Sperm whale family

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Sperm whales
Fossil range: Oligocene to Recent
Sperm Whale, Physeter macrocephalus
Sperm Whale, Physeter macrocephalus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Suborder: Odontoceti
Families and genera

Kogiidae Gill, 1871
  Kogia
Physeteridae Gray, 1821
  Physeter

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.

Contents

[edit] Characteristics

The Sperm Whale is the largest species of toothed whale, with adult 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 in relation 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.[1]

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.[1]

Another common characteristic is the spermaceti, a semi-liquid waxy white substance filling the 'case' or spermaceti organ in the whale's head, which has a role in diving and maintaining buoyancy. (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, although these are typically smaller in the kogiids.[1]

[edit] Evolution

The earliest sperm whale fossils are known from the late Oligocene - about 25 million years ago[2], [3], with an ancestry tracing back from the latest Eocene before diverging from the remainder of the Odontocetid line, leading to the dolphins, and porpoises.

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[3]. The earliest kogiids are known from the late Miocene, around 7 million years ago[4].

The close relationship between extant Physeteridae and Kogiidae is confirmed in recent molecular studies using mitochondrial cytochrome b[5], [6]; 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[7] 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).


[edit] Classification

[edit] References

  1. ^ a b c Lockyer, Christina (1984). in Macdonald, D.: The Encyclopedia of Mammals. New York: Facts on File, 204-209. ISBN 0-87196-871-1. 
  2. ^ Stucky, R. E. & McKenna, M. C. (1993). Mammalia. Pp. 739-771 in Benton, M. J. ed.: The Fossil Record 2. Chapman & Hall, London.
  3. ^ a b Mchedlidze. G. A. (2002). Sperm whales, evolution. Pp. 1172-1174 in Perrin, W. F., Würsic, B. & Thewissen, J. G. M. eds.: Encylopedia of Marine Mammals. Academic Press, San Diego.
  4. ^ Perrin, W. F., Würsic, B. & Thewissen, J. G. M. eds.: Encylopedia of Marine Mammals. Academic Press, San Diego, 1414 pp.
  5. ^ Verma, S. K., Sinha, R. K. & Singh, L. (2004). Phylogenetic position of Platanista gangetica: insights from the mitochondrial cytochrome b and nuclear interphotoreceptor retinoid-binding protein gene sequences. Molecular Phylogenetics and Evolution 33: 280-288.
  6. ^ May-Collado, L. & Agnarsson, I. (2006). Cytochrome b and Bayesian inference of whale phylogeny. Molecular Phylogenetics and Evolution 38: 344-354
  7. ^ Bianucci, G. & Landini, W. 2006. Killer sperm whale: a new basal physeteroid (Mammalia, Cetacea) from the Late Miocene of Italy. Zoological Journal of the Linnean Society 148: 103–131.