Rough-toothed dolphin
Rough-toothed dolphin (Steno bredanensis) Source: NOAA National Marine Fisheries Service
 Rough-toothed dolphins (Steno bredanensis) Source:Thomas Jefferson
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 Size comparison of an average human and a rough-toothed dolphin (Steno bredanensis). Source: Chris Huh
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| Conservation Status: Least Concern |
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Scientific Classification Kingdom: Animalia |
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Common Names: |
The Rough-toothed dolphin (Steno bredanensis') is a marine mammal within the family of oceanic dolphins, part of the order of cetaceans, which is found in deep, oceanic waters around the globe between 40°N and 35°S. It is only sometimes seen is coastal areas.
Contents
Physical description
Steno bredanensis have 20 to 27 teeth in each row. These teeth have slight, but detectable, vertical wrinkles or ridges. The ridges on their teeth are how the species received their English common name, the Rough-toothed dolphin (Jefferson, 1993).
Their colourings may vary geographically. The body of Steno bredanensisis dark grey with white or light coloured spots on their sides (Rouch, 1998). The belly, lips, and parts of the lower jaw are white (Jefferson, 1993). Rough-toothed dolphins have a distinctive colour pattern, consisting of a dark narrow cape which passes over the eyes and arches high on the sides of the body (HWDT, 1998). Some of these animals show white and yellowish scars, due to encounters with large squid, cookie-cutter sharks, other Rough-toothed dolphins, and interactions with boats (HWDT, 1998; Rouch, 1998).
Their nickname "slopehead" comes from their unique head shape. The beak blends into the head without a crease, unlike other dolphins with a more prominent beak. The slope of their head and large eyes give Steno bredanensisa slightly reptilian appearance. This species has large flippers and a centered dorsal fin. It can be confused at sea with Bottlenose dolphins, Spinner dolphins, and Spotted dolphins (Carwardine, 1995).
With a head that slopes smoothly down into a long beak, and large flippers that are set fairly far back on the body , this rather primitive-looking dolphin is sometimes said to be somewhat reptilian in appearance . Named for the subtle ridges and wrinkles on the teeth , the body of the Rough-toothed dolphin is patterned black, white and grey. It has a white underside, mid-grey sides, and a black to dark grey back. A darker region on the back, called a cape, runs narrowly from the top of the head to behind the tall, curved-back dorsal fin, where it widens . The body often bears the scars of bites from cookie-cutter sharks, leaving behind white patches, splotches and spots . Young Rough-toothed dolphins often lack these white marks, and are more subdued in colour.
Reproduction
Females reach sexual maturity at about ten years of age, whereas males reach maturity at 14 years of age (Rouch, 1998). The gestation and lactation periods are unknown (HWDT, 1998). Not much is known about their reproductive habits. In captivity, a bottlenose dolphin has been successfully mated with a rough-toothed dolphin to produce a hybrid offspring (Davis, 1997).
Parental care-giving behaviour has been observed off the coast of Brazil. According to reported sightings a dead dolphin was supported at the surface for over two hours by one of a group of seven other dolphins. The dead dolphin may have been a calf. If the supporting animal was still nursing, this could indicate a prolonged mother-calf association in Rough-toothed dolphins. Such behaviour has been observed in the tight social groups of other marine mammals (HWDT, 1998).
Lifespan
Maximum longevity in the wild is presumed to be approximately 32 years; however, researchers acknowledge that maximum longevity could be underestimated.
Behaviour
Although widespread, the Rough-toothed dolphin is not frequently encountered, and thus few studies have been conducted on its ecology and biology . Like many other dolphins, it is a sociable animal, commonly moving in groups of 10 to 20 individuals, although larger groups have also been observed, such as one consisting of up to 300 dolphins in Hawaii. In these groups, the Rough-toothed dolphin has been seen with other dolphin species, as well as often associating with flotsam, the rubbish and debris found floating in the ocean .
Often described as a sluggish or lethargic creature, the Rough-toothed dolphin often swims with its chin and head above the water's surface, skimming along with a distinctive splash . It is not the most acrobatic of dolphins, but will occasionally leap and ride the bow waves of boats.
Steno bredanensis are seen most commonly in groups of 10 to 30 individuals (Jefferson, 1993). Herds of up to 160 dolphins have been spotted together containing eight smaller groups (HWDT, 1998). These dolphins have been oberved with Pilot whales, Bottlenose dolphins, Spotted dolphins and Spinner dolphins.
Steno bredanensisare great divers and stay submerged for as long as 15 minutes. They are also rapid swimmers and may move just under the surface with their dorsal fins clearly visible. They enjoy "surfing" or bow-riding, but not as readily as some other tropical dolphins (Carwardine, 1995). This mammal produces both clicks and whistles. It is thought that the clicks are used in echolocation (Rouch, 1998).
Distribution
The Rough-toothed dolphin is found in all three major oceans of the world (the Atlantic, Pacific and Indian Oceans) , typically between 40 degrees north and 35 degrees south. .
Habitat
The distribution of Steno bredanensisis poorly known, probably because their populations dwell in areas that have not been well-studied (HWDT, 1998).
Steno bredanensis are rarely seen ranging north of 40 degrees latitude or south of 35 degrees latitude. They have been observed along the Atlantic coast of the United States, in the Caribbean Sea, Gulf of Mexico, Mediterranean Sea, eastern tropical Pacific, and Indian Ocean (Jefferson, 1993).
Food habits
This dolphin feeds on a range of fish and cephalopods, with its robust, rough teeth suggesting that some particularly large fish may be eaten. Algae have also been found in the stomachs of rough-toothed dolphins, although these organisms may have been consumed accidentally . It is known to dive to 70 metres to capture its prey and remain underwater for 15 minutes, although evidence suggests that this dolphin is actually capable of undertaking much deeper dives. With males reaching sexual maturity at 14 years, and females at 10 years, the Rough-toothed dolphin is known to live for up to 32 to 36 years .
The diet of Steno bredanensis primarily consists of fish and squid indigenous to the areas they inhabit. Mollusks and pelagic octopuses have been found in some animals which where stranded in Florida. Mahi mahi and algae have been found in the stomachs of beached animals in Hawaii. Steno bredanensisis an excellent diver, known to dive great depths in search of cephalopods and large fish. Cooperative foraging and "bait balls" have been reported for this species (HWDT, 1998).
Conservation status
The IUCN classifies this species as one of Least Concern.
Threats
The greatest threat to the Rough-toothed dolphin is likely to be incidental capture in fishing netsWhile this dolphin is directly hunted in several areas for its meat (including waters of Japan, Sri Lanka, Indonesia, the Solomon Islands and Papua New Guinea), relatively small numbers are taken; moreover, since the species inhabits offshore waters, it is unlikely to be affected by habitat degradation and water pollution to the same extent that coastal-dwelling dolphins are.
Detectable levels of pollutants have been found in the blubber of Rough-toothed dolphins that were stranded in Hawaii. Since much is unknown about this species, much more research is needed to properly assess conservation threats (HWDT, 1998).
Reference
- IUCN Red List (June, 2008)
- Jefferson, T.A. (2002) Rough-toothed dolphin. In: Perrin, W.F., Würsig, B. and Thewissen, J.G.M. Eds. Encyclopedia of Marine Mammals. Academic Press, London.
- Jefferson, T.A., Leatherwood, S. and Webber, M.A. (1992) FAO Species Identification Guide. Marine Mammals of the World. FAO, Rome.
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