The Amazon River dolphin (scientific name: Inia geoffrensis) is one of three species of River Dolphins in the family Inia. The other two are the Chinese River dolphin (now possibly extinct) and the Franciscana or La Plate dolphin.) It is also widely known as the Boto. This dolphin is found in the fresh water rivers and lakes of the Amazon and Orinoco drainage basins of South America, "from the deltas upstream to where impassable rapids, waterfalls, lack of water, and possibly low temperatures block their movement." The Amazon River dolphin is the most abundant river dolphin. During wet seasons, the Amazon River dolphin often moves from the main river channels into flooded forests and plains.
|Amazon river dolphin (Inia geoffrensis) Source: Thomas Jefferson|
Size comparison of an average human against the Amazon River dolphin. Source: Chris Huh
There are three subspecies:
- Inia geoffrensis geoffrensis in the Amazon River system of Brazil, Peru, and Ecuador;
- Bolivian River dolphin (I. g. boliviensis) in Bolivia; and,
- Inia geoffrensis humboldtiana in the Orinoco basin of Venezuela and Colombia
The Bolvian River dolphin, which occurs in the Madeira drainage in Bolivia above the Teotonio rapids is considered by some to be a distinct and separte species from the Amazon River dolphin. It is "smaller and a lighter grey in colour than the other species and has more teeth. It lives only in the Bolivian Amazon and is isolated from the other Amazon River dolphins, separated by a series of 18 rapids between Bolivia and Brazil."
Amazon River dolphins are the the largest of the freshwater dolphins. They grow to about 2 meters in length (maximum recorded is 2.55 meters for a male and 2.25 meters for a female) and 160 kg in weight (maximun recorded is 207 kg for a male and 153 kg for a female.) Males on aveage are 16% longer and 55% heavier than females.
An number of attributes of this dolphin given is considerable manouverability, which is very helpful in its habilitat. The attributes include long, braod flippers capable of circular movement, long flukes, a ridge on there back which forms a long, low "dorsal ridge", non-fused vertabrea allowing flexible head movement, and good vision in and out of water. The price of these evolutionary adaptations is speed; Amazon River dolphins are rather slow compared to other dolphin species; typically 1.5 - 3.2 km/h with burst of much higher speeds.
The color of the amazon river dolphin changes with age from dark grey as a new born, to light grey as an adolescent, to increasingly pick as an adult. The pinkness comes from scaring. Males in particular are agressive and typically more pink than females.
Their bodies appear to be rather fat and heavy, but they are very flexible. None of their cervical vertebrae are fused, which allows them to move their heads in all directions. They possess broad triangular flukes and wide pectoral flippers, which sometimes possess a sixth phalanx. Their long humeri enable their flippers to move in a circular motion, giving them exceptional maneuverability when navigating through vegetation in flooded forests. However, these characters also restrict the overall speed of swimming. (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)
The skulls of (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)are less asymmetrical than other odontocetes, but torsion of the prominent rostra and mandibles is not uncommon. Their eyes are small, yet they seem to have good vision both above and underwater. They also have small, flaccid melons on their foreheads that can be shaped by muscular control when used for echolocation.
Botos are distinguished from other river dolphins by several characteristics. On top of their rostrums, they have diagnostic stiff vibrissae. They possess heterodont dentition as well, with their anterior teeth being conical and their posterior teeth having flanges on the lingual portions of the crowns. They also have long, low dorsal keels (from 30 to 61 cm in length) rather than the typical triangular dorsal fins of other dolphins. (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)can be distinguished from Sotalia fluviatilis (tucuxis), a sympatric species of river dolphin, by their color, the mobility of their head and flippers, and their diving behavior.
