Өзен дельфиндері: Түрлер, эволюция және қорғау мәселелері

Superfamily of dolphins

River dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water. They are an informal grouping of dolphins, which itself is a paraphyletic group within the infraorder Cetacea. Extant river dolphins are placed in two superfamilies, Platanistoidea and Inioidea. They comprise the families Platanistidae (the South Asian dolphins), the recently extinct Lipotidae (Yangtze river dolphin), Iniidae (the Amazonian dolphins) and Pontoporiidae. There are five extant species of river dolphins. River dolphins, alongside other cetaceans, belong to the clade Artiodactyla, with even toed ungulates, and their closest living relatives the hippopotamuses, 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 5 feet (2 m) long South Asian river dolphin to the 8 feet (2 m) and Amazon river dolphin. Several species exhibit sexual dimorphism, in that the males are larger than the females. 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. River dolphins are not very widely distributed; they are all restricted to certain rivers or deltas. This makes them extremely vulnerable to habitat destruction. 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. as of 2020, there was only one river dolphin in captivity.

River dolphins are members of the infraorder Cetacea, which are descendants of land dwelling mammals of the order Artiodactyla (even toed ungulates). 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; thus, a grouping of all river dolphins is polyphyletic. Amazon river dolphins are actually more closely related to oceanic dolphins than to South Asian river dolphins. 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 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 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.

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 5 feet (2 m) long South Asian river dolphin to the 8 feet (2 m) and Amazon river dolphin. They all have female biased sexual dimorphism apart from Amazon river dolphin, with the females being larger than the males. River dolphins are polygynous, 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. 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 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. Unlike other cetaceans, their neck vertebrae are not fused together, meaning they have greater flexibility than other non terrestrial aquatic mammals, 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. Dolphins send out high frequency clicks from an organ known as a melon. 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. Most river dolphins have slightly flattened eyeballs, enlarged pupils (which shrink as they surface to prevent damage), slightly flattened corneas and a tapetum lucidum; 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. Both subspecies of South Asian river dolphins have been very adversely affected by human use of the river systems in the subcontinent. Irrigation has lowered water levels throughout both subspecies' ranges. Poisoning of the water supply from industrial and agricultural chemicals 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 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. 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 (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 sighting was in September 2004. One of these ecosystems is a floodplain, 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. 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 use of the Amazon river dolphin carcass as bait for the piracatinga dates back from 2000.

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.

A baiji conservation dolphinarium 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 82 feet (25 m) arc 23 feet (7 m) width and 11 feet (3 m) depth, 33 feet (10 m) diameter, 6.6 feet (2 m) deep and 39 feet (12 m) diameter, 11 feet (3 m) 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 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. In Chinese mythology, the baiji has many origin stories. For example, near the mouth of the Yangtze, the baiji 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.

Amazon river dolphins, 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 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. 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, insanity, and even death. Shamans often intervene in these situations. Kidnapping is also a common theme in such folklore. 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 canoes 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.