Rough-toothed Dolphin, Steno bredanensis
Rough-toothed Dolphin, Steno bredanensis. Underwater photograph taken in coastal waters off Kona, Hawaii, September 2015. Photography and identification courtesy of Bob Hillis, Ivins, Utah.
Phylogeny: The Rough-toothed Dolphin, Steno bredanensis (G. Cuvier, 1828), is a member of the Delphinidae family of Oceanic Dolphins. The genus Steno is one of eighteen genera in this family. This is the only species in this genus. They are known in Mexico as Delphin Dientes Rugosos.
Morphology: Rough-toothed Dolphins have a slender, streamlined appearance. The forehead slopes gradually from the blowhole to the slender beak. These Dolphins lack the crease that separates the melon from the beak that is found in similar species. The flippers and flukes are large. The dorsal fin is tall and moderately falcate. The dorsal fin is set at mid-back. The belly is white and the sides are medium gray. The back is dark gray. This dark gray “cape” narrows anterior to the dorsal fin. The lips are white, but the top of the beak is dark gray. The eyes are slightly bulging. There are often lighter and darker spots and blotches on the sides and belly, giving the dolphin a mottled appearance. The top and sides are often marked with scratch marks and round scars, from fights with other Rough-toothed Dolphins, squids, and Cookie-cutter Sharks. These Dolphins derive their name from the rough striations on their teeth. They are sexually dimorphic with the males reaching a maximum of 2.8 m (9 feet 2 inches) in length and 160 kg (350 lbs) in weight and females 2.55 m (8 feet 4 inches) in length.
Habitat and Distribution: Rough-toothed Dolphins are usually found in the open ocean, both nearshore and far offshore. They primarily feed in relatively shallow water, but have been recorded diving to 2,000 m (6,562 feet). They are normally found in pods of ten to twenty individuals, but sometimes the groups are as large as fifty individuals. Rough-toothed Dolphins often associate with other dolphins and whales. They are found worldwide in tropical and warmer temperate seas and in all Mexican waters of the Atlantic and Pacific Oceans.
Diet: Rough-toothed Dolphins are predatory carnivores that primarily consume fish, that include Dorado, Coryphaena hippurus that can be up to 5 kg (11 lbs) in weight and squid. It is thought that they hunt cooperatively and share their catch. They have been observed following False Killer Whales, Pseudorca crassidens, to pick up food scrapes. In captivity, these dolphins always behead and eviscerate fish before eating them.
Predators: Rough-toothed Dolphins are a poorly studied and understood species. Other than bites from Cookie-cutter Sharks, Isistius brasiliensis, there is no documentation of their being attacked by sharks or Killer Whales, Orcinus orca, though it seems likely based on other dolphin species.
Reproduction: As mammals, Rough-toothed Dolphins are gonochoric (male or female for life), and reproduce sexually, with internal fertilization. Very little information is available regarding the mating systems of rough-toothed dolphins in the wild, such as season or gestation period. The calves are born around 1.0 m (3 feet 4 inches) in length. Calves attempt to nurse within an hour after birth, but are unsuccessful for three days or so. They nurse for at least two months before they begin to catch fish on their own and decrease nursing time. Females reach sexual maturity between the ages of nine and ten years, and males at five to ten years.
Ecosystem Interactions: Rough-toothed Dolphins are a poorly studied and understood species. There is no documentation of them engaging in commensal or symbiotic relationships. They are known to host internal parasites such as cestodes, nematodes, and trematodes.
Human Interactions: Rough-toothed Dolphins are hunted and consumed in the West Indies, West Africa, Japan, and the Solomon Islands. They are also caught and used as shark bait. They face additional threats from entanglement in fishing gear and being caught in nets as by-catch. From a conservation perspective they are currently considered to be of Least Concern with stable, widely distributed populations.
Synonyms: Delphinorhynchus bredanensis, Delphinus (Steno) perspicillatus, Delphinus bredanensis, Delphinus chamissonis, Delphinus compressus, Delphinus frontatus, Delphinus oxyrhynchus, Delphinus planiceps, Delphinus reinwardtii, Delphinus rostratus, Steno bradanensis, Steno frontatus, Steno perspicillatus, and Steno rostratus.