CHANGES IN THE DEMOGRAPHIC TRENDS OF PINK RIVER DOLPHINS (INIA) AT THE MICROGEOGRAPHICAL LEVEL IN PERUVIAN AND BOLIVIAN RIVERS AND WITHIN THE UPPER AMAZON:MICROSATELLITES AND MTDNA ANALYSES AND INSIGHTS INTO INIA’S ORIGIN - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 180

commented, there is a significant correlation between S k and V, but it is only moderate. Why
didn't this correlation occur between the Ucayali and the Marañón River dolphin populations?
A possible explanation is in Figure 4 from Zhivotovsky et al. (2000). The initial size of a
population has opposite effects on the dynamics of S k and V. The lower the initial population
size, the lower the V value and the larger the S k value. Henceforth, although the three
Peruvian rivers showed a similar demographic dynamic, some subtle differences could be
claimed. The original Ucayali River dolphin population was slightly smaller than the original
Marañón population and the relative growth in the Ucayali River was higher than in the
Marañón River. Nonetheless, the absolute population expansion was higher in the Marañón
River and/or its population expansion was more recent than for the Ucayali River population
which had a higher initial population size. The Napo-Curaray river dolphin population
probably represented an original population slightly lower than the Ucayali and Marañón ones
but without any population expansion trend or, this one, was extremely recent. But the
differences in the three Peruvian river dolphin populations were quite small.
Zhivotovsky et al. (2000) showed a table (Table 5, page 761), where predicted values of
expansion time for humans were based on S k values with different initial population sizes. For
an initial population of 500 humans and for S k of 0.05, 0.20, 0.30 or 0.35, the population
expansions in humans began 10,000, 32,000, 77,000 and 171,000 years ago, respectively. For
an initial human population of 2,000 and for the same S k values, these expansion population
dates could be 17,000, 62,000, 141,000 and 272,000 years ago, respectively. For these
estimations, the human population was assumed to be at equilibrium prior to the expansion
and the growth was logistic. The generation time for humans was 25 years. The S k values for
some of our pink river dolphin populations, which showed positive values, were 0.164
(Putumayo river in Colombia), 0.175 (Marañón River), 0.208 (overall upper Amazon) and
0.350 (Ucayali River). Recall that a generation in Inia is about seven years. If I recalculate
these time expansions for river dolphins assuming the conditions for humans from
Zhivotovsky et al. (2000), although they obviously were not the same (it is only an exercise),
with S k around 0.20 (such as Putumayo, Marañón and overall upper Amazon) for initial
populations of 500 or 2,000 dolphins yielded expansion times of 9,000 or 18,000 years ago,
respectively. For the Ucayali River, with S k of 0.35, it could represent a temporal expansion
of 48,000-76,000 years ago. It is possible that the original population size for river dolphins
was yet smaller than the initial human population sizes. It would mean that the expansion
times were less than 9,000-76,000 years ago, but also that prior to expansion, the populations
were coming from an initial bottleneck which reduced the S k values during the population
expansion. It means that the real population expansion times could be higher than 9,000-
76,000 years ago. Even, Penny et al. (1995) and Zhivotovsky et al. (2000) claimed that more
attention should be paid to the lower confidence bounds of S k than to the estimation point. If
so, the referred river dolphin populations did not reach to expand or they expanded on the last
2,000 or 3,000 years ago. Anyway, it is clear that the temporal expansion of Inia was
relatively recent in the past (for instance, 10,000-100,000 years ago) but not several millions
of years ago, such as it was previously sustained by other authors.
Really, the number of possible Iniidae fossils is scarce (but see this topic, Barnes et al.,
2010; Cozzuol, 2010). Some genera as Anisodelphis , Ischyorhynchus and Saurodelphis
resemble Inia from the Miocene (Barnes et al., 1985). For instance, Ischyorhynchus is from
the late Miocene of Rio Acre. Also, another species named Plicodontina mourai seems to be
strongly related to Saurodelphis and therefore to Inia and was discovered in the Juruá River