Platanista gangetica

Geographic Range

Limited to southern Asia, Platanista gangetica inhabits the Ganges and Indus rivers and the many associated tributaries and connected lakes. This species is restricted to freshwater. There are two subspecies: Platanista gangetica gangetica , found in Eastern India, Nepal and Bangladesh in the Ganges, Meghna, Karnaphuli, Bramaputra, and Hooghly river systems, and Platanista gangetica minor , found in Pakistan in the Indus River system.

• Biogeographic Regions
• oriental oriental
  • native native

Habitat

Ganges River dolphins occupy freshwater river systems in southern Asia. They inhabit the Ganges and Indus River systems and their many tributaries, streams, and connecting lakes. They are found in tributaries that run through the hills and lowlands in Nepal (roughly 250 meters above sea level) and sometimes in flood plains and areas of rivers with heavy currents. These river dolphins prefer areas that create eddy countercurrents, such as small islands, river bends, and convergent tributaries. Since these animals occupy a vast area of river systems, they can tolerate a wide variance of temperatures; some as cold 8 degrees Celsius to warm waters above 33 degrees Celsius (46.4F to 91.4F). They inhabit depths from 3 to 9 meters and must surface every few minutes for air. In the monsoon season, Ganges River dolphins locally migrate to tributaries and then back to larger river channels in the dry, winter season. They also move along the coast of the Bay of Bengal when monsoons flush freshwater out along the southeastern coast of India.

• Habitat Regions
• tropical tropical
• freshwater freshwater

• Aquatic Biomes
• lakes and ponds
• rivers and streams
• coastal coastal
• brackish water brackish water

Physical Description

The two subspecies of Ganges River dolphins are virtually identical in physical appearance. They are readily identified by their elongated snout, which can reach lengths of 20% of total body length. Upon sexual maturity, females develop slightly longer snouts than males. This characteristic is useful in identifying sexually mature individuals. The beak is relatively flat and becomes widest at the tip. They bend slightly upward and can reach a length of 21 cm. On both the top and lower parts of the jaw they have long, sharp teeth, which are visible even when the mouth is closed. On the upper jaw, there are between 26 and 39 teeth on each side and on the lower jaw 26 to 35 teeth on each side. The lower teeth are typically longer than the teeth on the upper jaw. With age, the teeth eventually are worn down and become flat. Unlike other dolphins, Platanista gangetica lack snout hairs.

Well designed for aquatic life, Ganges River dolphins have long flippers that can be up to 18% of total body length. The tail fluke is quite large as well, reaching 46 cm or roughly a quarter of total body length. The dorsal fin resembles a fleshy hump on its back and is usually just a few centimeters in height. They are usually a grey to brown color, but may also have pink bellies and dark grey backs. Dorsal color is generally darker than ventral color.

The skull is highly asymmetrical and has a distinctly steep forehead and a longitudinal ridge. These river dolphins are unique in having long necks with unfused vertebrae. This makes them able to turn their heads from side to side with great flexibility. Ganges River dolphins are sometimes referred to as "blind river dolphins" since their eyes are extremely tiny and lack a lens. These animals are not reliant on vision as a primary sensory system, but the eye is thought to function as a light detector. Slightly larger than the eye and positioned just below it are the external ears. The blowhole is longitudinally positioned, which is unique in comparison to the horizontally positioned blowholes in most other toothed whales . Ganges River dolphins characteristically have several folds of skin that form a wattle. The exact function or purpose of this ornamentation is unknown.

Upon sexual maturity, females tend to be larger than males in overall body size and snout length. Unofficial records have adult females measuring 400 cm, but the average adult rarely exceeds 300 cm in length. At birth, young average 70 cm in length. Typical adult weights are between 51 and 89 kg.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• ornamentation ornamentation

Reproduction

Not much is known about mating systems in Ganges River dolphins. Further studies must be done to provide information regarding their mating behavior. They breed year round. Difficulty studying these species can in part be attributed to environmental conditions in their habitat due to the monsoon season. In addition the political and socioeconomic state of the area where these dolphins are found is not conducive to research.

