Hourglass dolphin
Hourglass Dolphins in Drake Passage (Source: Lomvi3, via Wikimedia Commons)
The Hourglass dolphin (Lagenorhynchus cruciger), named after its hourglass shaped colouration pattern, is most commonly found in the cold pelagic waters near Antarctica. Interestingly, while no specific predator has been found for Lagenorhynchus cruciger, the species have counter-shaded coloration - meaning that they have a lighter colored belly (so that they will blend with the light when viewed from below) and darker backs (so that they will blend with the dark water when viewed from above) which is a trait most commonly found in predated species. This marine mammal is a member of the familiy of Oceanic Dolphins, part of the order of cetaceans.
Contents
Physical description
Hourglass dolphins are small, robust, dolphins with a unique black and white color pattern. Pigmentation patterns vary greatly amongLagenorhynchus crucigerindividuals, but the sexes are monomorphic.
Hourglass dolphins have highly recurved, falcate dorsal fins with highly keeled tailstocks. The color pigmentation resembles that of an hourglass pattern for which the species gets its common name. It was first described by Qouy and Gaimard in 1824 and was called “cross bearer”.
Reproduction
Females nurse their young, who are able to swim along with their mothers from birth. In other memebers of the genus for which data have been collected, lactation can last from 12-18 months. Other information on parental care is lacking for these animals.
Lifespan
The lifespan ofLagenorhynchus crucigeris not known with certainty; however, it can be presumed to be similar to other species within its genus. The Atlantic white-sided dolphin,''L. acutus, can live 27 years, and the Pacific white-sided dolphin,'L. obliquidens, can live up to 46 years in the wild.'''
Behavior
Hourglass dolphins are social animals and travel in small groups. Social group size varies from one to 100 individuals, with an average group size of seven. It is not known if these groups are made up of related individuals, mixed sexes, or random dolphins. In 223 observed social groups, with a total of 1634 individuals, only three calves were observed. The low cow-calf ratio may be a result of ship avoidance by cows with calves, or may result from a winter birthing season, when little research is done and few observations are made.
There may be seasonal migrations that follow cold-water currents such as the West Wind Drift. In the summer months Lagenorhynchus cruciger is more often found in cooler southern waters. The species is found further north during the winter months.
Hourglass dolphins are attracted to large boats and ships and will often change their course of travel to intercept them. They enjoy riding in the bow waves and wakes created by the ships.
Lagenorhynchus cruciger is often seen playing in the vicinitiy of the larger rorqual whale. Hourglass dolphins commonly ride in the bow waves of these larger animals, periodically jumping out of the water. Whalers used these playing behaviors to locate the fin whale in their search efforts.
There are no recorded data on communication for this species. However, it is likely that like all odontocetes that have been studied, they communicate with high frequency sounds. They are likely to have some tactile and visual communication as well.
Distribution and habitat
Hourglass dolphin distribution in the world. Source: IUCN Red List
Widely distributed throughout their range,Lagenorhynchus crucigeris rarely seen near shore (pelagic) and prefers the colder waters of the open ocean.
Predation
There is no documentation of predation on these dolphins, however, they are likely preyed upon by killer whales,'Orcinus orca. Like many aquatic animals, these dolphins are countershaded. Countershading is widely thought to be an antipredator adaptation, as a light underbelly is difficult to see from below, and a darker dorsal surface is less readily detected from above.'
Ecosystem roles
In addition to traveling with conspecifics, these dolphins often associate with other species of cetaceans. In one study, hourglass dolphins were encountered 17% of the time traveling with the Fin Whale, Balaenoptera physalus; the Sei whale, B. borealis; the Minke whale, B. acutorostrata; Arnoux’s Beaked Whale, Berardius arnuxii; the Southern bottlenose whale, Hyperoodon planifrons; the long-finned pilot whale, Globicephala melaena, killer whales and the southern right whale dolphin, Lissodelphis peronii. There is one account ofLagenorhynchus cruciger swimming near southern right whales, Eubalaena australis.
