River Dolphin
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River dolphins are a
polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...
group of fully
aquatic mammal Aquatic mammals and semiaquatic mammals are a diverse group of mammals that dwell partly or entirely in body of water, bodies of water. They include the various ''marine mammals'' who dwell in oceans, as well as various freshwater species, such a ...
s that reside exclusively in freshwater or
brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuary ...
. They are an informal grouping of
dolphin A dolphin is an aquatic mammal in the cetacean clade Odontoceti (toothed whale). Dolphins belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontopori ...
s, which itself is a
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
group within the infraorder
Cetacea Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...
. Extant river dolphins are placed in two superfamilies, Platanistoidea and Inioidea. They comprise the families Platanistidae (the South Asian dolphins), the possibly extinct Lipotidae (Yangtze River dolphin), Iniidae (the Amazonian dolphins) and Pontoporiidae. There are five
extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...
species of river dolphins. River dolphins, alongside other cetaceans, belong to the clade
Artiodactyla Artiodactyls are placental mammals belonging to the order (biology), order Artiodactyla ( , ). Typically, they are ungulates which bear weight equally on two (an even number) of their five toes (the third and fourth, often in the form of a hoof ...
, with
even-toed ungulate Artiodactyls are placental mammals belonging to the order Artiodactyla ( , ). Typically, they are ungulates which bear weight equally on two (an even number) of their five toes (the third and fourth, often in the form of a hoof). The other t ...
s, and their closest living relatives the
hippopotamuses The hippopotamus (''Hippopotamus amphibius;'' ; : hippopotamuses), often shortened to hippo (: hippos), further qualified as the common hippopotamus, Nile hippopotamus and river hippopotamus, is a large semiaquatic Mammal, mammal native to su ...
, from which they diverged about 40 million years ago. Specific types of dolphins can be pink. River dolphins are relatively small compared to other dolphins, having evolved to survive in warm, shallow water and strong river currents. They range in size from the long
South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Indu ...
to the and
Amazon river dolphin The Amazon river dolphin (''Inia geoffrensis''), also known as the boto, bufeo or pink river dolphin, is a species of toothed whale Endemism, endemic to South America and is classified in the family Iniidae. Three subspecies are currently recogni ...
. Several species exhibit
sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...
, in that the females are larger than the males. They have streamlined bodies and two limbs that are modified into flippers. River dolphins use their conical-shaped teeth and long beaks to capture fast-moving prey in murky water. They have well-developed hearing that is adapted for both air and water; they do not really rely on vision since the water they swim in is usually very muddy. Instead, they tend to rely on echolocation when hunting and navigating. These species are well-adapted to living in warm, shallow waters, and, unlike other cetaceans, have little to no
blubber Blubber is a thick layer of Blood vessel, vascularized adipose tissue under the skin of all cetaceans, pinnipeds, penguins, and sirenians. It was present in many marine reptiles, such as Ichthyosauria, ichthyosaurs and plesiosaurs. Description ...
. River dolphins are not very widely distributed; they are all restricted to certain rivers or deltas. This makes them extremely vulnerable to
habitat destruction Habitat destruction (also termed habitat loss or habitat reduction) occurs when a natural habitat is no longer able to support its native species. The organisms once living there have either moved elsewhere, or are dead, leading to a decrease ...
. River dolphins feed primarily on fish. Male river dolphins typically mate with multiple females every year, but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. River dolphins produce a variety of vocalizations, usually in the form of clicks and whistles. River dolphins are rarely kept in captivity; breeding success has been poor and the animals often die within a few months of capture. , there was only one river dolphin in captivity.


