Biology Reference
In-Depth Information
Group Size
In Venezuela, mean group size was significantly associated with season (F = 6.74, df = 2,
478, P < 0.0129, N = 489). Mean group size was 1.7 dolphins during falling water, 1.9
dolphins during low water, and 2.3 during rising water (high water was not sampled). The
largest groups were found in confluences during rising water (mean group size = 2.7) and the
smallest groups were found in narrow (<50 m wide) river channels during rising water (mean
group size = 1.1).
In Peru,
Inia
were most often seen as singles in all rivers and lakes: 60% of all sightings
were of single dolphins, 30% were of pairs, 9% of triples, and only 1% of groups were larger
than 3 animals. Seasonal differences in group size were statistically significant for
Inia
in the
Marañón (Kruskal-Wallis H = 11.03, P = 0.01) and Samiria rivers (Kruskal-Wallis H = 12.48,
P = 0.006); other rivers were not sampled frequently enough across seasons for statistical
comparison. In the Marañón River, mean group size was greatest during low water (1.7
dolphins), and smallest during rising water (1.5 dolphins). In the Samiria River, mean group
size was largest during falling water (1.9 dolphins) and smallest during rising (1.5). Median
Inia
group size in lakes did not differ significantly according to season in two of the three
lakes surveyed. In lake Atun Cocha, the seasonal differences were significant (Kruskal-Wallis
H = 20.45, P = 0.0002) and the largest groups occurred during falling water (mean group size
of 2.1 dolphins). Regardless of season or habitat, mean
Inia
group size was never greater than
2.1 dolphins.
In Bolivia, solitary individuals comprised 41% of all observations. The frequency of
group size varied significantly by season (χ
2
= 73.67; df = 15;
P
= 0.00). Solitary individuals
were seen most frequently in all seasons, with the exception of low water, during which time
pairs were seen more often than other group sizes. Mean group size was 2.7 during falling
water, 2.9 during low water, 2.3 during rising water, and 2.1 during high water.
Seasonality of Reproduction
Inia
neonates were observed year-round in Peru and Bolivia, although diffuse seasonal
peaks occurred during falling water in both of these study areas. In Venezuela, neonates were
never observed during falling water, but began appearing at the end of the low water season
and peaked during rising water (high water was not sampled due to logistical constraints).
Significant seasonal differences in neonate sightings existed in all three study areas, even
after accounting for seasonal differences in sampling efforts and overall dolphin abundance
(Figure 4 & Figure 5; Peru χ
2
= 135, df = 3,
P
< 0.001; Bolivia χ
2
=17, df = 3,
P
< 0.001;
Venezuela χ
2
= 34, df = 2,
P
< 0.001) .
One
Inia
neonate was necropsied 13 August 1997 during low water in Peru. The animal
was a female, 82.5 cm total length (straight length, tip of snout to tail notch), with fetal folds.
The teeth had not yet erupted but were visible just below the surface of the gums. The
stomach was empty, although the intestinal contents indicated a milk diet. Based on its total
length and da Silva's (1994) estimates of neonate growth rates, it was estimated to be <
1month old, and born between falling and low water. Dead females that were pregnant or
lactating were not encountered.
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