Orcaella heinsohni

Geographic Range

Australian snubfin dolphins, Orcaella heinsohni , are native to Australian and Oriental coastal waters surrounding the Sahul shelf. They are found as far north as Manokwari, Indonesia and as far south as the Brisbane River in Australia. Their range in Australia is from Broome in Western Australia to Brisbane in Queensland.

• Biogeographic Regions
• oriental oriental
  • native native
• australian australian
  • native native

Habitat

Australian snubfin dolphins inhabit shallow coastal waters up to 30 m deep and 23 km from the coast. They are rarely seen in waters deeper than 20 m. Groups of Australian snubfin dolphins are often found adjacent to river and creek mouths. They exhibit preferential habitat selection for shallow areas with seagrass, over dredged channels, and areas with coral reefs.

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

• Aquatic Biomes
• coastal coastal

• Other Habitat Features
• estuarine estuarine

Physical Description

Australian snubfin dolphins are fusiform in body shape with round, highly mobile heads and small dorsal fins. It has been suggested they are sexually dimorphic but this has not been documented well. Adults are relatively small, varying in length from 2.1 to 2.3 m and weighing 114 to 133 kg. Australian snubfin dolphins are gray to blue gray with a white abdominal field between their flippers and genital region. This tri-partite coloration is the easiest way to distinguish them from their close relatives, Irrawady dolphins ( Orcaella brevirostris ). The dorsal fin is a crescent shape measuring 9.59 cm high, significantly higher than Irrawady dolphins. Their flippers are 16% of their body length with a smoothly curved anterior margin. These dolphins exhibit homodont dentition with up to 20 teeth in the upper right, 22 in the upper left, 18 in the lower right and 19 in the lower left. Beyond coloration, Australian snubfin dolphins have distinctive skeletal characteristics that separate them from Irrawady dolphins, including the number of bony ossicles, temporal fossa height, and pterygoid depth. Australian snubfin dolphins have three bony ossicles and nasal bones, while Irrawady dolphins only have two. Australian snubfin dolphins have a temporal fossa height of 61.2 mm compared to 45.8 mm in Irrawady dolphins. Australian snubfin dolphin pterygoid depth ranges from 19.4 to 31.2 mm, whereas Irrawady dolphin pterygoid depth, which ranges from 30.7 to 48.6 mm.

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

• Sexual Dimorphism
• sexes alike

Reproduction

Reproduction is understudied in this species. It is suggested that they are a monogamous species. This is based on the strong and temporally stable association patterns within groups. Groups range from 1 to 8 individuals, but groups of 5 are most common.

• Mating System
• monogamous monogamous

A breeding season in Australian snubfin dolphins has not been identified. However, the number of juveniles in groups is generally higher during the dry season from May through November than the wet season of December through April. A gestation length of 14 months suggests that more mating occurs during the wet season. Generally a single offspring is born. Birth mass is 10 to 12 kg. Due to their tight social structure, it is believed that the young are reared communally. Juveniles follow their mothers for as long as they are reliant on her for milk and protection. Adult size is reached between 4 and 6 years of age. It is suspected that once these dolphins reach adult size, they leave their mothers' social group.

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

Australian snubfin dolphins are born able to swim. They are weaned at about two years old and remain near their mother in their natal group until they are adult sized, between 4 and 6 years old.

• Parental Investment
• precocial precocial
• 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
• post-independence association with parents

Lifespan/Longevity

Australian snubfin dolphins can live for more than 20 years, but generally do not survive past 28 years of age.

Behavior

Australian snubfin dolphins are most frequently observed foraging and traveling in small groups during the dry season. Australian snubfin dolphins live sympatrically with Indo-Pacific humpback dolphins and dugongs . The majority of behavioral interactions between Indo-Pacific humpback dolphins and Australian snubfin dolphins are aggressive sexual behaviours. Male Indo-Pacific humpback dolphins will chase Australian snubfin dolphin females to make physical contact, females with juveniles are particular targets. Females react with submissive fleeing behavior or resistance using their rostrum. Interactions between Australian snubfin dolphins and dugongs are generally of an impartial nature.

• Key Behaviors
• natatorial natatorial
• motile motile
• nomadic nomadic
• social social

Home Range

Home range sizes in Australian snubfin dolphins are not reported in the literature.

Communication and Perception

Australian snubfin dolphins produce broadband clicks, three different types of pulsed sound, and two different types of whistles to communicate with one another. During foraging, Australian snubfin dolphins produce click trains, which are also used to a lesser extent during socializing. During social behavior, Australian snubfin dolphins often produce squeak sounds. The two whistle types are both short in duration and relatively low in frequency (1 to 8 kHz) and occur during socialization and foraging. There is also evidence of ultrasonic sound communication among Australian snubfin dolphins.