Taxonomy and evolution


Classification

Four families of river dolphins (Iniidae, Pontoporiidae, Lipotidae and Platanistidae) are currently recognized, comprising three superfamilies (Inioidea, Lipotoidea and Platanistoidea). Platanistidae, containing the two subspecies of
South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Indu ...
, is the only living family in the superfamily Platanistoidea. Previously, many taxonomists had assigned all river dolphins to a single family, Platanistidae, and treated the Ganges and Indus river dolphins as separate species. A December 2006 survey found no members of '' Lipotes vexillifer'' (commonly known as the baiji, or Chinese river dolphin) and declared the species
functionally extinct Functional extinction is the extinction of a species or other taxon such that: #It disappears from the fossil record, or historic reports of its existence cease; #The reduced population no longer plays a significant role in ecosystem function; #T ...
. The current classification of river dolphins is as follows: * Superfamily Platanistoidea ** Family Platanistidae *** Genus ''
Platanista South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Ind ...
'' ****
South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Indu ...
, ''Platanista gangetica'', with two subspecies *****
Ganges river dolphin The Ganges river dolphin (''Platanista gangetica'') is a species of freshwater dolphin classified in the family Platanistidae. It lives in the Ganges and related rivers of South Asia, namely in the countries of India, Nepal, and Bangladesh. It ...
(''susu''), ''P. g. gangetica'' *****
Indus river dolphin The Indus river dolphin (''Platanista minor'') is a species of freshwater dolphin in the family Platanistidae. It is endemic to the Indus River basin in Pakistan and Beas River in northwestern India. This dolphin was the first discovered side- ...
(''bhulan''), ''P. g. minor'' ** Family † Allodelphinidae (
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
-
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
) ** Family † Squalodelphinidae (
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
to Miocene) ** Family † Squalodontidae (Oligocene to Miocene) ** Family † Waipatiidae (Oligocene to Miocene) * Superfamily Inioidea ** Family Iniidae *** Genus '' Inia'' ****
Amazon river dolphin The Amazon river dolphin (''Inia geoffrensis''), also known as the boto, bufeo or pink river dolphin, is a species of toothed whale Endemism, endemic to South America and is classified in the family Iniidae. Three subspecies are currently recogni ...
(''boto''), ''Inia geoffrensis'' ***** ''Inia geoffrensis geoffrensis'' ***** ''Inia geoffrensis humbotiana'' **** Araguaian river dolphin, ''Inia araguaiaensis'' **** Bolivian river dolphin, ''Inia boliviensis'' *** Genus †'' Meherrinia'' (late Miocene) *** Genus †'' Isthminia'' (Miocene) ** Family Pontoporiidae *** Genus †'' Auroracetus'' **** †''Auroracetus bakerae'' *** Genus '' Pontoporia'' ****
La Plata dolphin The La Plata dolphin, franciscana or toninha (''Pontoporia blainvillei'') is a species of river dolphin found in coastal Atlantic waters of southeastern South America. It is a member of the Inioidea group and the only one that lives in the ocean ...
(''Franciscana''), ''Pontoporia blainvillei'' * Superfamily Lipotoidea ** Family Lipotidae *** Genus '' Lipotes'' **** ''
Baiji The baiji (''Lipotes vexillifer'') is a probably extinct species of freshwater dolphin native to the Yangtze river system in China. It is thought to be the first dolphin species driven to extinction due to the impact of humans. This dolphin is ...
'' (or Chinese river dolphin), ''Lipotes vexillifer'' In 2012 the
Society for Marine Mammalogy The Society for Marine Mammalogy was founded in 1981 and is the largest international association of marine mammal scientists in the world. Mission The mission of the Society for Marine Mammalogy (SMM) is to promote the global advancement of mari ...
began considering the Bolivian (''Inia geoffrensis boliviensis'') and Amazonian (''Inia geoffrensis geoffrensis'') subspecies as full species ''Inia boliviensis'' and ''Inia geoffrensis'', respectively; however, much of the scientific community, including the
IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...
, continue to consider the Bolivian population to be a subspecies of ''Inia geoffrensis''.Reeves, R.R., Jefferson, T.A., Karczmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E.R., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K. (2011). ''Inia geoffrensis''. The IUCN Red List of Threatened Species In October 2014, the Society for Marine Mammalogy took ''Inia boliviensis'' and ''Inia araguaiaensis'' off their list of
aquatic mammal Aquatic mammals and semiaquatic mammals are a diverse group of mammals that dwell partly or entirely in body of water, bodies of water. They include the various ''marine mammals'' who dwell in oceans, as well as various freshwater species, such a ...
species and subspecies and currently does not recognize these species-level separations.


