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

There has been debate and differing opinion as to the correct classification of the pygmy and dwarf sperm whales (see sperm whale family for details). The two were widely considered to be the same species, until 1966, when a scientist at the Smithsonian Institution definitively identified them as separate species. The pygmy sperm whale was first named by Henri Marie Ducrotay de Blainville in 1838.

The pygmy sperm whale is not much larger than many dolphins. They are about 1.2 metres (3 ft 11 in) at birth, growing to about 3.5 metres (11 ft) at maturity. Adults weigh about 400 kilograms (880 lb). The underside is a creamy, occasionally pinkish, colour and the back and sides are a bluish grey; there is, however, considerable intermixing 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. There is a whitish marking, often described as a "false gill", behind each eye.

The sperm whale (Physeter macrocephalus) is a marine mammal species, order Cetacea, a toothed whale (odontocete) having the largest brain of any animal. The name comes from the milky-white waxy substance, spermaceti, found in the animal's head. The sperm whale is the only living member of genus Physeter. The now outdated synonym Physeter catodon refers to the same species. It is one of three extant species in the sperm whale superfamily, along with the pygmy sperm whale and dwarf sperm whale.

A mature male can grow to 20.5 metres (67 ft) long. It is the largest living toothed animal. For large males, the head can represent up to one-third of the animal's length. It has a cosmopolitan distribution across the oceans. The species feeds primarily on squid but to some extent on fish, diving as deep as 3 kilometres (9,800 ft), which makes it the deepest diving mammal. Its diet includes giant squid and colossal squid. The sperm whale's clicking vocalization is the loudest sound produced by any animal. The clicking is used for sonar and may also be used for other purposes. These whales live in groups called social units. Units of females and their young live separately from sexually mature males. The females cooperate to protect and nurse their young. Females give birth every three to six years, and care for the calves for more than a decade. The sperm whale has few natural predators, since few are strong enough to successfully attack a healthy adult; orcas attack units and are capable of killing the calves. The sperm whale can live for more than 70 years.