By Judy Jensen
Geographic Range
Steno bredanensis, the rough-toothed dolphin, lives in tropical to subtropical deep oceanic waters (Jefferson, 1993). They are found from the western Pacific to the Mediterranean. They seem to avoid cold surface waters and cold currents (Carwardine, 1995).
Biogeographic Regions:
oceanic islands 
(native ); indian ocean (native ); atlantic ocean (native ); pacific ocean (native ).
Habitat
The distribution of Steno bredanensis is 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 spotted 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).
Physical Description
(220 to 330 lbs; avg. 275 lbs)
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 colorings may vary geographically. The body of Steno bredanensis is dark grey with white or light colored 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 color 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 bredanensis a 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).
Some key physical features:
endothermic ; bilateral symmetry .
Reproduction
Females reach sexual maturity at about 10 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).
Key reproductive features:
gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous .
Care-giving behavior 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 behavior has been observed in the tight social groups of other marine mammals (HWDT, 1998).
Parental investment:
pre-fertilization (provisioning, protecting: female); pre-hatching/birth (provisioning: female, protecting: female); pre-weaning/fledging (provisioning: female, protecting: female).
Behavior
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).
Steno bredanensis are great divers and stay submerged for as long as 15 minutes. They are also fast swimmers and may swim just under the surface with their dorsal fins clearly visible. They enjoy "surfing" or bow-riding, but not as readily as other tropical dolphins (Carwardine, 1995).
These dolphins have been spotted with pilot whales, bottlenose dolphins, spotted dolphins and spinner dolphins.
This species produces both clicks and whistles. It is thought that the clicks are used in echolocation (Rouch, 1998).
Key behaviors:
natatorial ; motile ; social .
Food Habits
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 bredanensis is 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).
Primary Diet:
carnivore (piscivore , molluscivore ).
Animal Foods:
fish; mollusks.
Plant Foods:
algae.
Economic Importance for Humans: Negative
There are no known negative effects of Rough-toothed Dolphins.
Economic Importance for Humans: Positive
Rough-toothed Dolphins are taken as food in some parts of the world.
Conservation Status
IUCN Red List: [link]:
Data Deficient.
US Federal List: [link]:
No special status.
CITES: [link]:
No special status.
Little is known of these beautiful animals. Many are still being hunted in small numbers by drive fisheries in Japan. They are still harpooned in Japan and West Africa. Some of these animals are being caught and killed in purse seine nets in the eastern tropical Pacific. Others have been caught in gillnet and driftnet fisheries in Sri Lanka, Brazil, and elsewhere.
Detectable levels of pollutants have been found in the blubber of animals that were stranded in Hawaii.
Since so little is known about these animals immediate conservation threats have not been identified (HWDT, 1998).
For More Information
Find Steno bredanensis information at
Contributors
Judy Jensen (author), Fresno City College.
Rodney Olsen (editor), Fresno City College.