Evolution

River dolphins are members of the infraorder
Cetacea Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...
, which are descendants of land-dwelling mammals of the order
Artiodactyla Artiodactyls are placental mammals belonging to the order (biology), order Artiodactyla ( , ). Typically, they are ungulates which bear weight equally on two (an even number) of their five toes (the third and fourth, often in the form of a hoof ...
(even-toed
ungulate Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...
s). They are related to the '' Indohyus'', an extinct chevrotain-like ungulate, from which they split approximately 48 million years ago. The primitive cetaceans, or archaeocetes, first took to the sea approximately 49 million years ago and became fully aquatic by 5–10 million years later. It is unknown when river dolphins first ventured back into fresh water. River dolphins are thought to have relictual distributions, that is, their ancestors originally occupied marine habitats, but were then displaced from these habitats by modern dolphin lineages. Many of the morphological similarities and adaptations to freshwater habitats arose due to
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
; thus, a grouping of all river dolphins is
polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...
. Amazon river dolphins are actually more closely related to oceanic dolphins than to
South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Indu ...
s. '' Isthminia panamensis'' is an extinct genus and species of river dolphin, living 5.8 to 6.1 million years ago. Its fossils were discovered near Piña, Panama. River dolphin has been considered a taxonomic description, suggesting an evolutionary relationship among the group, although it is now known that they form two distinct clades. 'True' river dolphins are descendants of ancient evolutionary lineages that evolved in freshwater environments. Some species of cetacean live in rivers and lakes, but are more closely related to oceanic dolphins or porpoises and entered fresh water more recently. Such species are considered facultative freshwater cetaceans as they can use both marine and freshwater environments. These include species such as the Irrawaddy dolphin, ''Orcaella brevirostris'', found in the
Mekong The Mekong or Mekong River ( , ) is a transboundary river in East Asia and Southeast Asia. It is the world's twelfth-longest river and the third-longest in Asia with an estimated length of and a drainage area of , discharging of wat ...
, Mahakam, the
Irrawaddy River The Irrawaddy River (, , Ayeyarwady) is the principal river of Myanmar, running through the centre of the country. Myanmar’s most important commercial waterway, it is about 1,350 miles (2,170 km) long. Originating from the confluence of the ...
s, as well as the Yangtze finless porpoise ''Neophocaena asiaeorientalis''. Some oceanic cetacean populations are known to live semi-permanently in river and estuarine systems, such as the Indo-Pacific bottlenose dolphin group resident in the Swan River of
Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...
which travel as far inland as Belmont. The tucuxi (''Sotalia fluviatilis'') in the Amazon River is another species descended from oceanic dolphins; however, it does not perfectly fit the label of 'facultative' either, as it occurs only in fresh water. The tucuxi was until recently considered conspecific with the Guiana dolphin (''Sotalia guianensis''), which inhabits marine waters. It may also be true for the Irrawaddy dolphin and the finless porpoise that, although the species may be found in both freshwater and marine environments, individual animals found in rivers may not be able to survive in the ocean, and vice versa. The tucuxi is currently classified as an oceanic dolphin (Delphinidae). The Franciscana (''Pontoporia blainvillei'') has shown a converse evolutionary pattern, and has an ancient evolutionary lineage in freshwater, but inhabits estuarine and coastal waters.


