AMAZON RIVER DOLPHINS
Amazon river dolphins (Scientific name: Inia geoffrensis) are the largest of the river dolphin species. They are toothed whale native to and found exclusively in South America, mostly in the Amazon area. There are three subspecies: 1) Amazon river dolphin (Inia geoffrensis geoffrensis), in the Amazon basin; 2) Bolivian river dolphin (Inia geoffrensis boliviensis), the upper Madeira River in Bolivia; and 3) Orinoco river dolphin (Inia geoffrensis humboldtiana), in the Orinoco basin. The position of Araguaian river dolphin (Inia araguaiaensis) within the clade is still unclear. [Source: Wikipedia]
Boto is the internationally-recognized common name for the Amazon River dolphin. Other common names include bufeo, pink river dolphin,Amazon River dolphin, bufeo colorado, inia, tonina and delfin rosado. There is no consensus on whether the ancestors of Amazon river dolphins 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. Their lifespan in captivity has reached 26 years. Their average lifespan in the wild is around years.
"The boto is a cantankerous pinkish loner of a dolphin," biologist Kenneth Norris told National Geographic, "so supple it can grasp its own tail.” There are no known records of a natural predator of botos, but black caimans, bull sharks, anacondas and jaguars are potentially capable of killing one. them. Some botos possess crescent-shaped wounds that have been attributed to catfish. Botos have formed mutualistic relationships with tucuxis (gray dolphins) and giant otters — forming coordinated hunting groups with them.
Amazon river dolphins have a long tube-like snout and sensory hairs in its back that assist them in the search for food. But other than this very is little is known about them that is still unknown.. They haven't been seriously studied until relatively recently and their remote habitats and the murky water in which they live make them difficult to observe.
Experts: Tony Martin of the University of Kent in England and his Brazilian colleague Vera da Silva have studied Amazon river dolphins for many years.
Websites and Resources: Britain-based Whale and Dolphin Conservation Society uk.whales.org ; Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures
River Dolphins in South America
The Amazon river dolphin (boto) and tucuxi (gray dolphin) are two species of freshwater dolphin that live in the Amazon and Orinoco river systems. River dolphins primarily eat fish and puff, snort and splash when they hunt. Like their ocean going cousins they sometimes enjoy following boats and leaping out of the water to do flips. They navigate through the murky waters in which the live using a highly developed system of sonar, or echolocation (the same method bats use), and scientist follow them by looking for trails of bubbles. [Source:Corinne Schmidt-Lynch of the Nature Conservancy]
Even though both species of dolphin live in rivers, they are not closely related. The tucuxi is more similar to marine dolphins species than it is to the Amazon river dolphin. Some scientists theorize that the Amazon river dolphin descended from primitive Pacific dolphins that were trapped in water that turned fresh when the Andes rose up. Other scientist suggest they entered the Amazon from the Atlantic between 2 million and 5 million years ago. The tucuxi, they believe began entering fresh water river in South America just a few hundred thousand years ago.
The Amazon basin was once a huge lake and the fresh water dolphins evolved when this lake was cut off from the sea. As the fresh water replaced the salt water over a long period of time, the dolphins were able to adapt. Today, lots of river dolphins are found around Puerto Nariño, Columbia and Pacaya-Samiria National Park in Peru. According to National Geographic: Tourists on the Rio Negro in Brazil watch Amazon dolphin lung for a baitfish dangled by the tour operator. Many of the numerous scratches on the dolphin’s skin are from brushes with other dolphins vying for bait. Hands-on wildlife encounters are popular in the region.
Indian Legends About River Dolphins
Botos play a big part in the folklore of many Amazonian peoples. 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. One myth speaks of the spirits of drowned people entering the bodies of botos. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
According to legend one Iquitos storyteller told Corinne Schmidt-Lynch of the Nature Conservancy, "dolphins were seen as gods of the water, and people from great underwater cities. They say that if dolphins lure a person down into their world, he never wants to leave, because it's so beautiful there. You see, dolphins can turn into people."
The reddish color of the Amazon river dolphin has give rise to stories about their relationship with redheaded people on land. "Once in the city of Iquitos," a story goes, "there was a redhead who loved to dance all night. But no matter what he was doing, no matter how much he danced, he always wore a straw hat. Eventually he wooed a girl and she fell in love with him. But every night, he would leave her at 2:00am. "One night he got very drink and passed out. The girl reached down and took his hat off. Underneath, she saw that he had a round hole on th top of his head that looked like a blowhole. He woke up and suddenly, grabbed his hat, ran down to the river and dived in. Out of the water where he had disappeared, a dolphin emerged."
