Inia geoffrensis (Blainville, 1817)

Russell A. Mittermeier & Don E. Wilson, 2014, Iniidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 364-379 : 377

1.

Amazon River Dolphin

Inia geoffrensis

French: Boto de Amazone / German: Amazonas-Delfin / Spanish: Delfin del Amazonas

Other common names: Boto, Bufeo, Pink River Dolphin; Orinoco Boto, Orinoco Bufeo (Orinoco population)

Taxonomy. Delphinus geoffrensis de Blainville, 1817 ,

“sur la cote du Brésil,” probably upper Amazon of Brazil.

Botos living in the Orinoco Basin have been recognized as a subspecies, the “Orinoco Boto” (humboldtiana), by some authors, but movement of dolphins between this and the adjacent Amazon Basin is very likely to occur. Monotypic.

Distribution. Most of the Orinoco and Amazon basins in Venezuela, Colombia, Brazil, Peru, and Ecuador.

Descriptive notes. Total length 219-255 cm (males) and 182-225 cm (females); weight 113.5-207 kg (males) and 72-154 kg (females). Botos are the largest of all river dolphins. Amazon River Dolphins are highly sexually dimorphic, with adult males longer and heavier than females. Mature adult males often have “cobblestoning” skin modifications on pectoral fins and tailstock. These are related to fighting and may be a form of scar tissue or a shield or weapon. Body of the Amazon River Dolphin is corpulent and heavy but extremely flexible, capable of bending and twisting. Young are dark gray and became lighter with age. Adults can be from uniform pink to uniform dark gray, but usually grayer above and pinker below. Adult males are more pink than females and become pinker with age. The difference of body color between sexes is greater than in any other cetacean, with pinkness of males perhaps advertising size and strength in sexual display. Rostrum is prominent and robust, with short bristles on top and below the jaw. Eyes are small and inconspicuous, but vision is good. Amazon River Dolphins have two different types of teeth: conical and molartype. Total number of upper jaw teeth range from 23-35 (mean 26-6) and the lower jaw from 24-35 (mean 27-1). Each upper and lower mandible has 7-9 molar-type teeth on both sides. Melon is relatively small and flaccid and can be altered in shape by muscular control. Pectoral fins are large, broad, thick, paddle-like, and capable of desynchronized movements, allowing botos to swim backward and in shallow places. Flukes are broad, thick, and triangular in shape. Dorsal fin is not falcate (curved), but laterally flattened and like a dorsal keel extending from the mid-body to the strongly keeled caudal peduncle.

Habitat. Freshwater, in waterways from the upper reaches of the Amazon and Orinoco to just above the brackish waters of their estuaries. Botos can swim in water shallower than 2 m and come very close to the shore c.1 m depth. Most Amazon River Dolphins live in areas of low gradient, where annual cycles of water depth produce profound habitat changes. For half the year, Amazon River Dolphins have access to vast areas of inundated floodplain, much of it consisting of a complex mosaic of forest, lakes, and channels. This habitat is characterized by low current and high fish density, providing a safe environment, especially for young individuals. Occupied rivers, lakes, and channels vary substantially in character. Some have acidic, black water emanating from flooded forests. Others have clear water originating in geologically old drainage basins. Many have white water laden with sediments from the Andes. Fish communities vary among these ecosystems, but Amazon River Dolphins feed on a wide variety of fish species. Seasonal changes in precipitation and water depth dominate habitats of botos. Major rivers and channels retain adequate water for Amazon River Dolphins throughout the year, but many of the smaller channels and lakes cannot be relied on to retain water in the dry season, so botos and most fish abandon them for several months of the year. The flexible body and broad, rotatable flippers of Amazon River Dolphins, together with the lack of a “normal” falcate dorsal fin, are adaptations for swimming in the tangled vegetation of the flooded forest.

