Commerson's dolphin
Underwater view of Commerson's dolphin with surface reflection. @ C.Michael Hogan
The Commerson's dolphin (scientific name: Cephalorhynchus commersonii), a marine mammal, is one of36 members of the family Delphinidae, part of the order of cetaceans. There are two main subpopulations of the playful Commerson's dolphin. One resides off the coast of South America, and the second resides off the coast of the Kerguelen Islands. It is also known as the Piebald dolphin.
Unlike most members of the Delphinidae family, the Commerson's dolphin lacks a well defined snout. In addition, different from most delphinids, the majority of female Cephalorhynchus commersonii will ovulate more frequently in the right than in the left ovary. The sex of this black and whitedolphin isdifferentiatedby a black patch covering the genital region. For males, the patch completely covers the region andis oval or heart-shaped; while in females the black patch onlyencircles the anterior (front) region.
Contents
Physical Description
The external shape of both groups is very similar. Neither has a well defined snout, although there is a distinct rostral depression in Kerguelen animals that is not present in the South American population. The dorsal fin is rounded and undercut on the posterior edge. Both flukes and flippers also have rounded tips.
There are frequently saw-tooth serrations on the leading edges of left flippers, and rarely on right flippers. This appears to be a genetic development, but the function of the serrations is not known
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 Size comparison of typical Commerson's dolphin to an average sized human adult. Source: Chris Huh |
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Conservation Status: |
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Scientific Classification Kingdom: Animalia (Animals) |
Commerson's dolphins exhibit sexual dimorphism, as the female is typically larger than the male. The black patch, which hightlights the genital region on the white belly, varies in outline between males and females. It is oval or heartshaped in males and entirely surrounds the penial slit. In females it only encircles the front half of the vaginal slit and may have two posterior extensions. (sometimes termed ears) on the outside of the mammary slits.
South American Population
The total lengths ranged from 64.0 to 146.5 cm. Maximum lengths were 140.5 cm (male) and 146.5 cm (female). The body mass of the adult of this species animals ranges from 26.0 to 44.5 kilograms.
The color pattern of the South American Population of Cephalorhynchus commersonii is black and white. The black color on the head extends to behind the blowhole and down the sides to include the flippers. Black also covers the dorsal fin, and runs back to encircle the tailstock behind the anus and flukes. The black color on the chest ends in a posterior-facing point. The rest of the body, including the tear-drop patch, is pure white.
Individuals can be recognized by the varying shape of the "widow's peak" behind the blowhole and pigmentation on the side of the tailstock. The fetus is dark grey on the surfaces that are white in the adult. This dark grey fades to a paler color in the first few months, and then to white within four to six months.
Kerguelen Islands Population
Specimens from the Kerguelen Islands are larger than those from the South American Population at all ages. The total lengths range from 124.6 to 174 cm. Maximum lengths are 167 cm (male) and 174 cm (female). The body mass ranges from 50 to 86 kg.
The coloring of Cephalorhynchus commersonii adults in the Kerguelen Islands is similar to that of South American juveniles. The surface in front of the dorsal fin is grey, as are the sides. The grey behind the blowhole is streaked with black and the widow's peak is not as well defined. The white throat patch is more asymmetrical than in South American animals. The chest always has a narrow white line in the center, a feature seldom found in South American animals.
Reproduction
Newborns in both populations are completely brown when born. In both populations, copulation has been observed in a vertical, belly to belly position. From hormone data and observations of preganancy and mating, gestation is though to be approximately 12 months.
It has also been noted that unlike most delphinids, some female Cephalorhynchus commersonii have more ovulations from the right than the left ovary.
Commerson's dolphins exhibit sexual dimorphism. The black patch, which hightlights the genital region on the white belly, varies in outline between males and females. It is oval or heartshaped in males and entirely surrounds the penial slit. In females,the black patchonly encircles the front half of the vaginal slit and may have two posterior extensions ("ears") on the outside of the mammary slits.
South American Population
Males are sexually mature by five to six years of age and 127-131 centimeters (cm), females by five to six years and about 130 cm.
Sexual activity has been noted in the same area from January to March. In animals in captivity, sexual activity has been observed during all seasons, but is intensified in January and February. Births are estimated to take place between early spring and late summer(October to March). Young have been observed from November to February in the wild. In captivity, a full term fetus was measured to be 50.5% of its mother's length.
Kerguelen Islands Population
In specimens from the Kerguelen Islands, males reached sexual maturity ateight years old and a body length of 165 cm. In females, sexual maturity was determined to be from five years of age and at a body length of 165 cm. Females are defined to be sexually mature by the presence of at least one ovulation scar on one ovary.
Calvinghas been seen to mostlytake place in summer.A lactating female has been captured in February, however, and behavior that has been interpreted as sexual display and matinghas beenobserved in mid-January.
Lifespan
South American population: Maximum ageis 18 years.
Kerguelen Islands population: Of the few animals examined, maximum age was ten years old.
