor and secure whale stocks. Whaling of 14 large species for commercial purposes was prohibited worldwide by this organization from 1985 to 2005, though some countries do not honor the prohibition. The stocks of species such as humpback and blue whales have recovered, though they are still threatened. The United States Congress passed the Marine Mammal Protection Act of 1972 to sustain the marine mammal population. Japanese whaling ships are allowed to hunt whales of different species for ostensibly scientific purposes. Aboriginal whaling is still permitted. About 1,200 pilot whales were taken in the Faroe Islands in 2017, and about 900 narwhals and 800 belugas per year are taken in Alaska, Canada, Greenland, and Siberia. About 150 minke are taken in Greenland per year, 120 gray whales in Siberia and 50 bowheads in Alaska, as aboriginal whaling, besides the 600 minke taken commercially by Norway, 300 minke and 100 sei taken by Japan and up to 100 fin whales taken by Iceland. Iceland and Norway do not recognize the ban and operate commercial whaling. Norway and Japan are committed to ending the ban.
The Tragedy of Captivity
Whales and dolphins have been kept in captivity for use in education, research and entertainment since the 19th century. Beluga whales were the first whales to be kept in captivity. Other species were too rare, too shy or too big. The first was shown at Barnum's Museum in New York City in 1861. For most of the 20th century, Canada was the predominant source. They were taken from the St. Lawrence River estuary until the late 1960s, after which they were predominantly taken from the Churchill River estuary until capture was banned in 1992. Russia then became the largest provider. Belugas are caught in the Amur Darya delta and their eastern coast and are transported domestically to aquaria or dolphinaria in Moscow, St. Petersburg and Sochi, or exported to countries such as Canada. They have not been domesticated. As of 2006, 30 belugas lived in Canada and 28 in the United States. 42 deaths in captivity had been reported. A single specimen can reportedly fetch up to US$100,000. The beluga's popularity is due to its unique color and its facial expressions. The latter is possible because while most cetacean smiles are fixed, the extra movement afforded by the beluga's unfused cervical vertebrae allows a greater range of apparent expression. The orca's intelligence, trainability, striking appearance, playfulness in captivity and sheer size have made it a popular exhibit at aquaria and aquatic theme parks. From 1976 to 1997, fifty-five whales were taken from the wild in Iceland, nineteen from Japan and three from Argentina. These figures exclude animals that died during capture. Live captures fell dramatically in the 1990s and by 1999, about 40% of the forty-eight animals on display in the world were captive-born. In captivity, they often develop pathologies, such as the dorsal fin collapse seen in 60 to 90% of captive males. Captives have reduced life expectancy, on average only living into their 20s, although some live longer, including several over 30 years old and two, Corky II and Lolita, in their mid-40s. In the wild, females who survive infancy live 46 years on average and up to 70 to 80 years. Wild males who survive infancy live 31 years on average and can reach 50 to 60 years. Captivity usually bears little resemblance to wild habitat and captive whales' social groups are foreign to those found in the wild. Critics claim captive life is stressful due to these factors and the requirement to perform circus tricks that are not part of wild orca behavior. Wild orca may travel up to 160 kilometers in a day and critics say the animals are too big and intelligent to be suitable for captivity. Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of stress. Orcas are well known for their performances in shows, but the number of orcas kept in captivity is small, especially when compared to the number of bottlenose dolphins, with only forty-four captive orcas being held in aquaria as of 2012. In 2013, SeaWorld's treatment of orcas in captivity was the basis of the movie Blackfish, which documents the history of Tilikum, an orca at SeaWorld Orlando, who had been involved in the deaths of three people. The film led to proposals by some lawmakers to ban captivity of cetaceans, and led SeaWorld to announce in 2016 that it would phase out its orca program.
The Silent Threat of Noise
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. The military and resource extraction industries operate strong sonar and blasting operations. Marine seismic surveys use loud, low-frequency sound that show what is lying underneath the Earth's surface. Vessel traffic also increases noise in the oceans. Such noise can disrupt cetacean behavior such as their use of biosonar for orientation and communication. Severe instances can panic them, driving them to the surface. This leads to bubbles in blood gases and can cause decompression sickness. Naval exercises with sonar regularly results in fallen cetaceans that wash up with fatal decompression. Sounds can be disruptive at distances of more than 100 kilometers. Damage varies across frequency and species. Since 2000, whale strandings frequently occurred following military sonar testing. In December 2001, the US Navy admitted partial responsibility for the beaching and the deaths of several marine mammals in March 2000. The coauthor of the interim report stated that animals killed by active sonar of some Navy ships were injured. Generally, underwater noise, which is still on the increase, is increasingly tied to strandings; because it impairs communication and sense of direction. Climate change influences the major wind systems and ocean currents, which also lead to cetacean strandings. Researchers studying strandings on the Tasmanian coast from 1920 to 2002 found that greater strandings occurred at certain time intervals. Years with increased strandings were associated with severe storms, which initiated cold water flows close to the coast. In nutrient-rich, cold water, cetaceans expect large prey animals, so they follow the cold water currents into shallower waters, where the risk is higher for strandings. Whales and dolphins who live in pods may accompany sick or debilitated pod members into shallow water, stranding them at low tide. Heavy metals, residues of many plant and insect venoms and plastic waste flotsam are not biodegradable. Sometimes, cetaceans consume these hazardous materials, mistaking them for food items. As a result, the animals are more susceptible to disease and have fewer offspring. Damage to the ozone layer reduces plankton reproduction because of its resulting radiation. This shrinks the food supply for many marine animals, but the filter-feeding baleen whales are most impacted. Even the Nekton is, in addition to intensive exploitation, damaged by the radiation. Food supplies are also reduced long-term by ocean acidification due to increased absorption of increased atmospheric carbon dioxide. The CO2 reacts with water to form carbonic acid, which reduces the construction of the calcium carbonate skeletons of food supplies for zooplankton that baleen whales depend on.