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Platanista minorIndus river dolphin

By Rachel Bricklin

Geographic Range

Indus River dolphins, Platanista minor, are found only in the Indus River in Pakistan. Platanista minor used to range throughout the river system, but the dolphins are now only found in the waters above the Kotri Barrage and below the Chasma, Trimmu, Sidhnai, and Islam Barrages. These human-created barriers, in addition to changes in rainfall patterns, have greatly limited the dolphins’ distribution. (Hamilton, et al., 2001; Mann, et al., 2000; Moreno, 2004)

Habitat

Platanista minor currently exists only in the freshwater Indus River. However, some paleontologists believe that river dolphins might have evolved from marine-dwelling relatives that eventually moved to estuaries and then rivers as seawater levels rose and fell during the Miocene. Though this species prefers water deeper than 3 meters, Indus River dolphins have special adaptations such as swimming on their sides that enable them to exist in shallower waters as well. The temperature of the water ranges from 8-33 degrees C. (Hamilton, et al., 2001; Mann, et al., 2000; Moreno, 2004)

  • Range elevation
    0 to 250 m
    0.00 to 820.21 ft
  • Range depth
    3 to 9 m
    9.84 to 29.53 ft

Physical Description

Indus River dolphins are roughly the same color as the river, gray or brown, though they sometimes are lighter on their undersides. Their “beaks” are distinctively swollen at the tip and very long, reaching 20% of the length of their bodies, with large, visible teeth. In contrast to their “beaks”, their dorsal fins are rather small and reduced compared to other river dolphins. Large flippers and flukes, combined with long and remarkably flexible necks, probably help the dolphins navigate effectively. Platanista minor has external ears located below their eyes, but their eyes are very small and probably can only see shadowy, unclear images. Though Platanista minor and Platanista gangetica barely differ physically except for slight differences in tail lengths, the two species are distinguishable by their ranges. Platanista minor lives only in the Indus River system, while Platanista gangetica only inhabits the Ganges River system. Females are larger than males. (Moreno, 2004)

  • Sexual Dimorphism
  • female larger
  • Average mass
    84 kg
    185.02 lb
  • Range length
    2 to 2.5 m
    6.56 to 8.20 ft

Reproduction

These dolphins probably do not mate seasonally, since calves are born at different times throughout the year. However, in a captive population of two females and a male in Switzerland, the male reportedly chased the females in the spring. Though scattered captive populations of Platanista minor exist, little is known about the mating behavior of any of the members of the Platanistidae family. (Herman, 1980; Moreno, 2004)

Platanista minor breeds throughout the year, and has a very long gestation period of 8-11 months. Though not mentioned in the literature, this species probably only gives birth to one offspring at a time since newborns are about a meter long when they are born, which is nearly half the length of an adult female. (Moreno, 2004)

  • Breeding interval
    Platanista minor probably does not breed more than once every two years or so due to such long gestation and nursing periods.
  • Breeding season
    Mating occurs throughout the year.
  • Range gestation period
    8 to 11 months
  • Range weaning age
    2 to 12 months
  • Average age at sexual or reproductive maturity (female)
    10 years
  • Range age at sexual or reproductive maturity (male)
    10 (low) years

Little information is available about parental investment in Platanista minor, however, they most likely spend much time and energy on their offspring. Since females are pregnant for up to 11 months, newborns are about half the size of their mothers at birth, calves nurse up to a year after birth, Indus River dolphin offspring are probably very costly. (Moreno, 2004)

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

Lifespan/Longevity

No data is available about the lifespan of Platanista minor. The dolphins probably live for a long time since they are relatively large and require 10 years to reach sexual maturity. However, recent poor water quality and reduced habitat might affect the current longevity of these animals in the wild. At least a few Indus River dolphins have been kept in captivity, however, their longevity data is unavailable. (Herman, 1980; Reeves and Chaudhry, 1998)

Behavior

Platanista minor is described as mainly solitary, though the dolphins have occasionally been found in groups consisting of as many as 30 individuals. In general, however, Indus River dolphins travel in groups of no more than 3. These dolphins are equipped to swim on their sides in very shallow water if necessary, yet except for juveniles, Platanista minor rarely exhibits the stereotypical dolphin aerial leaping behavior. On the other hand, like other dolphins these animals are highly vocal and perceive their environment through echolocation. These sounds are mainly used to navigate while swimming, with a very small percentage used for communication. (Moreno, 2004; Pilleri, 1974)

Home Range

No data is available about the home range of Platanista minor. Platanista gangetica, however, has been reported at densitites of 0.7-1.36 dolphins per kilometer. Perhaps Platanista minor also require about a kilometer of space each, though data is still absent to determine how far they travel. All that is known is that they appear to migrate seasonally along the river. (Smith, et al., 2001)

