Sotalia fluviatilis
gray dolphin
(Also: tucuxi)

Tucuxis, also known as gray dolphins and Guianian River dolphins, are neotropical dolphins that live exclusively in the Amazon and Orinoco basins and are thought to be endemic to this region of South America. The closest living relatives of Tucuxis are Costeros, dolphins that live in the shallow waters along the Atlantic Coast of South America. However, Tucuxis are sympatric with Amazon River dolphins of the family Iniidae. (Beneditto and Ramos, 2004; Caballero, et al., 2007; Cunha, et al., 2005; Ding, et al., 2001; Flores and da Silva, 2009; Martin, et al., 2004; Nowak, 1999; Oliveira, 2005; Secchi, 2010)

Tucuxis are found 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 2 km of the river relatively saline. Although some Tucuxi can be found within this area, they prefer the freshwater habitat found further inland. (Beneditto and Ramos, 2004; Caballero, et al., 2007; Martin, et al., 2004)

Sotalia fluviatilis and Sotalia guianensis are very similar in appearance and were once classified as a single species. However, phylogenetic evidence indicates that they diverged approximately 1.5 to 2 million years ago during the Pliocene or early Pleistocene. Despite their many similarities, three major differences help distinguish between these 2 species. First, S. fluviatilis largely prefers fresh water habitat, whereas S. guianensis prefers saline coastal habitat. Second S. fluviatilis is much smaller in size than S. guianensis. Finally, the haplotype and nucleotide sequences of S. fluviatilis are as diversified from S. guianensis as they are from other delphinids. In general, S. fluviatilis is smaller and has a shorter beak than most other members of the family Delphinidae. It ranges from blue to pearl-grey along the dorsal surface and from white to pale-pink along the ventral surface. 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 teeth. Sotalia fluviatilis is not sexually dimorphic and ranges from 86 to 206 cm long and weighs 55 kg on average. (Beneditto and Ramos, 2004; Caballero, et al., 2007; Cunha, et al., 2005; Ding, et al., 2001; Emmons, 1990; Nowak, 1999; da Silva, et al., 2007)

There is no information available regarding the mating system of Tucuxis.

Tucuxis breed during late summer and early fall. Gestation ranges from 10 to 11.6 months and results in one calf, which is born during the fall low-water season. Newborn calves range in size from 71 to 106 cm in length. Both sexes become sexually mature by six years of age, at which point males are around 180 cm long and females are around 160 cm long. Despite their differences in length at reproductive maturity, fully grown males and females are usually equal in length and weight. (Beneditto and Ramos, 2004; Flores and da Silva, 2009)

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. (Pivari and Rosso, 2005)

Evidence suggests that wild Tucuxis can live for up to 35 years. There is no information available indicating the average lifespan of captive individuals. They are susceptible to capture stress and often tangle and suffocate themselves within netting. In addition, Tucuxi do not respond well to extended periods of transportation. (Flores and da Silva, 2009)

Tucuxis are slow swimmers compared to other dolphins, but are mostly active and avoid resting. 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 120 cm out of the water and rise for air every 5 to 85 seconds. In a similar to fashion to certain ungulate species, males appear to herd females while traveling. They travel in small groups of 1 to 6 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 between conspecifics. Calves are often present in larger groups, which is thought to provide them with ample opportunity to learn a variety of social behaviors. (Emmons, 1990; Flores and da Silva, 2009; Nowak, 1999; Oliveira, 2005; Pivari and Rosso, 2005)

There is no information on the average home range size of Tucuxis. (Flores and da Silva, 2009)

Like most dolphins, Tucuxis use a variety of whistles and clicks to communicate with conspecifics. 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 (e.g., 16% 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 conspecifics to where food is abundant; suggesting a co-operative rather than competitive attitude between conspecifics. Like other delphinids, Tucuxis use echolocation to help them find prey. (Pivari and Rosso, 2005; Pivari and Rosso, 2005; Azevedo and Van Sluys, 2005; Mass and Supin, 1999; Pivari and Rosso, 2005)

Tecuxis 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. (Azevedo and Van Sluys, 2005; Mass and Supin, 1999; Pivari and Rosso, 2005)

Tecuxis are carnivorous, and primary prey includes marine ray-finned fishes, along with squids and octopuses. Confluence areas result in favorable pH levels for plankton growth, which attracts many species of ray-finned fish. As a result, Tucuxis are often seen travelling to confluences, likely in search of prey. (Beneditto and Ramos, 2004; Emmons, 1990; Martin, et al., 2004)

Other than illegal hunting by humans, Tucuxis have no known predators. (Flores and da Silva, 2009)

Although barnacles are commonly found Tucuxis, they are not considered parasitic, as they have no known negative impact on their host. Limited information exists regarding parasites of Tucuxis; however, stomach flukes and two species of nematode (Anisakis typica and Halocercus brasiliensis) are known to infect the gastrointestinal tissues of this species at various stages throughout their complex life cycle. (Beneditto and Ramos, 2004; Kane, et al., 2008)

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 (bait), oil (emulsion to protect boats from water), and various body parts that are used in traditional medicines or religious ceremonies. (Alves and Rosa, 2008; Nowak, 1999)

There are no known adverse effects of Tucuxis on humans.

Tucuxis are classified as "data deficient" on the IUCN Red List of Threatened Species. Currently, population trends are unknown and thus potential conservation and management needs cannot be determined. They are the only species of fresh-water delphinid currently known. Although there has never been a commercial fishery for this species, a significant number of deaths occur due to by-catch and incidental mortality in fishing gear. Other potentially important threats include damming, overfishing of prey, boat strikes, chemical pollution and noise pollution. Without further research on the demographics of this species throughout its geographic range, the significance of these threats cannot be established and potential conservation and management actions cannot be prioritized. (Cunha, et al., 2005; Pivari and Rosso, 2005; Secchi, 2010; Trujillo Gonzalez, 1994)