Food and Feeding. The heterodont (more than one type of tooth) dentition of all three species of botos, unique in cetaceans, allows them to capture and crush armored prey such as armored catfish, crabs, and, on occasion, turtles. Fish dominates the diet of all botos. More than 50 fish species, representing more than 20 families, have been found in the stomachs of Amazon River Dolphins in the Central Amazon area, Brazil, including freshwater fish of benthic, littoral, and pelagic habits. Fish of 5-80 cm in length (average c.20 cm) are consumed. Prey diversity is greater at high water levels when botos have access to flooded forests.

Breeding. Births of botos in Central Amazon, Brazil, occur throughout the year, with a small peak in the low-water season. Length of gestation is not accurately known, but it is probably 11-12 months. Littersize is always one. Neonates are 80-90 cm long and c.10-13 kg at birth. They remain with their mothers for at least two years. Simultaneous pregnancy and lactation are not uncommon, and reproductivciy active mature females spend most of their lives accompanied by a dependent offspring.

Activity patterns. There is limited evidence of diurnal activity patterns of Amazon River Dolphins from echolocation click density and movements, but presence or absence of light is arguably unlikely to be of profound importance because botos live in waters where visibility and light penetration are usually very low. Indeed,vision is not essential to their well-being, and a substantial number of healthy botos have opaque eye lenses (cataracts).

Movements, Home range and Social organization. Studies of Amazon River Dolphins in Brazil have relied on the resightings of individuals or, in one case, close-range detection of radio signals from marked individuals. Therefore, long-range movements are almost certainly underrepresented but are likely to occur. Daily movements of up to 20 km are normal, with sustained swimming speeds of 3-6 km/h. About half of Amazon River Dolphins at the entrance of the Mamiraua Lake system in the central Brazilian Amazon were “residents,” defined as being seen there in at least seven months of the year, and 90% of the botos seen inside the system met that definition. The “non-residents” demonstrated a range offidelity to the area, with some (among the hundreds marked permanently with freeze-brands) never being encountered again. Seasonal changes in water level and availabilities of habitats resulted in major local movements of botos on a scale of a few tens of kilometers, but they did not change overall numbers of botos in a local area. Similarly, no larger-scale migrations of Amazon River Dolphins are known or suspected, and there are no obvious reasons for such population-level movements.