Biology


Anatomy

River dolphins have a torpedo shaped body with a flexible neck, limbs modified into flippers, non-existent external ear flaps, a tail fin, and a small bulbous head. River dolphin skulls have small eye orbits, a long snout and eyes placed on the sides of the head. River dolphins are rather small, ranging in size from the long
South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Indu ...
to the and
Amazon river dolphin The Amazon river dolphin (''Inia geoffrensis''), also known as the boto, bufeo or pink river dolphin, is a species of toothed whale Endemism, endemic to South America and is classified in the family Iniidae. Three subspecies are currently recogni ...
. They all have female-biased
sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...
apart from Amazon river dolphin, with the females being larger than the males. River dolphins are
polygynous Polygyny () is a form of polygamy entailing the marriage of a man to several women. The term polygyny is from Neoclassical Greek πολυγυνία (); . Incidence Polygyny is more widespread in Africa than in any other continent. Some scholar ...
, meaning male river dolphins typically mate with multiple females every year, but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. River dolphins have conical teeth, used to catch swift prey such as small river fish. They also have very long snouts, with some measuring , four times longer than most of their oceanic counterparts. They have a two-chambered stomach that is similar in structure to that of terrestrial carnivores. They have fundic and pyloric chambers. Breathing involves expelling stale air from their blowhole, followed by inhaling fresh air into their lungs. They do not have the iconic spout, as this only forms when the warm air exhaled from the lungs meets cold external air, which does not occur in their tropical habitats. River dolphins have a relatively thin layer of
blubber Blubber is a thick layer of Blood vessel, vascularized adipose tissue under the skin of all cetaceans, pinnipeds, penguins, and sirenians. It was present in many marine reptiles, such as Ichthyosauria, ichthyosaurs and plesiosaurs. Description ...
. Blubber can help with buoyancy, protection from predators (they would have a hard time getting through a thick layer of fat), energy for leaner times, and insulation from harsh climates. The habitats of river dolphins lack these needs.


Locomotion

River dolphins have two flippers and a tail fin. These flippers contain five digits. Although river dolphins do not possess fully developed hind limbs, some possess discrete rudimentary appendages, which may contain feet and digits. River dolphins are slow swimmers in comparison to oceanic dolphins, which can travel at speeds up to ; the tucuxi can only travel at about . Unlike other cetaceans, their neck vertebrae are not fused together, meaning they have greater flexibility than other non-terrestrial
aquatic mammal Aquatic mammals and semiaquatic mammals are a diverse group of mammals that dwell partly or entirely in body of water, bodies of water. They include the various ''marine mammals'' who dwell in oceans, as well as various freshwater species, such a ...
s, at the expense of speed. This means they can turn their head without actually moving their entire body. When swimming, river dolphins rely on their tail fins to propel themselves through the water. Flipper movement is continuous. River dolphins swim by moving their tail fins and lower bodies up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. All species have a
dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...
.


Senses

The ears of river dolphins have specific adaptations to their aquatic environment. In humans, the
middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...
works as an impedance equalizer between the outside air's low impedance and the
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...
r fluid's high impedance. In river dolphins, and other cetaceans, there is no great difference between the outer and inner environments. Instead of sound passing through the outer ear to the middle ear, river dolphins receive sound through the throat, from which it passes through a low-impedance fat-filled cavity to the inner ear. The ear is acoustically isolated from the skull by air-filled sinus pockets, which allows for greater directional hearing underwater. Dolphins send out high frequency clicks from an organ known as a
melon A melon is any of various plants of the family Cucurbitaceae with sweet, edible, and fleshy fruit. It can also specifically refer to ''Cucumis melo'', commonly known as the "true melon" or simply "melon". The term "melon" can apply to both the p ...
. This melon consists of fat, and the skull of any such creature containing a melon will have a large depression. This allows river dolphins to produce biosonar for orientation. They are so dependent on echolocation that they can survive even if they are blind. Beyond locating an object, echolocation also provides the animal with an idea on the object's shape and size, though how exactly this works is not yet understood. The small hairs on the rostrum of the Amazon river dolphin are believed to function as a tactile sense, possibly to compensate for their poor eyesight. River dolphins have very small eyes for their size, and do not have a very good sense of sight. In addition, the eyes are placed on the sides of the head, so the vision consists of two fields, rather than a binocular view like humans have. When river dolphins surface, their lens and cornea correct the nearsightedness that results from the refraction of light. They have both rod and
cone In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the '' apex'' or '' vertex''. A cone is formed by a set of line segments, half-lines ...
cells, meaning they can see in both dim and bright light. Most river dolphins have slightly flattened eyeballs, enlarged pupils (which shrink as they surface to prevent damage), slightly flattened corneas and a
tapetum lucidum The ; ; : tapeta lucida) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It Reflection (physics), reflects visible light back through the retina, increas ...
; these adaptations allow for large amounts of light to pass through the eye and, therefore, a very clear image of the surrounding area. They also have glands on their eyelids and an outer corneal layer that act as protection for the cornea. Olfactory lobes are absent in river dolphins, suggesting that they have no sense of smell. River dolphins are not thought to have a sense of taste, as their taste buds are atrophied or missing altogether. However, some dolphins have preferences between different kinds of fish, indicating some sort of attachment to taste.


