The hourglass dolphin, Lagenorhynchus cruciger, is the world’s southernmost small dolphin. An oceanic species found in sub-Antarctic and Antarctic waters, it often bow rides ships and accompanies larger cetaceans but is one of the least studied cetaceans. Only three specimens were collected in the 136 years from the discovery of the species to 1960. Our knowledge of the biology of this species rests on 20 specimens (only 6 of them complete), 4 stranding observations and sightings at sea.
I. Taxonomy
The name Delphinus cruciger was based on a drawing from a sighting in the South Pacific in 1820. Synonyms include D. albigena, D. bivittatus, Electro clancula, E. cmcigera, D. su-perciliosus, Phocoena crucigera, P. d’Orbignyi. Lagenorhynchus wilsoni, L. latifrons, and L. Fitzroyi (with L. australis and L. obscurus). The accepted combination, L. cruciger; was made by Van Beneden and Gervais in 1880. Common names have included the crucigere, the albigena. grindhval, sea skunks, and springers: the name in Spanish is delfin cruzado.
II. Diagnostic Characters
A. Pigmentation
The hourglass dolphin is mainly black or dark with two elongated lateral white areas, in some animals joined with a fine white line, which give it its common name (Fig. 1). The forward patch extends onto the face above the eye. which is within the black surface but outlined with a large dark eve spot with a point forward and a thin white line. The dark pigment of the hps is of varying shape; a gape to flipper stripe may be gray to tan, beige, or even rose. One animal had a white half-moon mark outlining the blowhole. On the side below the white flank patch, there is a lobe of white projecting forward, which may form a sharp point, a blunt, curved shape, or a hook. The flank patches on some animals almost meet on the upper tail stock. Part of the underside of the flippers is white. The ventral region is generally white, with some dark areas forward from the tail stock to the genital region. The pigmentation of juveniles has not been described.
B. Size and Shape
The hourglass is a rather stocky dolphin with a large, recurved dorsal fin that is variable in shape from erect to hooked. The tail stock is often keeled. Total lengths (n=9) range from 142 to 187 cm. Females (h=5) measure 142-183 cm, males (n=3) 163-187. This is probably not the total range of length for the species. Weights are known for only three specimens.
Figure 1 Hourglass dolphins possess two lateral white areas along the flank that are united by a thin white line that resembles an hourglass. Schools of individuals can swell from the small group of 3 pictured, commonly around 6 or 7, and rarely up to 60 individuals.
Females of 163.5 and 183 cm weighted 73.5 and 88 kg. A 174-cm male weighed 94 kg.
III. Distribution
The hourglass dolphin is circumpolar in the Southern Ocean, in both Antarctic and sub-Antarctic waters, from about 45°S south to fairly near the ice pack (Fig. 2). Exceptional sightings were at 36°14′S in the South Atlantic and 33°40′S in the South Pacific off Valparaiso. Chile. The southernmost sighting was at 67°3S’S in the South Pacific. Most specimens were found between 45 and 60°S, the northernmost from New Zealand and the southernmost from the South Shetland Islands. Sightings and specimens are plotted in Fig. 2.
IV. Ecology
A. Habitat
This dolphin is pelagic and circumpolar in the Southern Hemisphere on both sides of the Antarctic Convergence and northward in cool currents associated with the West Wind Drift. Recorded water temperatures range from —0.3 to 13.4°C. Although oceanic, sightings of this dolphin are often made near islands and banks. Sightings reflect observer effort, with most in the Drake Passage, reflecting ship traffic between South America and the Antarctic Peninsula.
B. Prey and Predators
The stomachs of five specimens from different oceans have been examined; one was emptv. Prey items included unidentified small fish, the fish Krejftichtys andersonii (Mycophidae) of about 2.4 g and a length of 55 inm; small squid, including some from the families Onvchoteuthidae and Enoploteuthidae; and crustaceans. They often feed in large aggregations of sea birds and in plankton slicks.
No predators are known, although killer whales and leopard seals are possibilities.
Figure 2 Distribution of the hourglass dolphin compiled from incidental and dedicated sighting surveys and the location of specimens from 1824 to 1997. Small circles indicate single specimens, and large circles represent three specimens off New Zealand and four at Tierra del Fuego.
V. Behavior
Nothing is known of the migratory movements of this species. Hourglass dolphins are rapid swimmers with a forward, plunging movement. They commonly bow-ride ships, especially in rough weather. These dolphins were found so often with fin whales (Balaencrptera physalus), that whalers used them as “spotters” in finding whales. They have also been seen with sei and minke whales (B. borealis and B. bonaerensis), large bottlenose whales (Hyperoodon and Berardius), pilot whales (Globicephala melas), and southern right whale dolphins (Lissodelphis peronii).
During the Southern Hemisphere minke whale assessment cruises conducted by the International whaling commission, school sizes ranged from 1 to 60 animals (mean 7). Other studies reported mean sizes of 4 and 5.7 animals.
The sounds of this species have not been recorded.
VI. Internal Anatomy
The condylobasal lengths of 11 skulls ranged from 316 to 370 mm. Visible teeth numbered 26-34 upper and 27-35 lower in each jaw. Vertebral count is CV7, Thl2-13, LI8-22, and Ca 29-33 for a total of 69-72 (n=7). The first two cervicals are fused (n= 4). The vertebrae of L. cruciger are smaller than those of L. australis, but slightly larger than those of L. obscurus and are similar to the latter in shape. There are 12-13 ribs (n=7). One specimen had seven pairs of sternal ribs, another eight. The phalangeal formula (n=5) is 1=2-3, 11=8-11, 111=6-8, IV=2^, and V=0-2.
The internal organs were examined for one specimen and organ weights are known for another. The intestine length of one specimen was 18 m.
VII. Life History
A. Growth and Reproduction
Very little is known of growth and reproduction in this species. A 163.5-cm female was sexually immature and one of 183 cm was pubescent. Males of 174 and 187 cm were sexually mature. Based on fusion of the vertebral epiphyses, the 163.5-cm female was physically immature. The 183-cm female was subadult. In males, an animal of 163 cm was subadult, one of 187 cm was nearly mature, and one of 174 cm was physically mature. Nothing is known of the young, times of birth, and reproduction rates; only three calves, seen in January and February, have been reported. No studies on aging have been published.
B. Parasites and Disease
Nematodes (Anisakis sp.) were reported in the stomachs and intestines of two animals. The largest animal known, a male, had long-standing gastric trauma with extensive peritonitis. No other diseases have been reported.
VIII. Interactions with Humans
Several hourglass dolphins have been taken for scientific study. No other directed catches are known. The only incidental catches reported were three females from New Zealand and a drift net catch in the southern Pacific Ocean. No animals have been kept in captivity.
