Tucuxi (Smaller Amazon Dolphins) and Their Characteristics and Behavior

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Tucuxi (Scientific name: Sotalia fluviatilis) are one of the two species of freshwater river dolphins in the Amazon basin. The other is the Amazon river dolphins (boto, Scientific name: Inia geoffrensis). The smaller tucuxi lives in rivers and coastal water in Central America and in the Orinoco and Amazon river basin of northeastern South America. They are grey with a pink underside, and has a sickle-shaped dorsal fin. Its smaller than the Amazon river dolphin, reaching lengths of 1.2 to 1.8 meters (four to five feet) and weighing around a 45 kilograms (100 pounds) when fully grown. Their lifespan in the wild is estimated to be 35 years.

Tucuxis are shy animals that sometimes travel in schools just like oceanic dolphin species, filling the rivers where they live with the “sharp clicks of their echolocation." Also called gray dolphins, sotalia and Guianian River dolphins, they are neotropical dolphins that live exclusively in the Amazon and Orinoco basins and are thought to be endemic to this region of South America. Their closest living relatives are costeros, dolphins that live in the shallow waters along the Atlantic Coast of South America. Tucuxis also have overlapping ranges with with Amazon River dolphins. [Source: Michael Dobbin, Animal Diversity Web (ADW) /=]\

Tucuxis are generally found in tropical, freshwater environments, particularly rivers, but can also be found in brackish water and in estuaries and well in the sea. They reside throughout the Amazon and Orinoco River basins and are commonly found near low current confluences and river junctions where food is abundant and less energy has to be expended during foraging bouts. They avoid mud banks and flooded forest areas. The mouth of the Amazon River occurs at its junction with the Atlantic ocean, thus making the first two kilometers of the river relatively saline. Although some Tucuxi can be found within this area, they prefer the freshwater habitat found further inland.

Tucuxi feed on river and ocean fishes, even sometimes eat squids and octopuses. Confluence areas result in favorable pH levels for plankton growth, which attracts many species of fish they eat. As a result, Tucuxis are often seen travelling to confluences, likely in search of prey.

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

range of the tucuxi

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.

Tucuxi Physical Characteristics

Tucuxi range in length from 0.86 to two imeters (2.8 feet to 6.5 feet), with their average length being 1.5 meters (5 feet). Their average weight is 55 kilograms (121 pounds).Sexual dimorphism (differences between males and females) is not so apparent: Both sexes are roughly equal in size and look similar. 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). [Source: Michael Dobbin, Animal Diversity Web (ADW) /=]

size of the tucuxi

In general, tucuxi are smaller and have a shorter beak than most other dolphins. Their coloration ranges from blue to light-grey along their back side and from white to pale-pink along their bottom side. Most individuals have a white tipped beak. The dorsal fin has a prominent triangular shape that sometimes hooks toward the caudal fin. Adults have between 28 and 35 pairs of teeth.

Tucuxi and Guiana dolphins (Sotalia guianensis) are very similar in appearance and were once classified as a single species. However, DNA evidence indicates that they diverged approximately 1.5 to two million years ago during the Pliocene Period (5.4 million to 2.4 million years ago) or Early Pleistocene Period (2.6 million to 800,000 years ago).

Despite their many similarities, three major differences help distinguish between these two species. First, tucuxi prefers freshwater habitats, while Guiana dolphins prefers saline coastal habitats. Second Tucuxi is much smaller in size than Guiana dolphins. Finally, the haplotype and nucleotide DNA sequences of tucuxi are as diversified from Guiana dolphins as they are from other many other dolphin species.

Tucuxi Behavior

Tucuxis are diurnal (active mainly during the daytime), motile (move around as opposed to being stationary), sedentary (remain in the same area), and social (associates with others of its species; forms social groups). [Source: Michael Dobbin, Animal Diversity Web (ADW) /=]

Tucuxis are slow swimmers compared to other dolphins, but are faster than Amazon river dolphins. Tucuxis are very active and don’t rest much. They can perform many acrobatic maneuvers such as flips, summersaults, leaps, and rolls and often perform these movements in sync with other dolphins. They can leap as high as 1.2 meters out of the water and come up for air every five to 85 seconds.