According to another Iquitos storyteller. "When the Amazon river dolphin comes out onto land disguised as a man, he wears a white suit and a fine hat, the Panama kind. He has bulging eyes that can see 180 degrees, and a mouth that stretches from ear to ear. His guffaw shakes the woods and paralyzes teenage girls with fear on moonlight nights. He has an amorous, curving nose, arms so long that his hands reach his calves, nails that can tear to shreds anyone who dares fight him over a woman, knees that bend backwards like hind legs of a horse, and deformed feet that force him to use specially made shoes. the air around him smells like fish innards, the must of the swamp, the odor of a snake which has just shed its skin."
Ticunas Indian River Dolphin Tales
Mark Jenkins wrote in National Geographic: In Amazonian folk wisdom the Amazon river dolphin is an encantado—an enchanted being, a shape-shifter that sometimes takes on human form, coming out of the river to beguile men and women and lead them into its magical underwater city. Some say it wears a hat to hide its blowhole and its bulbous forehead. The stories are incredible to modern ears, and in a way, that's unfortunate. Because to survive in today's world, the Amazon river dolphin may need to enchant a wider audience. [Source: Mark Jenkins, in National Geographic, June 2009]
The Ticuna Indians in Brazil and Colombia have long believed that river dolphins possessed magical powers and that at one time they were human beings who lived a particularly active sex lives. They, like many Amazon Indians, regard river dolphins as equals who must be respected. A Ticuna spokesman old the New York Times: "They have their gods too....They live like us under the water, with cities and roads, stingrays as their hats, snakes for belts and cucha fish for shoes." [Source: New York Times]
According to the New York Times, "Ticunas tell or relatives or friends who have been enticed by the charms of the dolphin. Women who go swimming during menstruation are said to be made pregnant by these animals. And parents must beware lest dolphins disguised as handsome strangers carry off their daughters to the water to live with them.” Dolphins can both bring both good and bad luck. The Ticunas believe that if a dolphin's tooth falls into his hands it is good luck. If a thief then steals it he is likely to become the victim of some great misfortune. Dolphins are believed to have rescued drowning humans and stolen the souls of people who have tried to injure dolphins.
Ticunas have become avid supporters of dolphin conservation. They believed the dolphins are important beings whose preservation is inexorable bound with that of river bank dwellers. They have also revised some of their views about them, according to to the New York Times. “The belief, for example, that deformed babies were fathered by dolphins has lost strength."
Amazon River Dolphin Habitat and Where They Are Found
Amazon river dolphins 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 (2.7 million square miles), whose limits are mainly defined by seas, impassable rapids, waterfalls, and excessively shallow parts of the rivers. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
Of the three recognized subspecies: 1) Amazon river dolphins live in the central Amazon basin; 2) Bolivian river dolphins live in the upper Madeira River, cut off from the Aamzon by impassable rapids, in Bolivia; and 3) Orinoco river dolphins live in the Orinoco basin. The current distribution of this species does not appear to differ significantly from its estimated distribution in the past.
Amazon river dolphins can be found in nearly all types of microhabitats with their range: main rivers, small channels, mouths of rivers, lakes, and just below waterfalls and rapids. According to Animal Diversity Web: 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, Amazon river dolphins are 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, Amazon river dolphins 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 Amazon river dolphins 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.
