Sperm Whale  |  

Sperm Whales (Physeter macrocephalus) are the largest of the odontocetes (toothed whales) and the most sexually dimorphic cetaceans, with males considerably larger than females. Adult Sperm Whale females may grow to lengths of 11 m and weigh 15 tonnes, while adult males reach about 16 m and may weigh as much as 45 tonnes. The Sperm Whale is distinguished by its extremely large head, which takes up 25–35% of its total body length. The lower jaw is narrow and underslung. It is the only living cetacean that has a single blowhole asymmetrically situated on the left side of the head near the tip. Sperm Whales have the largest brain of any animal, however, compared to their large body size, the brain is not exceptional in size. Sperm Whales are mostly dark grey to brownish-grey. The interior of the mouth is often bright white. Some whales also have white patches on the belly. Sperm Whale flippers are paddle-shaped and small compared to the size of the body, and their flukes are very triangular in shape with a nearly straight trailing edge, rounded tips, and a deep notch. Sperm Whales have a small dorsal fin that is low, thick, and usually rounded, while a series of bumps, or crenulations, on the dorsal ridge of the tail stock tend to be more prominent in females. The body has a corrugated, or shrivelled, appearance posterior to the head. There are between 20–26 large conical teeth, up to 20 cm long, in each side of the lower jaw of the Sperm Whale. The teeth in the upper jaw rarely erupt and are often considered to be vestigial. It appears that teeth may not be necessary for feeding, since they do not break through the gums until puberty, if at all, and toothless but otherwise healthy Sperm Whales have been observed. Sperm Whales, like other toothed whales, are gregarious and live in groups of up to 50 individuals, although male Sperm Whales are sometimes solitary in high latitudes (above 40° N). The average Sperm Whale school sizes contain about 25 animals, although aggregations of such schools have been reported sometimes apparently numbering several thousand individuals. For example, a remarkable report is given in of ‘animals spouting from horizon to horizon…’ over seven to eight hours in a stream 70 miles wide in the Tasman Sea in February 1978. The social structure of Sperm Whales involves seasonal and geographical segregation of the populations by age, social and sexual state. Several types of schools can be recognised: matriarchal nursery schools; juvenile schools; bachelor schools; bull schools and lone bulls. Sperm Whale nursery schools of adult females and their offspring are found in lower latitudes all year round. Nursery schools are socially very tightly bonded, with individual Sperm Whales remaining in the same school for many years. On average, 10 to 30 mature females and their offspring are found within a Sperm Whale nursery group, although the group may be spread out over a large area. Female Sperm Whales are believed to remain in the same school throughout their life, but males leave to form bachelor groups that comprise older pubescent males and sexually, but not socially, mature males, all of similar size and age. Once they attain social maturity and can compete for access to females, male Sperm Whales leave these bachelor herds to associate with breeding schools, either solitarily or in small groups of usually less than six animals. Large bachelor Sperm Whales migrate seasonally to higher latitudes, probably because of better feeding conditions and reduced competition for food. Sperm Whales perform aerial behaviours including breaching and lobtailing. These behaviours often occur in a social context and when groups are splitting or joining, although single whales also breach and lobtail.