Orcaella brevirostris

Geographic Range

Irrawaddy dolphins ( Orcaella brevirostris ) are found throughout much of the tropical and subtropical Indo-Pacific region; along the coasts of India, Bangladesh, Myanmar (Burma), Thailand, Cambodia (Kampuchea), Lao Peoples' Democratic Republic (Laos), Socialist Republic of Vietnam, Malaysia, Singapore, Brunei Darrusalam, and Indonesia.

• Biogeographic Regions
• oriental oriental
  • native native
• indian ocean
  • native native
• pacific ocean pacific ocean
  • native native

Habitat

Irrawaddy dolphins prefer coastal areas, particularly muddy, brackish waters at river mouths and deltas, and do not appear to venture far offshore. Most sightings have been made within 1.6 km of the coastline, but some have been reported in waters greater than 5 km from shore. Some populations are apparently restricted to fresh water, e.g. Chilka Lake, India, and Songhkla, Thailand.

• Habitat Regions
• tropical tropical
• saltwater or marine saltwater or marine
• freshwater freshwater

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

• Other Habitat Features
• riparian riparian
• estuarine estuarine

Physical Description

Unlike other dolphins , Irrawaddy dolphins lack a beak and have flexible necks. The flexibility in the neck causes visible creases behind the head. Additionally, Irrawaddy dolphins have bulging heads, with the forehead extending past the mouth, broad triangular, paddle-like, pectoral fins, and small, triangular dorsal fins set approximately two-thirds of the body length along the back. Skin coloration varies from slate-blue to slate-gray with a lighter underside. The face and head are similar in appearance to beluga whales ( Delphinapterus leucas ). Irrawaddy dolphins have narrow, pointed, peg-like teeth about 1 cm in length in both the upper and lower jaws. Body mass varies from 114 kg to 143 kg and the length ranges from 146 cm to 275 cm. Males tend to be larger in mass and length, with a larger dorsal fin. Irrawaddy dolphins lack a cardiac sphincter and the stomach is subdivided into compartments.

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

• Sexual Dimorphism
• male larger

Reproduction

Little is currently known about mating systems in Irrawaddy dolphins. Since the animals live in small groups averaging between three to six individuals, it is presumed that breeding happens outside of those groups. The breeding season is between December and June; however, little is known about mating behavior. As in other dolphin species, it can be presumed that males mate with multiple females and compete over mates.

• Mating System
• polygynous polygynous

There is relatively little information on reproduction in Irrawaddy dolphins. The mating season in the northern hemisphere is reported to be December to June. The calving season seems to be from June to August, with noted exceptions. Data from Chilka Lake, India shows a low rate of breeding and production of only a single calf in three years. Additionally, gestation is approximately nine months, but data from two captive births estimate the gestation period at fourteen months. Captive born Irrawaddy dolphin calves measured 96 cm in length and weighed 12.3 kg. In the first seven months the calves increased in length by 59% and 266% in weight. Calves begin eating fish around six months and are fully weaned at about two years old. Adult length is achieved between three and five years old. Rate of sexual maturation is thought to be positively correlated with growth rate.

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

Irrawaddy dolphins are not completely weaned until 2 years. From birth to approximately seven months old, the calf survives solely on the nutrition from the mother. For the following seven months the calves stay within the pod and continue to receive nourishment from the mother while also eating fish. It is presumed the calves learn to prey on fish by copying behavior of the mother's and pod mates. There is little information on the rearing of calves. Currently it is not known if both males and females are involved in the upbringing of calves. Like most mammals, females invest heavily in their young.

• Parental Investment
• female parental care female parental care
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female
• pre-weaning/fledging
  • provisioning
    • female
  • protecting
    • female
• pre-independence
  • provisioning
    • female
  • protecting
    • female
• post-independence association with parents
• extended period of juvenile learning

Lifespan/Longevity

The oldest recorded Irrawaddy dolphins, all of which were found dead in fishing nets, were estimated to be 28 years old. Several individuals of this age have been found, all entangled in nets, so it is believed that Irrawaddy dolphins can live longer. Due to the small population size of Irrawady dolphins and subsequent difficulty in tracking the species, no further information is known regarding average lifespan.

Behavior

Irrawaddy dolphins stay in groups of three to six individuals and are social within their pods. They are also social outside of their group and mixing between groups has been reported. When scouting areas, Irrawaddy dolphins raise their heads out of the water and rotate around to see their surroundings. Irrawaddy dolphins swim slowly and display sluggish movements. When they surface to take a breath, only the top of the head is visible and it is done quickly; only 14% of all surfacings between long dives include rolling, splashing, or limb waving and slapping. Before an Irrawaddy dolphin dives, it usually surfaces two times. The longest recorded dive is over six minutes long.

Irrawaddy dolphins in Chilka Lake are known to swim onto a sand bar and roll around, then retreat if disturbed. Irrawaddy dolphins have also been seen waving or slapping their flippers and tail, breaching or partially leaping from the water, blowing bubbles, rolling sideways, and pausing at the surface. Water spitting has been observed on numerous occasions, but the reason for this behavior is unknown. Local fisherman report that they are able to identify individual dolphins based on unique behaviors.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• motile motile
• sedentary sedentary
• social social

Home Range

Groups of Irrawaddy dolphins inhabit discrete ranges of approximately 35 km in length, based on some observations.

Communication and Perception

Vocalizations of Irrawaddy dolphins in captivity within the Mahakam River, Indonesia included a single-component sonar signal, with the majority of frequencies captured at 60 kHz. Pulse trains were consistent with rates repeated at 40 to 60 kHz. Researchers theorize that Irrawaddy dolphins have a narrow sonar field. Little is known or recorded regarding courtship communication or other social signals.

