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Beaked whale
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Sowerby's Beaked Whale (on Faroese stamp) | |
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Family: | Ziphiidae |
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Beaked whales are creatures of the ocean deeps, feeding 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 (1,038 fathoms) 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).
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.
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Physical characteristics
Beaked whales are moderate in size, ranging from 4 to 13 metres (13 to 43 ft) and weighing from 1 to 15 tonnes (0.98 to 15 LT; 1.1 to 17 ST). Their key distinguishing feature is the presence of a 'beak', somewhat similar to 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 females. Beaked whale species are often sexually dimorphic—one or the other sex is significantly larger. The adult males often possess a dramatically bulging forehead.[2]
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, forehead shape and beak length.
Teeth
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' teeth has a characteristically unique shape. Females observe the teeth to help select mates, because the different species are otherwise quite similar in appearance.
The teeth also play an important role in competing to control a harem. This is believed to be the first known instance of a secondary sexual characteristic having shaped the evolution of a marine mammal.[3]
Taxonomy
Beaked whales comprise at least twenty species of small whale in the family Ziphiidae, which is one of the least-known families of large mammals: several species have been described only in the last two decades, and others may remain undiscovered. Six genera have been identified. Three of these, Indopacetus, the Hyperoodon and the Mesoplodon, are united in a single subfamily, the Hyperoodontinae.
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.
· ORDER CETACEA
· Suborder Odontoceti: toothed whales
o Family Kogiidae: Pygmy and Dwarf Sperm Whales
o Family Ziphiidae
§ Genus Africanacetus †
§ Genus Archaeoziphius †
§ Genus Berardius
§ Arnoux's Beaked Whale, Berardius arnuxii
§ Baird's Beaked Whale, Berardius bairdii
§ Genus Caviziphius †
§ Genus Hyperoodon: Bottlenose whales
§ Northern Bottlenose Whale, Hyperoodon ampullatus
§ Southern Bottlenose Whale, Hyperoodon planifrons
§ Genus Inlengesi †
§ Genus Indopacetus
§ Longman's Beaked Whale, Indopacetus pacificus
§ Genus Izikoziphius †
§ Genus Mesoplodon
§ Sowerby's Beaked Whale, Mesoplodon bidens
§ Andrews' Beaked Whale, Mesoplodon bowdoini
§ Hubbs' Beaked Whale, Mesoplodon carlhubbsi
§ Blainville's Beaked Whale, Mesoplodon densirostris
§ Gervais' Beaked Whale, Mesoplodon europaeus
§ Ginkgo-toothed Beaked Whale, Mesoplodon ginkgodens
§ Gray's Beaked Whale, Mesoplodon grayi
§ Hector's Beaked Whale, Mesoplodon hectori
§ Strap-toothed Whale, Mesoplodon layardii
§ True's Beaked Whale, Mesoplodon mirus
§ Pygmy Beaked Whale, Mesoplodon peruvianus
§ Perrin's Beaked Whale, Mesoplodon perrini
§ Stejneger's Beaked Whale, Mesoplodon stejnegeri
§ Spade Toothed Whale, Mesoplodon traversii
§ Genus Microberardius †
§ Genus Nenga †
§ Genus Pterocetus †
§ Genus Tasmacetus
§ Shepherd's Beaked Whale, Tasmacetus sheperdi
§ Genus Xhosacetus †
§ Genus Ziphius
§ Cuvier's Beaked Whale, Ziphius cavirostris
Extinct relatives
As many as ten genera predates humans. 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.
Ecology
Feeding
Beaked whales' feeding mechanism, suction feeding, is unique. Instead of catching prey with teeth, they suck it into their oral cavity. Their tongue can move very freely, and when suddenly retracted at the same time as the gular (throat) floor is distended, the pressure immediately drops within their mouth and the prey is sucked in with the water.[4]
Diet is primarily deep water squid, but also fish and some crustaceans.
Range and habitat
Beaked whales are found in all oceans and most species rarely venture into the relatively shallow water of the continental shelves.
Known areas where they congregate include the deep waters off the edge of continental shelves, and bottom features like seamounts, canyons, escarpments, and oceanic islands including the Azores and the Canaries.
Their range varies by species.
Life history
Beaked whales have been observed living in "harem-like" groups, where several females and young are accompanied by a single male. Beaked whales tend to associate in small family groups.
Conservation status
For many years, most 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 predator they are, like raptors, particularly vulnerable to build-up of biocontaminants. They frequently ingest plastic bags (which do not break down and can be lethal).
· They more and more frequently become trapped in trawl nets, due to the expansion of deepwater fisheries (particularly since the collapse of Atlantic Cod stocks late in the 20th century)
· They are assumed to be vulnerable to prey depletion.
· Beaked whales are especially vulnerable to modern ultra-loud sonar, 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 because their status is unknown.
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. ^ "Whale's teeth are aid to mating". BBC. December 2008. http://news.bbc.co.uk/2/hi/science/nature/7783517.stm.
4. ^ "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.
5. ^ Navy sonar blamed for death of beaked whales found washed up in the Hebrides - The Independent
External links
· Beaked whales - into the abyss - BBC News article
· Diving with Beaked Whales - A Whale Trackers documentary video.
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