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continuous fission-fusion process within a social lineage). Additionally, the social system in
Inia
is reinforced with other data. Kendall (1994) reported the existence of ―nurseries‖, where
calves were cared by one or several adults in the Colombian Amazon, although this event has
not been observed in other areas of
Inia's
geographic distribution. Another finding which
could help to demonstrate the existence of a social reproductive system in
Inia
was that
determined by Martin & da Silva (2004b). They found that, at the Mamirauá Reserve, the
proportion of males was highest in the main rivers and diminished towards the innermost
parts of lagoons within the flooded forest, reaching the highest male proportion at mid-rising
water of the main rivers. Males always presented higher numbers than females in main rivers
and their margins, meanwhile females greatly outnumbered males in low lying, remote open
areas within Várzea comprising temporary and permanent shallow lagoons (they are called
chavascal). The exception was during September-November in the lower water season where
the sex-ratio was identical in all habitats. They explained this possible sexual segregation by
three potential explanations. First, calves begin to take fish within their first year of life. The
abundance and diversity of small fishes is greater in shallow lagoons within várzea.
Therefore, calves and young could easily obtain more food. Secondly, lactating females can
receive shelter and seek refuge from strong riverine currents, and thus maintain low energetic
cost. A refuge would provide an easier situation for resting, and nursing, as well as provide an
area for calves and young to learn how to catch fish. This explanation has been proposed for
Platanista gangetica
(Smith, 1993) and it is possible for
Inia
. Thirdly, sexual segregation
may protect calves from the attacks of adult males, which can be extremely aggressive with
other males. These activities frequently wound young and potentially lactating females and
their calves. Nonetheless, we observed a different situation in some Peruvian lagoons. For
instance, we appreciated in the tiphisca Urarinas at the Puhinauva channel (Ucayali River),
how an adult male protected a female and her young (another male) when we captured these
animals during high water. In identical sense, in Cocha Paucar, another lagoon in the
Puhinauva channel (Ucayali River), we assisted in the spectacle of several adult and young
individuals emitting agonistic display against us and following our wood boat when we
captured one exemplar and transported it for 30-40 meters to release in another river area. In
whatever case, these behaviors agree quite well with Chesser's simulations showing a social
reproductive system within and between small reproductive size units in this species but does
not support the claims of those authors which consider this as a solitary species. All of these
attributes found in the pink river dolphins could enhance the possibility of both the interdemic
selection model (Wright, 1982) and trait-group model (Wilson, 1977) acting on this and other
cetacean species. Genetic differences over small geographic distances could be the rule rather
the exception (Smith et al., 1978).
Could the Social Reproductive Parameters found in this Study be Typical for
all the
Inia
Populations?
It is recognized that some lake systems contain numerically important populations of pink
river dolphins (for example, Tipischa San Pablo in Marañón River in Perú or Mamirauá in the
Japurá (Caquetá) River in Brazil). Martin & da Silva (2004a) estimated, at least, 100
individuals used the main Mamirauá lake as the center of their residence. This depends on the
density, the temporal and spatial distribution, and fish species diversity in these lakes. But it's
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