Status and Conservation. CITES Appendix I. Classified as Data Deficient on The IUCN Red Lust. Population sizes and trends are inadequately known. The Amazon River Dolphin is distributed throughout most of the Amazon Basin, and it occurs in and near the main stem of the Orinoco River and its largest tributary, the Apure River. It has been reported in smaller rivers of the Orinoco River system and may occur at low densities throughout this basin. Amazon River Dolphins do not occupy salty or brackish waters, but they are tolerant of a wide variety of types of freshwater and habitats. Shallow water, impassable rapids, and availability of food likely limit distribution of the Amazon River Dolphin. Total population size is unknown, but it is likely tens of thousands or more. In the year 2000, 13,000 Amazon River Dolphins were estimated to occur in an area of ¢.11,240 km? in Mamiraua Sustainable Development Reserve in the central Brazilian Amazon. Encounter rates are ¢.0-6 ind/km of waterway, and most botos in the major rivers occur near the margins. Estimated density of Inia in the Amazon River in Colombia (Loretoyacu and Javari rivers) was 1-8 ind/km? in an area of 593 km?, giving a total population estimate of 1115 botos. In Peru, in the Samiria River and a stretch of the Maranon River between the city of Iquitos and the confluence with the Samiria River, densities were 2.7 ind/km? and 5-9 ind/km? in an area of 554 km?, and estimated population size of 917 botos. In the Ecuadorian Amazon, in the rivers Napo, Cuyabeno, Yasuni, Lagartococha, and Aguarico in a surveyed area of 144 km?®, the population size estimate was 147 botos; encounter rates ranged 0-03-0-4 ind/km®. In the Orinoco River estimated density was 1-1 ind/km?* and population size 1779 in a surveyed area of 1684 km. The Amazon River Dolphin has been widespread and abundant, despite loss of many individuals (probably thousands) per year to entanglement and drowning in monofilament gillnets,first introduced in the 1960s and now used by almost every riverside household in the region. Young and juvenile botos were, and remain, especially vulnerable. Since 2000, standardized monitoring of abundance of Amazon River Dolphins in Mamiraua Sustainable Development Reserve detected a marked increase in mortality and the beginning of a steady decline in boto abundance. This coincided with reports of the deliberate killing of botos for use as fish bait. Botos are used as bait to catch an abundant scavenging catfish, the zamurito (Calophysus macropterus, Siluriformes), which is widely distributed in the Amazon River Basin. One person can catch hundreds of catfish in a single night, with no equipment other than a large box to hold the catch and a supply of suitable bait. As stocks of catfish diminished in Colombia, this method offishing was exported downriver into Brazil, and today botos from the Brazilian Amazon are harpooned and often sold specifically for bait in Colombia, Venezuela, Peru, and Brazil. Hunting botos is illegal, clandestine, and almost impossible to quantify. Nevertheless, the approximate size of the catfish catch is known, and calculations of the ratio of botos used per ton of catfish landed, derived from the communities involved, indicates that across its distribution, thousands of Amazon River Dolphins are killed each year in pursuit of this one fishery. In some places, the impact of this hunt on the overall population of Amazon River Dolphins appears to be profound. The hunt continues unabated, and the population continues to decline at an annual rate of ¢.10%. The Amazon River Dolphin is steeped in folklore legends. River-dwelling communities in Brazil fear and revere the Amazon River Dolphin, which is reputed to transform into a man under cover of darkness and seduce women, thereby explaining unexpected pregnancies in human fishing communities. Supposed body parts of Amazon River Dolphins are bought as love charms in Amazonian markets, but DNA analyses have revealed that these are rarely accurately labeled, at least in recent times. Most derive from domestic pigs and sheep, and the Guiana Dolphin (Sotalia guianensis), an ocean dolphin.

Bibliography. Aliaga-Rossel et al. (2006), Araujo (2010), Araujo & da Silva (2014), Araujo & Wang (2012), Banguera-Hinestroza et al. (2002), Best (1984), Best & da Silva (1983, 1984, 1989a, 1989b, 1993), de Blainville (1817), Costa et al. (2013), Cuvier (1823), Desmarest (1822), Dutra et al. (2013), Gervais (1855), Gomez-Salazar et al. (2012), Gravena et al. (2008), Hrbek et al. (2014), Lailson-Brito et al. (2008), Latrubesse & Stevaux (2006), Martin & da Silva (2004a, 2004b, 2006), Martin, da Silva & Rothery (2008), Martin, da Silva & Salmon (2004), May-Collado & Wartzok (2007), McGuire & Aliaga-Rossel (2010), McGuire & Winemiller (1998), Mead & Koehnken (1991), Mintzer et al. (2013), d'Orbigny (1834), Podos et al. (2002), Rapp (1837), Reeves & Martin (2009), Reeves, Jefferson et al. (2008c), Romagnoli (2009), Romagnoli et al. (2011), Sasaki (2013), Serrano et al. (2007), da Silva (1983, 1994, 2009), da Silva & Best (1990,1996), da Silva & Martin (2000, 2007, 2010), da Silva, Goulding & Barthem (2008), da Silva, Martin & do Carmo (2011), Sioli (1984), Slater (1994), von Spix & von Martius (1831), Tavera et al. (2010), Tregenza et al. (2007), Trujillo et al. (2010), Vidal et al. (1997).

Figures

1. Amazon River Dolphin (Inia geoffrensis), 2. Bolivian Boto (Inia boliviensis), 3. Araguaian Boto (Ima araguaiaensis)

Distribution. Most of the Orinoco and Amazon basins in Venezuela, Colombia, Brazil, Peru, and Ecuador.