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

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

Food Habits

Australian snubfin dolphins are opportunistic and generalist feeders. They feed on a variety of fish and invertebrates found in their coastal and estuarine habitat. They have been recorded eating anchovies , sardines , eels , halibut , breams , and grunters . Squid , cuttlefish , and other cephalopods are also important in the diet. They occasionally eat decapod crustaceans , and isopods .

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • molluscivore molluscivore

• Animal Foods
• fish
• mollusks
• aquatic crustaceans

Predation

Humans pose the greatest predation risk to Australian snubfin dolphins. Other important predators are sharks, particularly tiger sharks ( Galeocerdo cuvier ). Australian snubfin dolphins are also at risk of death by Indo-Pacific humpback dolphin ( Sousa chinensis ) aggression. They escape predation through social behavior, grouping to improve vigilance, agility in the water, and their countershaded color pattern.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Australian snubfin dolphins are dominant predators of fish and cephalopod species in coastal areas of Australia and Indonesia. There is little else known about their ecosystem roles.

Economic Importance for Humans: Positive

Australian snubfin dolphins are an endemic species and their level of importance within the ecosystem has not been determined. Australian snubfin dolphins and Irrawady dolphins are an interesting example of allopatric speciation.

Economic Importance for Humans: Negative

There is no negative impact of Australian snubfin dolphins on humans.

Conservation Status

The International Union for Conservation of Nature and Natural Resources categorize Australian snubfin dolphins as near threatened. There are currently less than 10,000 mature individuals. The major threats include entanglement in gill nets set in shallow waters for fishing, entanglement in shark nets set for human protection, and habitat degradation (coastal zone development, pollution, boat traffic, and overfishing of prey resources).

Other Comments

Orcaella heinshoni was separated from Orcaella brevirostris in 2005 after genetic research and other evidence suggested that they are evolutionarily significant populations.

Additional Links

Encyclopedia of Life

Contributors

Maureen Hanzel (author), University of Manitoba, Jane Waterman (editor), University of Manitoba, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Beasley, I., P. Arnold, G. Heinsohn. 2002. Geographical variation in skull morphology of the Irrawaddy dolphin, Orcaella brevirostris (Owen in Gray, 1866). Raffles Bulletin of Zoology , 10: 15-34.

Beasley, I., K. Robertson, P. Arnold. 2005. Discription of a New Dolphin, The Australian Snubfin Dolphin Orcaella Heinsohni Sp. N. (Cetacea, Delphinidae). Marine Mammal Science , 21/3: 365-400.

Gribble, A., G. McPherson, B. Lane. 1989. Effect of the Queensland Shark Control Program on non target species: whale, dugong, turtle and dolphin: a review. Marine and Freshwater Research , 49: 645-651.

Heinsohn, G. 1979. Biology of small cetaceans in north Queensland waters. . Townsville, Queensland: Unpublished report to the Great Barrier Reef Marine Park Authority. 23 pp.

Marsh, H., R. Lloze, G. Heinsohn, Kasuya. 1989. Irrawaddy dolphin Orcaella brevirostris (Gray, 1866). Pp. 101-118 in Handbook of Marine Mammals , Vol. 6. New York: Academic Press.

Parijs, S., G. Parra, P. Corkeron. 2000. Sounds produced by Australian Irrawaddy dolphins, Orcaella brevirostris . Journal of Acoustical Society of America , 108/4: 1938-1940.

Parra, G., M. Jedensjö. 2009. Feeding habits of Australian Snubfin ( Orcaella heinsohni ) and Indo-Pacific humpback dolphins ( Sousa chinensis ). The Great Barrier Reef Marine Park Authority, Townvsille and Reef & Rainforest Research Centre LImitred, Cairns: 1-22.

Parra, G., P. Corkeron, H. Marsh. 2006. Population sizes, site fidelity and residence patterns of Australian snubfin and Indo-Pacific humpack dolphins: Implications for conservation. Biological Conservation , 129: 167-180.

Parra, G., A. Preen, P. Corkeron, H. Marsh. 2002. Distribution of Irrawaddy dolphins, Orcaella brevirostris , in Australian waters. Raffles Bulletin of Zoology , 10: 141-154.

Parra, G. 2005. Behavioural ecology of Irrawaddy, Orcaella brevirostris (Owen in Gray, 1866), and Indo-Pacific humpback dolphins, Sousa chinensis (Osbeck, 1765), in northeast Queensland, Australia: A comparative study . Cairns: James Cook University.

Parra, G. 2006. Resource partitioning in sympatric delphinids: space use and habitat preferences of Australian snubfin and Indo-Pacific humpback dolphins. Journal of Animal Ecology , 75: 862-874.

Reeves, R., M. Dalebout, A. Jefferson, L. Karczmarski, K. Laidre, G. O'Corry-Crowe, L. Rojas-Bracho, E. Secchi, E. Slooten, B. Smith, J. Wang. 2009. " Orcaella heinsohni " (On-line). IUCN Red List of Threatened Species. Accessed October 20, 2011 at www.iucnredlist.org .