Amazon River Dolphin (marine mammals)



I. Genus and Species: Common Names and Taxonomy

The Amazon River dolphin. Inia geoffrensis, is known by different names throughout its distribution: boto in Brazil; bufeo and bufeo Colorado in Colombia, Ecuador, and Peru; and tonina and delfin rosado in Venezuela. It is also known in English as pink dolphin, although the Brazilian name “boto” is considered the international common name.

The boto belongs to the superfamily Platanistoidea. The genus Inia is monospecific, with three currently recognized subspecies: Inia geoffrensis geoffrensis, I. g. boliviensis, and I. G. humoldtiana.

II. Distribution, Abundance, and Density

The boto has an extraordinarily wide distribution, occurring almost everywhere it can physically reach without venturing into marine waters. It occurs in six countries of South America— Bolivia, Brazil, Colombia, Ecuador, Pern, and Venezuela—in a total area of about 7 million km” (Fig. 1). It can be found along the entire Amazon River and its principal tributaries, smaller rivers and lakes, from the delta near Belem to its headwaters in the Ucayali and Maranon Rivers in Peru. Its principal limits are impassable falls such as those of the upper Xingu and Tapajos Rivers, and the Teotonio falls in the upper Madeira River in the southern part of the Amazon basin. The boto is also found throughout the Orinoco river basin, with the exception of the Caroni River and upper Caura River above Para falls in Venezuela. An isolated population occurs above Teotonio and Abulia falls in the upper Madeira River and in the Beni/Mamore basin of Bolivia.

The boto is the most common river dolphin. Its current distribution and abundance apparently do not differ from in the past, although relative abundance and density are highly seasonal and appear to vaiy among rivers. During the dry season the dolphins are concentrated in the main channels of the rivers, whereas during the flooded season they dispense into the flooded forest (igapo) and river floodplains (varzea).

No quantitative estimation of the relative abundance of the boto between rivers or basins exists. Differences in survey methodology used by different authors and lack of effort make the comparison between the results of the different surveys available in the literature very difficult. The only long-distance surveys of the species were carried out on the Solimoes-Amazon River, from Manaus to Santo Antonio do Iga-Tabatinga over a total of ca. 1200 km. The number of sightings per unit effort gave an average number of 332 ± 55 botos per survey in = 9), and the estimated density was of 0.08-0.33 botos/km in the main river and 0.49-0.98 botos/km in the smaller channels. Another boat survey along ca. 120 km of the Amazon River bordering Colombia, Peru, and Brazil carried out by Vidal and collaborators (1997) estimated 345 (CV = 0.12) botos in the study area with a density per square kilometer of 4.8 in tributaries, 2.7 around islands, and 2.0 along the main banks. These figures suggest that the boto shows the highest densities among any cetacean.

III. External Characteristics

The boto (Fig. 2) is the largest of the river dolphins, with a maximum recorded body length of 255 cm and mass of 185 kg for males and 215 cm and ca. 150 kg for females. The body is corpulent and heavy but extremely flexible. Nonfused cervical vertebrae allow the movement of the head in all directions. The flukes are broad and triangular: the dorsal fin is long, low, and keel-shaped, extending from the midbody to the strong laterally flattened caudal peduncle. The flippers are large, broad, and paddle-like and are capable of circular movements. Although most of these characteristics restrict speed during swimming, they allow this dolphin to maneuver between trees and submerged vegetation to search for food in the flooded forest. The rostrum and mandible are prominent, long, and robust. Short bristles on the top of the rostrum persist during adulthood. The melon is small and flaccid, but the shape can be altered by muscular control. The small, round eves are lunctional and the vision is good, both under and above water.

Map showing the general distribution of the boto (Inia geoffrensis.) in South America.

Figure 1 Map showing the general distribution of the boto (Inia geoffrensis.) in South America.

