The Amazon River dolphin is endemic to South America and is the most studied of all the species of river dolphin. Currently the IUCN recognises three subspecies; I. g. geoffrensis in the Amazon River system of Brazil, Ecuador and Peru; I. g. humboldtiana in the Orinoco basin of Colombia and Venezuela; and I. g. boliviensis in Bolivia. However, recent genetic evidence has brought this classification into question. The Committee on Taxonomy (2015) list of marine mammal species and subspecies from the Society for Marine Mammalogy (SMM) initially accepted the evidence for an additional species, Inia boliviensis, the Bolivian river dolphin, found solely in the river systems of Bolivia (and rivers along the Bolivia-Brazil border) however the theory was not supported by further genetic sampling and for the moment it remains a sub-species. Another new species, Inia araguaiaensis was described from the Araguaia River, which is not connected to the Amazon, however for several reasons the authors conclusions are not currently thought to be very convincing and for the moment official recognition of the species has not been given. The remaining two subspecies seemingly present no morphological differences and it is possible that they are in fact one species.
Appearance[]
The Amazon River dolphin is a fairly robust animal with small but functioning eyes and a long slender beak that curves down slightly towards the tip. They are the only toothed whales to have different types of teeth in their jaw, those in the front are the usual simple conical shape whilst those in the back are designed to aid in the crushing of prey items. The melon is bulbous and abrupt, and the dolphin is able to change the shape of its melon. The crescent shaped blowhole is set left of centre and the neck is very flexible due to unfused cervical vertebrae, and has a well defined crease. The Amazon River dolphin has a long dorsal ridge and very low dorsal fin. The flippers are triangular, broad and have blunt tips, while the flukes are triangular, broad and have pointed tips. Both flukes and flippers have ragged trailing edges. One of the most striking characteristics of this species is its colouration, ranging from white/grey to pink on the dorsal surface, and a paler colour below. Some individuals however are a vivid pink colour. The colouration is highly variable and affected by age and viewing conditions. As the only other cetacean to share the Amazon River dolphin's range is the tucuxi, it is almost impossible to confuse it with any other species.
Behavior[]
Another feature of this primitive looking dolphin are the 'chubby cheeks' which it is believed may obstruct its downward vision and may be the reason they are often seen swimming upside down. Amazon River dolphins are typically slow swimmers, more active in the early morning and late afternoon, although when feeding they can show short bursts of speed. They are generally seen alone or in pairs, but during the dry season or in the presence of abundant prey they will congregate in groups of some 10 to 15 individuals. Amazon River dolphins are not an acrobatic species and rarely leap from the water. They also breathe at a rather shallow angle, showing beak, melon and dorsal fin all at the same time. Their diet consists of fish, shellfish, and even small turtles.
Distribution[]
The Amazon River dolphin is found throughout the Orinoco and Amazon River basins, in the mainstems of the rivers, their tributaries and lakes, although in some localities its natural range has been restricted by development and the building of dams. During the wet season its range extends into the flooded forests. The major threat to this species is directed hunts where the animals are either used for bait or seen by fishermen as competition. Other threats to the species include human disturbance, entanglement in fishing gear, prey depletion, and chemical pollution. The IUCN Red List categorises this species as Data Deficient.
Diet[]
The diet of the Amazon river dolphin is the most diverse of the toothed whales. It consists of at least 53 different species of fish, grouped in 19 families. The prey size is between 5 and 80 centimetres (2.0 and 31.5 in), with an average of 20 centimetres (7.9 in). The most frequently consumed fish belong to the families Sciaenidae (croakers), Cichlidae, and Characidae (tetras and piranhas). The dolphin’s dentition allows it to access shells of river turtles and freshwater crabs. The diet is more diverse during the wet season, when fish are spread in flooded areas outside riverbeds, thus becoming more difficult to catch. The diet becomes more selective during the dry season when prey density is greater.
Usually, these dolphins are active and feeding throughout the day and night. However, they are predominantly crepuscular. They consume about 5.5% of their body weight per day. They sometimes take advantage of the disturbances made by boats to catch disoriented prey. Sometimes, they associate with the distantly-related tucuxi (Sotalia fluviatilis), and giant otters (Pteronura brasiliensis) to hunt in a coordinated manner, by gathering and attacking fish stocks at the same time. Apparently, there is little competition for food between these species, as each prefers different prey. It has also been observed that captive dolphins share food.
Echolocation[]
In order to navigate and find prey in the often highly murky river waters Amazon river dolphin produce echolocation clicks and then listens for returning echoes. Echolocation in shallow waters and flooded forests may result in many echoes to keep track of and many echoes are likely to return to the animals almost on top of each other which makes object discrimination difficult. This may be why the Amazon river dolphin produce less powerful clicks compared to other similar sized toothed whales. By sending out clicks of lower amplitude only nearby objects will cast back detectable echoes, and hence fewer echoes need to be sorted out. Toothed whales generally do not produce a new echolocation click until all relevant echoes from the previous click have been received, so if detectable echoes are only reflected back from nearby objects, the Amazon river dolphin is able to produce clicks at a high rate. This in turn allow the animals to have a high acoustic update rate on their surroundings which may aid prey tracking when echolocating in shallow rivers and flooded forests with plenty of hiding places for the prey.
Communication[]
Like other dolphins, river dolphins use whistling tones to communicate. The issuance of these sounds is related to the time they return to the surface before diving, suggesting a link to food. Acoustic analysis revealed that the vocalisations are different in structure from the typical whistles of other species of dolphins.
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