Breeding in Platanista gangetica occurs year round, as does birthing. Most births are from October to March, with a peak in December and January, preceding the beginning of the dry season. Gestation is typically about 10 months but can be from 8 to 12 months. Ganges river dolphins bear a single offspring from 70 to 90 cm long. Weaning can begin as early as 2 months or as late as 12 months, typical time to weaning is at 9 months old. Once offspring have been weaned, they disperse and become independent. Males and females typically reach sexual maturity at 10 years of age, although growth continues into their 20's.

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• viviparous viviparous

The main form of parental care in Platanista gangetica , besides gestation, is provisioning in the form of lactation until weaning. Offspring are weaned no later than 1 year old. Once weaning occurs both male and female offspring disperse. Platanista gangetica are solitary animals so, upon leaving, the offspring is entirely on its own.

• Parental Investment
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female
• pre-weaning/fledging
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

Very little is known about the lifespan and longevity of Ganges River dolphins. Few specimens have been observed for the entirety of their lives, but a handful of estimates exist. The oldest male on record lived to be 28 years of age, while the oldest female reached 17.5 years of age. Based on crude estimates, dolphins reaching 18 to 22 years of age may not be uncommon. Few successful efforts have brought Platanista gangetica individuals into captivity for study.

Behavior

Ganges River dolphins are solitary animals but they occasionally congregate in groups of 3 to 10 individuals. Groups of up to 30 animals have been reported. Mothers and calves stay together until the infants are weaned. Despite their mostly solitary nature, these river dolphins are found in loose aggregations, especially at tributary junctions where prey congregate. Some consider Ganges River dolphins semi-gregarious. There are some indications of territoriality, as chasing behaviors in males have been observed. Generally, these animals are shy towards humans, even in captivity. Their elusive nature has made them difficult subjects to study.

Ganges River dolphins have a peculiar method of swimming, in that they swim on their sides when submerged. At roughly a 10 degree angle, they swim a few centimeters from the bottom, constantly nodding the head, allowing it to sweep the bottom in search of food. For unknown reasons, most individuals swim on their right sides. Side swimming also positions their eye at an angle appropriate to sensing light, perhaps providing orientation for the animal while diving. Their tails are always positioned higher than the head in side swimming, allowing them to swim in water as shallow as 30 cm. When they surface for air, they level out and swim laterally. In captivity, measured swimming speed reached 5.4 km/hr, but this may not be representative of possible speeds in the wild, where habitats are more open. They have been recorded swimming upwards of 27 km/hr in the wild. Captive animals swim and vocalize continuously over a 24 hour period, with only brief interruptions lasting a few seconds. When swimming intensity relaxed, for example in drifting locomotion, the intensity of vocalizations emitted also relaxed. Dives are typically short, the longest wild dive was 3 minutes. Average dive times in the wild are between 1 minute 10 seconds and 1 minute 40 seconds. Dives in captivity are shorter than wild dive times, with the longest being 1 minute and 35 seconds. In many cetaceans , myoglobin concentrations are generally high to cope with the stress of diving, but due to their relatively shallow river habitats and short dive periods, myoglobin concentrations in heart and muscle tissues are considerably lower.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• nocturnal nocturnal
• motile motile
• nomadic nomadic
• migratory migratory
• solitary solitary
• territorial territorial

Home Range

Little is known about home range sizes in Platanista gangetica . Aerial and shoreline observations have proven inadequate at determining the given range of an individual. These animals travel extensively throughout river ecosystems, moving from mainland channels to coastlines and tributaries as the seasons change.

Communication and Perception

Ganges River dolphins have poor vision. They lack lenses in their eyes, making it impossible for them to resolve images, they are likely to only be able to detect the presence or absence of light. Ganges River dolphins have highly developed sonar systems. They use pulse sounds not whistles to navigate. This allows them to perceive objects, specifically prey, in murky water. Over a 24-hour period there is almost always a constant emission of sound, 87% of these sounds are clicks for echolocation, the remaining sounds are sounds used in communication. There have not been enough studies to determine what the significance is of these communicative sounds.

