The hourglass dolphin (Cephalorhynchus cruciger) is a small dolphin in the family Delphinidae that inhabits offshore Antarctic and sub-Antarctic waters. It is commonly seen from ships crossing the Drake Passage but has a circumpolar distribution.
The species was identified as a new species by Jean René Constant Quoy and Joseph Paul Gaimard in 1824 from a drawing made in the South Pacific in 1820. It is the only cetacean to have been widely accepted as a species solely on witness accounts.
Description
left|thumb|Necropsy of an hourglass dolphin. The markings on the side are a certain character for identification of the species.
The hourglass dolphin is colored black on top and white on the belly, with white patches on the sides and sometimes variations of dark grey. For this reason, it was colloquially known by whalers as a "sea cow" (although it does not belong to the taxonomic order Sirenia) or "sea skunk". The absence of a dorsal fin in right whale dolphins, in contrast to the generally tall and curved dorsal fin of hourglass dolphins makes confusion of the two species very unlikely. The dorsal fin in hourglass dolphins is variable and the curvature may be particularly pronounced in older animals. The hourglass dolphin has disk-shaped vertebrae and other inclined processes which gives them higher stability.
An adult male is about in length and weighs over 90 kilograms (about 200 lbs). Juvenile females range from in length and weigh from . There have been no verified sightings of calves and their coloration, size, and diet remains unknown.
Geographic range and distribution
The range is circumpolar from close to the Antarctic pack ice to about 45°S. The overall circumpolar distribution is about 45°S to 67°S, with few sightings made near islands and the majority in the Southern ocean. Sightings have been made most commonly from the south of New Zealand, around the South Shetland Islands and off Tierra del Fuego, Argentina.
The hourglass dolphin life-span is estimated to be similar to that of Atlantic white-sided dolphin and Pacific white-sided dolphin, around 27-46 years.
Taxonomy
The species was first named Delphinus cruciger by Quoy and Gaimard (1824) after their sighting in January 1820. and it was suggested that it should be moved to a new genus Sagmatias.
Population status
Sighting surveys were conducted in 1976–77 and 1987–88. Abundance was estimated to be 144,300 individuals, based on line transect sightings in January 1977 and January 1988 in northern Antarctic waters. While the population of the hourglass dolphins has slightly declined since 1988, they are not considered endangered. They are also listed in Appendix II of the Convention on International Trade in Endangered Species (CITES). Although they have not been studied extensively, there are no known major threats to hourglass dolphins, and the species is listed as Least Concern on the IUCN Red List.