The Hourglass Dolphin (Lagenorynchus cruciger) is a small dolphin found in Antarctic and sub-Antarctic waters.
The dolphin was for a long time rarely seen. It was first identified
as a new species by Qouy and Galmard in 1824 from a drawing made in the South
Pacific in 1820. It is only the cetacean to have been widely accepted as a
species on eye accounts alone 1. Indeed by 1960, despite decades of whaling
in the Southern Ocean, only three specimens had been brought to the attention
of scientists. Even now only 6 complete and 14 partial specimens have been
examined. Further information has been obtained from 4 strandings and boats
which have deliberately set out to observe the dolphins in areas rarely otherwise
visited by ships.
Physical description:
The Hourglass Dolphin is coloured black and white and for this reason was
colloquially known by whalers as the "sea skunk". On each flank
there is a white patch at the front of the dolphin, above the beak, eye and
flipper and a second patch at the rear. These two patches are connected by
a thin strip of white, creating, loosely speaking, an hourglass shape and
hence the common name of the dolphin. The scientific name cruciger is from
the Latin for "cross-carrier". That the white patches are supposed
to resemble both an hourglass and a cross is perhaps indicative of how inexact
these descriptions are! The remainder of the body is black.
In its usual range the dolphin is easily identifiable. Only the Southern Right Whale Dolphin is of comparable size and found so far south. The Right Whale Dolphin doesn't have a dorsal fin and so the two species are trivially distinguished from one another. The fin varies from one individual to another quite considerably. Broadly speaking it is tall and curved, and the curve may be particularly pronounced in older animals.
A fully grown adult is about 1.8m in length and weighs 90-120kg. Females are probably slightly smaller and lighter than males (from a sample size of nine specimens).
Population and distribution:
The range is circumpolar from close to the Antarctic ice pack to about 45°S.
The most northerly confirmed sightings were 36°S in the South Atlantic
Ocean and 33°S near Valparaiso, Chile in the Pacific. Sightings have been
made most commonly from the south of New Zealand around the South Shetland
Islands and off Tierra del Fuego, Argentina. The species is unlikely to be
particularly densely populated close to these lands. One survey estimated
that there was a minimum of 140,000 individuals alive today.
Behaviour:
Hourglass Dolphins tend to move in groups of about 5-10 in number. One International
Whaling Commission study recorded a group of 60. They share feeding grounds
with other cetaceans such as Sei, Pilot, Bottlenose and Minke Whales and Southern
Right Whale Dolphins. They are found very regularly with Fin Whales. In fact
whalers hunting these much larger whales used to Hourglass Dolphins as "look-outs"
to aid them in their hunt.
Hourglass Dolphins are keen bow-riders.
Examinations of the stomach contents of the few analysed specimens indicated the it eats various (unrecorded) types of squid and small fish.
Conservation:
The Hourglass Dolphin is believed to be locally common. It has never been
taken into captivity or deliberately hunted. The number of by-catches is very
small. The effects of pollution on the dolphin is likely to be negligible
given its distant distribution. The species is one of few cetaceans essentially
unaffected by humans.
References:
National Audubon Society: Guide to Marine Mammals of the World .
Encyclopedia of Marine Mammals.