This site is indebted to the writings of Lyall
Watson (amongst others), for his amazing and well-researched book Whales
of the World, illustrated by Tom Ritchie.
The
Rough-toothed Dolphin (Steno bredanensis)
is a fairly large dolphin that can be found
in deep warm and tropical waters around the world. The Dolphin was first
described by Cuvier in 1823. The genus name Steno, of which this species
is the only member, comes from the Greek for 'narrow', referring to the
species nose - which is a diagnostic characteristic of the species. The
specific name honours van Breda who studied Cuvier's writings. The common
name refers to the thin lines of enamel that run vertically down the
dolphins teeth. Physical
description
Its proportions are perfect and it should be beautiful, but somehow
it is put together in a way that makes it ungainly. The long beak runs
directly into a sloping forehead which continues in a clear sweeping
line all the way through to the tail. The beak is very slender, giving
the whole head the sharp conical, streamlined look of a cetacean Concorde.
In many ways the outline is reminiscent of one of the sleeker marine
dinosaurs or ichthyosaurs, but the dolphin has peculiar bulging eyes.
The flippers and dorsal fin are smoothly curved and pointed, almost
identical in shape and size, and the same balanced contours are repeated
on the flukes. Adults, however, lose something of this aesthetic
appeal by developing thickenings or keels on the upper and lower surfaces
of the tailstock.
The colour is dark grey with a ruddy underlay which shows through as a pinkish hue on the white belly. The throat, chin
and the tip and sides of the beak are white. There are always numerous streaks, and star-shaped yellow or white scars all over the body, which are probably the result of injuries, tussles with squid and the attachment of external parasites
When travelling normally, Roughtooth Dolphins break the water cleanly with the top of the head and the sharklike fin, usually every 7-10 seconds. They sometimes school with tuna and have been taken, along with Bridled
Dolphins and Spinner Dolphins in purse-seine nets. They have been known to associate with both Bottlenose
Dolphins and Shortfin Pilot Whales.
The species is societal. Groups sizes are commonly as large as fifty and groups as large as 100 have been reported. The Dolphin has not been observed to bow-ride but does "skim" - swim with their heads and chin above the surface of the water.
Population and distribution
The distribution and population of the Rough-toothed
Dolphin is poorly understood. Most of the research activity concerning the dolphin has been directed in the eastern Pacific Ocean where a population estimate of 150,000 has been obtained. There have been numerous reports from other warm seas, usually as a result of by-catches. Populations of unknown sizes exist in the Mediterranean and Caribbean Seas and the Atlantic, Indian and Pacific Oceans. Live sightings are almost universally made far off-shore beyond the continental shelf.
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