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(Figure 2) also indicates that the Orinoco basin has been settled by a diverse set of mtDNA
haplotypes. Although the NJ tree (Figure 4) could suggest the monophyly of Orinoco
haplotypes, this clade is not well supported by bootstrap and it does not remain clustered in
some maximum parsimony analyses (data not shown). The Orinoco basin has a unique
communication with the Amazon basin in the Casiquiari Channel (Venezuela) and, if we
assume that
Inia
has an origin in the Amazon as suggested by our phylogenetic analysis
(Figure 4), it could indicate a previous colonization event of the Orinoco bringing together
distinct haplotype groups in one or more dispersion waves.
Although the phylogeographic pattern reveals a strong geographic correlation (Figures 2
& 4, Table 4), all Mantel tests using pairwise
F
ST
'
s and distances along the rivers revealed no
significant correlation (data not shown). Thus, geographic distances may not be the only
factor that is significantly affecting the present distribution of mtDNA haplotypes.
The strong phylogeographic structure observed in
Inia
indicates a high phylopatry, at
least for the females. In mammals the females are usually more phylopatric than males.
Inia
probably does not often migrate long distances, or the migrations may be only made by
males. However, the significant differentiation observed between the two closely located (45
km) Brazilian populations (potentially in contact), requires another explanation. Sampling
bias is not a likely explanation as capture of individuals has not involved members of a single
family, and has occurred in different times. Although small, sample sizes look representative
of the sampled areas considering that a population around 250-300 resident individuals was
estimated for the Mamirauá Lake system (Martin & da Silva, 2004a). According to the
authors, there is an average of 1.16
Inia
per km
2
of
várzea
(only the flooded area) or 0.84 per
km
2
in the total area. Extrapolating this value to the 125 km
2
of Tefé Lake system, we could
predict a total population of 105 to 145
Inia
individuals, although lower values would be
expected in the Tefé Lake due to higher human impact and because it is less productive when
compared to Mamirauá. Thus, our current sample sizes would correspond to about 4.5% (n =
11) and 8.5% (n = 9) of the total resident population, in Mamirauá and Tefé respectively.
Besides, our conclusions are based on the presence of significant differentiation observed
between our samples, not on the lack of data that is expected to produce non-significant
comparisons.
Considering the significant differentiation between the two adjacent Brazilian
subpopulations, our explanatory alternative is based on the observation that Mamirauá and
Tefé are ecologically very distinct water environments, typically found in the Amazon Basin
(Sioli, 1984).
Inia
populations may have local adaptations to dark, transparent, low pH water
that are different from a turbid water environment such as the Amazon (Solimões) River,
which separates Mamirauá and Tefé. Locally adapted populations tend to isolate from their
neighbors adapted to other environments, thus these
Inia
populations may have been
accumulating genetic differentiation separated by ecological barriers.
Indeed, unique adaptations related to the water environment are present in
Inia
. The
flexible vertebral column and neck (nonfused cervical vertebrae) and large, broad, and
paddle-like flippers which are capable of circular movements, allowing to move between
trees and submerged vegetation to search for food in the flooded forest (da Silva, 2002).
Furthermore, like other river dolphins,
Inia
are endowed with a sophisticated sonar system,
but also have good vision, different from Indus and Ganges' dolphins, which are functionally
blind (Reeves, 2002). All adaptive characteristics would be very dependent on the water
characteristics in the Amazon basin. The Amazon rivers are characterized by very distinct
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