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External appearance

The sperm whale is the largest toothed whale and is among the most sexually dimorphic of all cetaceans. Both sexes are about the same size at birth, but mature males are typically 30% to 50% longer and three times as massive as females.

Newborn sperm whales are usually between 3.7 and 4.3 meters (12 and 14 ft) long. Female sperm whales are sexually mature at 8 to 9 meters (26 to 30 ft) in length, whilst males are sexually mature at 11 to 12 meters (36 to 39 ft). Female sperm whales are physically mature at about 10.6 to 11 meters (35 to 36 ft) in length and generally do not achieve lengths greater than 12 metres (39 ft). The largest female sperm whale measured up to 12.3 meters (40 ft) long, and an individual of such size would have weighed about 17 tonnes (19 short tons). Male sperm whales are physically mature at about 15 to 16 meters (49 to 52 ft) in length, and larger males can generally achieve 18 to 19 meters (59 to 62 ft). An 18 meters (59 ft) long male sperm whale is estimated to have weighed 57 tonnes (56 long tons; 63 short tons). By contrast, the second largest toothed whale (Baird's beaked whale) measures up to 12.8 meters (42 ft) and weighs up to 14 tonnes (15 short tons).

There are occasional reports of individual sperm whales achieving even greater lengths, with some historical claims reaching or exceeding 80 feet (24 m). One example is the whale that sank the Essex (one of the incidents behind Moby-Dick), which was claimed to be 85 feet (26 m). However, there is disagreement as to the accuracy of some of these claims, which are often considered exaggerations or as being measured along the curves of the body.

An individual measuring 20.7 metres (68 ft) was reported from a Soviet whaling fleet near the Kuril Islands in 1950 and is cited by some authors as the largest accurately measured. It has been estimated to weigh 80 tonnes (79 long tons; 88 short tons). In a review of size variation in marine megafauna, McClain and colleagues noted that the International Whaling Commission's data contained eight individuals larger than 20.7 metres (68 ft). The authors supported a 24-metre (79 ft) male from the South Pacific in 1933 as the largest recorded. However, sizes like these are rare, with 95% of recorded sperm whales below 15.85 metres (52.0 ft).

In 1853, one sperm whale was reported at 62 feet (19 m) in length, with a head measuring 20 feet (6.1 m). Large lower jawbones are held in the British Natural History Museum and the Oxford University Museum of Natural History, measuring 5 metres (16 ft) and 4.7 metres (15 ft), respectively.

The average size of sperm whales has decreased over the years, probably due to pressure from whaling. Another view holds that exploitation by overwhaling had virtually no effect on the size of the bull sperm whales, and their size may have actually increased in current times on the basis of density dependent effects. Old males taken at Solander Islands were recorded to be extremely large and unusually rich in blubbers.

The sperm whale's unique body is unlikely to be confused with any other species. The sperm whale's distinctive shape comes from its very large, block-shaped head, which can be one-quarter to one-third of the animal's length. The S-shaped blowhole is located very close to the front of the head and shifted to the whale's left. This gives rise to a distinctive bushy, forward-angled spray.

The sperm whale's flukes (tail lobes) are triangular and very thick. Proportionally, they are larger than that of any other cetacean, and are very flexible. The whale lifts its flukes high out of the water as it begins a feeding dive. It has a series of ridges on the back's caudal third instead of a dorsal fin. The largest ridge was called the 'hump' by whalers, and can be mistaken for a dorsal fin because of its shape and size.

In contrast to the smooth skin of most large whales, its back skin is usually wrinkly and has been likened to a prune by whale-watching enthusiasts. Albinos have been reported.