Interactions with humans


Threats


Development

Development and agriculture have had devastating impacts on the habitats on river dolphins. The total population of Araguaian river dolphins is estimated to be between 600 and 1,500 individuals, and
genetic diversity Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species. It ranges widely, from the number of species to differences within species, and can be correlated to the span of survival for a species. It is d ...
is limited. The ecology of their habitat has been adversely affected by agricultural, ranching and industrial activities, as well as by the use of dams for hydroelectric power. The inhabited section of the Araguaia River probably extends over about out of a total length of . The
Tocantins river The Tocantins River ( , Parkatêjê dialect, Parkatêjê: ''Pyti'' ɨˈti is a river in Brazil, the central fluvial artery of the country. In the Tupi language, its name means "toucan's beak" (''Tukã'' for "toucan" and ''Ti'' for "beak"). It ...
habitat is fragmented by six hydroelectric dams, so the population there is at particular risk. Its probable eventual
IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...
status is vulnerable or worse. Both subspecies of
South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Indu ...
s have been very adversely affected by human use of the river systems in the
subcontinent A continent is any of several large geographical regions. Continents are generally identified by convention rather than any strict criteria. A continent could be a single large landmass, a part of a very large landmass, as in the case of A ...
. Irrigation has lowered water levels throughout both subspecies' ranges. Poisoning of the water supply from industrial and
agricultural chemicals An agrochemical or agrichemical, a contraction of ''agricultural chemical'', is a chemical product used in industrial agriculture. Agrichemical typically refers to biocides (pesticides including insecticides, herbicides, fungicides and nematicid ...
may have also contributed to population decline. Perhaps the most significant issue is the building of more than 50 dams along many rivers, causing the segregation of populations and a narrowed
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survi ...
in which the dolphins can breed. Currently, three subpopulations of Indus river dolphins are considered capable of long-term survival if protected. As China developed economically, pressure on the baiji river dolphin grew significantly. Industrial and residential waste flowed into the Yangtze. The riverbed was dredged and reinforced with concrete in many locations. Ship traffic multiplied, boats grew in size, and fishermen employed wider and more lethal nets. Noise pollution caused the nearly blind animal to collide with propellers. Stocks of the dolphin's prey declined drastically in the late 20th century, with some fish populations declining to one thousandth of their pre-industrial levels. In the 1950s, the population was estimated at 6,000 animals, but declined rapidly over the subsequent five decades. Only a few hundred were left by 1970. Then the number dropped down to 400 by the 1980s and then to 13 in 1997 when a full-fledged search was conducted. On December 13, 2006, the
baiji The baiji (''Lipotes vexillifer'') is a probably extinct species of freshwater dolphin native to the Yangtze river system in China. It is thought to be the first dolphin species driven to extinction due to the impact of humans. This dolphin is ...
(''Lipotes vexillifer'') was declared "functionally extinct", after a 45-day search by leading experts in the field failed to find a single specimen. The last verified and widely accepted sighting was in September 2004, but it has been allegedly seen and photographed by Chinese citizens on four occasions since then.