Communication and Perception

Indus River dolphins have extremely poor eyesight, perhaps since vision is nearly useless to navigate the murky rivers in which they live. They instead rely on echolocation to perceive their environment. Indeed, one of the common names for Platanista minor is “blind river dolphin”. Their external ears might help receive echolocation signals, which are intermittent pulses rather than continuous whistles. Though Indus River dolphins are very vocal, they use sounds for communication only about 5% of the time that they vocalize. (Moreno, 2004)

Food Habits

Indus River dolphins use their echolocation abilities combined with their highly toothed, long snouts to forage for many bottom-dwelling animals including fish and invertebrates. Platanista minor has been known to eat some species of catfish, herring, carp, gobies, mahseers, prawns, and clams. Captive individuals reportedly consume about a kilogram of food each day. (Moreno, 2004)

  • Animal Foods
  • fish
  • mollusks
  • aquatic crustaceans
  • other marine invertebrates

Predation

Platanista minor has few if any natural predators, however, they are often hunted by local people. (Moreno, 2004)

Ecosystem Roles

Indus River dolphins each eat about a kilogram of benthic fish and invertebrates daily, it is not clear how strongly they impact any of their prey populations. (Moreno, 2004)

Economic Importance for Humans: Positive

Some people hunt Indus River dolphins for their meat and oil. People in some areas eat the dolphin meat, while others use it as a fishing lure, though studies indicate that fish flesh is just as effective. The dolphin’s oil is used for medicinal purposes, its supposed effectiveness as an aphrodisiac, and to rub on one’s skin. (Moreno, 2004; Sinha, 2002)

Economic Importance for Humans: Negative

There are no known adverse affects of Platanista minor on humans, though no one knows what might happen to the river ecosystem if these highly-endangered animals eventually become extinct. At the very least, however, Pakistan will lose part of its biodiversity forever if the country does not take steps to protect this unique dolphin. (Moreno, 2004; Reeves and Chaudhry, 1998)

Conservation Status

Platanista minor is a species of great concern, and a combination of human-created barriers such as dams and barrages, hunting, and a limited natural range have resulted in a dangerously-low total population of only several hundred individuals. These dolphins are classified as endangered, and have been so since the 1970's. Their extremely low population size may also restrict their gene pool, thus they might soon have many problems associated with low genetic variation within a population. (Moreno, 2004; Reeves and Chaudhry, 1998)

Other Comments

Some authors consider Platanista minor to be a subspecies of Platanista gangetica, others consider them the same species, and others consider them distinct species. Genetic analysis has not yet answered the taxonomic mysteries of the Platanistdae family. (Moreno, 2004; Hamilton, et al., 2001; Moreno, 2004)

Contributors

Matthew Wund (editor), University of Michigan-Ann Arbor.

Rachel Bricklin (author), University of Michigan-Ann Arbor, Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

acoustic

uses sound to communicate

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

echolocation

The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

piscivore

an animal that mainly eats fish

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

solitary

lives alone

tactile

uses touch to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

year-round breeding

breeding takes place throughout the year

References

Biswas, S., S. Boruah. 2000. Ecology of the River Dolphin (Platanista gangetica) in the Upper Brahmaputra. Hydrobiologia, 430: 97-111.

Hamilton, H., S. Caballers, A. Collins, R. Brownell. 2001. Evolution of river dolphins. Proc. R. Soc. Lond., 268: 549-558.

Herman, L. 1980. Cetacean Behavior: Mechanisms and Functions. New York: John Wiley & Sons, Inc..

Mann, J., R. Connor, P. Tyack, H. Whitehead. 2000. Cetacean societies: field studies of whales and dolphins. Chicago: University of Chicago Press.

Moreno, P. 2004. Ganges and Indus dolphins (Platanistidae). Pp. 13-17 in M Hutchins, D Kleiman, V Geist, M McDade, eds. Grzimek's Animal Life Encyclopedia, Vol. Volume 15: Mammals IV, Second Edition Edition. Detroit: Thompson Gale.

Pilleri, G. 1974. Side-swimming, vision, and sense of touch in Platanista indi (Cetacea, Platanistidae). Experientia, 30: 100-104.

Reeves, R., A. Chaudhry. 1998. Status of the Indus River dolphin Platanista minor . Oryx, 32: 35-44.

Sinha, R. 2002. An alternative to dolphin oil as a fish attractant in the Ganges River: conservation of the Ganges River dolphin. Biological Conservation, 107: 253-257.

Smith, B., B. Ahmed, M. Edrise Ali, G. Braulik. 2001. Status of the Ganges river dolphin or shushuk Platanista gangetica in Kaptal Lake and the southern rivers of Bangladesh. Oryx, 35: 61-72.

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