What does the sperm whale eat? Can it really eat a giant squid?

The battle between the sperm whale and the giant squid fed the imagination of sailors and writers for centuries. Jules Verne devotes a famous chapter of Twenty Thousand Leagues Under the Seas to it. In Moby Dick, Herman Melville evokes those “gelatinous masses” that sometimes rise to the surface after being disgorged by a dying sperm whale. For a long time nothing precise was known: all that was available were scars on the skin of sperm whales, perfect circles, the characteristic marks of squid suckers, and the chitinous beaks of giant squid found in stomachs. The first photograph of a live giant squid in the wild dates only from 2004 (Kubodera & Mori), at 900 metres depth off the Bonin Islands. Science has since filled in a fascinating picture.

An exclusive, and well documented, diet

The sperm whale consumes about one tonne of food a day, a few per cent of its body mass. More than 95% of that diet consists of cephalopods. The rest is made up of deep-sea fish, grenadiers, deep-water anglerfish, sleeper sharks, and incidentally a few crustaceans.

The great majority of the squid consumed are modest in size: between one kilo and a few kilos, living in shoals at depths of between three hundred and one thousand two hundred metres. They are abundant prey, easy to catch, and they allow a satisfactory energy return. But the sperm whale stomachs that have been analysed also regularly contain the chitinous beaks of much larger squid, sometimes reaching several tens of kilos.

How is this known? Because everything of the squid is digested apart from its beak. Chitin, a protein also found in the shell of insects and in the external skeleton of arthropods, is indigestible for mammals. The beak therefore stays intact in the sperm whale’s stomach for weeks, even months, building up into a deposit that then goes on to form ambergris. In studies on stranded or (formerly) hunted sperm whales, scientists count and measure these beaks. An adult male sperm whale may hold several hundred beaks in its stomach, a record of the last weeks of hunting.

Architeuthis: the giant squid of the textbooks

The giant squid, Architeuthis dux, is the best known of the giant cephalopods. It commonly reaches eight to ten metres with the tentacles extended, exceptionally thirteen metres. The largest documented specimens weigh around two hundred and fifty kilos. It lives mainly between four hundred and one thousand metres depth, in all the temperate and tropical waters of the world.

For a long time Architeuthis was known only from stranded carcasses or from specimens disgorged by dying sperm whales. The Kubodera and Mori photograph of 2004, followed by the first video in the wild in 2012 by NHK and Discovery, at last made it possible to observe the animal alive. The squid appears surprisingly slow and fragile there for an animal of that size: its tentacles coil around prey (fish, other squid) with a targeted violence, but its body remains relatively static. It is an ambush predator, not a fast swimmer.

The colossal squid, more impressive still

Less well known but more massive, the colossal squid (Mesonychoteuthis hamiltoni) lives in Antarctic waters. The largest whole specimen ever recovered, in 2007 by a New Zealand fisherman, weighed 495 kilos for a mantle of about 4.2 metres. With the tentacles extended it reached ten metres. Its eyes, enormous, measure about twenty-seven centimetres across, the largest in the animal kingdom, larger than a basketball. This is probably an adaptation for detecting the backlit silhouettes of sperm whales descending to hunt them.

In addition to ordinary suckers, the colossal squid has swivelling hooks at the tip of its tentacles, which allow it to grip its prey firmly. The southern sperm whales that hunt them carry particularly deep scars, evidence of the resistance these squid put up against capture.

The fight itself: what is known of it

No encounter has ever been filmed at great depth. But a good part of the scene can be reconstructed by triangulating between scars, stomach contents and acoustic profiles recorded on sperm whales fitted with DTAG tags (Mark Johnson and his team at Woods Hole, in the 2000s).

The sperm whale descends to a thousand metres or more, diving in silence on pure echolocation. When it detects a large squid, the rhythm of its clicks shifts into buzz mode: several hundred clicks per second, the signature of the final attack phase. At that depth the darkness is total. The squid does not see the sperm whale coming. It may perhaps feel its vibrations in the water. At the moment of seizure its tentacles coil around the attacker’s head, hence the circular scars that are so distinctive.

The asymmetry of strength and mass is heavily in the sperm whale’s favour. An adult male of 16 metres weighs 45 tonnes; the largest squid barely exceeds half a tonne. The sperm whale does not chew its meal: it sucks in the squid with a violent movement of the tongue and the pharynx, and swallows it whole. The squid is then digested in the sperm whale’s multi-chambered stomach, which can in this way process in parallel several prey items taken during a single dive sequence.

Modern conflicts with fishing

In the Gulf of Alaska and around the Azores, some sperm whales have learned to raid commercial longlines. They follow the boats that set their lines for halibut, wait for the fish to be hooked, then come and tear them off the hooks before the fisherman hauls his line in. This behavioural innovation, documented since the 1990s, has spread between populations by learning. Fishermen have had to develop anti-depredation devices (“masks”, protective nets), which are only partly effective. The conflict illustrates both the adaptive intelligence of sperm whales and the complexity of contemporary interactions between humans and cetaceans.

Sources

• Clarke, M.R. (1996). “Cephalopods as prey. III. Cetaceans”. Philosophical Transactions of the Royal Society B, 351.
• Kubodera, T. & Mori, K. (2005). “First-ever observations of a live giant squid in the wild”. Proc. R. Soc. B, 272.
• Watwood, S.L. et al. (2006). “Deep-diving foraging behaviour of sperm whales (Physeter macrocephalus)”. Journal of Animal Ecology, 75.
• Rosa, R., Sundberg, F. et al. (2017). “Recent advances in cephalopod-cetacean interactions”. Marine Mammal Science.
• Santos, M.B. et al. (2002). “Sperm whales Physeter macrocephalus in the Mediterranean Sea: An updated review”. Journal of the Marine Biological Association of the UK.

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