Taxonomy & Nomenclature
Physical Description / Field Identification
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 maturity. In some animals (possibly adult males), the dorsal fin also possesses a sharp backward bend about halfway up. The tail stock is often keeled.
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 rostrum, 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.
A tooth count of approximately 28 teeth on each side of each jaw has been recorded.
Less than a dozen specimens have been measured; the maximum length and weight so far recorded were 1.87 m and 94 kg. Length at birth is assumed to be about 1 m.
Can be Confused With
As the only small oceanic dolphin with a pointed dorsal fin in subantarctic and antarctic waters, the strikingly marked hourglass dolphin should not be confused with other species.
Hourglass dolphins are distributed in a circumpolar pattern in the higher latitudes of the southern oceans. They range to the ice-edges in the south, but the northern limits are not well-known. Hourglass dolphins appear to be oceanic; however, some sightings have been made in waters of 200 m or less, near islands and banks.
Ecology and Behavior
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 60 have been seen, groups of 1-6 are more common. Hourglass dolphins have been encountered with several other species of cetaceans, and are often seen with large whales. 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 life history of this dolphin species.
Feeding and Prey
The stomach contents of the handful of hourglass dolphins that have been examined contained small fish (including myctophids), squids, and crustaceans.
Threats and Status
The only known exploitation has been several individuals taken for scientific research, and three specimens killed in a gillnet operation in the South Pacific Ocean. Because of its habitat in one of the most remote marine habitats known, this species is not thought to be threatened. It is currently “Not Listed” (IUCN); “Not Listed” (ESA).
Brownell, Jr., R.L. and M.A. Donahue. 1999. Hourglass dolphin Lagenorhynchus cruciger (Quoy and Gaimard, 1824). pp. 121-136 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Vol. 6: The second book of dolphins and the porpoises. Academic Press.
Goodall, R.N.P. 1997. Review of sightings of the hourglass dolphin, Lagenorhynchus cruciger, in the South American sector of the Antarctic and the sub-Antarctic. Reports of the International Whaling Commission 47:1001-1014.
Goodall, R.N.P. 2002. Hourglass dolphin Lagenorhynchus cruciger. pp. 583-585 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.
Goodall, R.N.P., A.N. Baker, P.B. Best, M. Meyer and N. Miyazaki. 1997. On the biology of the hourglass dolphin, Lagenorhynchus cruciger (Quoy and Gaimard, 1824). Reports of the International Whaling Commission 47:985-1000.