What is a pygmy sperm whale?

When the words “sperm whale” come up, the image that follows at once is the giant of the deep, 18 metres and 50 tonnes, able to dive to 2,000 metres. Yet there are two other sperm whale species, much smaller, much more discreet and largely unknown to the public at large: the pygmy sperm whale (Kogia breviceps) and the dwarf sperm whale (Kogia sima). The first is one of the most mysterious cetaceans in the ocean.

A cetacean of modest size

The pygmy sperm whale measures between 2.7 and 3.4 metres long and weighs 320 to 400 kilograms. Set against the sperm whale (Physeter macrocephalus), it is a featherweight. Its general outline nonetheless recalls that of its giant cousin: a massive head, a streamlined body, a dark back and a pale belly.

The detail that stands out most is its short, square head, hence its scientific name breviceps (“short head”). That head is proportionally enormous: it accounts for close to one sixth of the total body length. Like its giant cousin, it houses a spermaceti organ, though on a reduced scale. The precise role of this organ in the pygmy sperm whale is still debated: perhaps linked to buoyancy, perhaps to echolocation, perhaps to both.

A further distinctive feature: its single blowhole is, as in the sperm whale, set to the left and towards the front, an anatomical character shared by all physeterids. This asymmetry is one of the most distinctive evolutionary signatures of the family.

A specialised dentition

Like all sperm whales, the pygmy sperm whale has functional teeth only in the lower jaw. Its upper jaw carries nothing but vestigial, unerupted teeth. Its lower jaw is narrow, almost underdeveloped compared with the rest of the skull, which gives it that curious profile in which the mouth looks very small against the head.

Those teeth are perfectly suited to its specialised diet: cephalopods mainly, supplemented by a few fish and crustaceans. Like the sperm whale, it hunts on deep dives, but at more modest depths, generally between 200 and 1,000 metres.

A global distribution, complete discretion

The pygmy sperm whale is present in every temperate and tropical ocean in the world, but it is so inconspicuous that most records come from stranded animals rather than from encounters at sea. It is generally met alone or in very small groups (2 to 6 individuals), and it keeps away from most vessels.

Its outline at the surface is so unobtrusive that it can be confused with other small cetaceans, or even with a shark (because of its rounded dorsal fin). It is one of the least studied cetaceans in the world, despite its wide distribution.

The defensive “sperm whale ink”

The pygmy sperm whale has an extraordinary defensive feature, a rare one among mammals: it can release a reddish faecal fluid when threatened. This dark “cloud”, released through the rectum, acts as a protective ink that darkens the water and lets the animal get away.

It is the biological equivalent of cephalopod ink, a convergent mechanism that shows how similar evolutionary solutions can emerge in very different lineages faced with comparable selective pressures. This defensive ability is unique among cetaceans and illustrates how singular the pygmy sperm whale is.

A species vulnerable to human activities

The conservation status of the pygmy sperm whale remains poorly assessed, for want of sufficient data. The IUCN lists it as “Data Deficient”. Several threats have nonetheless been identified: ingestion of plastic debris (necropsies of stranded animals frequently reveal plastic bags mistaken for cephalopods), bycatch in certain fishing gear, and noise pollution, which disturbs its echolocation.

Every stranded pygmy sperm whale is a valuable scientific opportunity for biologists: it is one of the rare chances to study this enigmatic cetacean. Its very existence, a few centimetres away from the giant that is the sperm whale but following an entirely different life strategy, illustrates the fascinating diversity of the physeterid family.

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