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Inia geoffrensisAmazon river dolphin(Also: boto; pink river dolphin)

By Ryan Bebej

Geographic Range

Inia geoffrensis (boto or Amazon River dolphin) can be found 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. Three subspecies are recognized, with each subspecies occupying a different area of these river systems: I. g. geoffrensis occupies the central Amazon River basin; I. g. humboldtiana resides in the Orinoco River basin; and I. g. boliviensis can be found in the upper Madeira River, cut off from the Amazon River by impassable rapids. 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)

Habitat

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, Inia geoffrensis 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)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools

Physical Description

Inia geoffrensis is the largest of the river dolphins, with males achieving a length of up to 2.55 m (average: 2.32 m) and a mass of up to 207 kg (average: 154 kg). Females are smaller, getting up to 2.18 m (average: 2.00 m) in length and 154 kg (average: 100 kg) in mass. This difference in size marks this species as one of the most sexually dimorphic cetaceans, and having larger males makes it unique among river dolphins, where females are generally the larger sex. (Best and da Silva, 1993; da Silva, 2002; Martin and da Silva, 2006)

Body color varies with age, with young individuals being dark gray and adults possessing a solid or blotched pink hue, although males have been found to be significantly pinker than females. Some adults are darker on their dorsal surface than others, and it is thought that coloration may depend on temperature, clarity of water, and geographic location. (Best and da Silva, 1993; da Silva, 2002; Martin and da Silva, 2006)

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 I. geoffrensis 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. (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)

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. Inia geoffrensis 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. (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)

  • Sexual Dimorphism
  • male larger
  • male more colorful
  • Range mass
    98.5 to 185 kg
    216.96 to 407.49 lb
  • Range length
    1.24 to 2.55 m
    4.07 to 8.37 ft

Reproduction

Little is known about the mating system of Inia geoffrensis. 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. (Best and da Silva, 1984; Caldwell, et al., 1989; Martin and da Silva, 2006; McGuire and Winemiller, 1998)

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. Inia geoffrensis 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. (Best and da Silva, 1984; Best and da Silva, 1989a; Best and da Silva, 1993; Brownell, 1984; da Silva, 2002; Harrison and Brownell, 1971)

  • Breeding interval
    Botos breed once every year
  • Breeding season
    Botos breed between June and August
  • Range number of offspring
    1 to 1
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    11 months
  • Average gestation period
    287 days
    AnAge
  • Average weaning age
    12 months
  • Average time to independence
    2-3 years
  • Average age at sexual or reproductive maturity (female)
    5 years
  • Average age at sexual or reproductive maturity (male)
    5 years

The rather long periods of lactation and calving observed in Inia geoffrensis signify a strong mother-calf bond. Most boto pairs seen in the wild are mothers with their calves, and one pair in captivity was inseparable for three years. Some authors have suggested that this long period of parental care may be for learning and development of the young, as seen in bottlenosed dolphins (Tursiops truncatus). (Best and da Silva, 1989b; Best and da Silva, 1993; McGuire and Winemiller, 1998)

  • Parental Investment
  • precocial
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female
  • extended period of juvenile learning

Lifespan/Longevity

The longevity of Inia geoffrensis 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. (Best and da Silva, 1993; Caldwell, et al., 1989)

Behavior

Inia geoffrensis is 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, et al., 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, I. geoffrensis is less timid than bottlenose dolphins (Tursiops truncatus), yet they have been more difficult to train than most other dolphins. (Best and da Silva, 1989a; Best and da Silva, 1993; da Silva, 2002)

Home Range

Apparently occupying the same area for more than a year, most botos are rather sedentary. They display no obvious defense of home ranges, but if they do, the ranges are likely large and overlapping. This species does undertake seasonal migrations correlated with water level and fish abundance, but these shifts are minor excursions from the area they occupy during the rest of the year. (Best and da Silva, 1989a; Best and da Silva, 1989b; Best and da Silva, 1993)