Competition

The region of the Amazon in Brazil has an extension of containing diverse fundamental ecosystems. One of these ecosystems is a
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...
, or a várzea forest, and is home to a large number of fish species which are an essential resource for human consumption. The várzea is also a major source of income through excessive local commercialized fishing. Várzea consist of muddy river waters containing a vast number and diversity of nutrient-rich species. The abundance of distinct fish species lures the Amazon River dolphin into the várzea areas of high water occurrences during the seasonal flooding. In addition to attracting predators such as the Amazon river dolphin, these high-water occurrences are an ideal location to draw in the local fisheries. Human fishing activities directly compete with the dolphins for the same fish species, the
tambaqui The tambaqui (''Colossoma macropomum'') is a large species of freshwater fish in the family Serrasalmidae. It is native to tropical South America, but kept in aquaculture and Introduced species, introduced elsewhere. It is also known by the names ...
(''Colossoma macropomum'') and the pirapitinga (''Piaractus brachypomus''), resulting in deliberate or unintentional catches of the Amazon river dolphin. The local fishermen overfish, and when the Amazon river dolphins remove the commercialized fish from the nets and lines, it damages the equipment and the capture and causes a negative reaction from the local fishermen. The Brazilian Institute of Environment and Renewable Natural Resources prohibit fishermen from killing the Amazon river dolphin, yet they are not compensated for the damage to their equipment and the loss of their catch.


Bycatch

During the process of catching the commercialized fish, the Amazon river dolphins get caught in the nets and exhaust themselves until they die, or the local fishermen deliberately kill the dolphins that become entangled in their nets. The carcasses are discarded, consumed, or used as bait to attract a scavenger catfish, the piracatinga (''Calophysus macropterus''). The use of the Amazon river dolphin carcass as bait for the piracatinga dates back from 2000. The increasing consumption demand by the local inhabitants and Colombia for the piracatinga has created a market for distribution of the Amazon river dolphin carcasses to be used as bait throughout these regions. For example, of the 15 dolphin carcasses found in the Japurá River in 2010–2011 surveys, 73% of the dolphins were killed for bait, disposed of, or abandoned in entangled gillnets. The data does not fully represent the actual overall number of deaths of the Amazon river dolphins, whether accidental or intentional, because a variety of factors make it extremely complicated to record and medically examine all the carcasses. Scavenger species feed upon them and the complexity of the river currents makes it nearly impossible to locate all the carcasses. More importantly, the local fishermen do not report these deaths out of fear that legal action will be taken against them, as the Amazon river dolphin and other cetaceans are protected under the Brazilian federal law, prohibiting any takes, harassments, and kills of the species.


Conservation


International River Dolphin Day

The ''Global Declaration for River Dolphins'' was signed by nine countries on 24 October 2023, a date chosen as it is known as the International River Dolphin Day. This pact is intended to promote research and cooperation between countries with river dolphin populations. It is hoped that five further countries will join.


Amazon and Orinoco Rivers

Marine biologist Fernando Trujillo co-founded the Omacha Foundation in 1993 to protect the pink river dolphins in the
Amazon Amazon most often refers to: * Amazon River, in South America * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon (company), an American multinational technology company * Amazons, a tribe of female warriors in Greek myth ...
and
Orinoco The Orinoco () is one of the longest rivers in South America at . Its drainage basin, sometimes known as the Orinoquia, covers approximately 1 million km2, with 65% of it in Venezuela and 35% in Colombia. It is the List of rivers by discharge, f ...
Rivers. Since 2013, Trujillo and his team have been measuring the mercury levels in river dolphins. Mercury, leeched into the river through illegal gold mining processes, is harmful to both dolphins and humans.


Climate Change

As global temperatures rise, the Amazon has seen record level of drought and river temperatures. In 2023, within just two months (September–October), 330 river dolphins were found dead in the Amazon region due to extreme drought.


