Botos

(Iniidae)

Class Mammalia

Order Cetacea

Suborder Odontoceti

Family Iniidae

Thumbnail description
Pink or grayish dolphin with narrow, long beak and prominent forehead; long flippers and low dorsal crest

Size
Maximum length is 8.4 ft (2.55 m) for males and 6.6 ft (2 m) for females; weight is 345 lb (156.5 kg) for males and 217 lb (98.5 kg) for females

Number of genera, species
1 genus; 1 species

Habitat
Rivers, tributaries, floodplains, and inundated forests

Conservation status
Vulnerable

Distribution
South America in the Orinoco and Amazon river systems

Evolution and systematics

Ischyrorhyncus, a fossil from the late Miocene found in Argentina, is a confirmed ancestor of Inia geoffrensis that was present in freshwater. Precisely how ancestors of boto entered the Amazon remains unsolved. Two theories posit the timing and origin of colonization in freshwater: one suggests an earlier colonization from the Pacific Ocean about 15 million years ago (mya), while the second proposes a more recent one (1.8–5 mya) from the Atlantic Ocean. In spite of contentious issues of phylogeny and taxonomy, a growing body of morphological,

genetic, and fossil evidence position I. geoffrensis closest to franciscana (Pontoporia blainvillei), and suggest that these two also are strongly related to Delphinoidea. Compared to other river dolphins, I. geoffrensis is the most recent taxa. Presently, I. geoffrensis is divided into three subspecies: I. g. geoffrensis (mainstem Amazon River); I. g. boliviensis (Amazon in eastern Bolivia); and I. g. humboldtiana (Orinoco River). This analysis is supported by morphologic studies and, in the case of the first two subspecies, also by molecular evidence.

The taxonomy for this species is Inia geoffrensis (Blainville, 1817), probably Upper Amazon. Other common names include: English: Amazon River dolphin, pink river dolphin; French: Dauphin de l'Amazone, inia; Spanish: Bufeo.

Physical characteristics

Bulky, with a prominent forehead and a long snout with sparse hair. The dorsal fin is reduced to a long dorsal crest. The flippers are large, paddle-shaped, and can rotate freely. The peduncle is compressed laterally and the flukes have a concave trailing edge with a medial notch. Botos are very flexible, largely due to free cervical vertebra.

Their eyes are very small but, unlike the Ganges and Indus dolphin, they provide good vision, both underwater and above it.

Coloration varies, apparently related to age and environmental factors such as water clarity. Calves are dark gray, while adults are pink. Dolphins living in "black waters" are generally darker than those from "white waters."

Distribution

Widely distributed in the main rivers of Amazon and Orinoco basins throughout Venezuela, Colombia, Ecuador, Guyana, Peru, Bolivia, and Brazil.

Fast-moving water such as Teotonio rapids in the Madeira River constitute barriers to the species' dispersal.

Current or historical abundance estimates for their entire range are unavailable. A growing number of surveys have provided local densities. This is the most abundant of the river dolphins, but population numbers are generally unknown.

Habitat

Exclusively a freshwater species, the boto occupies rivers, tributaries, and lakes, but does not tolerate the brackish waters of estuaries. It has an affinity for high turbulence zones, often where tributaries merge and where there are higher prey concentrations. The Amazon and Orinoco drainages undergo seasonal flood cycles, causing the water level to rise or fall up to 33 ft (10 m). The rainy season is from November to May and the dry season is from June to October. During flooding, dolphins disperse into floodplains and inundated forests where plentiful seeds and fruits attract fish. When the waters recede, the dolphins migrate back into the main channels of rivers or into deep pools. Water temperature where boto occurs ranges from 73.4 to 86°F (23–30°C). It shares its habitat with another river dolphin, Sotalia fluviatilis (tucuxi), which is not strictly a freshwater dolphin.

Behavior

Typically, sightings consist of single individuals, pairs, or a few individuals to a few dozen. Botos swim slowly but are extremely maneuverable, negotiating shallow waters and making their way around obstacles such as woody debris. A common mode of surfacing is ascending horizontally and showing only the top of the head. A much briefer surfacing, associated with deep-diving, occurs when a boto breaks the water with the snout and then exposes the dorsal crest by arching and rolling forward. Leaping out of the water is rare. Sometimes, the dolphins' presence can be detected even without seeing them because of the loud blowing sound produced when they exhale.

Botos are very curious and playful, using almost any object available (logs, turtles, or paddles) as toys. This phenomenon is also observed in captivity.

Nearly 40 dolphins were tracked with radio transmitters in Mamiraua System, Brazil, showing some individuals to be year-round residents. There is evidence that the boto uses echolocation for navigation and to capture prey. The sounds they produce are known as "clicks," which are predominantly between 85 and 100 kHz, well above the frequency audible to human beings. The boto also produces low-frequency sounds. Some researchers have recorded only pulsed sounds, while others have also recorded whistling. Further research is needed to determine whether the boto produces whistles and, if so, whether these sounds have similar functions to those made by oceanic species.

Feeding ecology and diet

Their agility allows the boto to pursue and capture fish in shallow or deep channels and in densely vegetated areas. Feeding tends to occur more often during early morning or in the afternoon. However, nocturnal fish species have also been found among stomach contents, suggesting that night feeding may occur.

The boto diet includes over 40 species of fish, mainly sciaenids, cichlids, and characins. The size of fishes varies 2–31 in (5–80 cm), but is typically 8 in (20 cm). Crustaceans, mollusks, and turtles have also been found in stomachs. Their teeth are differentiated into conical and molar-like teeth that facilitate masticating prey with hard exteriors. This dentition is unique among dolphins, which commonly have equal-sized conical teeth suited for grasping.

Reproductive biology

Reproduction is seasonal. Peak of births varies with river systems, but is generally between May and August. It is still unclear how calving season relates to the flood cycle of the rivers. Best and da Silva (1989) reported that in the Amazon, births appeared to be associated with seasonally receding waters and suggested that this timing would be favorable for pregnant and lactating females because prey fish would become more concentrated as surface waters diminished. However, in a tributary of the Orinoco River, McGuire and Winemiller (1998) observed calves when water levels began to rise, but never witnessed them during the period of falling waters. Lactation lasts more than one year, and females give birth every two to three years.

Sexual maturity (about five years old) is reached at total body lengths of 6.6 ft (2 m) and 5.2–5.9 ft (1.6–1.8 m) in males and females, respectively. Gestation is estimated at 10–11 months. The length at birth is approximately 2.6 ft (0.8 m).

The species' mating system is unknown. This is partly because their behavior and morphology hinder identification of individuals.

Conservation status

Although its range does not appear to be shrinking, the boto faces many serious threats to its habitat. Thus, it is classified as Vulnerable. National protective regulations are still incomplete, and most are recent. In Brazil, botos became protected in 1986.

Hydroelectric dams are common throughout the Orinoco and Amazon Rivers, and construction of many more are planned. Dams isolate dolphin populations into ever-smaller groups, making them increasingly vulnerable to environmental changes because of reduced genetic diversity. Moreover, dams and extensive deforestation reduce fish productivity. Pollution is another concern: mercury used in gold mining and pesticides, which tend to bio-accumulate in animals

higher on the food chain, are particularly detrimental to top predators like dolphins. Females transfer pollutants to their calves while nursing. Very few studies have reported on levels of toxic substances in fish and dolphins. Pesticides and high concentrations of mercury—close to levels considered toxic for humans—were detected in milk of botos. Pulp mills are also an important source of pollution. As the use of gill nets and seine nets increases, so does the incidence of entanglements. Although illegal, dynamite fishing still occurs and attracts dolphins to feed on stunned fish, exposing them to subsequent blasts. Preservation of boto habitat will require implementation of environmental impact assessment for new dams and monitoring pollution levels.

Significance to humans

During the 1960s and 1970s, about 100 dolphins were captured for exhibition at aquaria and oceanaria. Of these, about 70% went to the United States. However, high mortality occurred. Presently, only a handful are still exhibited worldwide. Many of these deaths resulted from aggression between the dolphins. Considering that botos do not generally occur in large groups, it is likely that confining several botos in small enclosures prevented them from maintaining minimum necessary spacing.

The boto once was a strong cultural influence on indigenous people—the focus of many folk tales and superstitions. It was both respected and feared for its supposed supernatural powers and its reputed ability to bring misfortune. Currently, dolphins interfere with fisheries, damaging nets while stealing fish. There are no reports of direct dolphin killings other than to stop destruction of fishing gear. However, when they are killed or found dead, genitalia and eyes are sometimes traded as love charms.

Resources

Books

Klinowska, Margaret. Dolphins, Porpoises and Whales of the World. Gland, Switzerland: World Conservation Union (IUCN), 1991.

Ridgway, Sam H., and Richard J. Harrison. Handbook of Marine Mammals. Vol. 4. London: Academic Press, 1989.

Periodicals

Ding, Wang, Bernd Würsig, and Stephen Leatherwood. "Whistles of Boto, Inia geoffrensis, and Tucuxi, Sotalia fluviatilis." Journal of the Acoustical Society of America 109, no. 1 (January 2001): 407–411.

Hamilton, H., S. Caballero, A. G. Collins, and R. L. Brownell Jr. "Evolution of the River Dolphins." Proceedings of the Royal Society of London 268 (2001): 549–558.

Kamminga, C., M. T. Van Hove, F. J. Engelsma, and R. P. Terry. "Sonar X: A Comparative Analysis of Underwater Echolocation of Inia spp. and Sotalia spp." Aquatic Mammals 19, no. 1 (1993): 31–43.

McGuire, Tamara L., and Kirk O. Winemiller. "Occurrence Patterns, Habitat Associations, and Potential Prey of the River Dolphin, Inia geoffrensis, in the Cinaruco River, Venezuela." Biotropica 30, no. 4 (1998): 625–638.

Messenger, Sharon L., and Jimmy A. McGuire. "Morphology, Molecules, and the Phylogenetics of Cetaceans." Systematic Biology 47, no. 1 (1998): 90–124.

Perrin, W. F., R. L. Brownell Jr., K. Zhou, and J. Liu. "Biology and Conservation of the River Dolphins." Occasional Papers, IUCN Species Survival Commission, no. 3(1989).

Podos, Jeffrey, Vera M. F. da Silva, and Marcos R. Rossi-Santos. "Vocalizations of Amazon River Dolphins, Inia geoffrensis: Insights into the Evolutionary Origins of Delphinid Whistles." Ethology 108 (2002): 601–612.

Rosas, Fernando C. W., and Kesae K. Lehti. "Nutritional and Mercury Content of Milk of the Amazon River Dolphin, Inia geoffrensis." Comparative Biochemistry and Physiology 115A, no. 2 (1996): 117–119.

Yang, G., K. Zhou, W. Ren, G. Ji, and S. Liu. "Molecular Systematics of River Dolphins Inferred From Complete Mitochondrial Cytochrome-B Gene Sequence." Marine Mammal Science 18, no. 1 (2002): 20–29.

Organizations

Instituto Nacional de Pesquisas da Amazonia (INPA). Alameda Cosme Ferreira 1796, Aleixo, Manaus, 69011-970 Brazil. Phone: (092) 643-3184. Fax: (092) 643-3292. E-mail: [email protected] Web site: <http://www.cnpq.br>

Omacha Foundation. Web site: <http://www.omacha.org>

The World Conservation Union (IUCN), Species Survival Commission. Rue Mauverney 28, Gland, 1196 Switzerland. Phone: 41 (22) 999-0152. Fax: 41 (22) 999-0015. E-mail: [email protected] Web site: <http://www.iucn.org>

Paula Moreno, MS