One Commerson's Dolphin, born in the wild, was reported as still living in captivity at 25.8 years.
Behavior
The sound production of both populations has been studied. The South American Population has a narrow band of five to ten kilohertz (kHz) and high frequency (125-135 kHz) pulses of short duration (0.1-1.2 milliseconds) at low levels. Thisnarrow band maylead to Entanglement in some fishing nets because the sound does not reflect well off the nets, and are therefore more difficult for the dolphin to detect and avoid. The composit clicks of Kerguelen Island Commerson's dolphins have a broader frequency band spectrum.
South American Population
Herds are usually made up of one to three individuals. There have been sightings of solitary individuals as well as reports of groups of 100 or more individuals.These groups may consist ofseasonal, feeding, or breeding congregations. There is much questionabout whether these large herds remain together for a long time, or are short term congregations of smaller herds.
Some groups arecontrolled by one dominant male, whileother less agressive males have been seen relocating toadjacent pools. A calf becomes synchronized to its mother's swimming immeditaely after birth. When another female tried to "adopt" the calf, the mother appeared defensive of her young.
Fishermen claim that most dolphins move out of the shore area during the winter, andreturn in November. It has been speculated that the reason for the move is because the dolphins follow the fish, which also move offshoreduring winter.
These dolphins exhibit playful behavior. Cephalorhynchus commersonii have been seen pushing objects, such as inflated inner tubes, around the bay and onto the beach. They have also been observed surfacing under and nudging birds off of a rig. There are various accounts of aerial acrobatics, such as vertical leaps. Sometimes, Cephalorhynchus commersonii swims on its back. Commerson's dolphins also engage inriding wind waves at sea, breaking shore waves, bow waves of vessels and stern wakes.
Kerguelen Islands Population
Groups are usually composed of two to three individuals. On rare occasions, groups of 20-30 and of 100s have been reported. The majority of sightings occured when the dolphins approached boats.
Dolphins in the Kerguelen Population also exhibit playful behavior. There have been reports of dolphins spinning underwater on their longitudinal axis as they ride the pressure waves from vessels. Frequently, these dolphinshave beenseen swimming on their backs.
Distribution
The "South American," and "Kerguelen Island"are thetwo populations of Commerson's dolphins;which are geographically separated by about 8500 kilometers.
There have been at least six episodes of live capture of Cephalorhynchus commersonii for public display. At this time, Commerson's dolphins can be found in aquariums in Germany, Japan and the United States (Goodall et al., 1988a).
Distribution of Commerson's dolphin in the world. Source: IUCN
Habitat
South American Population
Most sightings are in the [[coast]al regions] near the mouths of bays and estuaries, or over the wide shallow continental shelf, where the tidal range is great.
Kerguelen Islands Population
All sightings of Cephalorhynchus commersonii have been over the Kerguelen shelf. Sightings are most common within the Golfe du Morbihan. The dolphins are found in areas with open waters, kelp-ringed [[coast]lines], and protected areas between islets.
Ecosystem Roles
South American Population
In South America, Commerson's dolphins have been seen swimming near and interacting with numerous birds and other marine mammals. Like other dolphins, their presence is often announced by flocks of birds overhead (usually terns). Commerson's dolphins associate most commonly with Peale's dolphins, Lagenorhynchus australis, and South American sea lions, Otaria flavescens. With Peale's dolphins, Cephalorhynchus commersonii often swim synchronously for long periods, ride the waves of the same vessels or feed in the same areas.
Kerguelen Islands Population
No associations with other species have been reported for Cephalorhynchus commersonii in the Kerguelen Islands.
Commerson's dolphin. Source: Georges Declercq/Encyclopedia of Life
Feeding Habits
Ingeneral, both of the populations seem to utilize a large variety of food resources.
South American Population
By analyzing the stomachs of Cephalorhynchus commersonii caught in fishing nets, it has been determined that these dolphins are opportunistic coastal feeders. The South American Population of dolphins eat mysid shrimp (22.5% of total diet), three species of fish (20.4%), squid (14.1%), algae, isopods and other benthic (bottom dwelling) invertebrates. A total of 25 food items were found in 53 stomachs, as well as miscellaneous plant remains, seeds, sand, and pebbles in the stomachs.
In areas of high tides, Commerson's dolphins seem to feed in the shallow areas in order to feed on sardines or anchovies. In other areas, Cephalorhynchus commersonii remain for long periods and feed in kelp beds, in open waters, around submarine banks, and near artificial structures such as piers and oil rigs.
Commerson's dolphins are sometimes seen feeding singly,however, it is more common to find them feeding cooperatively by herding fish. Two main types of herding are used. In one type, a group of 15 or fewer dolphins form a half circle and drive a school of fish against the shore. Sometimes, the dolphins are temporarily stranded on shore as well, but they are usually able to return to the sea on their own. The other type of herding does not involve the use of the shore as a barrier. In groups of two to six, Cephalorhynchus commersonii circle around a group of fish and take turns passing through the center of the circle, feeding and then returning to the perimeter of the circle.