Communication and Perception

Inia geoffrensis uses echolocation to catch prey, navigate, and perceive its environment. The frequency of these clicks does not appear to be significantly different from that of Tursiops truncatus, with 45 kHz being a dominant frequency. These clicks, which range from 16-170 kHz, are also used to communicate between individuals. Botos in captivity have been shown to make 10 distinct calls, including echolocation-like burst click runs, barks, whimpers, squeaks, and cracks. They also appear to use open mouths when communicating, as suggested by some tooth rake scars seen on all individuals. (Best and da Silva, 1993; Martin and da Silva, 2006)

Food 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 Serrasalmidae (piranhas), but their heterodont dentition allows them to crush armored prey as well, including river turtles (Podocnemis sextuberculata) and crabs (Poppiana argentiniana). 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. (Best and da Silva, 1989a; Best and da Silva, 1989b; Best and da Silva, 1993; da Silva, 2002; McGuire and Winemiller, 1998)

Botos are usually solitary feeders, most active between 0600-0900 hours and 1500-1600 hours 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)

  • Animal Foods
  • reptiles
  • fish
  • aquatic crustaceans

Predation

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 Cetopsidae and Trichomycteridae. (Best and da Silva, 1989a; Best and da Silva, 1993)

Ecosystem Roles

The diverse diet of Inia geoffrensis causes it to have an impact on a number of different species. Of its prey items, botos may have the largest effect on the family Sciaenidae, since they seem to prefer these species. They have also formed mutualistic relationships with Sotalia fluviatilis (tucuxis) and Pteronura brasiliensis (giant otters) by forming coordinated hunting groups with them. Botos have several parasitic trematodes and nematodes, many of which are host-specific. If the crescent-shaped wounds seen on botos can indeed be attributed to catfish from the family Trichomycteridae, then they have an ectoparasite as well. (Best and da Silva, 1989a; Best and da Silva, 1993)

Mutualist Species
Commensal/Parasitic Species

Economic Importance for Humans: Positive

There is little direct hunting of botos by native people, although Portuguese colonists may have hunted them to obtain oil for lamps. If botos are found dead, native people may use the fat as a cure for asthma and the oil to treat rheumatic pains or even infections in their cattle. They sometimes use the eyes, genitalia, and teeth as love charms and amulets as well. However, they never use the meat or skin. In addition, fishermen have been known to use botos to lead them to schools of fish. (Best and da Silva, 1989a; Best and da Silva, 1989b; Best and da Silva, 1993)

Economic Importance for Humans: Negative

While there is little overlap between the fish that Inia geoffrensis prefers and the species that fisheries seek, botos have been known to tear fish from nets, causing damage to expensive fishing gear and, in some cases, a drastic reduction in fish catch. (Best and da Silva, 1989b; Culik, 2000)

Conservation Status

Human activities are exerting a lot of pressure on Inia geoffrensis populations. There have been many negative interactions with fisheries. As fishing technologies have improved, the incidental catching of botos has greatly increased. They have also been harpooned, shot, and poisoned for stealing fish out of nets and damaging the fishing equipment. A greater human demand for fish decreases the abundance of potential prey items for botos as well. (Best and da Silva, 1993; Culik, 2000; da Silva, 2002; Vidal, 1993; Best and da Silva, 1989b; Best and da Silva, 1993; Culik, 2000; da Silva, 2002; Vidal, 1993)

Hydroelectric dams have been problematic in several ways. They decrease the available food supply by preventing various species of fish from migrating downstream, while also decreasing the oxygen level downstream. Dams split up populations of I. geoffrensis, potentially reducing gene pools in these subpopulations to levels where they may not have enough genetic diversity to survive, thereby increasing the risk of extinction. (Best and da Silva, 1989b; Best and da Silva, 1993; da Silva, 2002; Vidal, 1993)

Deforestation for agriculture in floodplains reduces fish populations by eliminating the fruits and seeds in the flooded forests that they feed upon, thus decreasing the potential food supply for botos. The rivers inhabited by I. geoffrensis are polluted by pesticides from agricultural fields and heavy metals (including mercury) from gold refining, which negatively affect both botos and their prey items. (Best and da Silva, 1989b; Best and da Silva, 1993; Culik, 2000; da Silva, 2002; Vidal, 1993)

Inia geoffrensis is classified as vulnerable by the IUCN. They have traditionally been difficult to keep in captivity, due to aggression and fairly short longevity. If boto numbers begin to dwindle to dangerously low levels in the wild, it would be alarming because populations may not be able to be maintained for long in captivity. (Caldwell, et al., 1989; da Silva, 2002)