Captivity

A baiji conservation
dolphinarium A dolphinarium is an aquarium for dolphins. The dolphins are usually kept in a pool, though occasionally they may be kept in pens in the open sea, either for research or public performances. Some dolphinariums (sometimes called dolphinaria in plu ...
was established at the Institute of Hydrobiology (IHB) in Wuhan in 1992. This was planned as a backup to any other conservation efforts by producing an area completely protected from any threats, and where the baiji could be easily observed. The site includes an indoor and outdoor holding pool, a water filtration system, food storage and preparation facilities, research labs and a small museum. The aim is to also generate income from tourism which can be put towards the baiji plight. The pools are not very large, only kidney shaped tanks with dimensions of arc width and depth, diameter, deep and diameter, deep, and are not capable of holding many baijis at one time. Douglas Adams and Mark Carwardine documented their encounters with the endangered animals on their conservation travels for the BBC programme '' Last Chance to See''. The book by the same name, published in 1990, included pictures of a captive specimen, a male named Qi Qi (淇淇) that lived in the Wuhan Institute of Hydrobiology
dolphinarium A dolphinarium is an aquarium for dolphins. The dolphins are usually kept in a pool, though occasionally they may be kept in pens in the open sea, either for research or public performances. Some dolphinariums (sometimes called dolphinaria in plu ...
from 1980 to July 14, 2002. Discovered by a fisherman in Dongting Lake, he became the sole resident of the Baiji Dolphinarium (白鱀豚水族馆) beside East Lake. A sexually mature female was captured in late 1995, but died after half a year in 1996 when the Shishou Tian-e-Zhou Baiji Semi-natural Reserve (石首半自然白鱀豚保护区), which had contained only finless porpoises since 1990, was flooded. The Amazon river dolphin has historically been kept in dolphinariums. Today, only three exist in captivity: one in Acuario de Valencia in Venezuela, one in Zoologico de Guistochoca in Peru, and one in Duisburg Zoo in Germany. Several hundred were captured between the 1950s and 1970s, and were distributed in dolphinariums throughout the US, Europe, and Japan. Around 100 went to US dolphinariums, and of that, only 20 survived; the last (named Chuckles) died in Pittsburgh Zoo in 2002.


Mythology


Asia

In
Hindu mythology Hindu mythology refers to the collection of myths associated with Hinduism, derived from various Hindu texts and traditions. These myths are found in sacred texts such as the Vedas, the Itihasas (the ''Mahabharata'' and the ''Ramayan ...
, the
Ganges river dolphin The Ganges river dolphin (''Platanista gangetica'') is a species of freshwater dolphin classified in the family Platanistidae. It lives in the Ganges and related rivers of South Asia, namely in the countries of India, Nepal, and Bangladesh. It ...
is associated with Ganga, the deity of the
Ganges The Ganges ( ; in India: Ganga, ; in Bangladesh: Padma, ). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international which goes through India, Bangladesh, Nepal and China." is a trans-boundary rive ...
river. The dolphin is said to be one of the creatures which heralded the goddess' descent from the heavens, and Ganga's mount, the Makara, is sometimes depicted as a dolphin. In Chinese mythology, the baiji has many origin stories. For example, near the mouth of the
Yangtze The Yangtze or Yangzi ( or ) is the longest river in Eurasia and the third-longest in the world. It rises at Jari Hill in the Tanggula Mountains of the Tibetan Plateau and flows including Dam Qu River the longest source of the Yangtze, i ...
, the
baiji The baiji (''Lipotes vexillifer'') is a probably extinct species of freshwater dolphin native to the Yangtze river system in China. It is thought to be the first dolphin species driven to extinction due to the impact of humans. This dolphin is ...
was a princess that had lost her parents and had lived with her stepfather, whom she had longed to get away from. The stepfather wanted to trade her since she would be sold for a great sum of money, but as they were crossing the river to get to the trader, a storm rolled in. The enraged stepfather tried to take her, but she plunged herself into the river, was transformed into a dolphin before she drowned, and swam away from her abusive stepfather, who also fell in and was transformed into a
porpoise Porpoises () are small Oceanic dolphin, dolphin-like cetaceans classified under the family Phocoenidae. Although similar in appearance to dolphins, they are more closely related to narwhals and Beluga whale, belugas than to the Oceanic dolphi ...
. In another story, the baiji was the daughter of a general deported from the city of
Wuhan Wuhan; is the capital of Hubei, China. With a population of over eleven million, it is the most populous city in Hubei and the List of cities in China by population, eighth-most-populous city in China. It is also one of the nine National cent ...
during a war who ran away while her father was in duty. Later, the general met a woman who told him how her father was a general. When he realized that she was his daughter, he threw himself into the river out of shame, and his daughter ran after him and also fell into the river. Before they were drowned, the daughter was transformed into a dolphin and the general into a porpoise.


