Pygmy sperm whale

The pygmy sperm whale (Kogia breviceps) is one of two extant species in the family Kogiidae in the sperm whale superfamily. They are not often sighted at sea, and most of what is known about them comes from the examination of stranded specimens.

The pygmy sperm whale was first described by naturalist Henri Marie Ducrotay de Blainville in 1838. He based this on the head of an individual washed up on the coasts of Audierne in France in 1784, which was then stored in the Muséum d'histoire naturelle. He recognized it as a type of sperm whale and assigned it to the same genus as the sperm whale (Physeter macrocephalus) as Physeter breviceps. He noted its small size and nicknamed it "cachalot a tête courte"–small-headed sperm whale; further, the species name breviceps is Latin for "short-headed". In 1846, zoologist John Edward Gray erected the genus Kogia for the pygmy sperm whale as Kogia breviceps, and said it was intermediate between the sperm whale and dolphins.

In 1871, mammalogist Theodore Gill assigned it and Euphysetes (now the dwarf sperm whale, Kogia sima) to the subfamily Kogiinae, and the sperm whale to the subfamily Physeterinae. Both have now been elevated to the family level. In 1878, naturalist James Hector synonymized the dwarf sperm whale with the pygmy sperm whale, with both being referred to as K. breviceps until 1998.

The pygmy sperm whale is not much larger than many dolphins. They are about 1.2 m (3 ft 11 in) at birth, growing to about 3.5 m (11 ft) at maturity. Adults weigh about 400 kg (880 lb). The underside is a creamy, occasionally pinkish colour and the back and sides are a bluish grey; however, considerable intermixing occurs between the two colours. The shark-like head is large in comparison to body size, given an almost swollen appearance when viewed from the side. A whitish marking, often described as a "false gill", is seen behind each eye.

The lower jaw is very small and slung low. The blowhole is displaced slightly to the left when viewed from above facing forward. The dorsal fin is very small and hooked; its size is considerably smaller than that of the dwarf sperm whale and may be used for diagnostic purposes.

Like its giant relative, the sperm whale, the pygmy sperm whale has a spermaceti organ in its forehead (see sperm whale for a discussion of its purpose). It also has a sac in its intestines that contains a dark red fluid. The whale may expel this fluid when frightened, perhaps to confuse and disorient predators.

Dwarf and pygmy sperm whales possess the shortest rostrum of current day cetaceans with a skull that is greatly asymmetrical.

Pygmy sperm whales have from 50 to 55 vertebrae, and from 12 to 14 ribs on either side, although the latter are not necessarily symmetrical, and the hindmost ribs do not connect with the vertebral column. Each of the flippers has seven carpals, and a variable number of phalanges in the digits, reportedly ranging from two in the first digit to as many as 10 in the second digit. No true innominate bone exists; it is replaced by a sheet of dense connective tissue. The hyoid bone is unusually large, and presumably has a role in the whale's suction feeding.