Thursday, 24 March 2011

The hourglass dolphin

Fig. 1   The hourglass dolphin (Tidman, 2011)

Common name: Hourglass dolphin              
Latin name: Lagenorhynchus cruciger (Quoy & Gaimard, 1824)

Cruciger is Latin for “cross-bearing” and denotes the area of black and white pigmentation on the back of the dolphin which when viewed from above, resembles an hourglass (Brownell & Donahue, 1999).

Kingdom:          Animalia
Phylum:            Chordata
Class:               Mammalia
Order:              Cetacea
Family:             Delphinidae
Genus:             Lagenorhynchus

Size and Weight
Adult length:
Male: 1.63m long (Nichols, 1908)
Female: 1.66m - 1.83m long (Fraser 1966)

There is possibly sexual dimorphism in the Hourglass dolphins, as male specimens are usually slightly shorter in length. However this is disputed to some extent due to a limited number of specimens (Brownell & Donahue, 1999).

Adult weight:
90-120 kg (Miyazaki, 1986)
The small and robust hourglass dolphin has inherited the nickname the “sea cow” due to its characteristic black and white colouring (Brownell & Donahue, 1999). The common name refers to the two white patches connected by a thin white strip on each flank, which bear a resemblance to an hourglass (Brownell & Donahue, 1999). Despite the species having no known predators, the hourglass dolphin exhibits counter-shading colouration (Brownell & Donahue, 1999). Countershading is an anti-predator defence where the belly is light in colour (blending with the light sky when viewed from below) and the back is dark in colour (blending with the dark depths when viewed from above) (Oxford Dictionaries, 2010).

The fin varies significantly between individuals, but it is generally tall and curved (Brownell & Donahue, 1999). It is thought that the curve is perhaps more pronounced in older individuals (Fraser 1966). The hourglass dolphin is easily distinguished from its similarly sized neighbour, the southern right whale dolphin (Lissodelphis peronii), as unlike the right whale, it has a dorsal fin (Brownell & Donahue, 1999).

The hourglass dolphin has a circumpolar distribution, in the higher latitudes of the southern oceans (Goodall, 1997; Goodall et al, 1997; Brownell & Donahue, 1999).

Fig. 2   Hourglass dolphin distribution, dominating the cold waters of 
the Southern Hemisphere (Hammond et al, 2008).

The southern range is to the Antarctic ice-edges, with the most southerly sightings occurring near 68°S in the South Pacific (Goodall 1997; Brownell & Donahue, 1999). The distribution in the north is less well known, however they are found to at least 45°S. Some sightings have occurred as far north as 36°S in the South Atlantic Ocean and 33°S near the Chilean city of Valparaíso (Goodall, 1997).

Hourglass dolphins are most commonly seen around the Antarctic Convergence or between South America and Macquarie Island (Goodall, 1997). They can also be seen off the south coast of New Zealand, near the South Shetland Islands and around the Tierra del Fuego province (Goodall, 1997). The hourglass dolphin is the only small delphinid species which is regularly found south of the Antarctic Convergence (Goodall et al, 1997). Hourglass dolphins are usually found in southern waters during the summer and northern waters during the winter, this suggests that they migrate seasonally following the cold-water currents (Goodall, 1997).

The hourglass dolphin is found in the cold, deep waters of the Antarctic; however some sightings have occurred in relatively shallow waters near South America and the Antarctic Peninsula (Goodall, 1997). It can be found within 160km of the ice edge within the Southern range (Jefferson et al, 1993). It appears to have a preference for surface water temperatures of between 0.6°C and -13°C (Goodall, 1997), but can even inhabit waters as cold as -0.3°C (Goodall, 2002).

Hourglass dolphins are sociable and usually observed travelling in small groups of around 7 individuals (Kasamatsu et al, 1990). Group sizes, however, can range from 1 to 100 individuals (Kasamatsu et al, 1990). The ratio of males to females within these groups is unknown; however it is unusual to see a calf travelling with larger groups (Kasamatsu et al, 1990). It has been suggested that this could be due to ship evasion from females with calves, or perhaps the species undergoes synchronised breeding in the winter (Kasamatsu et al, 1990). There is a lack of information with regards to the parental care behaviour exhibited in the species, however, we do know that females nurse their young from birth and based on data from other species in the genus, it is thought that lactation lasts for 12-18 months (Brownell& Donahue, 1999).

Hourglass dolphins enjoy riding bow waves and wakes, and have been observed altering their direction of travel just so they can catch the waves created by travelling boats and ships (Bowles et al, 1994). They also like to ride the bow waves of fin whales, Balaenoptera physalus, regularly jumping out of the water as they play around the larger animals (Fraser, 1964). Whalers historically searched for this characteristic behaviour in order to locate the fin whale in their hunt. Hourglass dolphins also like to socialise with other species such as the minke whale (Balaenoptera acutorostrata), pilot whales (Globicephala melas and Globicephala macrorhynchus), beaked whales (Ziphiidae), sei whale (Balaenoptera borealis) and southern right whale dolphins (Lissodelphis peronii) (Kasamatsu et al, 1988).
Fig. 3   Hourglass dolphins porpoising (Pitman, 2011)

Other areas of uncertainty include the lifespan of the hourglass dolphin; however it is likely to be comparable to that of other species within its genus (Brownell & Donahue, 1999). The closely related Atlantic white-sided dolphin (Lagenorhynchus acutus) has a lifespan of 27 years, whereas the Pacific white-sided dolphin (Lagenorhynchus obliquidens) can live to 46 years (Klinowska, 1991).

Little is known about the feeding habits of the hourglass dolphin, but scientists have recorded small fish, cetaceans and squid (from the Onychoteuthidae and Enoploteuthidae families) from the stomach contents of several specimens (Clarke, 1986; Ash 1962). The species has also been observed feeding in plankton swarms and seabird aggregations (Clarke, 1986).

Like all toothed whales, hourglass dolphins use echolocation for orientation and prey location (Khyn et al, 2009). It is also likely that they also communicate using visual and tactile cues (Brownell & Donahue, 1999). A recent study showed that they produce very high-pitched clicks, which allow them to detect prey at more than twice the distance of other dolphin species (Khyn et al, 2009). It was suggested that this could be due to the deep and open nature of their habitat, meaning that the hourglass dolphin needs to cover larger areas to locate prey (Khyn et al, 2009).

There are currently no known threats to the hourglass dolphin (Brownell & Donahue, 1999). In 1995, the first and only study to date (March, 2011) was conducted on the population size of the species, combining data from surveys occurring from 1976 to 1988. This study estimated that there were 144,300 individuals for waters south of the Antarctic convergence (Kasamatsu and Joyce, 1995).

It is thought that the species is probably preyed upon by killer whales, Orcinus orca, but there hasn’t been any documented evidence of predation. The hourglass dolphin is not commercially hunted and accidental by catch is limited (Brownell & Donahue, 1999).

The Hourglass dolphin is classified as Least Concern (LC) on the IUCN Red List (Hammond et al, 2010) and listed on Appendix II of CITES. There have been no attempts to bring the hourglass dolphin into captivity, apart from several specimens being collected for scientific research (Brownell & Donahue, 1999). This is most likely due to their remote distribution (Brownell & Donahue, 1999). Increasing ecotourism in the Antarctic will likely lead to further knowledge of this species (Brownell & Donahue, 1999).

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