River Dolphins
 

Common Name: River Dolphins

Class: Mammalia

Order: Cetacea

Family: Platanistidae

Genus: Four genera

Species: Four species

Yangtze River dolphin, Lipotes vexillifer

River Dolphins: Family Platanistidae

The family Platanistidae includes four species of dolphins, collectively called “river dolphins”, although only three species inhabit river systems.  Platanistidae belongs to the Mammalian Order Cetacea, in the suborder Odontoceti.  All toothed whales belong to the suborder Odontoceti, which is Latin for "toothed whales".  Platanistidae is derived from the Greek word platus, meaning "flat".  A Latin derivation of the word, platanista, was used by the Roman historian Pliny to describe a fish he observed in the Ganges River.  The beaks the dolphins in this family are generally broader than those of the cetaceans in Delphinidae and Ziphiidae. The species of this family are the La Plata dolphin or Fanciscana, Pontoporia blainvillei, the Ganges and Indus River dolphins, Platanista gangetica, the Amazon River dolphin or Boto, Inia geoffrensis, and the Yangtze River dolphin or white fin dolphin, Lipotes vexillifer.  Some researchers place each genus in a separate family, Iniidae includes Inia, Platanistidae includes Platanista, Pontoporiidae includes Pontoporia, and Lipotidae includes Lipotes.  In fact, so little is known about the evolution of these species, that some researchers question the monophyly, or common ancestry, of the group.

Yangtze River dolphin, Lipotes vexillifer

Generally, Platanistids are recognized from other dolphins by a long, slender rostrum and tiny eyes.  Their neck vertebra are unfused, which allows these dolphins to rotate their heads in a manner not associated with most cetaceans.  There is evidence that all species use echolocation to navigate and find prey, but the use of their long snouts to probe riverbeds and ocean bottoms to find prey is also common.  Fish, squid and shrimp are the common components of their diet.

Genus:  Pontoporia

The generic name Pontoporia, is derived from the Greek words pontos, meaning "sea", and poreuo, meaning "to carry".  This probably refers to the dolphin's coastal, ocean habitat.

 

La Plata dolphin or Franciscana Pontoporia blainvillei

The specific name, blainvillei, is named for Henri Marie Ducrotay de Blainville (1777-1850), a French zoologist and anatomist.  Blainville became the chair of anatomy and zoology in the Faculty of Sciences at Paris.  The common name, La Plata, means "silver" in Spanish and Portuguese.  This probably refers to the dolphin's gray coloration.  The only member of Platanistidae to prefer saltwater, the La Plata dolphin resides in the coastal waters and estuarine habitats of southeastern South America.  Adults reach 4-6ft in length and 45-130lbs in weight.  Females are generally larger than males.  La Plata dolphins are gray dorsally and light yellow ventrally.  Gestation lasts 10-11 months.  Calves are 35 inches in length and 15lbs in weight at birth.  They are weaned at nine months, reaching sexual maturity at 2-3 years.  La Plata dolphins, along with members of Phocoenidae, are the only Odontocetes with symmetrical skulls.

 

Genus:  Inia

The generic name Inia, is a Bolivian word for dolphin.

 

Amazon River dolphin or Boto Inia geoffrensis

The specific name, geoffrensis, is named for the French zoologist Etienne Geoffroy Saint-Hilaire (1772-1844).  The common name, boto, is the Portuguese name for this animal.  Portuguese is the principal language of Brazil, the region in which this animal is found.  This species prefers freshwater, inhabiting slow moving streams and waterways of the Amazon and Orinoco River systems .  Adults reach 5-9ft in length and 110-195lbs in weight.  Males are generally larger than females.  Amazon River dolphins range from pink to dark blue in coloration.  Dolphins found in cloudy waters are more pinkish, while those in clearer waters have more dark blue coloration.  Unlike all other members of Platanistidae, Amazon River dolphins have bristly hair on their snouts.  This hair may be used to locate prey.  Gestation lasts 10-11 months.  Calves are two feet at birth.  They are weaned at one year.  Groups of 12-15 individuals have been observed, although solitary animals and mother with calf pairs are more common.

 

Amazon River dolphin, Inia geoffrensis

 

Genus:  Lipotes

The generic name Lipotes, is Latin, derived from the Greek words leipo, meaning "left behind".  This refers to the morphological similarities of this species with fossilized remains of prehistoric dolphin species, as though this animal was "left behind" as a relic of the past.

 

Yangtze River dolphin or whitefin dolphin Lipotes vexillifer

The specific name, vexillifer, is derived from the Greek words vexillum, meaning "flag", and feros, meaning "to bear".  This may refer to the dorsal fin of the species, which is observed at the surface of the water.  This species prefers freshwater, inhabiting the Yangtze River system of China.  Males of this species reach 4.5-6ft in length and 100-275lbs in weight.  Females grow 5-8ft in length and 140-350lbs in weight.  Yangtze River dolphins are pale gray dorsally and white ventrally.  Gestation lasts 10-11 months.  Calves are about four feet at birth.  Group size ranges from 3-10 individuals.

 

Yangtze River dolphin, Lipotes vexillifer

 

Genus:  Platanista

The generic name Platanista, is derived from the Greek word platus, meaning "flat".  A Latin derivation of the word, platanista, was used by the Roman historian Pliny to describe a fish he observed in the Ganges River.  This refers to the broad beaks of the animals in this genus.

 

Ganges River Dolphin Platanista gangetica

The specific name, gangetica, is Latin for "belonging to the Ganges", one of the Indian rivers in which it is found.  This species prefers freshwater, inhabiting the Brahmaputra-Meghna River system and the Karnaphuli River of India, Bangladesh and Nepal.  Some scientists classify the population of Platanista gangetica living in the Indus River system as a separate species, Plantanista minor, commonly called the Indus River dolphin.  Adults reach 6-9ft in length and 110-195lbs in weight.  Females are generally larger than males.  Ganges River dolphins are dark gray dorsally and lighter gray ventrally.  The Ganges River dolphin's eye lacks a lens.  Light can only be received at a certain angle, resulting in sideways swimming.  Gestation lasts 11-12 months.  Calves are 30 inches at birth.  They are weaned at one year, reaching sexual maturity at ten years.  Groups of 3-10 individuals have been observed.

 

Amazon River dolphin, Inia geoffrensis

Conservation

All species of river dolphins are endangered.  The Yangtze River dolphin is critically endangered, numbering about 20 individuals.  Ganges River dolphins arguably number 4000-5000, with the population of Indus River dolphins estimated at 500 individuals.  Local peoples in all regions may hunt river dolphins for food, bait, or fat.  Fisherman may accidentally drown dolphins with their nets.  The construction of damns on the river systems in which some species are found may negatively impact river dolphin populations by altering their ecosystem.  Pollution from urban development and boat traffic have adverse effects on river dolphins as well.

 

Amazon River dolphin, Inia geoffrensis


 

Content provided by Canisius College students under the direction of Michael Noonan, PhD.