Kerguelen Islands Population
The diet of this population was primarily found to be 15-25 centimeters semipelagic chaennichthyid fish, as well as pelagic and benthic crustaceans.
Conservation Status
Because of a lack in data, the threat level for this species is unknown.
Threats
Fishing and water pollution threaten this species.
South American Population
In the first half of this century, Commerson's dolphin was hunted for its meat and oil. More recently, Cephalorhynchus commersonii was harpooned for crab bait in the Strain of Magellan in Chile. As a result, the dolphins are no longer common in this area.
Commerson's dolphin is the cetacean species most frequently taken in fishing nets off the coast of southern South America. In Tierra del Fuego alone, at least five to thirty dolphins die each year as bycatch in nets set perpendicular to the shore.
Kerguelen Islands Population
There are no recorded incidents of dolphins being taken for bait or any dolphin mortality through fishing activities in the Kerguelen Population. Low levels of chlorinated hydrocarbons (DDT, PCB and HCB) were found in the blubber of Commerson's dolphins in this area. Contamination varied with age and sex; as males' levels rise with age, whereas in females the levels decrease, perhaps through transfer to the fetus.
References
- Encyclopedia of Life. Species curator: C.Michael Hogan. 2011. Cephalorhynchus commersonii. (Lacépède, 1804)
- Cephalorhynchus commersonii, Animal Diversity Web, Baxi, S. 1999, (accessed November 16, 2009)
- Commerson's dolphin- IUCN Redlist
- AnAge entry for Cephalorhynchus commersonii (accessed November 16, 2009)
- Bastida, R., Lechtschein, V. and Goodall, R.N.P. 1988. Food habits of Cephalorynchus commersonii off Tierra del Fuego. Pages 143-161 in R. Brownell, Jr. and G.P. Donovan, eds. Biology of the genus Cephalorynchus. Reports of the International Whaling Commission (Special Issue 9).
- Brown, S.G. 1988. Records of Commerson's dolphin (Cephalorynchus commersonii in South American waters and around South Georgia. Pages 85-92 in R. Brownell, Jr. and G.P. Donovan, eds. Biology of the genus Cephalorynchus. Reports of the International Whaling Commission (Special Issue 9).
- Collet, A. and Robineau, D. Data on the gential tract and repoduction in Commerson's delphin, Cephalorhynchus commersonii (Lacepede, 1804), from the Kerguelen Islands. Pages 119-142 in R. Brownell, Jr. and G.P. Donovan, eds. Biology of the genus Cephalorynchus. Reports of the International Whaling Commission (Special Issue 9).
- Goodall, R.N.P. 1994. Commerson's dolphin. Pages 241-267 in S.H Ridgway and R. Harrison, eds. Handbook of Marine Mammals. Volume 5: The First Book of Dolphins. Academic Press Limited, San Diego.
- Goodall, R.N.P, Galeazzi, A.R., Leatherwood, S., Miller, K.W., Cameron, I.S., Kastelein, R.K. and Sobral, A.P. 1988. Studies of Commerson's dolphins, Cephalorynchus commersonii, off Tieraa del Fuego, 1976-1984, with a review of information on the speices in the South Atlantic. Pages 3-70 in R. Brownell, Jr. and G.P. Donovan, eds. Biology of the genus Cephalorynchus. Reports of the International Whaling Commission (Special Issue 9).
- Leatherwood, S., Kastelein, R.A. and Miller, K.W. 1988. Observations of Commerson's dophin and other cetaceans in south Chile, Janurary-February 1984. Pages 71-84 in R. Brownell, Jr. and G.P. Donovan, eds. Biology of the genus Cephalorynchus. Reports of the International Whaling Commission (Special Issue 9).
- Leatherwood, S., Kastelein, R.A. and Hammond, P.S. Estimate of numbers of comerson's dolphins in a portion of hte northeaster strait of Magellan, January-February 1984. Pages 93-102 in R. Brownell, Jr. and G.P. Donovan, eds. Biology of the genus Cephalorynchus. Reports of the International Whaling Commission (Special Issue 9).
- Leatherwood, S. and Reeves, R.R. 1983. The Sierra Club Handbook of Whales and Dolphins. Sierra Club Books, San Francisco ISBN: 0871563401
- Lockyer, C., Goodall, R.N.P., and Galeazzi, A.R. Age and body-length characteristics of Cephalorynchus commersonii from incidentally-caught specimens off Tierra del Fuego. Pages 103-118 in R. Brownell, Jr. and G.P. Donovan, eds. Biology of the genus Cephalorynchus. Reports of the International Whaling Commission (Special Issue 9).
- Weigl, Richard, Longevity of Mammals in Captivity; from the Living Collections of the World, Kleine Senckenberg-Reihe 48: Stuttgart (2005), ASIN: B002EDDA0I
