By Aaron Marine, Alicia Pulchinski, Scott Gagliardi

 A paper for MSCI 375, Biology of Marine Mammals, submitted December 2, 1998

Location and distribution

River dolphins are divided into 5 species in 4 genera in a superfamily known as the Platanistoidea. They are known as the Ganges, Indus, Chinese or Baiji, Amazon or Boto, and La Plata River dolphins (May 1990). The Ganges River Dolphin lives in the Ganges and Brahmaputra river systems of India, Nepal, Bhutan, and Bangladesh. There are only about 5,000 surviving individuals today. The Indus River Dolphin lives only in a 105 mile stretch of the Indus River in Pakistan and there are between 400-500 individuals left (May 1990). The Chinese River Dolphin lives in the Yangtze River of China in an 875-mile stretch. It is the most endangered dolphin species with less than 300 individuals surviving and some estimates go as low as less than 50. The Amazon River Dolphin lives in flooded forests of the Amazon and Orinoco river systems of Columbia, Peru, Brazil, Ecuador, and Venezuela. (Whale times 1995). It is the largest in size and abundance of the five species. Finally, the La Plata River Dolphin, an uncommon species is found in the coastal waters of Brazil, Uruguay, Argentina, and the La Plata estuary (May 1990).

What makes most river dolphins so unique is that they never leave fresh water. Their small size allows them to exist in shallow waters and even in flooded forests. River dolphins live in ancient and major waterways that gave that species plenty of time and opportunity to evolve within their limited areas almost making it imperative to learn the history of a river system in order to study the species. For example, in geological history, the Amazonia was a huge shallow marine gulf that opened into the Pacific Ocean. About 20 million years ago the Andean mountain chain was raised along the western edge of South America and changed the gulf into an island sea. The enclosed body of water grew due to its tropical location. The water then broke through near Obidos Brazil and drained into the Atlantic Ocean. Nothing of the sea remains today and the area now makes up the largest river system on earth today more commonly known as the Amazon (Ritchie, 1993). Tremendous speciation occurred in the amazon many families of marine fishes including soles, stingrays, needlefish, and puffers have a fresh water representative living in the amazon (Ritchie, 1993).

A number of species of marine dolphins and porpoises habitually enter freshwater systems—most notably the Irrawaddy dolphin and the finless porpoise of Asia. And the Tucuxi whose range matches the Amazon River dolphin but it also frequents coastal waters (Morell, 1997).

Individual Characteristics

The Ganges and Indus River Dolphins are often categorized together because of their various similarities and their indistinguishable appearance. Both are members of the Platanistidae family having scientific names of Platanista gangetica and Platanista minor respectively. In terms of relative size, males can grow to 7 feet long and weigh 185 pounds and females can reach lengths of just over 8 feet and 184 pounds (May 1990). Their most striking feature is the long, slender beak, which can measure up to 1/5 of the total body length. The body is small and plump with large paddle-like flippers and wide pointed flukes. Although there is no dorsal fin, there is a long dorsal keel. They appear uniform gray in color and have very poor eyesight. Their eyes are severely reduced to the size of pinholes, and therefore, they rely heavily on their echolocation abilities. They swim on their sides scanning the water horizontally moving their heads up and down emitting a constant stream of echolocation clicks.

The Chinese River Dolphin is also more commonly known as the Baiji, meaning white dolphin in Chinese. The scientific name for the Chinese river dolphin is Lipotes vexillifer. As stated before, it is the most endangered dolphin species and could become the first extinct cetacean if proper measures are not taken (May 1990). They are shy animals with long, thin beaks and are light to dark gray fading to white on their bellies. They tend to be seen in pairs and dive only briefly. There are now preserves set up in China promoting its preservation.

