River Dolphins

1. Amazon River Dolphin (Boto)
2. Baiji
3. Indus and Ganges River Dolphins
4. Franciscana

River dolphins are only very distantly related to oceanic dolphins. They are not classified under the family Delphinidae. In fact they comprise several families, including Iniidae, Platanistidae, Lipotidae, and Pontoporiidae. Most of them live in some of the world's largest and most complex river systems: the Amazon, the Indus and Ganges, and the Yangtze. One species, the Franciscana, is actually found in the estuaries on the southeastern coast of South America. They are perplexing creatures, having both some of the most advanced characteristics of all living cetaceans and some evolutionary holdovers from many eons past. River dolphins most likely descend from marine cetaceans, as some fossils of the family Iniidae have been found in oceanic environments. It is believed that all odontocetes are at least primitively marine. They probably came about through convergent evolution, with similar habitats selecting similar characteristcs. They may not be closely related to each other at all.

All river dolphins fall into the superfamily Platanistoidae. It appears that this superfamily separated from Delphinoidae at a very early stage, right at the time of the first odontocetes. It is possible that they evolved from some taxon within the family Agorophiidae, which is now extinct. Agorophiidae consisted of many long-snouted genera, possibly related to the highly derived platanistids from Asia. It was most diverse during the Miocene.

Living river dolphins do not very closely resemble their oceanic counterparts. Their sizes range from very small to medium-sized. They all have extremely long beaks, sometimes reaching one fifth of the total body length, and many pointed teeth. Their extremely flexible necks allow them to navigate dense, swampy water basins, and their pronouced melons betray extraordinary echolocation abilities, allowing them to operate in the most murky, dense waters. Their brains are extremely large and well developed. They have broad, short flippers, with visible "fingers" -- an obvious evolutionary holdover. They also have very small dorsal fins, often only a couple of centimeters tall. As the waters they navigate are so muddy, they have little need for vision, and as a result they are almost blind. The Indus and Ganges river dolphins have lost their lenses altogether.

Being very easily frightened and usually impossible to approach by boat, Baiji has not been an easy dolphin to study and is thus little known. It's habitat consists of the Yangtze River. Once classified as Iniidae, it now falls under the family Lipotidae. The family Pontoporiidae has also been suggested, since fossiles from the late Miocene and Pliocene have been discovered that are intermediate between the Pontoporia and the Lipotes. Baiji are sometimes found with finless porpoises and are mostly active at night. When active, its swimming is very rapid and many variations in style are found. Individuals change directions often and dives are frequent and short. Conversely, when not active, swimming is slow, smooth, and usually unidirectional. Dives are also fewer in number and longer in duration. The species has been under legal protection since 1949, but despite the efforts of the Chinese government to protect these dolphins, the population appears to be declining. It appears to be the most endangered of all cetaceans.

The Indus and Ganges River dolphins are almost identical in anatomy and behavior, but are separated geographically. For many years, it was thought that they belonged to the same species, but craniological differences, as well as differences in the blood protein were found. In appearance and habits they are nearly identical. They are the only living members of the family Platanistidae. They are small and have several primitive characteristics, including flexible flippers and necks. However, they have many derived cranial characteristics, and their skulls are some of the most modified of any cetacean. Members of this species are usually seen living alone or in pairs, although groups of up to ten individuals have been observed. These observations may merely be a result of diminishing population size, since in the 19th century, reports of large schools were quite common. They are the only cetaceans with no crystalline eye lens, making them effectively blind. They can probably detect the direction, and possibly the intensity, of light, but they navigate and find food with a very advanced system of echolocation. Individuals are active at all hours. Compared to other river dolphins, they are more active at the surface. They often swim with their beaks out of the water, and females sometimes lift their calves completely above the water. When distressed, they breach. Their beaks are long and narrow, sometimes reaching one fifth of the body length. Their teeth are not entirely homogeneous; those at the tip of the beak are longer. Instead of a dorsal fin, Indus and Ganges River dolphins only have triangular humps.

The family Pontoporiidae, which used to be Stenodelphinae or Stenodelphininae, consists of the only modern odontocetes that retain a symmetrical skull. Fossils have been found in the South American marine deposits, and they appear to be coastal marine mammals. The only modern species of this family is the franciscana. Very few records of this species in the wild exist. It is inconspicuous and easy to overlook except in very calm conditions. Although closely related to river dolphins, this species lives at sea, occupying the estuaries of southeastern South America. It prefers very shallow coastal waters. One of the smallest cetaceans, franciscana's beak is very long -- relatively speaking, the longest of any cetacean. It most likely prefers a solitary existance, but groups of up to five individuals have been occasionally reported. It has been known to lie on the sand at the bottom of the water on hot days, coming to the surface periodically to breath. When approached by a predator, it remains completely stationary. Movement is very smooth, as it seldom rolls and shows little of itself when breathing. The major threat to this species is entanglement in fishing nets, which may very well have depleted the species.

Bibliography

Carwardine, Mark. Eyewitness Handbooks: Whales, Dolphins, and Porpoises. New York: Dorling Kindersley Ltd., 1995.

Evans, Peter G. H. The Natural History of Whales and Dolphins. New York: Facts on File Publications, 1987.