FREE ESSAY ON DOLPHINS OF THE AMAZON RIVER: HOW SOTALIA FLUVIATILIS AND INIA GEOFFRENSIS COEXIST IN THEIR HABITAT

DOLPHINS OF THE AMAZON RIVER: HOW SOTALIA FLUVIATILIS AND INIA GEOFFRENSIS COEXIST IN THEIR HABITAT

Kristi Simpson
Biosc 491-14/Tropical Biology
April, 1999
Dr. E. Pivorun
Dolphins of the Amazon River: How Sotalia fluviatilis and Inia geoffrensis coexist in
their habitat
The Amazon River and its lush, beautiful forest are surely among the most amazing
ecosystems in the world. The ever-present, primordial cacophony that echoed in my ears as
I stood breathlessly watching saddle-backed tamarins (Saguinus fuscicollis) leap from
tree to tree is what I will forever crave to hear again. As a biology student, I have
always read about the "great biodiversity" of the neotropics, as the importance of
habitat conservation and protection is beaten into my brain at every turn. Of course, as
a naturalist, I agree with the prevailing opinion of today that our world is in dire need
of help. However, I couldn't appreciate the true beauty and magic of the tropical Amazon
until I was literally swimming in the middle of it, breathing in the wet, fragrant air
and seeing the misting, puffing backs of the pink river dolphins (Inia geoffrensis)
circling around me. 
As a lover of all cetaceans, I was very interested in learning more about the dolphins of
the Amazon River, but it was a surprise to find that the pink river dolphins (also called
botos) are not the only species of cetacean in their habitat. Tucuxis (Sotalia
fluviatilis) are smaller, sleeker inhabitants that share the waters with the botos,
although they are also found in coastal waters of the South American Atlantic Ocean. The
tucuxi looks like a miniature bottlenose dolphin, with its short beak and sleek design.
It is important to make note that tucuxis are considered to be in two forms: the marine
form and the freshwater form (Borobia et al. 1026). Borobia hypothesizes that the two
forms, which vary for the most part in size, may be due to temperature differences, so
that the marine form, as it inhabits colder seasonal temperatures than its riverine
counterpart, is the larger of the two (1035). 
The tucuxi in any form, however, is much smaller than the boto, which, in fact, looks
anything but graceful with its chubby body, small eyes, elongated beak, and bulbous
forehead or "melon." The boto also has a flexible neck due to unfused neck vertebrae
(Morell 29), adding to its seemingly awkward design; it is also the world's largest
freshwater dolphin, as the males reach lengths of three meters and weights of 16o
kilograms (Henningsen and Hempel 12). The most distinguishing characteristic, of course,
is the pink hue that the botos are famous for; the tucuxis are dark grey. 
These morphological differences begin to explain why the botos and tucuxis coexist in
seemingly identical habitats; I could not help but wonder how they utilize their
respective food sources and keep from outcompeting each other. Their body designs allow
for different feeding methods; for example, the pink dolphins' flexible neck as well as
the ability to paddle forward with one flipper and backwards with another gives it more
maneuverability in the flooded forests, where the trees and other vegetation can be dense
(Morell 29). Botos will swim close to the bank, single out one fish, and chase it until
it is caught. The tucuxi is not as maneuverable in the shallow, flooded forest, but
instead is a fast swimmer; "[t]hey whip through the water, often leaping high into the
air, to drive small shoals of fish before them. Groups of Tucuxis often coordinate their
operations, herding a shoal of fish towards the river bank or into a small bay..." then
take turns in groups catching them (Henningsen and Hempel 15).
Choice of prey is another difference between the two species. Botos will eat almost any
kind of fish; Vera da Silva, biologist and boto researcher, "found that they prey on
nearly 50 species from 19 families (Morell 29)." However, according to Ritchie,
"[s]tudies indicate that Amazon river dolphins eat a variety of fishes, but they seem to
prefer catfish and characins, including piranhas (33)." By contrast, the tucuxis' diet
consists primarily of pelagic and schooling fish (Henningsen and Hempel 14, Borobia et
al. 1035).
Although I have separated morphology and choice of prey as differences between Inia and
Sotalia, they are interwoven and connected, and hard to separate concretely. The tucuxis
are characteristic of deep, open waters; so characteristic, in fact, "that river pilots
rely on observing them for navigational aids (Krichner 206)." Their morphology does not
allow them to take advantage of the shallow waters, as I have described, so it would
stand to reason that they would benefit by staying in the open waters and preying upon
open-water fish species. Likewise for the boto, since their body design allows them to
exploit the flooded forest, it follows that they will favor whatever prey species may be
found there. Herein lies the old "chicken and the egg" question: is the morphology of
each species a result of favored prey or vice versa? It seems that evolution has worked
its magic, shaping Inia as it adapted to its riverine environment, and thus its prey,
while Sotalia chased its prey-type, invading freshwater from the sea. So, in fact, the
tucuxi has adapted itself, not really in a morphological sense like the boto, but more
from a behavioral standpoint, to the fish of the open, deep river waters. This also
explains why the boto has a much more varied diet than the tucuxi: it has evolved to
survive in the Amazon River, changing its diet along with the greatly speciating fishes.

Thus, my original question - how do Sotalia and Inia share the same habitat and food
resources? - is answered: they do not! They have instead carved out entirely different
niches for themselves, coexisting yet not competing. How delicate, yet strong and
flexible, Nature is!
Bibliography
Works Cited
Borobia, Monica et al. "Distribution of the South American dolphin Sotalia
fluviatilis." Canadian Journal of Zoology. 69:1025-1039, 1991.
Henningsen, Thomas, and Gotthilf Hempel. "Botos, Tucuxis, and their Life in
the Amazon: River Dolphins in their Natural Habitat." Reports of the
DFG. n 1: 12-15, 1998.
Krichner, John. A Neotropical Companion. Princeton University Press;
Princeton, NJ, 1997.
Morell, Virginia. "Looking for Big Pink." International Wildlife. 26-31,
November/December 1997.
Ritchie, Tom. "Pink Dolphins Could Lose Their Charm: Superstitions Help
Protect the Amazon River Dolphins." Sea Frontiers. 39 (6): 30-33,52,
1993.