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that
Pontoporia
is a remainder of those species of dolphins that penetrated the Paraná basin
via rivers or areas situated in the current La Plata River. Or, it's also possible that
Pontoporia
(or an ancestor) returned to the coastal habitats when the geological, hydrological and
climatological conditions were adverse inside the continental South America and the internal
water interconnection between Paraná and Amazon basins was lost. The presence of
Pontistes
, a Late Miocene Pontoporidae, in the marine sediments of the Paraná Basin,
together with the Iniidae fossils ratify the strong connection of these dolphins and their
introduction to the continental South-America via the Paraná Basin. Another alternative
hypothesis is that the ancestor of the current
Inia
penetrated to the Amazon Basin prior to the
close of the Guayaquil gate. It could be correlated with the fact that the upper Amazon pink
river dolphin populations seem to have higher gene diversity than the
Inia
populations of
other Amazon tributaries such as the Putumayo and Caquetá rivers. However, a lot of other
Inia
populations should be analyzed to estimate their respective gene diversity levels in other
Amazon areas (like the Juruá, Purús, Madeira, Tapajós, lower Xingú and Tocantins-Araguaia
rivers, all in Brazil) to support this Pacific origin of
Inia
. Other mammals seem to have their
original foci of dispersion from the upper Amazon. This could be the case of
Ateles
(Ruiz-
García et al.,2006),
Cebus apella
(Ruiz-García & Castillo, 2010), the jaguar (
Panthera onca
)
(Ruiz-García et al., 2010), the lowland tapir (
Tapirus terrestris
) (unpublished results) and
maybe even
Saimiri
(Lavergne et al., 2009) and partially
Lagothrix
(Ruiz-García & Pinedo,
2009). Nevertheless, with the current data it's easier to understand that
Inia
has an Atlantic
origin rather than a Pacific one.
Inia
population sizes is another noteworthy topic. Presently our research group is using
gene diversity and coalescence methods (Ruiz-García, 2010b) to address questions regarding
this topic. These new results could give new insights about
Inia's
precise geographic origin.
At the present, I don't know the overall size of the
Inia
population. A maximum improbable
size could be derived from some data proportioned by Martin & da Silva (2004). They
estimated 13,000 pink river dolphins for 11.240 km
2
of várzea at the Mamirauá Reserve
(which is an extreme favorable habitat for this species) at the central Brazilian Amazon. If we
assume that the entire Amazon basin is about six millions km
2
, and assuming that the entire
basin was in condition to maintain pink river dolphin populations (a fact that is not certain), a
potential maximum total of 6.9 million of pink river dolphins could exist in the Amazon
basin. Obviously, the real size must be considerably lower than this value. Futures studies on
the demographic historical evolution and estimations of effective population sizes in diverse
regions of the Orinoco and Amazon are needed for a complete and compressive evolutionary
perspective of this species as well as to take correct measures in biological conservation.
A
CKNOWLEDGMENTS
Economic resources to carry out this study were obtained from Colciencias (Grant 1203-
09-11239; Geographical population structure and genetic diversity of two river dolphin
species,
Inia boliviensis
and
Inia geoffrensis
, using molecular markers) and the Fondo para la
Accion Ambiental (US-Aid) (120108-E0102141; Structure and Genetic Conservation of river
dolphins,
Inia
and
Sotalia
, in the Amazon and Orinoco basins). Main thanks go to Pablo
Escobar-Armel (Colombia), Dr. Diana Alvarez (Colombia), Ariel Rodriguez (Colombia),
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