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confluences than in the main river channels (Alliaga-Rossel 2002; Martin et al., 2004).
Therefore, our captures were carried out in the areas where the dolphins were more frequently
concentrated, with the exception of river confluences where the currents were so intense that
no nets could be used to capture animals. As I will briefly comment, these are the sampling
points where the social reproductive structure, if it exists in this species, can be recorded.
Wright (1940) was the first to consider the dispersal of animals from their natal areas. But
this author considered this process to be random with regard to both sex and distance.
Nonetheless, many studies have demonstrated that sexual differences in dispersal trends exist
(Chesser 1991a,b; Chesser et al., 1993; Prout, 1981) and that this affects the genetic structure
of the species. For example, Chesser (1991a,b) and Chesser et al. (1993) demonstrated that
migrant sex is the vector of gene flow and a factor of global genetic homogeneity, while
philopatric sex introduces internal homogeneity within the lineages (at least, promotes gene
correlation between individuals in the same lineage or close geographic lineages) and genetic
heterogeneity among the most distant lineages. The genetic heterogeneity among the lineages
in the population considered (F
LS
= 0.26) was higher than those usually published for other
mammal species (
Mus musculus
from different farms, F
ST
= 0.047, Selander & Kaufman,
1975;
Macaca mulatta
among different social groups, F
ST
= 0.035, Melnick et al., 1984;
Cynomys ludovicianus,
between wards within populations, F
ST
= 0.045-0.065, Chesser, 1983,
to cite a few cases). However, this genetic heterogeneity was of a similar magnitude to the
differences found by Chesser (1983), for the quoted rodent, among coteries within wards (F
ST
= 0.227). However, we must take into account, following the theoretical models and
simulations made by Rothman et al. (1974) and Fix (1978) that the generation and splitting of
new lineages have an inflationary effect on the F
ST
estimations, even when the population was
a single reproductive one and the gene flow is extensive. However, the F
LS
obtained was
similar to that obtained for other molecular marker set (RAPD), we applied to the different
lagoon dolphin samples that we obtained in the Napo-Curaray, Ucayali and Marañón rivers
(F
ST
= 0.2302; Ruiz-García et al., 2007). Lewontin (1972) was the first researcher that
determined that a large proportion of population genetic variance exists within small
population units in humans. The same has been demonstrated for other wild primates
(Melnick, 1987), rodents (Chesser, 1983; van Staadent et al., 1994) or in small or large cats
(
Felis catus
, Ruiz-García, 1998, 1999;
Panthera onca
and
Puma concolor
, Ruiz-García et al.,
2006a, 2009).
Philopatry and Gene Flow
The results obtained showed that all the animals studied belonged to a unique gene pool
(AMOVA). Although the gene flow among different lagoons existed, this gene flow was
highly restricted (assignation analysis and Chesser's social structure analysis) and only one
sex was responsible for this gene flow. Martin & da Silva (2004a) annually determined the
existence of around 260 pink river dolphins in 225 km
2
of the main Mamirauá lake system, in
the Japurá (Caquetá) river in Brazil, where half were permanent residents. However, 90 % of
the dolphin sightings that these authors marked within the lake system between November
2001 and November 2002 were of permanent residents, or their dependent offspring. They
also found a possible cline in site fidelity between those that always lived in the lake system
and those where dolphins occasionally visited the lake system. These observational results
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