Body color varies with age. Fetuses, neonates, and young animals are dark gray. Juveniles and subadults are uniform medium gray to pinkish, and older botos are completely pink or blotched pink. When adult botos are dark on the dorsum, the flanks and underside are pinkish. One albino was captured and maintained in captivity for more than 1 year in an aquarium in Germany.

IV. Behavior and Life History

The boto is at times solitary and is not often seen in cohesive groups of more than three individuals; most groups of two are mother-calf pairs. Loose aggregations may be seen at the mouth or in bends of rivers and canals due to the large concentrations of fish or for purposes of courtship and mating. The boto is known to react protectively to injured or captured individuals.

Often characterized by pink body color, Amazon River dolphins (Inia geoffrensis) are the largest of the platanistoid dolphins.

Figure 2 Often characterized by pink body color, Amazon River dolphins (Inia geoffrensis) are the largest of the platanistoid dolphins.

The boto is a slow swimmer with a normal speed of 1.5-3.2 km/hr, but bursts of >14-22 km/hr have been recorded. The boto is capable of strong swimming for some length of time. When surfacing, the melon, tip of the rostrum, and long dorsal keel are out of the water simultaneously in a very conspicuous way. The boto does a high-arching roll in which these parts appear sequentially thrust well out of the water. The tail is rarely raised out of the water prior to a dive. Botos also wave a flipper, show the head or tail above the surface, lob-tail, and rarely jump clear of the water.

Studies in captivity indicated that botos are less timid and show less social contact, aggressive behavior, play, and aerial behavior than bottlenose dolphins (Tursiops truncatus). However, botos in captivity may not show their true range of behaviors. The boto is very curious and playful, rarely showing fear of strange objects. Wild botos grasp fishermen’s paddles, rub against canoes, pull grass under water, throw sticks, and play with logs, clay, turtles, and fish. Several observers have reportedly seen botos in a stationary position, often upside down with the eyes closed.

The boto is active day and night. The greatest fishing activity occurs at 0600-0900 and 1500-1600 hrs. It feeds on over 43 species of fish belonging to 19 families. Stomach content analysis has revealed up to 11 fish species in one animal. The mean size of consumed fish is 20 cm (range 5-80 cm), with larger fish torn to pieces. In captivity, food sharing has been recorded. Daily consumption is about 2.5% of the body weight. The botos diet is unique among cetaceans in that its heterodont dentition allows it to tackle and crush armored prey.

Males attain sexual maturity much later than females at about 200 cm in length. In females, sexual maturity occurs at around 5 years of age at body lengths between 160 and 175 cm. Reproductive events are seasonal. Gestation time has been estimated at about 11 months, and the calving season is apparently long, with most births occurring at the peak of the river’s flood level. Length at birth is about 80 cm. Lactation lasts more than 1 year and the birth interval is 2 to 3 years. Studies of marking and recaptures carried out by da Silva and Martin in Central Brazil have shown that some individuals are resident in a particular area during the entire year.

V. Human Effects and Interactions

The boto is part of the folklore and culture of Amazonian people, and several legends and myths are commonly known throughout its distribution. Because of these legends, often giving the boto supernatural powers, the boto was protected and respected in the past, although body parts of incidentally captured animals have been used by local people for medical purposes and as love charms. With increased use of nylon gill nets, machine-made lampara seines, and other new fishing techniques, the incidental catching of botos has become more common. With greater demand for fish due to rapid increases in human populations, the boto s food sources are being reduced. Other threats to the species are the construction of hydroelectric dams on major tributaries affecting the abundance and presence of some species of fish. Dams separate and isolate populations and may reduce the gene pool, and thereby increase chances of extinction. Analysis of milk from botos from the upper Amazon River (Letfcia) and Central Amazon (Man-aus) revealed that chemical pollution of the river systems by pesticides and mercury poses serious threats to the species.

VI. Status and Conservation

Inia geoffrensis of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and is classified by the IUCN as Vulnerable because of serious threats throughout its range.

Next post:

Previous post: