Why the sperm whale has a huge head

The profile of the sperm whale, Physeter macrocephalus, is unmistakeable. The creature’s huge, blunt head makes up at least a third of its body mass. Part double-decker bus, part whale, it must burn a huge amount of energy as it shoves through the water. What purpose could it serve for the animal? Why is it so?

Early whalers, to their great delight, discovered the sperm whale’s head to be a treasure trove of the finest quality oil, called spermaceti (literally whale seed). Some 2,000 litres of it lay in a huge sac called the spermaceti organ. Unlike the oil the whalers got from rendering the blubber, spermaceti oil would not go rancid on storage and remained sweetly scented. It was used for cosmetics, pharmaceuticals and as a lubricant for clocks and watches.

Spermaceti was a godsend for all these human needs, but what function did it serve for the whale?

For more than 100 years it was believed that the sac of oil helped the whale adjust its buoyancy when it dived into the depths after its favourite prey – the giant deep-sea squid. The idea was that by reducing the blood flow to the spermaceti organ the oil would cool down, become denser and allow the whale to sink down after its prey.

It was an ingenious idea, but scientists now favour a different hypothesis. They believe the sac of oil is the focusing lens for the whale’s enormously powerful echolocation equipment.

Whalers knew that whales made complex sounds and assumed they were communicating with each other. But during World War II the US navy started using radar to detect submarines. Radar works by sending out pulses of radio waves and detecting the echoes that bounce back. If the radio waves hit an object, the sound of the echo changes. People realised that whales and bats used the same pulse and echo technique to “see” their environment.

What scientists do not agree on is just how the animals produce and aim these sounds. It is very hard to study the spermaceti organ in a living whale. That leaves dead whales to study and without the buoyancy of water, this huge sac, which almost entirely fills the whale’s head, becomes a very soft, floppy tissue making it very difficult for scientists to study its structure to determine how the sound is produced.

Whales vocalise by blowing air across structures resembling vocal cords. These lie in the head and are called monkey lips. After the monkey lips generate sound waves, the oil-filled organ in the whale’s head focuses them, researchers believe. Acting like a giant acoustic lens, muscles alter the organ’s shape to focus the sound and perhaps scan the surrounding waters.

It’s not only sperm whales that have this oil-filled organ. Think of the high-domed “forehead” of dolphins. Like all toothed whales, their domed forehead contains an oil-filled sac, which is used as part of their echolocation equipment.

But the sperm whale’s sac of oil is by far the biggest, taking up almost a quarter of its entire body, whereas in a dolphin it takes up just 5% of its body volume. The most likely reason it has evolved to such an extreme size is to help the sperm whale chase down giant squid.

As the squid adapted to live deeper in the ocean to avoid the sperm whale, the whale evolved a larger and more powerful echolocation organ to detect the squid at greater distances.

The whale’s powerful radar may be able to do more than just locate its prey. Scientists have found smaller and faster prey in the whale’s stomach, and they don’t show bite marks. How does this lumbering giant catch them?

One possibility is that the sound pulses focused through the huge head stun the fish and make them easy prey: in other words the spermaceti organ is the focus for an acoustic stun gun. Scientists are proposing to attach cameras and data recorders to the heads of the whales to test this hypothesis.