• Communication Channels
• visual visual
• acoustic acoustic
• chemical chemical

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

Food Habits

Irrawaddy dolphins feed on fish, cephalopods, and crustaceans. Irrawaddy dolphins sometimes spit water while feeding, which may be used to herd fish.

Researchers have documented that in the Irrawaddy (or Ayerarwady) River, Myanmar, these dolphins engage in cooperative fishing with cast-net fishermen. Fishermen search for dolphins and call them by tapping a lahai kway, wooden key, on the sides of their boats. One or two lead dolphins then swim in smaller and smaller semi-circles herding the fish towards the shore. During cooperative fishing, the dolphins often dive deeply with their flukes aloft just after the net is cast and create turbulence under the surface around the outside of the net. The dolphins seem to benefit from the fishing by preying on fish that are confused by the sinking net, and those trapped around the edges of the lead line or stuck in the mud bottom just after the net is pulled up.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • eats non-insect arthropods
  • molluscivore molluscivore

• Animal Foods
• fish
• eggs
• mollusks
• aquatic crustaceans

Predation

Other than humans, there are no known natural predators of Irrawaddy dolphins. Humans are responsible for a large number of deaths because Irrawaddy dolphins are often caught in nets or harmed through destructive fishing practices (e.g., dynamite fishing). Otherwise, they are typically considered the top predator in their river ecosystems.

Ecosystem Roles

Irrawaddy dolphins are top ecosystem predators, feeding on fish, cephalopods, and crustaceans. They are hosts for parasites including roundworms ( Anisakis simplex ), flukes ( Braunina cordiformis ), and tapeworms ( Monorygma delphini ). Healthy dolphin populations indicate a healthy marine ecosystem.

• Ecosystem Impact
• keystone species keystone species

Economic Importance for Humans: Positive

In the Irrawaddy River these dolphins engage in cooperative fishing with cast-net fishermen. Irrawaddy dolphins increase the amount of fish the fishermen catch; therefore, they are of economic value. Additionally, Irrawaddy dolphins have brought ecotourism to communities in Indonesia and India.

• Positive Impacts
• food food
• ecotourism ecotourism

Economic Importance for Humans: Negative

The only way that Irrawaddy dolphins may have a negative economic impact on humans is that they share a common food source. Some fisherman believe the dolphins are pests because they reduce their catch, although this is unlikely.

Conservation Status

Currently, the most immediate threat facing Irrawaddy dolphins is drowning in gill nets. The threat of gill net entanglement occurs primarily during the dry season (December to May), when dolphins settle in deep water pools. Dynamite and electric fishing occur in some important habitats. These activities are causing depletion of the dolphin's fish supply and noise from the explosions is potentially dangerous to dolphins. Due to the small population size and their narrow distribution, it is quite possible that dam construction anywhere within their habitat might critically endanger populations. Furthermore, uncontrolled tourism can harass dolphins in important habitats during the dry season and interfere with normal activities, such as feeding, resting, and socializing. Overfishing, collisions with boats and injuries from boat propellers are also threats to their survival.

Additional Links

Encyclopedia of Life

Contributors

Melissa Koss (author), San Diego Mesa College, Lucretia Mahan (author), San Diego Mesa College, Sam Merrill (author), San Diego Mesa College, Paul Detwiler (editor), San Diego Mesa College, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

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Dawblin, W. 1972. Whales and Dolphins. Pp. 274 in Encyclopedia of Papua New Guinea , Vol. 1, 1 Edition. Melbourne: Melbourne University Press.

Kamminga, C., H. Wiersma, W. Dudok Van Heel. 1983. Sonar sounds from Orcaella brevirostris of the Mahakam River, East Kalimantan Indonesia. Aquatic Mammals , 10: 83-95.

Smith, B., T. Mya. 2007. Status and conservation of Irrawaddy Dolphins in Ayeyarwady river of Myanmar. Status and conservation of freshwater populations of irrawaddy dolphins , 31: 21-40.

Stacey, P., S. Leatherwood. 1997. Asian Marine Biology: 1997 . Hong Kong: Hong Kong University Press.

Sutaria, D. 2005. "Irrawaddy dolphin - India" (On-line). Whale and Dolphin Conservation Society. Accessed April 24, 2010 at http://www.wdcs.org/submissions_bin/consprojectirr.pdf .

2004. "Facultative river dolphins : conservation and social ecology of freshwater and coastal Irrawaddy dolphins in Indonesia" (On-line). University Van Amsterdam Digital Academic Repository. Accessed March 08, 2010 at http://dare.uva.nl/document/115596 .

2006. "Irrawaddy Dolphin" (On-line). Ocean Park and Recreation Foundation Hong Kong. Accessed March 08, 2010 at http://www.opcf.org.hk/pdf/species/Irrawaddy_Dolphin.pdf .

1999. Mammalian Species Orcaella brevirostris . American Society of Mammologist , 616: 1-8. Accessed March 08, 2010 at http://www.science.smith.edu/msi/pdf/i0076-3519-616-01-0001.pdf .

2010. " Orcaella brevirostris (Gray, 1866)" (On-line pdf). Convention on Migratory Species. Accessed March 08, 2010 at http://www.cms.int/reports/small_cetaceans/data/O_brevirostris/O_brevirostris.PDF .

2002. "Status and Conservation of Facultative Freshwater Cetaceans in Asia" (On-line pdf). Accessed March 08, 2010 at http://137.110.142.7/uploadedFiles/Divisions/PRD/Publications/Smith_Jefferson2002(66).pdf .

2007. "Trey Psaut Cambodia’s Irrawaddy Dolphin" (On-line pdf). Accessed March 08, 2010 at http://faculty.washington.edu/banasn/animals/student_work/Julia%20Boubel,%20Trey%20Psaut.pdf .