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 family Kogiidae and genus Kogia. The two species were not regarded as separate until 1966. 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 species of dolphin. Specimens often make slow, deliberate movements with little splash or blow and usually lie motionless when at the sea's surface. Consequently, they are usually observed only in very calm seas (Beaufort 0–1).
The dwarf sperm whale is similar in appearance and behavior to its relative, 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 possible yellowish vein-like streaks. A white false gill is 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, sharp teeth (none to six in the upper jaw and 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 has a spermaceti organ in its forehead. Like the pygmy, it 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 pods.
The brain of the dwarf sperm whale is roughly 0.5 kg in mass.
Dwarf sperm whales feed mainly on squid and crab.
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.
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 2017 a live specimen was for the first time ever sighted in the North Sea (Belgium, Ostend). 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 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 1.0 m long. Males reach puberty at 2.0 m 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 exist 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. 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).