Biology Reference
In-Depth Information
Seasonality of Reproduction
Our field observations and review of the published and unpublished literature from
throughout the approximately 8 million km
2
range of
Inia
indicates geographic variation in
reproductive seasonality, with some areas exhibiting year-round reproduction with diffuse
seasonal peaks (McGuire, 1995; McGuire & Winemiller, 1998; Aliaga-Rossel, 2002;
McGuire & Aliaga-Rossel, 2007). Seasonality of peaks in births varied according to study
area, and may be more closely associated with local environmental and prey conditions than
with relative seasonal differences between high and low water levels of the study areas, or
distribution according to river basin or latitude.
Inia
births in the central Brazilian Amazon have been reported as highly seasonal,
occurring during the local period of high water and the beginning of falling water (Best, 1984;
Best & da Silva, 1984; Brownell, 1984; Best & da Silva, 1989
;
da Silva, 1994; da Silva &
Best, 1996). Best & da Silva (1989) hypothesized these births coincided with increased access
to prey fish, which become vulnerable as habitat and cover from previously inundated
vegetation decrease with falling water levels.
Because the taxonomy of
Inia
is determined by distribution according to river basin, the
possible effects of differences in phylogeny are essentially indistinguishable from differences
in broad geographic distribution. When we associated study area with species (
i.e.,
I.
boliviensis
in Bolivia,
I.g. humboltiania
in Venezuela, and
I.g.geoffrensis
in Peru and Brazil)
no clear patterns emerged with respect to synchrony of reproduction. In fact, there is more
similarity in the patterns of seasonality of reproduction in dolphins from the Peruvian and
Bolivian study areas (
I.g. geoffrensis
and
I.bolivensis
, respectively) than between
I.g.geoffrensis
from the Peruvian and Brazilian study areas from the same river basin.
Although sample sizes are small, two separate studies from Venezuela (McGuire, 1995;
Caranto & Gonzalez-Fernandez, 1998) suggest
I.g.humboltania
may differ from the other
subspecies in that the calving peaks during low water, and does not occur year-round.
Based on patterns of reproductive seasonality found in other odontocetes, we were
interested in exploring if degree of seasonality of reproduction in
Inia
increases with distance
from the equator. If so, we would predict little to no seasonality in the da Silva Brazilian
study site (3
o
S) and strong seasonality in the Bolivian study site (14
o
S), with intermediate
levels in Peru and Venezuela (5
o
S and 6
o
N, respectively); however this was not the case, and
no clear patterns emerged.
Is degree of reproductive seasonality in
Inia
related to relative differences between high
and low water levels? Are differences in reproductive seasonality more pronounced where
seasonal differences in aquatic habitat are most extreme? In general, differences between high
and low water levels not only increase with latitude, but also along the headwater-to-
confluence course of a river. Compared to the rest of South America, seasonality is minimal
in the northwestern Amazon Basin (Lewis et al., 1995). The Amazon River is just a few
degrees south of the equator, where its seasonal flux is generally not as extreme as those
rivers located at higher latitudes. In addition, there is an east/west gradient of seasonal
changes in water level. For example, the difference between maximum and minimum waters
in the Peruvian study area is 7 m, but downriver in central Brazil it is almost twice that
amount (Martin & da Silva, 2004a,b);
Inia
reproduction is year-round in the first study site
and seasonal in the second. This model does not readily apply to our other study sites
however, as Venezuela had the smallest difference between high and low waters, yet
Inia
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