Dwarf sperm whale
|Dwarf sperm whale|
|Size compared to an average human|
|Dwarf sperm whale range|
The dwarf sperm whale (Kogia sima, formerly Kogia simus) is one of three extant species in the sperm whale family. They are not often sighted at sea, and most extant information comes from the study of stranded carcasses.
Today, the dwarf sperm whale is generally classified as one of two species, along with the pygmy sperm whale, in the Kogiidae family and Kogia genus. It was not until 1966 that the two species were regarded as separate. Most taxonomists regard the family Kogiidae as belonging to the superfamily Physeteroidea, though some consider this taxon to be a subfamily (Kogiinae) of the family Physeteridae.
The dwarf sperm whale is the smallest species commonly known as a whale. It grows up to 2.7 m (8.9 ft) in length and 250 kilograms (550 lb) in weight— making it smaller than the bigger dolphins. The species makes slow, deliberate movements with little splash or blow and usually lies motionless when at the sea's surface. Consequently, it is usually observed only in very calm seas (Beaufort 0-1).
The dwarf sperm whale is similar in appearance and behavior to its cousin the pygmy sperm whale. Identification may be close to impossible at sea – however, the dwarf is slightly smaller and has a larger dorsal fin. The body is mainly bluish gray with a lighter underside with slightly yellow vein-like streaks possibly visible. There is a white false gill behind each eye. The flippers are very short and broad. The top of the snout overhangs the lower jaw, which is small. Dwarfs have long, curved and sharp teeth (0–6 in the upper jaw, between 14 and 26 in the lower). These teeth led to the species being described as the "rat porpoise" in the Lower Antilles.
Like other sperm whales, the dwarf sperm whale has a spermaceti organ in its forehead. Like the pygmy, the dwarf is able to expel a dark reddish substance when frightened or attacked—possibly to put off any predators.
Dwarf sperm whales are usually solitary but have occasionally been seen in small groups.
The brain of the dwarf sperm whale is roughly half a kilogram in mass.
Dwarf sperm whales and pygmy sperm whales are unique among cetaceans in using a form of "ink" to evade predation in a manner similar to squid. Both species have a sac in the lower portion of their intestinal tract that contains up to 12 L of dark reddish-brown fluid, which can be ejected to confuse or discourage potential predators.
Population and distribution
The dwarf sperm whale prefers deep water, but is more coastal than the pygmy sperm. Its favorite habitat appears to be just off the continental shelf. In the Atlantic, strandings have been observed in Virginia, United States in the west and Spain and the United Kingdom in the east, and as far south as southern Brazil and the tip of Africa. In the Indian Ocean, specimens have been found on the south coast of Australia and on many places along the Indian Ocean's northern coast - from South Africa right round to Indonesia. In the Pacific, the known range includes the Japanese coast and British Columbia. No global population estimates have been made. One survey estimated a population of about 11,000 in the eastern Pacific.
The calving season lasts four to five months. Pregnancy lasts for 9 to 11 months. The newborn calf is about one meter long. Males reach puberty at 2.0m and become sexually mature at 2.1 to 2.2 m.
The dwarf sperm whale was actively hunted by commercial whalers. Occasional harpoon kills are still made by Indonesian and Japanese fishermen. Since the dwarf is more coastal than the pygmy, it may be more vulnerable to human activities such as fishing and pollution. Insufficient data exists as to whether such activities threaten the species survival.
The dwarf sperm whale is covered by the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS). The species is further included in the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU) and the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU).
- Mead, J.G.; Brownell, R.L., Jr. (2005). "Order Cetacea". In Wilson, D.E.; Reeder, D.M. Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. p. 737. ISBN 978-0-8018-8221-0. OCLC 62265494.
- Taylor, B.L.; Baird, R.; Barlow, J.; Dawson, S.M.; Ford, J.K.B.; Mead, J.G.; Notarbartolo di Sciara, G.; Wade, P.; Pitman, R.L. (2012). "Kogia sima". IUCN Red List of Threatened Species. Version 2014.3. International Union for Conservation of Nature. Retrieved 2015-01-11.
- L Marino, K Sudheimer, DA Pabst, WA McLellan, and JI Johnson (2003). Magnetic resonance images of the brain of a dwarf sperm whale (Kogia simus). Journal of Anatomy, 203(1): 57–76
- "Dwarf sperm whale (Kogia sima)". NOAA Fisheries Office of Protected Resources web site. NOAA. 2014-10-20. Archived from the original on 2013-10-03. Retrieved 2015-01-11.
- Kristin Petrie. Dwarf Sperm Whales. pg 16
- David Nagorsen (1985). Kogia simus. Mammalian Species 239
- Official website of the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area
- Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia
- Official webpage of the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region
- Pygmy and Dwarf Sperm Whales by Donald F. McAlpine in Encyclopedia of Marine Mammals (2002), San Diego: Academic Press, pp. 1007–1009, ISBN 0-12-551340-2
- Whales Dolphins and Porpoises, Mark Carwardine, Dorling Kindersley Handbooks, ISBN 0-7513-2781-6
- National Audubon Society Guide to Marine Mammals of the World, Reeves, Stewart, Clapham and Powell, ISBN 0-375-41141-0
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