The ecological niche of hourglass dolphins is not known. They are social animals and will often travel and feed with other whales and dolphins. Based upon their diet, Lagenorhynchus crucigerare most likely secondary or tertiary level consumers. They therefore may play some role in regulating prey populations.
Feeding habits
Hourglass Dolphins feed primarily on fish, squid (OnychoteuthidaeandEnoploteuthidae), and crustaceans. Squid beaks from these families were found in the stomach of one specimen, and the remains ofKrefftichtys andersonii, a mesopelagic lantern fishwere found in another. They are often seen feeding in large congregations near the surface, that attract albatross, petrels and other sea birds. Researchers will often focus in on these large aggregations of birds to locateLagenorhynchus cruciger.
Conservation status
Currently Lagenorhynchus cruciger is not listed as threatened or endangered; rough population estimates indicates the species numbers are greater than 140,000. They are not exploited commercially, and attempts to bring them into captivity have never been made. This is most likely due to the distribution of the species and the remoteness of the species range. A few specimens were collected during commercial whaling operations for scientific research. Accidental bycatch from commercial fisheries are limited.
Threats
There is no documentation of predation on these dolphins, however, they are likely preyed upon by killer whales, Orcinus orca. Like many aquatic animals, these dolphins are counter-shaded. Hourglass dolphins are not commercially harvested, but some are taken annually along with the Dusky Dolphin, to be used as crab bait by local fishermen in Chile. Increasing ecotourism in the Antarctic also allows for further observations of this species.
References
- IUCN Red List
- Encyclopedia of Life
- Brownell Jr., R., M. Donahue. 1999. Hourglass Dolphin, *Lagenorhynchus cruciger*. Pp. 121-135 in S. Ridgeway, H. Harrison, eds. Handbook of Marine Mammals: Vol. 6: The Second Book of Dolphins and Porpoises. San Diego: Academic Press.
- Goodall, R. 1997. Review of Sightings of the Hourglass Dolphin, *Lagenorhynchus cruciger*, in the South American Sector of the Antarctic and Sub-Antarctic. Report of the International Whaling Commission, 47: 1001-1013.
- Goodall, R., A. Baker, P. Best, M. Meyer, N. Miyazaki. 1997. On The Biology of the Hourglass Dolphin, *Lagenorhynchus cruciger* (Quoy and Gaimard, 1824). Report of the International Whaling Commission, 47: 985-999.
- Gordon, D. (Ed.) (2009). New Zealand Inventory of Biodiversity. Volume One: Kingdom Animalia. 584 pp
- IUCN (2008) Cetacean update of the 2008 IUCN Red List of Threatened Species.
- Klinowska, M. 1991. Dolphins, Porpoises and Whales of the World. The IUCN Red Data Book. Gland, Switzerland and Cambridge, U.K.: IUCN.
- Leatherwood, S., R. Reeves. 1983. The Sierra Club Handbook of Whales and Dolphins. San Francisco, CA: Sierra Club Books.
- Mead, James G., and Robert L. Brownell, Jr. / Wilson, Don E., and DeeAnn M. Reeder, eds. 2005. Order Cetacea. Mammal Species of the World: A Taxonomic and Geographic Reference, 3rd ed., vol. 1. 723-743
- Nowak, R. 1999. Walker's Mammals of the World, Vol. 2, Sixth edition. Baltimore and London: The Johns Hopkins University Press.
- Perrin, W. (2010). Lagenorhynchus cruciger Quoy & Gaimard, 1824. In: Perrin, W.F. World Cetacea Database. Accessed through: Perrin, W.F. World Cetacea Database 2011-02-05
- Rice, Dale W. 1998. Marine Mammals of the World: Systematics and Distribution. Special Publications of the Society for Marine Mammals, no. 4. ix + 231
- UNESCO-IOC Register of Marine Organisms
- Wilson, Don E., and DeeAnn M. Reeder, eds. 1993. Mammal Species of the World: A Taxonomic and Geographic Reference, 2nd ed., 3rd printing. xviii + 1207
- Wilson, Don E., and F. Russell Cole. 2000. Common Names of Mammals of the World. xiv + 204
