Beaked whale

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Beaked whales
Fossil range: Miocene–Recent
Sowerby's Beaked Whale (on Faroese stamp)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Subclass: Theria
Infraclass: Eutheria
Order: Cetacea
Suborder: Odontoceti
Family: Ziphiidae
Gray, 1850
Genera

Berardius
Hyperoodon
Indopacetus
Mesoplodon
Tasmacetus
Ziphius

A beaked whale is any of at least twenty species of small whale in the family Ziphiidae. They are one of the least-known families of large mammals: several species have only been described in the last two decades, and it is entirely possible that more remain as yet undiscovered. Six genera have been identified. Three of these, Indopacetus, the Hyperoodon and the Mesoplodon, are united in a single subfamily, the Hyperoodontinae.

Beaked whales are creatures of the ocean deeps, feeding, so far as is known, on or near the sea floor. They have an extraordinary ability to dive for long periods—20 to 30 minutes is common, and 85 minute dives have been recorded—and to great depths: 1,899 metres and possibly more.[1] They are currently the only marine mammals whose evolution is believed to have been shaped by a secondary sexual characteristic (the male's teeth).

Contents

[edit] Extinct relatives

As many as ten genera existed before humans existed. Some included ancestors of giant beaked whales (Berardius), such as Microberardius. The Cuvier's beaked whale (Ziphius) had many relatives, such as Caviziphius, Archaeoziphius, and Izikoziphius. They were probably preyed upon by predatory whales and sharks, including Carcharocles megalodon.

[edit] Physical characteristics

Beaked whales are moderately large in size, ranging from 4 to 13 metres in length, and weighing from 1 to 15 tonnes. Their key distinguishing feature is the presence of a 'beak', somewhat similar to that of many dolphins. Other distinctive features include a pair of converging grooves under the throat, and the absence of a notch in the tail fluke. Although Shepherd's Beaked Whale is an exception, most species have only one or two pairs of teeth, and even these do not erupt in the females. Beaked whale species are often sexually dimorphic, with one or the other sex being significantly larger, and the adult males often possessing a dramatically bulging forehead.[2]

[edit] Classification

The beaked whales are the second-largest family of Cetaceans (after the dolphins) and were one of the first groups to diverge from the ancestral lineage. The earliest known beaked whale fossils date to the Miocene, about 20 million years ago.

[edit] Ecology

Beaked whales possess a unique feeding mechanism known as suction feeding. Instead of catching their prey with teeth, it is sucked into their oral cavity. Their tongue can move very freely, and when suddenly retracted at the same time as the gular floor is distended, the pressure immediately drops within their mouth and the prey is sucked in with the water.[3] Beaked whales are found in all oceans and most species rarely venture into the relatively shallow water of the continental shelves. They are very difficult to identify in the wild: body form varies little from one species to another, and the observer must rely on often subtle differences in size, color, shape of forehead, and length of beak.

Beaked whales tend to associate in small family groups and avoid shallow water. Known areas where they congregate include the deep waters off the edge of continental shelves, and close to bottom features like seamounts, canyons, escarpments, and oceanic islands including the Azores and the Canaries. Diet is primarily deep water squid, but also fish and some crustaceans.

Because of their preferred habitat and their inclination to make long dives, they are very difficult to observe, and little is known of most species. Several have yet to be formally described or named; others are known only from remains and have never been sighted alive. Only three or four of the 20-odd species are reasonably well-known. Baird's and Cuvier's Beaked Whales were subject to commercial exploitation off the coast of Japan; and the Northern Bottlenose Whale was extensively hunted in the northern part of the North Atlantic late in the 19th and early in the 20th centuries.

In December 2008, researchers from the Marine Mammal Institute at Oregon State University completed a DNA tree of all 21 known species of beaked whale. Among the results of this study was the conclusion that the male's teeth are actually a secondary sexual characteristic, similar to the antlers of male deer. Each species has characteristically unique shaped teeth; because the different species are otherwise quite similar in appearance, the females use the teeth to help select the proper males with whom to mate. Also, the whales have been observed living in "harem-like" groups, where several females and young are accompanied by a single male. The teeth then also play an important role in fighting among males for control of a harem. This is believed to be the first known instance of a secondary sexual characteristic having shaped the evolution of a marine mammal.[4]

[edit] Conservation status

For many years, most of the beaked whale species were insulated from human impact because of their remote habitat. However there are now clear issues of concern: studies of stranded beaked whales show rising levels of toxic chemicals in their blubber (as a top-order predator they are, like raptors, particularly vulnerable to build-up of biocontaminants) and they frequently have ingested plastic bags (which do not break down and can be lethal). With the ongoing worldwide expansion of deepwater fisheries (particularly since the collapse of Atlantic Cod stocks late in the 20th century), beaked whales are more and more frequently trapped in trawl nets, and are also assumed to be vulnerable to prey depletion.

Beaked whales are proving especially vulnerable to modern ultra-loud submarine seeking sonars, which may force them to surface too quickly and die from the bends.[5]

Four of the more than 20 beaked whale species are classified by the IUCN as "lower risk, conservation dependent": Arnoux's and Baird's Beaked Whales, and the Northern and Southern Bottlenose Whales. None of the remaining species are classified - not because they are considered secure, but because their status is simply unknown.

[edit] References

  1. ^ Lewis Smith (October 17, 2006). "It's official: New free-diving record is 1,899 meters (6,230 ft)". CDNN. http://www.cdnn.info/news/industry/i061017a.html. 
  2. ^ Christensen, Ivar (1984). Macdonald, D.. ed. The Encyclopedia of Mammals. New York: Facts on File. pp. 210–211. ISBN 0-87196-871-1. 
  3. ^ "Suction feeding in beaked whales: Morphological and experimental evidence". Natural History Museum of Los Angeles County. 1996. http://www.nhm.org/research/mammals/suction.htm. 
  4. ^ "Whale's teeth are aid to mating". BBC. December 2008. http://news.bbc.co.uk/2/hi/science/nature/7783517.stm. 
  5. ^ Navy sonar blamed for death of beaked whales found washed up in the Hebrides - The Independent

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/Beaked_whale"