When the Amazon spills into the forest in the wet season floods, Amazon river dolphins often swim many kilometers away from the main channel to feed among the trees. Mark Jenkins wrote in National Geographic: Every spring the Amazon dolphins get to leave the confines of their river channel for a taste of their former habitat. At the Mamirauá Reserve in western Brazil, two Amazon tributaries flood thousands of square miles of forest for half the year, converting it into a vast tree-canopied sea.Female dolphins in particular stray far into the forest—perhaps to take refuge from aggressive, bright pink males.[Source: Mark Jenkins, in National Geographic, June 2009]
Amazon River Dolphin Physical Characteristics
Amazon river dolphins are the largest of the river dolphins. They range in length from 1.24 to 2.55 meters (4 to 8.4 feet). They range in weight from 98.5 to 185 kilograms (217 to 407.5 pounds). Sexual Dimorphism (differences between males and females): Males are larger than females and more colorful. Males reach the length of 2.55 meters (with an average: 2.32 meters) and weigh up to 207 kilograms (average: 154 kilograms). Females reach the length 2.2 meters (with an average of two meters) and weigh up 154 kilograms (average: 100 kilograms). The difference in size make this species one of the most sexually dimorphic cetaceans (whales and dolphins. It is also unsual that males are so large. Among most most sexually dimorphic cetaceans females are larger. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
The Spanish name for Amazon river dolphins is “bufeo colorado,” or pink dolphin. These mammals range in color from grey to pink. Some males are bright pink males. Females are mostly gray. The males' pink color, scientists say, is scar tissue. Body color varies with age, with young individuals being dark gray and adults possessing a solid or blotched pink hue.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./=\
Instead of a dorsal fin, Amazon river dolphins have a low ridge that runs along their back. Underneath it bulbous forehead is a long tube-shaped beak with upwardly curving "smile," which is characteristic of many species of dolphin found in the ocean. These dolphins are endothermic (use their metabolism to generate heat and regulate body temperature independent of the temperatures around them) and homoiothermic (warm-blooded, having a constant body temperature, usually higher than the temperature of their surroundings). /=\
According to Animal Diversity Web: 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. The skulls of Amazon river dolphins are less asymmetrical than other toothed whales, 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..
Amazon river dolphins 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 centimeters in length) rather than the typical triangular dorsal fins of other dolphins. Amazon river dolphins can be distinguished from tucuxis, another species of river dolphin which also live in the Amazon basin, ,, by their color, the mobility of their head and flippers, and their diving behavior. /=\
Amazon River Dolphin Behavior and Swimming
Amazon river dolphins are motile (move around as opposed to being stationary), migratory (make seasonal movements between regions, such as between breeding and wintering grounds), sedentary (remain in the same area) and solitary. They are active during both day and nighttime hours and they are known to associate with other animals, including tucuxis and giant otters when pursuing prey. Most Amazon river dolphins are rather sedentary. They often occupy the same area for more than a year. They display no obvious defense of home ranges, but if they do, the ranges are likely large and overlapping. These dolphin undertake seasonal migrations correlated with water level and fish abundance, but these migration are generally not so far from areas they occupy during the rest of the year. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
Amazon river dolphins are rather slow and sluggish swimmers especially when compared to the zippier tucuxi. Botos normally swim at about 1.5 to 3.2 kilometers per hour (one to two miles per hour), but they are capable of speed bursts of 14 to 22 kilometers per hour (8.5 to 15 miles per hour). 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.
Amazon river dolphins often travels alone, especially in the rainy season when the fish disperses into the flood plain. Other times they move in small family groups led by a dominant male. They rarely seen in tight groups of more than three individuals. Pairs are usually mothers with their calves. Loose aggregations associated with either feeding or mating occur periodically. Amazon river dolphins 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.
Botos are quite playful and curious in the wild. They have been observed rolling, waving flippers, and lob-tailing. 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, Amazon river dolphins are less timid than bottlenose dolphins, but are more difficult to train than most other dolphins. /=\
Amazon River Dolphin Perception and Communication
Amazon river dolphins communicate with vision, touch and sound and sense using vision, touch, sound, echolocation (emitting sound waves and sensing their reflections to determine the location of objects) and chemicals usually detected by smell. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
Amazon river dolphins 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.
Amazon river dolphins in captivity have been shown to make 10 distinct calls, including echolocation (emitting sound waves and sensing their reflections to determine the location of objects)-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. /=\
Amazon River Dolphin Food and Eating Behavior
Amazon river dolphins eats about three to five kilograms (6 to 10 pounds) of fish a day. Describing a group of four fishing bofos, Schmidt-Lynch wrote, "Occasionally one does a slow side roll, while another swooshes along on its side, a flipper languidly stroking the air. The fish jump madly out of the water to escape the dolphins. Experts have seen as many as 35 [bofos] cooperatively herding and banking fishes, often in association with tucuxi.
According to Animal Diversity Web: A single Amazon river dolphin’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 five to 80 centimeters (two inches to 2.7 feet), with an average of 20 centimeters (eight inches). 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 and crabs 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. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
Botos are usually solitary feeders, most active between 6:00am and 9:00am and 3:00pm and 4:00pm. They consuming about 2.5 percent 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 form loose aggregations with tucuxis and 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. Food sharing has actually been observed between Amazon river dolphins in captivity.