Other Comments

Boto is the internationally-recognized common name of Inia geoffrensis, but other common names include the Amazon River dolphin, bufeo, bufeo colorado, tonina, delfin rosado, and pink dolphin. (da Silva, 2002)

Botos are part of the folklore of Amazonian people. There are several legends giving botos supernatural powers, which is why they are typically respected and protected. Some myths tell of botos turning into beautiful men or women during the night and luring members of the opposite sex down into the river, never to return. Another myth speaks of the spirits of drowned people entering the bodies of botos. (Best and da Silva, 1993; da Silva, 2002)

There is not consensus as to whether the ancestors of I. geoffrensis entered the Amazon River basin from the Pacific Ocean before the Andean orogeny 15 million years ago or from the Atlantic Ocean much more recently. (Best and da Silva, 1993)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Ryan Bebej (author), University of Michigan-Ann Arbor, Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

echolocation

The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

freshwater

mainly lives in water that is not salty.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

piscivore

an animal that mainly eats fish

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

young precocial

young are relatively well-developed when born

References

2004. "Boto (Amazon River Dolphin), Inia geoffrensis fact sheet" (On-line). American Cetacean Society. Accessed January 27, 2006 at http://www.acsonline.org/factpack/Boto.htm.

Best, R., V. da Silva. 1989. Amazon River Dolphin, Boto, Inia geoffrensis (de Blainville, 1817). Pp. 1-23 in S Ridgway, R Harrison, eds. Handbook of Marine Mammals: River Dolphins and the Larger Toothed Whales. London: Academic Press.

Best, R., V. da Silva. 1989. Biology, Status and Conservation of Inia geoffrensis in the Amazon and Orinoco River Basins. International Union for Conservation of Nature and Natural Resources (IUCN), Species Survival Commission, Occasional Paper 3: 22-34.

Best, R., V. da Silva. 1993. Inia geoffrensis. Mammalian Species, 426: 1-8.

Best, R., V. da Silva. 1984. Preliminary Analysis of Reproductive Parameters of the Boutu, Inia geoffrensis, and the Tucuxi, Sotalia fluviatilis, in the Amazon River System. Report of the International Whaling Commission, Special Issue 6: 361-369.

Brownell, R. 1984. Review of Reproduction in Platanistid Dolphins. Report of the International Whaling Commission, Special Issue 6: 149-158.

Caldwell, M., D. Caldwell, R. Brill. 1989. Inia geoffrensis in Captivity in the United States. International Union for Conservation of Nature and Natural Resources (IUCN), Species Survival Commission, Occasional Paper 3: 35-41.

Culik, B. 2000. "Inia geoffrensis (de Blainville, 1817)" (On-line). Convention on Migratory Species. Accessed January 27, 2006 at http://www.cms.int/reports/small_cetaceans/data/I_geoffrensis/I_geoffrensis.htm.

Harrison, R., R. Brownell. 1971. The Gonads of the South American Dolphins, Inia geoffrensis, Pontoporia blainvillei, and Sotalia fluviatilis . Journal of Mammalogy, 52: 413-419.

Martin, A., V. da Silva. 2004. River dolphins and flooded forest: seasonal habitat use and sexual segregation of botos (Inia geoffrensis) in an extreme cetacean environment. Journal of the Zoological Society of London, 263: 295-305.

Martin, A., V. da Silva. 2006. Sexual dimorphism and body scarring in the boto (Amazon River dolphin) Inia geoffrensis . Marine Mammal Science, 22: 25-33.

McGuire, T., K. Winemiller. 1998. Occurrence Patterns, Habitat Associations, and Potential Prey of the River Dolphin, Inia geoffrensis, in the Cinaruco River, Venezuela. Biotropica, 30: 625-638.

Vidal, O. 1993. Aquatic Mammal Conservation in Latin America: Problems and Perspectives. Conservation Biology, 7: 788-795.

da Silva, V. 2002. Amazon River Dolphin. Pp. 18-20 in W Perrin, B Würsig, J Thewissen, eds. Encyclopedia of Marine Mammals. San Diego: Academic Press.

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