The Dwarf Sperm Whale (Kogia sima) is one of three species of whale in the sperm whale family. They are not often sighted at sea and most of our understanding of the creatures comes from the study of washed-up specimens.
Taxonomy: Nowadays 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, however it was not until 1966 that the two species were regard as separate and even more recently Kogiidae was regarded as a subfamily (Kogiinae) of Physeteridae.
Physical description: The Dwarf Sperm Whale is the smallest of all whales. It grows up to 2.7 m in length and 250 kg in weight – making it smaller than the bigger dolphins. The species makes slow, deliberate movements with little splash or blow and will usually lie motionless when at the sea's surface. Consequently it can only be observed in very calm seas.
The Dwarf Sperm Whale is physically very similar to and has the same behavioural characteristics as its cousin the Pygmy Sperm Whale. Identification may be close to impossible at sea – however the Dwarf is slightly smaller and has a considerably larger dorsal fin. The body is mainly bluish grey 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. Dwarves have long, curved and sharp teeth (0–6 in the upper jaw, between 14 and 26 in the upper). These teeth have led the species to be described as the "rat porpoise" in the Lower Antilles.
Like the other Sperm Whales the Dwarf Sperm Whale has a spermaceti organ in its forehead. Like the Pygmy Sperm, the Dwarf is able to expel a dark reddish substance when frightened or attack – possibly to put off any predators.
Dwarf Sperm Whales are usually solitary creatures but have occasionally been seen in small groups. They feed mainly on squid and crab.
Population and distribution: The Dwarf Sperm Whale prefers deep water but is still more coastal than the Pygmy Sperm. Its favourite 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 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.
Human interaction: The Dwarf Sperm Whale has been actively hunted by commercial whalers. Occasion harpoon kills are made by Indonesian and Japanese fishermen. As the Dwarf Sperm Whale is more coastal than the Pygmy it may be more vulnerable to human interference such as fishing and pollution. No data exists at this time as to whether such activities are threatening the long-term survival of the species.
References: Pygmy and Dwarf Sperm Whales by Donald F. McAlpine in Encyclopedia of Marine Mammals pp. 1007-1009
Whales Dolphins and Porpoises, Mark Carwardine, Dorling Kindersley Handbooks
National Audubon Society Guide to Marine Mammals of the World, Reeves, Stewart, Clapham and Powell