What do sperm whales eat, and how do we know?

The sperm whale (Physeter macrocephalus) is one of the most powerful predators on the planet. At the top of the deep-sea food chain, it consumes close to a tonne of food a day, the mass equivalent of an average car. Its diet, long mysterious for want of direct observation at great depth, is now well documented thanks to several converging sources: stomach content analysis, tissue isotopes, biomarkers and, more recently, acoustic tags attached to the animals.

A diet made up almost entirely of cephalopods

The sperm whale is described as teuthophagous, a term derived from the Greek teuthis (squid). Cephalopods, mainly squid and to a lesser extent octopus, make up more than 80% of its food intake. This is a remarkable dietary specialisation for a predator of this size, and it has profoundly shaped its anatomy: an enormous head filled with acoustic structures for hunting in the dark, an extreme diving capacity, teeth used mainly for gripping.

Most of the prey are medium-sized squid, generally between a few hundred grams and a few kilograms, belonging to families little known to the general public: Octopoteuthidae, Histioteuthidae, Onychoteuthidae, Architeuthidae… These squid live in the mesopelagic and bathypelagic layers, between 300 and 1,500 metres deep, where the sperm whale descends each day to hunt them.

The giant squid myth: true or not?

Accounts of battles between sperm whales and giant squid have fed marine mythology since the 19^th century. Is there anything to them? Yes, but they do not account for the bulk of the diet. Sperm whales occasionally eat two remarkable species. The giant squid (Architeuthis dux), which can reach 13 metres with tentacles extended and about 275 kg. And the Antarctic colossal squid (Mesonychoteuthis hamiltoni), more massive still, up to 14 metres and more than 500 kg.

The evidence: the well-known circular scars on the skin of sperm whales, the characteristic marks of the hooked suckers of these giant squid, together with the indigestible chitinous beaks found in their stomachs. But these exceptional prey remain rare in the diet, the sperm whale spends most of its time hunting far more modest squid.

The menu of large solitary males

Large adult males, which travel alone towards high latitudes (the Norwegian Sea, Alaska, the Antarctic), show a more varied diet than that of females and young animals. It includes more fish: tuna, cod, deep-sea anglerfish, rays, barracuda. More surprisingly, these large males sometimes take sharks of several species: sleeper shark, basking shark, blue shark, and even megamouth shark in the most voracious cases.

More rarely, remains of crustaceans and even of small marine mammals have been found. This dietary flexibility in males is probably explained by their greater size and strength, which allow them to capture a wider range of prey, and by the conditions of the cold waters they frequent, where cephalopods may be less abundant in some seasons.

How do we know all this?

The investigation of sperm whale diet rests on four complementary methods. The oldest is stomach content analysis of stranded or (historically) hunted animals. The sperm whale does not digest the chitinous beaks of squid: these beaks accumulate in its forestomach for weeks. By counting and identifying them, the animal’s diet over its last weeks can be reconstructed. A single stomach may contain several hundred beaks.

The second method is isotope analysis of tissues (skin, blubber, teeth). Carbon and nitrogen isotope ratios vary with the prey consumed and their trophic level. These analyses give an average dietary signature over several months, and over several years in the case of teeth.

More recently, DTAG tags have transformed the field. These miniaturised sensors, attached temporarily to the animal by suction cup, record continuously the sounds emitted and received, the depth and the acceleration. A hunt can literally be “listened to”: the echolocation clicks shift into a rapid buzz as the animal closes on prey, and then the characteristic sound of the capture is heard. Mark Johnson and his team at Woods Hole have in this way documented dozens of real hunting sequences in detail.

Finally, environmental DNA (eDNA) and the analysis of faeces, when they are found floating at the surface, make it possible to identify the species consumed precisely, by genetic sequencing. Combined, these approaches now draw an extremely detailed picture of a diet that was still largely a matter of speculation thirty years ago.

---

Further reading