The sperm whale family or simply the sperm whales is the collective name given to three species of whale found worldwide; the Sperm Whale, in the genus Physeter, and the Pygmy Sperm Whale and Dwarf Sperm Whale, in the genus Kogia. In the past these genera have sometimes been united in the single family, Physeteridae, with the two Kogia species in a subfamily (Kogiinae), however recent practice is to allocate the genus Kogia to its own family, Kogiidae, leaving Physeteridae as a monotypic (single extant species) family, although additional fossil representatives of both families are known (see "Evolution"). The name Sperm Whale comes from sailors of whaling boats who thought that the spermaceti on the whales head was actual sperm from the reproductive system.
The Sperm Whale is the largest species of toothed whale, with adult bulls(males) growing to be about 15-18 metres (50-60 feet) long, and weighing about 45-70 tonnes. The two kogiid species are much smaller, at only around 2.5 to 3.5 metres (9-11 feet) in length, and weighing 350-500 kilograms (770-1,100 pounds).
The body of sperm whales is robustly proportioned, with paddled-shaped flippers. The lower jaw is always relatively small and thin relative to the upper jaw. The nasal bones of sperm whales are distinctly asymmetrical, with the blowhole being located on the left side of the head; in the Sperm Whale this is near the top of the head, while on the kogiids it is further forward. All species have a large number of similar, and relatively simple, teeth. In the kogiids, and sometimes also in the Sperm Whale, the teeth in the upper jaw do not erupt, and are sometimes altogether absent.
The eyes of sperm whales are unable to swivel in their sockets, and possess only a vestigial anterior chamber. It is likely that echolocation is a far more important sense to these animals than vision.
Another common characteristic is the spermaceti, a semi-liquid waxy white substance filling the 'case' or spermaceti organ in the whale's head, which plays the role of a ballast in diving and maintaining buoyancy. This is done by flushing cold water though the nose, hardening the spermaceti, to dive, and pumping warm blood to melt the spermaceti to surface. (See the individual species articles for further details). All three species dive to great depths to find food, although the Sperm Whale is believed to dive much deeper than either of the kogiids. Members of both families eat squid, fish, and even sharks.
Gestation lasts from nine to fifteen months, depending on species. The single calf remains with the mother for at least two years, before being weaned. Sperm whales do not reach full sexual maturity for several years. All species congregate together in 'pods' or herds, consisting of mostly females, calfs, and adolescent males, although these pod sizes are typically smaller in the kogiids.
The fossil record suggests that sperm whales were more common in the Miocene, where basal lineages (such as Zygophyseter and Naganocetus) existed; other fossil genera assigned to the Physeteridae include Ferecetotherium, Helvicetus, Idiorophus, Diaphorocetus, Aulophyseter, Orycterocetus, Scaldicetus, and Placoziphius, while Kogiid fossil genera include Kogiopsis, Scaphokogia, and Praekogia. The earliest kogiids are known from the late Miocene, around 7 million years ago.
The close relationship between extant Physeteridae and Kogiidae is confirmed in recent molecular studies using mitochondrial cytochrome b, ; on the basis of these analyses, their nearest relatives appear to be the Ziphiidae on one hand, and the Mysticeti and Platanistidae on the other. The last cited paper also favours the grouping of Physeteridae and Kogiidae in a single superfamily, Physeteroidea, as has sometimes previously been suggested. Bianucci & Landini, 2006 suggest that Diaphorocetus, Zygophyseter, Naganocetus and Aulophyseter pre-date the inferred split of Kogiidae and Physeteridae and thus would restrict the family Physeteridae to those genera that post-date this split (a cladistic view).