River dolphin


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 possibly extinct Lipotidae, Iniidae 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 long South Asian river dolphin to the and Amazon river dolphin. Several species exhibit sexual dimorphism, 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 their eyes are very small and 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., there was only one river dolphin in captivity.

Taxonomy and evolution

Classification

Four families of river dolphins are currently recognized, comprising three superfamilies. Platanistidae, containing the two subspecies of South Asian river dolphin, 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 and declared the species functionally extinct.
The current classification of river dolphins is as follows:
File:Arktocara NT small.jpg|thumb|Life reconstruction of Arktocara yakataga, an allodelphinid
In 2012 the Society for Marine Mammalogy began considering the Bolivian and Amazonian subspecies as full species Inia boliviensis and Inia geoffrensis, respectively; however, much of the scientific community, including the IUCN, continue to consider the Bolivian population to be a subspecies of Inia geoffrensis.
In October 2014, the Society for Marine Mammalogy took Inia boliviensis and Inia araguaiaensis off their list of aquatic mammal species and subspecies and currently does not recognize these species-level separations.

Evolution

River dolphins are members of the infraorder Cetacea, which are descendants of land-dwelling mammals of the order Artiodactyla. 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 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, Mahakam, the Irrawaddy Rivers, 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 which travel as far inland as Belmont.
The tucuxi 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, 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.
The Franciscana 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 to the 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.
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 can help with buoyancy, protection from predators, 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 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.