South America

Amazon river dolphin The Amazon river dolphin (''Inia geoffrensis''), also known as the boto, bufeo or pink river dolphin, is a species of toothed whale Endemism, endemic to South America and is classified in the family Iniidae. Three subspecies are currently recogni ...
s, known by the natives as the boto, encantados or toninhas, are very prevalent in the mythology of the native South Americans. They are often characterized in mythology with superior musical ability, seductiveness and love of sex that often results in illegitimate children, and attraction to parties. Despite the fact that the Encante are said to come from a
utopia A utopia ( ) typically describes an imagined community or society that possesses highly desirable or near-perfect qualities for its members. It was coined by Sir Thomas More for his 1516 book ''Utopia (book), Utopia'', which describes a fictiona ...
full of wealth and without pain or death, the encantados crave the pleasures and hardships of human societies. Transformation into human form is said to be rare, and usually occurs at night. The encantado will often be seen running from a festa, despite protests from the others for it to stay, and can be seen by pursuers as it hurries to the river and reverts to dolphin form. When it is under human form, it wears a hat to hide its blowhole, which does not disappear with the
shapeshift In mythology, folklore and speculative fiction, shapeshifting is the ability to physically transform oneself through unnatural means. The idea of shapeshifting is found in the oldest forms of totemism and shamanism, as well as the oldest exist ...
. Besides the ability to shapeshift into human form, encantados frequently wield other magical abilities, such as controlling storms, hypnotizing humans into doing their will, transforming humans into encantados, and inflicting
illness A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...
,
insanity Insanity, madness, lunacy, and craziness are behaviors caused by certain abnormal mental or behavioral patterns. Insanity can manifest as violations of societal norms, including a person or persons becoming a danger to themselves or to other ...
, and even
death Death is the end of life; the irreversible cessation of all biological functions that sustain a living organism. Death eventually and inevitably occurs in all organisms. The remains of a former organism normally begin to decompose sh ...
. Shamans often intervene in these situations.
Kidnapping Kidnapping or abduction is the unlawful abduction and confinement of a person against their will, and is a crime in many jurisdictions. Kidnapping may be accomplished by use of force or fear, or a victim may be enticed into confinement by frau ...
is also a common theme in such
folklore Folklore is the body of expressive culture shared by a particular group of people, culture or subculture. This includes oral traditions such as Narrative, tales, myths, legends, proverbs, Poetry, poems, jokes, and other oral traditions. This also ...
. Encantados are said to be fond of abducting humans with whom they fall in love, children born of their illicit love affairs, or just about anyone near the river who can keep them company, and taking them back to the Encante. The fear of this is so great among people who live near the Amazon River that both children and adults are terrified of going near the water between dusk and dawn, or entering water alone. Some who supposedly have encountered encantados while out in their
canoe A canoe is a lightweight, narrow watercraft, water vessel, typically pointed at both ends and open on top, propelled by one or more seated or kneeling paddlers facing the direction of travel and using paddles. In British English, the term ' ...
s have been said to have gone insane, but the creatures seem to have done little more than follow their boats and nudge them from time to time.


References


Further reading

* Reeves, Randall R. et al. (2002). ''National Audubon Society Guide to Marine Mammals of the World''. Alfred A. Knopf. 527 pp. {{Portal bar, Cetaceans, Mammals Extant Oligocene first appearances Animals that use echolocation Mammal common names Polyphyletic groups