The Amazon River Dolphin, scientific name Inia geoffrensis, is more commonly known as the Boto or pink dolphin. It is the largest of the river dolphins reaching a maximum length of 8ft 4in and 353 pounds in males and 6ft 3in and 213 pounds in females (May 1990). It has a long dorsal keel and large rounded flippers that curve to a point. The eye openings are larger than in other river dolphins and seem to be somewhat functional. The young are usually slate gray but as they mature the dark pigment fades to reveal the blood flow through their semi-translucent skin. When excited they may temporarily turn a bright pink as well. They are the only river dolphin with two types of teeth, being conical and molars (Vidal 1994). This means that they can chew their food. They are also one of the two species that has six "fingers" of their flippers. The Gulf of California harbor porpoise only shares this trait (Vidal 1994). They are also solitary animals and slow swimmers. They have often been observed resting on the bottom of the rivers.

The last of the species of river dolphins, the La Plata river dolphin is not a "true river dolphin", but is related so closely to and similar in appearance to the river dolphins that they are placed in the river dolphin family (May, 1990). The scientific name is Pontoporia blainvillei. Males reach length of just over 5 feet and weigh 104 pounds and the females reach 5ft 10in long and weigh 115 pounds (May, 1990). They have a long, slender beak and the front of the melon is rounded and have a crescent-shaped blowhole. It is the only river dolphin with a dorsal fin, which has a rounded peak, and flippers are serrated. They are brown-gray in color, darker on their backs than on their bellies. They feed near the bottom of the ocean and don’t form schools.

Unique characteristics or fun facts

Calves are bluish or gray but as they mature a ruddy color spreads from the undersides up to the back until in most cases they become completely pink (Morell, 1997). Most adults either keep or redevelop a bluish or gray patch on the top of their heads or backs. Some cetologists believe sunlight may influence an adults color like a sun tan.

Mobility

River dolphins are relatively small in size allowing them to live in fresh water. They are not as energetic as marine dolphins nor are they designed to sustain speeds and acrobatics (Cousteau, 1975). A unique characteristic that sets them apart from marine dolphins is that they have unfused neck vertebrae which enables them to turn their necks. This added mobility allows them to catch fish. Studies indicate that they eat a variety of fishes but prefer catfish and some develop a taste for crabs and shrimp (Carwardine, 1998). They have short stiff hairs on the snout which acts as a tactile organs that gives them an added advantage in muddy waters. River dolphins can hunt for food in no more than 8 inches of water where they move by swimming on their side. They hunt in small groups and breathe every 30 seconds rarely staying under for more than 2 minutes (Ritchie 1993).

It was believed that none of the river dolphins whistled until Ding Wang recorded simple whistles from the Baiji (Cousteau, 1975). A characteristic of many non-whistling species is that they tend to be less gregarious than many whistling odontocets. They travel in small groups rather than in large herds. Sound emissions in both species are generally higher than salt water counterparts because they have reduced eyes. The Ganges river dolphin continually emits a high frequency of sounds of 40-50 kHz sometimes going as high as 380KHz (Herman, 1980). The Indus river dolphin communicates with a range of 800 Hz to 16Khz with a sonar frequency ranging between 1 and 300 kHz. Both species emit clicks between 10 and 1000 clicks per min. Platanista species in captivity continuously emit clicks at 40 to 80 pulses/sec regardless, of the time of day or aquarium clarity, whereas the Inia species only echolocate in the presence of an unknown object, or food, or in the absence of light.

The most extreme adaptations are found in the eyes of river dolphins, where vascularization provides for rapid cellular repair and defense against pathogens, where microorganisms and particulate matter are a big threat to their eyes. The adaptations in Plantanista species are so extensive that image formation is most likely impossible (Herman, 1980). Tursiops and other marine dolphins have superior visual apparatus to any river dolphin, with Inia sp.having intermediate vision, and Plantanista sp. having the most regressed systems (Herman, 1980). Plantanista sp. tend to swim on their sides with a pectoral fin gliding over the bottom of the river. A theory was formulated by Purves and Pilleri that the eye is a light detector, and in cooperation with the gliding pectoral and echolocation, provides for depth perception and navigation in very turbid waters (Herman, 1980).

