How Sperm Whales Hunt in the Deep Ocean

A sperm whale’s head is unlike that of any other animal, and much of it is given over to sound production rather than brain space. Inside sits the spermaceti organ, a large chamber of oil that whalers once prized and that scientists now believe helps focus and amplify the clicks the whale produces for echolocation. Air is passed back and forth through structures in the nasal passages, generating a sharp click that is then shaped and directed forward by the spermaceti organ and a lower chamber known as the junk. The result is one of the loudest sounds produced by any animal, aimed like a beam into the water ahead of the whale.

This adaptation exists for one reason above all others, which is to find food in an environment where eyesight is close to useless. Below a few hundred metres, sunlight barely penetrates, and by a thousand metres there is none at all. A sperm whale does not need to see a squid to catch it. It needs only to hear the echo bouncing back.

As a sperm whale descends, it produces a steady rhythm of loud clicks, sometimes described as sounding like a slow-ticking clock. These are known as usual clicks, and they are used for long-range echolocation, sweeping the water ahead and around the whale for the echo of a returning signal. Researchers have found that these clicks are directional and powerful enough to detect prey at a considerable distance, giving the whale time to adjust course as it hunts.

Once a whale detects something promising, the pattern of clicking changes. The clicks speed up dramatically into what is known as a creak, or buzz, similar in principle to the terminal buzz produced by an echolocating bat closing in on an insect. These buzzes typically last between one and five and a half seconds and occur at depths of roughly 300 to 1,200 metres, marking the final, fast-paced phase of an attempted capture. Data collected from tagged whales suggests that a sperm whale may attempt as many as 17 prey captures during a single deep dive, each one preceded by this acceleration in clicking as the whale closes the distance.

Sperm whales are extraordinary breath-hold divers, though the numbers vary depending on the individual and the purpose of the dive. Typical foraging dives last around 45 minutes and reach depths of 1,000 to 2,000 metres, though sperm whales are capable of diving considerably deeper and holding their breath for close to two hours in exceptional circumstances. Between dives, a whale spends time at the surface breathing and recovering, building up the oxygen stores it will need for the next descent.

This is a costly way to feed. Every dive represents a significant investment of energy and time, which is presumably why sperm whales have evolved such a sophisticated sonar system in the first place. A hunting method this expensive only makes sense if it reliably locates enough squid to justify the dive, and for sperm whales, whose diet is dominated by squid of various sizes, it evidently does.

The appetite involved is considerable. Estimates generally put a sperm whale's daily intake at somewhere around three per cent of its body weight, which for an adult female of fifteen tonnes works out at roughly 450 to 500 kilograms of squid a day, and rather more again for a large male. Since most of that is made up of squid averaging 20 kilograms each, a whale may need to catch several hundred individual animals in a day, which explains why it makes ten or more deep dives in every twenty-four hours and spends the greater part of its life either hunting or recovering between hunts. Taken across the species as a whole, sperm whales are thought to remove tens of millions of tonnes of squid from the deep ocean each year, a figure comparable to the entire global fishing catch. It is worth bearing in mind while watching a whale rest at the surface off Dominica, because the calm is temporary and the animal is almost certainly about to go back down to forage.

Contrary to what many people assume, the fight between a sperm whale and a squid is not always one-sided. Squid cannot hear the low-frequency clicks a whale produces, which removes any early warning the squid might otherwise have. Once contact is made, however, the squid does not go quietly. Its eight arms and two long feeding tentacles are lined with hundreds of suction cups, many of which carry sharp, chitinous rings capable of gripping tightly under the extreme pressure found at depth.

The evidence of these encounters is written on the skin of sperm whales themselves. Circular scars, often clustered around the head and jaw, are a common sight on individuals that have hunted squid over many years, and they are widely understood to be the marks left by suckers during a struggle before the whale subdues its prey. Where a whale has encountered a colossal squid rather than a giant squid, the scarring can look markedly different, since the colossal squid’s suckers are armed with hooks rather than smooth rings. These scars are, in effect, a visible record of a contest that plays out far beyond the reach of any camera, most of the time.