Whale of the Month - Sperm Whale

Species fact file

Common names: sperm whale, cachalot, pot whale, spermaceti whale

Icelandic name: búrhvalur

Scientific name: Physeter macrocephalus

Family: Physeteridae (sperm whales)

Max length: 11m (females), 20m (males)

Distribution: All oceans, predominantly offshore

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Sperm whales are the largest toothed animal on Earth. They are unusual among cetaceans: the only member of their family, sperm whales appear to have separated from other toothed whales very early in their evolution. They have a distinctively huge, square head with a comparatively small mouth. They only have teeth on their lower jaw, which apparently play little part in the capture of prey since healthy, well-fed sperm whales have been found with no teeth or even missing their lower jaw.

The sperm whale’s massive head contains a brain which is commonly cited as the largest brain of any animal. In fact, they are tied for this position with male killer whales – even though male sperm whales are almost double the length of male killer whales, and more than 6 times their mass. Compared to other odontocetes, sperm whales actually have quite small brains relative to their body size. We currently have little information on sperm whale intelligence other than what can be gleaned from their ecology; in particular, their social system. Sperm whales have one of the most sophisticated and complex societies in the whole animal kingdom, suggesting that their relatively small brain size has not hindered their cognitive abilities.

Most of the space in their head is taken up not by their brain but by their impressive nasal complex, the spermaceti organ, and the so-called ‘junk’. Most of these organs aid sperm whales in their search for something to eat.

Sperm whales can dive to over 1000m and spend more than an hour underwater. Their main prey is squid, including the giant squid – a true leviathan of the deep ocean and the real-life inspiration of the legend of the kraken. However, sperm whales are not picky eaters; these opportunistic hunters will take almost anything that comes their way, and in Iceland they seem to feed mostly on fish. As sperm whales hunt in the depths of the ocean with no natural light to guide them, they rely almost entirely on their natural sonar system, echolocation, to find their food.

Echolocation clicks are created in an organ at the front of the head and directed back through the spermaceti, then bounce off an air sac in front of the skull and are beamed through the junk into the water in front of the whale. Their echolocation clicks are incredibly loud and create a narrow, focused beam of sound which gives the whale detailed information on their environment.

Surprisingly, sperm whales have very good eyesight which is well adapted to the underwater environment. Their eyes are flattened rather than round and the lens is spherical, which allows them to focus underwater, and they also have excellent vision in low light. Sperm whales have clusters of special, giant nerve cells which need less light entering the eye to send a nerve impulse to the brain.

Male and female sperm whales live very different lives. For no other cetacean is there such a dramatic difference in body size between the sexes: adult females are about 11m long and weigh 14 tonnes, but the average male will be 16m and 45 tonnes at maturity. There are reports of much longer and much heavier males, although there is some debate over the reliability of these measurements; even so, the maximum size of a male sperm whale is generally cited as being between 18 and 20 metres long and up to 80 tonnes.

As well as this, male and female sperm whales spend most of their lives in completely different parts of the world. Females live in tropical and warm temperate regions year-round. They form tight-knit social groups with other females and their calves – this social structure likely evolved as a means of protecting their calves from hungry killer whales. Within these social groups, adult females share the role of caring for calves, sometimes even providing milk for calves that are not their own. Since young calves cannot dive as deep or as long as their mothers, the females stagger their dives so that there is always at least one adult accompanying the calves at the surface.

Adult males live more solitary lives in cold temperate, sub polar and polar seas. They are seldom seen in groups, but do form strong social bonds with other males which can last for many years. Males migrate to warmer regions in order to mate, but this isn’t the same kind of regular, whole-population migrations that baleen whales do. We still don’t fully understand what factors trigger male sperm whales to return to the tropics to breed.

Sperm whales have impressively slow birth and growth rates. Females reach maturity at about 10 years old and give birth on average every five years, but often with a much greater gap in between calves. Gestation lasts a staggering 16 months – longer than any mammal except the African and Asian elephants. Calves remain strongly bonded to their mothers until very late in life and females may care for multiple offspring at once, often continuing to provide milk to adult offspring. Males may leave their mother’s social unit at anywhere between 4 and 20 years old and although they generally reach maturity by 20, most do not reproduce until they are in their late twenties.

Sperm whales were hunted extensively during the peak of commercial whaling and were almost wiped out, but have been protected from whaling for several decades and nowadays are less at risk than most other large whales – their prey is not currently exploited by human fisheries, and there are probably over 100,000 sperm whales worldwide. But few places remain on Earth that are completely untouched by human activity and as we encroach further into the open oceans, sperm whales are becoming increasingly threatened by ship strikes, entanglement, plastic pollution and noise. Because of their naturally low reproductive rates, sperm whale populations recover very slowly, and the threats they face now are only going to become more intense with time.

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Learn more about...

How communal care of calves is a central part of sperm whale society

Sperm whale diving behaviour and use of echolocation