Eyes are reduced, but functional in Inia sp. and appear to be nearly non-functional in Plantanista sp. (Sylvesttre, 1993). Pupils lack an operculum, a lens covering the eyeball like a third eyelid on a dog; and the central optical region, where the pupil admits light, is partially obscured by small blood vessels in the cornea, allowing less light to enter the pupil. No tapetum was found in any species of river dolphins, but a partially reflective tapetum was found in Tursiops (Herman, 1980).

Lenses are spherical, transparent and colorless, except for Inia sp., which have the appearance of a small, yellow sphere. Plantanista sp. have no lens at all. A flat membrane is all that remains. Without a lens, light isn’t being bent through the eye so image formation is impossible, so maybe this proves Purves and Pilleri’s idea that their eye is a light detector (Herman, 1980).

No epthalmic rete was found in any species of river dolphins. The epthalmic rete is responsible for stabilizing the temperature of the retina and optic nerve in deep-diving mammals. Instead, a large orbital flat pad similar to those found in many terrestrial mammals occupies the rete location (Herman, 1980).

Studies on Plantanista sp. have found a hyaloid artery in mature dolphins. This is bizarre because it feeds the core of the eye during development and usually disappears with time. Because of this, it is thought that Plantanista eyes are in a very regressed state of development, unlike other marine mammals (Herman, 1980).

Studies on two Amazon River Dolphins (Inia sp.) in captivity have shown similar behavior to Tursiops in captivity. More conspicuous behavior included play, orientation to humans, sexual activity and various solitary swimming and surface activities. Other contrasts include a lack of aerial behavior or fast swimming and rarity of coordinated swimming or breathing behavior among pairs of captive Inia sp. (Herman, 1980).

Studies on Ganges River Dolphins (Plantanista sp. ) in captivity showed an apparent establishment of exclusive territories among male-male pairs as well as female-male pairs. Complex swimming patterns developed as Indus River Dolphins (Plantanista sp.) adapted to their tanks and included chase behavior among pairs, especially during their spring mating season. Unison swimming was common throughout the year as was contractual behavior between the dolphins and manipulation of objects in their tanks (Herman, 1980).

Riverine habitats and the dolphins, which live in them, are highly vulnerable to the effects of a wide variety of human activities. The river dolphins are under increasing threats from entanglements in fishing nets, pollution through pesticides, and construction of dams for irrigation and hydroelectric power. The latter of the problems seems to be of particular concern (May, 1990). Construction of dams may reduce the ability to spawn in certain waters or follow food supplies. Much more effectively dams restrict the natural range of the dolphin. There are no easy solutions to most of the problems effecting river dolphins. It seems that unless captive breeding is accompanied by equally energetic efforts to address the interlinking problems affecting the habitat, small numbers may survive. Unfortunately there is very little hope in rescuing certain species such as the Yangtze and it will most likely become extinct in our times.

Herman, Louis M. 1980. Cetacean Behavior: Mechanisms and Functions. John Wiley and Sons, Inc. University of Hawaii, Honolulu. pp. 462.

Sylvesttre, Jean-Pierre. 1993. Dolphins and Porpoises: A Worldwide Guide. Sterling Publishing Co., Inc. New York. pp.160.

Cousteau, Jacques-Yves. 1975. Dolphins. Doubleday and company, inc. Garden city, New York pp. 272

Carwardine, Mark and Erich Hoyt. 1998. The nature company guides to whales dolphins and porpoises. Time life books. San Francisco, California. pp.166-170.

May, John. 1990. The Greenpeace book of dolphins. Sterling publishing co.,inc. New York, New York. pp. 111-117.

Morell, Virginia. 1997. Looking for big pink. International wildlife. Nov-Dec vol27 p26

Ritchie, Tom. 1993. Pink dolphins could lose their charm. Sea frontiers, Nov-Dec vol39. pp30-35.

Vidal, Omar. 1994. "A fresh look at river dolphins." Americas. March-April v46 n2 p44(4).

Whale Time Whale Facts. Jakenmax Productions. 1995. http://www.whaletimes.com/whandl.HML

Biologists see little hope for Chinese pink dolphin. 1996. http://www.cais.net/publish/stories/0996mar3.htm.

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