(Quoy & Gaimard, 1824) - Hourglass dolphin
Hourglass dolphins are robust, with extremely short and stubby (but well-defined) beaks. The moderately tall dorsal fin is set midway along the back. The markedly hooked fins seen on some individuals probably develop at the onset of physical maturity.
Hourglass dolphins are strikingly marked; black above and white below. The black sides are broken by a bold white flank patch that covers most of the tail stock in a wedge shape, tapering as it rises towards the fin. There, it meets the vertex of a white dorsal-spinal blaze that widens over the flippers, passes above the eye to cover the sides of the face and finally converges at the gape with the white of the chest and throat. These white markings resemble an hourglass in shape and give the dolphin its common name. The black rostrum is typical for the genus. The forehead and top of the head are also black. A white, hook-shaped mark curves up to the black side below the flank patch, near the genital aperture. The flippers, dorsal fin, and flukes are all black.
There are approximately 28 small, sharp teeth on each side of each jaw.
Can be confused with
The hourglass dolphin has distinctive markings and is the only small oceanic dolphin with a pointed dorsal fin in subantarctic and antarctic waters; therefore, it is difficult to confuse with other species.
Few animals have been measured; a 1.63 m male and a 1.83 m female have been reported. Length at birth is assumed to be about 1 m.
Hourglass dolphins are circumpolar in the higher latitudes of the southern oceans. They range to the ice-edges in the south, but the northern limits are not known. Hourglass dolphins appear to be oceanic; however, some sightings have been made in waters of 200 m or less, near islands and banks.
Biology and Behaviour
Very little is known about hourglass dolphins. Groups tend to be small, which is unusual for a small oceanic delphinid. Although herds of up to 40 have been seen, groups of 1 to 6 are more common. Hourglass dolphins have been encountered with several other species of cetaceans. These dolphins are enthusiastic bowriders, often leaping as they race towards the bow or stern. They can also move rapidly without leaping, usually when avoiding a vessel; at such times they cause a highly visible “rooster tail” spray.
Almost nothing is known of the reproductive biology of this species.
The stomach contents of one hourglass dolphin contained a mass of partially digested small fish.
It may be more fair to describe the hourglass dolphin as poorly known rather than rare. It is likely that their numbers are at or near original levels. There has never been any systematic exploitation.