The sperm whale (Physeter macrocephalus) is often pictured in 19th-century images of whaling, and was made famous by Herman Melville in his novel Moby Dick. In terms of survival, this species is one of the more successful: it has survived for millions of years and remains one of the most numerous of the large whales, despite centuries of hunting.
The sperm whale has a huge square head, with a small underslung jaw. Its skin is dark grey to dark brown and corrugated. In place of a dorsal fin, it has a hump and a series of knuckles. It has triangular flukes which it raises before diving. The blowhole is off to the left and when the whale blows, the blast shoots forward at a 45º angle.
The sperm whale was named by the early whalers who discovered whitish oil in the whale’s head and thought the fluid looked like semen. It is possible that this spermaceti oil helps to focus the sound emitted during echolocation, and to stun the whale’s prey.
The sperm whale is by far the largest of the toothed whales, and the size difference between males and females is much more marked than in any other whale species. Males are about 18 metres long and weigh 32–45 tonnes, and are up to 40% longer and 300% heavier than females. The sperm whale’s brain weighs almost 10 kilograms, and is the heaviest of any animal.
Sperm whales are found at Kaikōura in the South Island, to the west of Stewart Island, off East Cape and North Cape, and in pockets to the west of New Zealand. In the 19th century the major hunting grounds were north-east of New Zealand near the Kermadec Islands.
The sperm whale captures its prey by diving deep into the ocean’s trenches. It reaches depths of 3,000 metres, descending at a rate of over 100 metres a minute. The whale can stay there up to an hour and, in the dark, find its prey by echolocation. On surfacing, the whale takes 45–50 breaths to re-oxygenate.
The social behaviour of sperm whales is unusual. Females and males live mostly separate lives. Small groups of up to 50 closely related whales, usually consisting of several females and immature whales of both sexes, live together for up to 10 years. Their need for cooperation is vital: when a cow spends an hour away on a deep fishing expedition, her vulnerable calf requires babysitting. Young males leave the whale nursery school at between 7 and 10 years of age to form bachelor schools before starting to breed at about 25. As adults, the males become increasingly solitary and begin to feed in Antarctic waters.
‘Schools and schoolmasters’
Herman Melville gave this title to one chapter of Moby Dick. He explained that a school of female sperm whales was usually accompanied by one male, who would swim at its rear and was known by the whalers as the ‘schoolmaster’.
Sperm whales at Kaikōura
Kaikōura, on the east coast of the South Island, is the only place in the world with a deep-water canyon close to shore that attracts sperm whales. Just 4.5 kilometres from shore, the continental shelf suddenly falls away and the ocean plunges 1,000 metres. Here the whales dive for the famed giant squid, as well as groper and ling.
Kaikōura is a foraging ground for about 80 bachelor whales. About half of those that have been identified remain at Kaikōura for more than one season. The others move on. These are probably older males, passing through on their way to Antarctic waters, or expelled by the younger whales.
While Kaikōura sperm whales eat squid, fish also make up a significant proportion of their diet. Studies in the 1960s of sperm whales killed in the Cook Strait region showed that squid accounted for 65% of their food, and groper and ling the remaining 35%. Depending on their size, sperm whales eat anywhere between 400 and 1,000 kilograms of food a day, or 3% of their body weight.
Pygmy sperm whale
The pygmy sperm (Kogia breviceps) is a small grey whale up to 4 metres long, which is more often seen after stranding than at sea. It too has spermaceti oil in its head. The pygmy is hard to distinguish from the very much rarer dwarf sperm whale (Kogia simus).