In a similar to fashion to certain large mammal species, males appear to herd females while traveling. They travel in small groups of one to six individuals but have been witnessed in groups as large as 40. Larger groups can coordinate hunting strategies. Whistling is used as a means of communication. Calves are often present in larger groups, which is thought to provide them with ample opportunity to learn a variety of social behaviors.

Tucuxi Perception and Communication

tucuxi in a river in Peru

Tucuxis communicate with sound and sense using vision, touch, echolocation (emitting sound waves and sensing their reflections to determine the location of objects) and chemicals usually detected by smell. [Source: Michael Dobbin, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web: Like most dolphins, Tucuxis use a variety of whistles and clicks to communicate with members of their own species. Among delphinids, evidence suggests that species' size has a linear effect on whistle pitch, with smaller species having higher pitched whistles and larger species having lower pitched whistles. Tucuxis align with this relationship, and as one of the smallest extant dolphin species, they are known to have some of the highest pitched whistles when compared to other dolphins (such as, 16 percent of whistles exceed 24Hz). Whistle frequency tends to ascend rather than descend during a single whistle. Whistling increases while foraging, and is thought to attract members of their own species to where food is abundant; suggesting a co-operative rather than competitive attitude between members of their own species. Like other delphinids, Tucuxis use echolocation (emitting sound waves and sensing their reflections to determine the location of objects) to help them find prey.

Tucuxis use their vision to perceive the local environment, and despite living in a freshwater environment, have a number of ocular features that are similar to those of many marine dolphins. Tucuxis have two high density ganglion areas, a feature which is common among delphinids and helps them process visual information more efficiently than Amazon River dolphins, which have only one. Being originally adapted for life in a saline environment, however, may negatively affect their ability to see objects in freshwater at high resolution. Evidence suggests that although their clarity of vision is less than that of true river dolphins, it is better than that of marine dolphins. What they lack in eye sight, however, they make up for in their ability to echolocate prey and potential predators.

Tucuxi Mating, Reproduction and Offspring

Tucuxis 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 from August to October., with the average number of offspring being one. The gestation period ranges from 10 to 11.6 months. On average females and males reach sexual maturity at age six years at which point males are around 1.8 meters long and females are around 1.6 meters long.. [Source: Michael Dobbin, Animal Diversity Web (ADW) /=]

Information is lacking regarding tucuxi mating behavior. Calves are born during the fall low-water season. They range in size from 71 to 106 centimeters (2.3 to 3.5 feet) in length. Despite their differences in length at reproductive maturity, fully grown males and females are usually equal in length and weight.

Parental care is provided by females. Little is known of parental care in Tucuxis; however, mothers are known to whistle at their calves once they have found food. As mammal, mothers likely nurse their young until weaning is complete. During the pre-birth and pre-weaning stages provisioning and protecting is done by females.

Tucuxi, Humans and Threats

Currently, tucuxi population sizes trends are unknown and thus its is difficult to determine whether they are endangered or threatened or not. They are listed “Data Deficient” on the International Union for Conservation of Nature (IUCN) Red List. The Convention on International Trade in Endangered Species (CITES) lists them in Appendix I, which lists species that are the most endangered among CITES-listed animals and plants.

Indigenous South Americans hold Tucuxis in high regard as protectors that carry the drowned to shore for burial. Despite Brazilian law protecting them, Tucuxis are illegally hunted for their meat (mainly for bait), oil (emulsion to protect boats from water), and various body parts that are used in traditional medicines or religious ceremonies. [Source: Michael Dobbin, Animal Diversity Web (ADW) /=]

Tucuxis are susceptible to capture stress and do not respond well to extended periods of transportation. They often become entangled and suffocate in fishing nets or are injured or killed by vessel strikes. Other threats include damming, overfishing of prey, chemical pollution and noise pollution. /=\

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

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