Amazon River dolphins are typically solitary and is rarely seen in tight groups of more than three individuals (pairs are usually mothers with their calves). However, loose aggregations associated with either feeding or mating do occur periodically. Botos do not appear to establish a social hierarchy through aggression in captivity, but violent acts are not uncommon and have even resulted in the death of some individuals. They have also been known to react protectively to individuals that have been captured or injured. They are active during both day and nighttime hours, and they are known to associate with other animals, including tucuxis (Sotalia fluviatilis) and giant otters (Pteronura brasiliensis), when pursuing prey items. (Best and da Silva, 1989b; Best and da Silva, 1993; Caldwell, Caldwell, and Brill, 1989; da Silva, 2002)
Botos are slower swimmers than most other dolphins (normally about 1.5-3.2 km/hr), but they are capable of speed bursts (14-22 km/hr). They are often found above moderate river rapids, indicating that they are capable of sustaining strong swimming for a long period of time. They do not dive very deep, and they rarely raise their flukes out of the water. When they come to the surface, the tips of their rostrums, their melons, and their dorsal keels emerge simultaneously, and they have been observed rolling, waving flippers, and lob-tailing. (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)
Botos are quite playful and curious in the wild. It is not unusual for them to rub against canoes and grasp canoe paddles of fishermen in the rivers, and they have been observed pulling grass under water, throwing sticks, and playing with logs and smaller animals (including fish and turtles). In captivity, (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)is less timid than bottlenose dolphins (Tursiops truncatus), yet they have been more difficult to train than most other dolphins.
Females reach sexual maturity at 6-7, while males reach maturity later. Mating typically occurs when water levels are low and bithing after about elen months when water levels are higher. Young are nursed for about a year. Females give birth every 2-3 years.
Little is known about the mating system of (Best and da Silva, 1984; Caldwell, Caldwell, and Brill, 1989; Martin and da Silva, 2006; McGuire and Winemiller, 1998). Before it was determined that this species exhibited sexual dimorphism, some workers postulated that botos were monogamous. However, males are now known to be larger than females, and very aggressive sexual behavior in males has been observed. Some authors have observed hostility between pink botos in the wild, while others have noted extremely aggressive activity during copulation in captivity. Males also have a higher degree of damaged fins, flukes, and blowholes due to biting and abrasion, in addition to more abundant scarring due to tooth-raking. This evidence suggests that there may be intense competition for access to females. This might indicate a polygynous mating system, but polyandry and promiscuity cannot be ruled out.
Courting and foreplay have been observed for botos in captivity. Males seem to initiate sexual activity by nibbling at the flippers or flukes of females, but if the females are not receptive, they might respond aggressively. This might not deter the males, however, who may still try and copulate with her. Copulation has been observed to be very frequent (one pair in captivity copulated 47 times in less than 3.5 hours) and to occur in three different positions: facing ventrally at right angles, lying parallel head-to-head, and head-to-tail. (Best and da Silva, 1989a; Best and da Silva, 1993)
Male botos reach sexual maturity at about 2.0 m in length, while females attain sexual maturity when they are 1.60-1.75 m long. Reproduction is seasonal, with births occurring between May and July. This birthing period corresponds with peak water levels in rivers, and since females remain in flooded areas longer than males, this offers several advantages. As water levels begin to decrease, the density of prey items in flooded areas begins to increase due to loss of habitat, offering easy access to nourishment for fueling the high energy demands of giving birth and nursing. The gestation period is estimated to be about 11 months, and births in captivity took from 4-5 hours. Mothers give birth to single calves, and once the umbilical cords break, they help their calves to the surface for air. (Best and da Silva, 1984; Best and da Silva, 1989a; Best and da Silva, 1993; Brownell, 1984; da Silva, 2002; Harrison and Brownell, 1971)calves are about 0.80 m long at birth and have been shown to grow about 0.21 m per year in captivity. Mothers lactate for well over a year, and several individuals are known to have been lactating and pregnant simultaneously. The interval between births is estimated as being between 15-36 months, and the calving period is 2-3 years.
The longevity of (Best and da Silva, 1993; Caldwell, Caldwell, and Brill, 1989)in the wild is unknown, but healthy individuals in captivity can live from 10-26 years. However, the average longevity of captive botos has been reported to be only about 33 months.
Distribution & Movements
South America, in rivers and lakes in the Amazon and Orinoco river basins and their main tributaries in Bolivia, Brazil, Colombia, Ecuador, Peru, and Venezuela. Their distribution covers approximately 7 million square kilometers and is limited mainly by marine waters, impassable rapids, waterfalls, and excessively shallow parts of the rivers. The current distribution of this species does not appear to differ significantly from its estimated distribution in the past. (Best and da Silva, 1993; da Silva, 2002)
The IUCN Red List provides a more details description of the distribution:
Besides the Amazon mainstem, I. g. geoffrensis has been recorded in the Tocantins (and das Mortes and Verde affluents), Xingu, Tapajós, Madeira (below the Teotonio rapids), Purus, Juruá, Ucayali, and Marañon (and Samiria affluent) tributaries flowing generally north, and in the Negro, Caqueta (Japurá), Apaporis, Putumayo, Napo, and Tigre tributaries flowing generally south (partial list from Best and da Silva 1989a,b; Leatherwood 1996).
