Dolphins Of The Amazon River: How Sotalia Fluviatilis And Inia Geoffrensis Coexist In Their Habitat Essay, Research Paper

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!

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.