• Communication Channels
• tactile tactile
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• ultrasound ultrasound
• echolocation echolocation
• chemical chemical

Food Habits

Ganges River dolphins are top predators in their river ecosystems. Side swimming and a flexible neck allow them to search river bottoms to stir up hiding prey. Their formidable speed and ability to swim in shallow water allows them to chase and herd schools of fish. They feed on a variety of aquatic animals. Their physical appearance demonstrates how well equipped they are to catch fish and crustaceans. They are strictly carnivorous, although some vegetation has been found in their stomachs, most likely as a result of messy foraging in the river bed or left over plant remains inside the fish the dolphins have consumed. Their teeth and long snouts are designed to catch and hold fish. They have been observed shaking prey in their jaws and manipulating it to be swallowed head first so that the scales on the fish do not move against the animals throat. As these dolphins do not use vision as a sensory system to catch prey, they rely on echolocation to find food hidden in the mud and river bottom. Once prey are located, they grab it with their long snouts.

In the Indus river, catfish ( Wallago attu , Macrones aor ) and carp ( Catla buchanani ) make up a majority of the Ganges River dolphin's diet. Other fish, such as a gobies ( Glossogobius giuris ), herring ( Clupea telara ), and freshwater sharks ( Saccobranchus fossilis ) are frequently taken. In addition to freshwater fish, crustaceans such as prawn ( Palaemon and Penaeus ) and mollusks, such as Indonia coerulea , are eaten.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore

• Animal Foods
• fish
• mollusks
• aquatic crustaceans

Predation

Other than humans, there are no known natural predators of Ganges River dolphins. Humans have exploited these animals for oil, meat, and as bait for catching catfish. Otherwise, they are typically considered the top predator in their river ecosystems.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Ganges River dolphins are top predators in their river ecosystems. They are important in controlling and maintaining healthy fish and crustacean populations, their primary sources of food. Unfortunately, these river dolphins are experiencing the adverse effects of human environmental impacts and are highly endangered.

While little is known about parasites that use Platanista gangetica as a host, there are reports of Cyclorchis campula , Echinochasumus andersoni , Anisakis simplex , and Contracaecum lobulatum parasitizing these dolphins.

Economic Importance for Humans: Positive

Ganges River dolphins have historically been important as a source of oil and meat. The oil is used or as an ingredient in traditional medicines. The oil can be used to lure a specific species of catfish. The meat is used as bait to attract fish. However, dolphin meat does not attract fish any more than other fish scraps, so local fishermen must be educated to use other fish scraps due to the endangered status of Ganges River dolphins. Many top predators, including Platanista gangetica , serve as key indicators of water and environmental quality. In recent decades, due to heightened awareness of human impact on these freshwater ecosystems, many researchers are beginning to understand how extreme the pollution and toxin build up in these river systems has become.

• Positive Impacts
• food food
• body parts are source of valuable material
• source of medicine or drug source of medicine or drug

Economic Importance for Humans: Negative

There are no known adverse effects of Platanista gangetica on humans.

Conservation Status

Ganges River dolphins are among the most endangered of all cetaceans . With rising human populations in Southern Asia, the natural habitat of Ganges River dolphins has been extensively modified and degraded. Agricultural and industrial discharges are polluting the river systems in which these dolphins live and feed. In certain populations, the accumulation of heavy metals and organochlorides is posing serious health risks to the animals. In addition, dangerously high levels of arsenic in the water is a serious health threat to every animal using the water system, including Ganges River dolphins. Human modifications to river systems are also impacting the habitat of the dolphins. Over fifty dams affect populations of Ganges River dolphins, cutting populations off from one another. Dams have caused the gene pools of Ganges River dolphins to shrink, which could pose detrimental effects in future generations. Some engineering efforts are underway to construct channels around dams for aquatic wildlife, including dolphins. Ganges River dolphins are becoming more and more restricted to a smaller range. In Pakistan, a few hundred river dolphins are restricted to roughly 1200 square kilometers of water. Many local peoples regard these dolphins as a source of meat, oil and bait. Hunting has certainly impacted the numbers of dolphins in the Ganges and Indus river systems. Also, Ganges River dolphins are caught and drowned in fishing lines and nets, causing considerable fatalities. Bull sharks that make their way into South Asian river systems are known to attack waders and fishermen and are highly aggressive. Many of these attacks on local peoples are wrongly blamed on Ganges River dolphins. While it is highly unlikely these dolphins would ever attack a human, their similar size and color to bull sharks results in their persecution by local peoples.

Additional Links

Encyclopedia of Life

Contributors

Jonathan Swinton (author), University of Michigan-Ann Arbor, Whitney Gomez (author), University of Michigan-Ann Arbor, Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor, Tanya Dewey (editor), Animal Diversity Web.

References

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