Amazon River Dolphin Mating, Reproduction and Offspring
Amazon river dolphin are viviparous, meaning they give birth to live young that developed in the body of the mother, and engage in seasonal breeding. The breeding season is between June and August. The interval between births is estimated at being between 15 and 36 months, and the calving period is every two to three years. The number of offspring is one. The average gestation period is 11 months. On average females and males sexual maturity at age five years. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
Little is known about the mating habits of Amazon river dolphin. It was once thougt that they may be monogamous. However, males are much larger than females, and known to have very aggressive sexual behavior. Some scientists have observed hostility between male dolphins in the wild, and others have noted extremely aggressive activity during copulation in captivity. Males often considerable scarring and damaged fins, flukes, and blowholes due to biting and abrasion due to tooth-raking, presumably from other male Amazon river dolphins. This aggressive activity suggests that there may be intense competition for access to females and may indicate a polygynous mating system (males having more than one female as a mate at one time), but polyandry (females having more than one male as a mate at one time) and promiscuity (both sexes having multiple partners) cannot be ruled out.
Male Amazon river dolphins reach sexual maturity when they are about two meters (6.5 feet) in length, while females attain sexual maturity when they are 1.6 to 1.75 meters (5.2 to 5.7 feet) long. Courting and foreplay have been observed for Amazon river dolphins 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 and essentially rape the female. Copulation has been observed to be very frequent. One pair in captivity copulated 47 times in less than 3½ hours. There are three different sex positions: 1) face to face; 2) lying parallel head-to-head, and 3) head to tail.
Amazon River Dolphin Offspring and Development
Amazon river dolphin young are precocial. This means they are relatively well-developed when born. The average weaning age is 12 months and the average time to independence is 2-3 years. During the pre-birth, pre-weaning and pre-independence stages provisioning and protecting are done by females. There is an extended period of juvenile learning. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
The relatively long periods of nursing and parental care by females observed in Amazon river dolphins indicates a strong mother-calf bond. Most Amazon river dolphin pairs seen in the wild are mothers with their calves, and one pair in captivity was inseparable for three years. Some scientists have suggested that this long period of parental care may be for learning and development of the young, as it is with bottlenose dolphins/=\
Amazon river dolphins calves are about 80 centimeters (2.6 feet) long at birth and have been shown to grow about 0.21 meters per year in captivity. Births in captivity have taken four to five hours hours. After the umbilical chord is broken, mothers help their calves to the surface for air. Mothers lactate for well over a year. Several individuals have been observed lactating and being pregnant at the same time. According to Animal Diversity Web: The 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.
Amazon River Dolphins, Humans and Threats
Amazon river dolphins are not particularly threatened or endangered but their numbers are difficult to ascertain as they are widely dispersed in remote areas. They are classified as vulnerable on the International Union for Conservation of Nature (IUCN) Red List. They have no special status according to the Convention on the International Trade in Endangered Species (CITES).
River dolphins are affected by deforestation which takes away the fruit and seeds that supply the fish they eat with food. Fishing nets drown them. Pesticides from agriculture and mercury from gold mining, and chemicals from cocaine production also hurt the dolphins. Proposed hydroelectric dams may submerge their habitats. The dolphin’s teeth, eyes and genitalia are sometimes used as love charms, amulets and aphrodisiacs. Indigenous people never used the meat or skin. Fishermen sometimes follow Amazon river dolphins to places with lots of fish. River dolphins have traditionally been difficult to keep in captivity, due to aggression and fairly short longevity.
There has traditionally been little direct hunting of Amazon river dolphins by native people, who held them in deep respect, but Portuguese colonists may have hunted them to obtain oil for lamps. When Amazon river dolphins were found dead, native people sometimes used their fat as a cure for asthma and the oil to treat rheumatism pains and infections in their cattle. [Source: Ryan Bebej, Animal Diversity Web (ADW) /=]
Human activities are putting a lot of pressure on Amazon river dolphins populations. According to Animal Diversity Web: There have been many negative interactions with fisheries. As fishing technologies have improved, the incidental catching of Amazon river dolphins 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 Amazon river dolphins as well. 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 Amazon river dolphins, 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. /=\
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated June 2023