The boto, or Amazon River dolphin, is probably the best-known of the river dolphins. These animals are moderately robust, with long beaks and steep bulbous foreheads capable of changing shape. There is no true dorsal fin, but only a dorsal ridge that is low and wide-based. The flippers are large and triangular, with blunt tips, and the flukes have a concave trailing edge that is often ragged. The eyes are small, but not as small as those of the susu or bhulan.
Botos are gray to pink above and lighter below; some individuals are totally pink. In general, young animals are mostly uniform dark gray; they become progressively more pinkish with age. The extreme color is so unique that the boto is often called the pink dolphin.
The mouth is lined with 23-35 stout teeth in each row. This is the only species of cetacean with differentiated teeth; those at the front of the jaw are typically conical, but those near the rear are flanged on the inside.
Adult size ranges to 2.3 m (females) or 2.8 m (males). Males can reach maximum weights of 160 kg. At birth, botos are about 80 cm long.
Can be Confused With
The only other dolphin that inhabits the range of the boto is Sotalia. This latter species is much smaller, has a true dorsal fin, and more spritely dolphin-like movements.
Botos are endemic to the Amazon and Orinoco drainage basins of South America. Their distribution extends to the upper reaches (impassible falls or rapids) of these rivers and their tributaries in Guyana, Colombia, Ecuador, Peru, and Bolivia, as well as the lower reaches in Brazil and Venezuela. Mark/recapture studies have shown that some individuals are resident in specific areas year-round.
Ecology and Behavior
Boto are widely found in main river channels and are also found in smaller tributaries, lakes and (seasonally) the flooded forest. Loose aggregations of up to 12-15 have been observed, generally at confluences, but most botos are seen singly or in small groups. They generally move slowly, and surface at a shallow angle, showing the top of head and the dorsal ridge. Their responses to humans can range from shyness to curiosity, although they do not ride vessel bow waves. Aerial behavior is rare. Botos swim in to the flooded forest in the high-water season, and can often be heard searching for prey among the roots and trunks of partially-submerged trees.
In Brazil, most births apparently occur in May to July, the season of peak flooding. Females mature sexually at about 5 years of age, and males do so much later.
Feeding and Prey
These animals feed on a large variety of fishes, generally near the bottom. Some of their prey have hard outer shells, and dolphins have been observed breaking up their larger prey before swallowing them. They sometimes feed in a coordinated manner, occasionally with other species (such as Sotalia).
Threats and Status
Main threats include:
• Incidental catches in fishing gear
• Prey depletion
• Damming of rivers (although this is, at present, much less of a problem than for Platanista gangetica)
• Environmental pollution from organochlorines and heavy metals
The boto is unquestionably the species of river dolphin in least danger of extinction. It is still widespread and relatively numerous in many portions of its range. The current conservation status is “Vulnerable” (IUCN) and “Not Listed” (ESA).
Best, R.C. and V.M.F. da Silva. 1989. Amazon River dolphin, boto Inia geoffrensis (de Blainville, 1817). pp. 1-24 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Vol. 4 River dolphins and the larger toothed whales. Academic Press, London, UK.
Best, R.C., and V.M.F. Da Silva. 1993. Inia geoffrensis. Mammalian Species 426:8 pp.
Da Silva, V.M.F. 2002. Amazon River dolphin Inia geoffrensis. pp. 18-20 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press, San Diego, CA.
Leatherwood, S. 1996. Distributional ecology and conservation status of river dolphins (Inia geoffrensis and Sotalia fluviatilis) in portions of the Peruvian Amazon. Ph.D. dissertation, Texas A&M University.
Reeves, R.R., T.L. McGuire and E.L. Zuniga. 1999. Ecology and conservation of river dolphins in the Peruvian Amazon. IBI Reports 9:21-32.
Vidal, O., J. Barlow, L.A. Hurtado, J. Torre, P. Cendon, and Z. Ojeda. 1997. Distribution and abundance of the Amazon river dolphin (Inia geoffrensis) and the tucuxi (Sotalia fluviatilis) in the upper Amazon River. Marine Mammal Science 13:427-445.