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several seasonal changes because the animals sampled were in the reproductive epoch. Martin
& da Silva (2004a) demonstrated that there is no clear influx or efflux of pink river dolphins
in Mamirauá at any time of the year. In fact, although no animal spent the entire year within
this lake system (Martin & da Silva, 2004a,b), an important fraction of them probably
traveled no farther than the area near to the entrance of the lake system. Nonetheless, Alliaga-
Rossel (2002) showed that group size frequency significantly varied by season (31 %; p =
0.001) in the Tijamuchí river in Bolivia. Solitary individuals were more frequent in all the
seasons with exception of low water, where pairs were seen more frequently. But in falling
(19 individuals) and low waters (14 individuals) (from April to September), this author
observed the highest groups of pink river dolphins, coinciding with the greatest fish
abundance. He even commented that he observed more than 30 individuals in the same
lagoon. I saw similar sized groups in Peruvian and Bolivian rivers. In fact, the same has been
observed by other authors such as Vidal et al. (1993), Best & da Silva (1993) or Schmidt-
Lynch (1994), who observed aggregation of 20-35 individuals. In the falling and low waters
is the reproductive time for
Inia
and it is when the group size increases and probably reflects
more precisely, which is the social and reproductive size of the
Inia
's lineages. This author
observed a reproductive activity season day with 12 individuals involved in the event. One
day after, one animal appeared dead probably due to sexual aggression (a male of 2.25 m). In
my experience, I have witnessed many reproductive activities (and many seasons) for this
species and participation from 5 to 15 animals was quite common. The most frequent number
of animals ranged from 7 to 10. The social Chesser's model simulations yielded seven
females, as the most probably number within each lineage, and one male breeding with four
females. Therefore, 2 or 3 males could be the fathers of all the new offspring. If I sum these
quantities, 9-10 animals are the effective lineage sizes for the minimal social reproductive
unit for
Inia
which coincides with those numbers seen in other reproductive seasons by
Aliaga-Rossel (2002) and me. The polygyny for this species, obtained by the Chesser's
simulations, agrees quite well with certain sexual dimorphism determined in
Inia
. Although
some females could reach a total length similar to the largest males (Ruiz-García et al.,
2006b; Ruiz-García et al., 2010b), the oldest and largest males are more robust than the oldest
and longest females (for instance, girth in front of pectoral flippers, width of the pectoral
flippers or weight) (da Silva, 1994; Martin & da Silva, 2006; Ruiz-García et al., 2010b).
Therefore, Chesser's social simulations offer the possibility of polygyny in
Inia,
which
supports sexual dimorphism in favor of males because they actively fight for the sexual
access to females (Clutton-Brock, 1989). If so, this is one of the clearer cases of polygyny in
cetaceans, together with the possible case of
Physeter catodon
. A possible close kinship
within maternal lineages, could generate kin clusters identified by a greater geographic
proximity between the members of these clusters than between members of clusters far away.
Although greater geographic physical overlap of home ranges (lagoons within Várzea, for
instance) could occur among kin females than with non-kin females. Kin female lineages
could not develop cooperative social behavior with other close kin female lineages, with each
female lineage retaining the best possible area in floodplain lagoons (although the best core
habitat for this species is dependent on fish movements and the rainy season). Such behaviors
could enhance a population consisting of small philopatric groups, creating genetic patches
within the overall population. Henceforth, the comparisons of my molecular results and the
observational data can indicate that most solitary animals could be part of social reproductive
groups and that they regroup for fishing or breeding and later they again disperse (like a
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