In addition to the Madeira mainstem above the Teotonio rapids, I. g. boliviensis has been reported from the Beni (and Orton and Madre de Dios affluents), Iténez or Guaporé (and Verde and Iporuporé affluents), and Mamoré (and Ibaré and Ichilo affluents) tributaries (Pilleri and Gihr 1977).
In the Orinoco system, besides the mainstem, I. g. humboldtiana has been recorded in the Apuré (and Portuguesa and Guanmar affluents), Capanaparo, Cinaruco, Meta, Bita, Vichada, Tomo, Tuparro, Guaviare (and Guayabero affluent), Inírida, and Atabapo (and Temi affluent) tributaries flowing south and east, and in the Aro, Caura, Parquaza, Ventauri (San Juan affluent) tributaries flowing north and west, as well as in the Casiquiare Canal, which connects the Orinoco with the Negro River (a tributary of the Amazon), above and below the two sets of rapids at Puerto Ayacucho, which are the principal barriers that may (or may not) separate the Amazon and Orinoco populations (as summarized in Pilleri and Gihr 1977; Best and da Silva 1989a,b; Meade and Koehnken 1991). Botos have been seen crossing the first set of rapids at Puerto Ayacucho (Atures) during high water (Fernando Trujillo pers. comm. to B.D. Smith).
Within the aforementioned river systems, botos can be found in nearly all types of microhabitats, including in main rivers, small channels, mouths of rivers, lakes, and just below waterfalls and rapids.
The water level cycle exerts the strongest influence on habitat use by these dolphins during different parts of the year, both directly, by determining which areas are navigable, and indirectly, by dictating where fish are most abundant. During the dry season,is most abundant in the main river channels because smaller water channels are too shallow and prey items are concentrated along the margins of these rivers.
During the wet season, botos can easily navigate smaller tributaries, and individuals even venture into river floodplains and flooded forests. Males and females appear to have different habitat preferences, with males returning to main river channels while water levels are still rising and females and their calves continuing farther inland. Females and calves may remain in the floodplains longer for several reasons.
The calmer waters could prevent young botos from getting drawn away by strong river currents, allowing them to rest, nurse, and catch fish in a calmer environment. They may also be at a lower risk of aggression from adult males and predation from other species. (Best and da Silva, 1993; da Silva, 2002; Martin and da Silva, 2004)
The IUCN Red List adds:
Botos are generally concentrated below channel confluences (Magnusson et al. 1980; Mead and Koehnken 1991; Leatherwood 1996; Vidal et al. 1997; Leatherwood et al. 2000), with mixing of white and black waters (Martin et al. 2004). Their affinity for confluences diminishes during the high water season, probably because the animals move into appended lakes and flooded forests (Leatherwood 1996; Leatherwood et al. 2000; Martin and da Silva 2004b). They occur most often within 150 m of the edges of rivers, with lower densities in the centers of large rivers (Martin et al. 2004).
Food & Feeding Habits
A single boto’s stomach may contain more species of fish than the total number of prey species seen in other dolphins. Their very diverse diet includes at least 43 different species of fish in 19 families, with prey items ranging in size from 5-80 cm (average: 20 cm). They apparently prefer fish from the families Sciaenidae (drums or croakers), Cichlidae (cichlids), Characidae (characins and tetras), and Characidae (piranhas), but their heterodont dentition allows them to crush armored prey as well, including river turtles ( (Best and da Silva, 1989a; Best and da Silva, 1989b; Best and da Silva, 1993; da Silva, 2002; McGuire and Winemiller, 1998)) and crabs ( ). Their diet is most diverse during the wet season, when the fish spread out into the floodplain and are more difficult to catch, and becomes more selective during the dry season when fish densities are higher.
