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territoriality and were solitary. In a 550 km transect of the Amazon River, Magnusson et al.
(1980), observed that the majority of pink river dolphins were solitary (81 %), followed by
couples (15 %), whereas other Amazon dolphin,
Sotalia fluviatilis
, presented greater numbers
of groups than did
Inia
(55 % in pairs or more). Similarly, this aforementioned work showed
that the groups of
Sotalia
were more clumped than a Poisson (random) distribution, whereas
the groups of
Inia
did not differ significantly from random. This was interpreted that
Sotalia
need more specialized food and are less adaptable to a variety of river conditions than
Inia
as
well as showing that
Inia
are
more territorial than
Sotalia
. But it was not clear if this
territoriality was in favor of a social structure or in favor of an individual and solitary species.
Schnapp & Howroyd (1992) tried to study whether
Inia
is solitary and if grouping behavior is
only the result of aggregation in favorable habitats. They found that individual dolphins were
in patch distributions and the groups did not differ significantly from a Poisson distribution.
Fifty-eight of their sightings were clusters of two or more individuals (2 to 7 dolphins
forming groups). They found a significant correlation between group size and stream velocity.
Most large groups were found in slow running sections of the rivers and lagoons. These
authors reached the conclusion that
Inia
occupied an undefended home-range. Vidal et al.,
(1997) reported a mean group size of 2.9 dolphins in approximately 120 km of the Amazon
River bordering Colombia, Brazil and Peru that they analyzed. The highest mean group size
was in the tributaries (4.1), whereas the smaller mean group was determined in lakes (2.0).
McGuire & Winemiller (1998) determined an average group size of two individuals and that
the number of sightings were higher in the habitat with the higher degree of heterogeneity and
that the frequency of sightings most frequently occurred during the dry season than during the
rainy season (41 % vs. 24 %). The mean group and the relative frequency of group size
changed by season. The largest group they observed was composed of eight individuals.
Denkinger et al., (2000) showed population density estimations in the Cuyabeno River at the
Ecuadorian Amazon ranging from 0.01 dolphins/linear km to 0.47 dolphins/linear km
depending on the rainy or dry seasons, respectively. Aliaga-Rossel (2002) found on average,
1.12 dolphins per linear km in 185 km transect conducted along the Tijamuchí River in
Bolivia. He found 41 % of the animals to be solitary animals, 32 % were in pairs, 15 % were
in groups of three animals and that the maximum group size was 19 animals, which suggests
that there is a certain sociability within this species. Martin et al. (2004) determined that 99 %
of pink river dolphins groups observed consisted of one to four specimens with a mean of
1.42 individuals in the confluence area of the Amazon and Japurá rivers in the Brazilian
Amazon. However, the work of Martin & da Siva (2004b, 2006) showed strong evidence in
favor of social components in the behavior and ecology of the pink river dolphins. One
difficulty here is that a continuous distribution of this species in many Amazon rivers can
mask any population structure that may exist (Martin & da Silva, 2004a). Additionally, since
this is a top predator in the Amazon and Orinoco river basins and because many fish-nets
converge on them (Ruiz-García et al., 2007), population extinctions of this species could
result and consequently reveal extreme alteration in the ecology of the aforementioned rivers.
Denkinger et al. (2000), for example, determined an
Inia
population density decline in the
Cuyabeno and Lagartococha rivers within the Cuyabeno Reserve in the north east of the
Ecuadorian Amazon from 1996 to 1998. This declination seemed to be due to the
contamination caused by six oil spills and waste water effluent from oil fields. In fact, since
1990, the other Amazon dolphin (
Sotalia fluviatilis
) has disappeared from the Lagunas
Grandes in the upper Cuyabeno River, an affluent of the Aguarico River.
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