Inia araguaiaensis, Hrbek et al., 2014

Russell A. Mittermeier & Don E. Wilson, 2014, Iniidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 364-379 : 378-379

3.

Araguaian Boto

Inia araguaiaensis

French: Boto de I'Araguaia / German: Araguaia-Delfin / Spanish: Delfin del Araguaia

Taxonomy. Inia araguaiaensis Hrbek et al., 2014 ,

“entrance of Lake Jurumirim” Araguaia River, State of Goias.

This species is monotypic.

Distribution. Araguaia-Tocantins Basin in the Brazilian states of Goias, Tocantins, Mato Grosso, and Para.

Descriptive notes. Few data available, but body measurements from individuals below and above Tucurui Dam fall into the range of other botos; the largest individual male recorded was 236 cm long (n = 9) and the largest female 201 cm (n = 12). One individual found at Curuca Estuary was 252 cm in total length. Like the other species of botos, the Araguaian Boto is strongly sexually dimorphic with adult males larger, more robust, more pink and with a more “ragged” appearance than females. Newborn and young individuals are dark gray; adults are lighter gray and with pinkish coloration below. The main differences between the skull of the Araguaian Boto and that of the two other species of botos are the larger biparietal width: males 8:1.9-2 cm (mean 8-7 cm, n = 2) and females 9.6-11.5 cm (mean 10-6 cm, n = 2), and larger greatest width of maxillaries (females 11.3-11.8 cm, mean 11:6 cm, n = 2). The length of the rostrum is 68% oftotal length of the skull in males and 66% in females, but based on measurements ofjust two animals of each sex. Number of teeth may be slightly less than in the Amazon River Dolphin, with total number per row ranging 24-28 (n = 3) teeth. Each upper and lower tooth row has 6-8 molar-type teeth on both sides.

Habitat. The Araguaia River is a floodplain river with sinuous bed and muddy waters from the substantial sediment load. During the peak of the dry season (July/August) the depth of the river is dramatically reduced, exposing extensive sandbanks. The Tocantins River, however, is embanked, with relatively clear water, several rapids and a faster current. Botos occurin all habitats in this river, but highest densities occur (both during the seasons of low and falling water) in confluences and bays.

Food and Feeding. Very little information is available for this species, but accounts of fishermen and observations of Araguaian Botos stealing fish from hooks and nets and pursuing schools of fish in an apparently cooperative way confirm the piscivorous diet of the species, similar to that of the two other species of botos. One study reported three species of fish seen in the mouths of Araguaian Botos: Hydrolycus armatus (Characiformes), and Pimelodus blochii and Oxydoras niger (both Siluriformes) of estimated size of 80 cm, 40 cm, and 60 cm respectively. Large catfish (over 60 cm) were found floating dead with boto teeth marks and missing chunks of the posterior part of the body torn out by botos.

Breeding. There is no specific information available for this species, but it is likely similar to the Amazon River Dolphin. Lactating females were recorded in the months ofJuly, August, and September without reference to the size of their young. Two halfgrown fetuses were collected in August, several years apart. Apparent neonates have been observed year-round.

Activity patterns. There is no specific information available for this species, but due to the strong seasonal variation in the water level, and fish movements during the transition from high to low waters, Araguaian Botos must move from lagoons and flooded areas to main river channels, as do the other species of botos.

Movements, Home range and Social organization. Group size varies seasonally and usually is between one and three individuals. In the Araguaia River singletons are predominant but 3-4 dolphins are not uncommon, and larger aggregations can occur around a resource or at a confluence or bay. During low water, groups of botos drive schools of fish toward sandbanks and into very shallow water, where they are captured. Surveys above the Tucurui Dam revealed that even after the area was flooded, botos tended to aggregate in places where confluences and bays previously existed, suggesting a strong fidelity to the area. Other characteristics of their movements and social organization are likely similar to the Amazon River Dolphin.

Status and Conservation. Because of its recent description, the Araguaian Boto has not yet been formally evaluated on The IUCN Red List. However, due to population-level threats and fragmentation of the population by dams, the species conforms to IUCN criteria designating it as Vulnerable. The Araguaian Boto has a restricted distribution, with fragmented habitat and population, and low population size when compared to the Amazonian River Dolphin. Animal densities of only 0-8-0-98 ind/km* have been reported, and the species appears to have low levels of genetic diversity. In addition to the seven large dams that have been built in the Tocantins River (Tucurui, Luiz Barreto, Lajeado, Peixe Angical, Sao Salvador, Cana Brava, and Serra da Mesa) and two new large dams planned (Serra Quebrada and Maraba) which divide the boto population into nine fragments, agricultural and ranching activities cause cumulative negative effects on several biotic and abiotic aspects of the river basin ecosystem. After 1984, the Tucurui Dam, 11 km long and 78 m high, totally isolated the Araguaian Boto population, stopping its contact with botos from the Para River and estuary about 350 km below. Before the construction of the dam, botos were recorded above and below the rapids and falls that existed in the area, all flooded bythe reservoir. So, potentially, the species can also occur below the Tucurui Dam and its confluence with the Para River to near Soure on the east side of Marajo Island. Recent analysis of mtDNA for a specimen from Para State revealed a different and isolated geographic form of boto, genetically distant from the Amazonian River Dolphin and the Bolivian Boto. In the Tocantins River, the most upstream and southerly records were well above the rapids near Jatobal as well as below and above the hydroelectric dam of Serra da Mesa, near Uruacu and rivers Lajeado and Manoel Alves, among others. In the Araguaia River, the Araguaian Boto has been recorded between the city of Aragarcas/Barra do Garcas and the southern part of Bananal Island past the city of Luis Alves to Santa Isabel rapids and down to Maraba, and in tributaries such as the Formoso, Cristalino, Vermelho, Peixe, Crixas Acu, and Agua Limpa rivers, as well at lakes dos Tigres and Rico in Goias State, up to the confluence with the Tocantins River, and also in Lake Montaria in Mato Grosso State. Interactions with fisheries, including net entanglement and dolphins being shot for taking fish from nets and offlines, result in direct mortality. Anotherthreat is the large concentration oftourists on the Araguaia River from May to October (dry season) when camping facilities are established on riverine sandbanks, motorized boats, and other high-speedvehicles are common, and intensive sport fishing results in botos ingesting hooks and lines.

Bibliography. Araujo (2010), Araujo & da Silva (2014), Araujo & Wang (2012), Best & da Silva (1989a), do Carmo & da Silva (2010), Costa et al. (2013), Hrbek et al. (2014), Latrubesse & Stevaux (2006), Martin et al. (2004), Natale (2013), da Silva (1994), da Silva et al. (1998), Rebelo & do Carmo (2008).

Figures

1. Amazon River Dolphin (Inia geoffrensis), 2. Bolivian Boto (Inia boliviensis), 3. Araguaian Boto (Ima araguaiaensis)

Distribution. Araguaia-Tocantins Basin in the Brazilian states of Goias, Tocantins, Mato Grosso, and Para.