Botos are usually solitary feeders. They appear to be most active in early morning and late afternoon and consuming about 2.5% of their body weight every day. They often hang out near waterfalls and river mouths where river currents disrupt schools of fish and make them easier to catch. They also make use of disturbances made by canoes to catch disoriented prey. Sometimes they even form loose aggregations with Sotalia fluviatilis (tucuxis) and Pteronura brasiliensis (giant otters) to hunt fish in a coordinated fashion, herding and attacking shoals of fish together. Apparently, there is little competition between these species, as each prefers different types of fish. In addition, food sharing has actually been observed between botos in captivity. (Best and da Silva, 1989a; Best and da Silva, 1989b; Best and da Silva, 1993; da Silva, 2002)
There are no known records of a natural predator of botos, but black caimans (Melanosuchus niger), bull sharks (Carcharhinus leucas), anacondas (Eunectes), and jaguars (Panthera onca) are potentially capable of handling them. Some botos also possess crescent-shaped wounds that have been attributed to catfish of the families (Best and da Silva, 1989a; Best and da Silva, 1993)and Trichomycteridae.
Threats & Conservation Status
The IUCN Red List reports:
Threats include incidental mortality in fishing gear, deliberate killing for fish bait and attractant and for predator control, damming of rivers (although this is, at present, less of a problem than for the river cetaceans in Asia), and environmental pollution from organochlorines and heavy metals (see da Silva 2002; IWC 2007; Reeves et. Al. 1999).
The Mura Indians hunted botos near Barro do Rio Negro, Brazil during the nineteenth century (Pilleri and Arvy 1981) and the Tucuna and Cocama Indians may have killed them in the Amazon near Leticia (Allen and Neil 1958; Layne and Caldwell 1964) and the lower Ucayali River (Mohr 1964), respectively, until at least the 1950s.
Between 1956 and the early 1970s, more than 100 botos were live-captured and exported mostly to the U.S. and Europe (Brownell 1984) and (a few) to Japan (Tobayama and Kamiya 1989). However, in recent decades, there have been no reported live-captures and exports.
Incidental mortality has not been studied systematically in most areas. However, similar to other small cetaceans, botos are vulnerable to entanglement in a variety of nets (lampara seine nets, fixed gill nets, drifting gill nets; Best and da Silva 1993; Martin et al. 2004) as well as to drop traps intended to catch large fish or manatees in the flooded forest in Peru (Leatherwood 1996)
There is an emergent, but already large-scale, problem involving the deliberate killing of botos in Brazil for fish attractant (IWC 2007). Botos are also killed deliberately in some areas because fishermen regard them as competitors and because the dolphins damage fishing nets (F. Trujillo pers. comm. to B.D. Smith).
Fishing with explosives, although illegal in most areas, is common in some areas of the Amazon Basin (Goulding 1983; Smith 1985). This fishing technique threatens botos due to the concussive effects of explosions. Fishermen also reportedly attempt to kill dolphins that are attracted to prey on the stunned or dead fish (Best and da Silva 1989a).
Water development projects have not been as extensive in the Amazon and Orinoco basins as in Asian rivers inhabited by cetaceans. A population of botos was isolated above the Tucuruí dam in the Tocantins River, but there is no information on their current status. The Balbina dam in the Uatuma River may have isolated botos but there is no information on historic or current occurrence of dolphins in this river. Both of these dams in the Amazon basin, and the Guri dam in the Caroní, an Orinoco tributary, have probably degraded downstream habitat due to their effects on flow and temperature regimes (see Ward and Stanford 1989; Ligon et al. 1995; Kondolf 1997). There are many more proposed dams, especially for northward flowing Amazon tributaries that, if built, would restrict dolphin movements (Best and da Silva 1989a) and probably degrade their habitat.
Mercury is often used to separate gold from soil and rock in mining operations along the Amazon (Pfeiffer ,i>et al. 1993). In the Madeira River almost 60% of children under five years old had accumulated levels of mercury, presumably from eating contaminated fish, high enough to cause neurological damage (Boischio and Henshel 1996). A study of mercury in the sediments and floating plants in the Tucuruí Reservoir of the former Tocantins River, Brazil, emphasized the risk of mercury accumulation in the bed of non-flowing waters (Aula et al. 1995). The effects of the bioaccumulation of mercury in botos are unknown but the high levels recorded in the Amazon ecosystem give reason for concern.
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