|
The gharial (''Gavialis gangeticus''), also known as the gavial, and the fish-eating crocodile, is a crocodilian of the family Gavialidae, native to the northern part of the Indian Subcontinent.〔 The global gharial population is estimated at fewer than 235 individuals, which are threatened by loss of riverine habitat, depletion of fish resources, and entanglement in fishing nets. As the population has declined drastically in the past 70 years, the gharial is listed as ''Critically Endangered'' on the IUCN Red List. The gharial is one of the longest of all living crocodilians, measuring up to , though it should be noted that this is an extreme upper limit, as the average adult gharial is only in size.〔 With 110 sharp, interdigitated teeth in its long, thin snout, it is well adapted to catching fish, its main diet.〔 The male gharial has a distinctive boss at the end of the snout, which resembles an earthenware pot known in Hindi as ''ghara''. The gharial's common name is derived from this similarity.〔 Gharials once inhabited all the major river systems of the Indian Subcontinent, from the Irrawaddy River in the east to the Indus River in the west. Their distribution is now limited to only 2% of their former range. They inhabit foremost flowing rivers with high sand banks that they use for basking and building nests. They usually mate in the cold season. The young hatch before the onset of the monsoon.〔 The gharial is one of three crocodilians native to India, the other two being the mugger crocodile and the saltwater crocodile.〔Choudhury, B.C. (ed.) (2006). (''West Asia Regional Report'' ). Crocodile Specialist Group Steering Committee Meeting, 19 June 2006.〕 == Characteristics == The gharial is characterised by its extremely long, thin jaws, which are regarded as an adaptation to a primarily piscivorous diet. Males reach up to with an average weight of around .〔Stevenson, C. and Whitaker, R. (2010). (Gharial ''Gavialis gangeticus'' ) pp. 139–143 in: Manolis, S. C. and C. Stevenson. (eds.) ''Crocodiles. Status Survey and Conservation Action Plan''. Third Edition. Crocodile Specialist Group, Darwin.〕 It is dark or light olive above with dark cross-bands and speckling on the head, body and tail. Dorsal surfaces become dark, almost grey-black, at about 20 years of age. Ventral surfaces are yellowish-white. The neck is elongated and thick. The dorsal ridges are more or less restricted to the median regions of the back. The fingers are extremely short and thickly webbed. Males develop a hollow bulbous nasal protuberance at the tip of the snout upon sexual maturity.〔Brazaitis, P. (2001). ''A Guide to the Identification of the Living Species of Crocodilians''. Science Resource Center, Wildlife Conservation Society〕 The name gharial is derived from the resemblance of the nasal growth to an earthen pot known locally as 'ghara'.〔 Gharials are the only extant crocodilian with visible sexual dimorphism.〔 Although the function of the nasal boss is not well understood, it is apparently used as a visual sex indicator, as a sound resonator, or for bubbling or other associated sexual behaviours. The average size of mature gharials is . The largest recorded length is , and the largest recorded weight is . Hatchlings approximate .〔 Young gharials can reach a length of in 18 months. The average body weight ranges from . Males commonly attain a total length of , while females are smaller and reach a body length of up to .〔GCA (2009). (''Gharial biology'' ). Gharial Conservation Alliance〕 The elongated, narrow snout is lined by 110 sharp interdigitated teeth and becomes proportionally shorter and thicker as an animal ages.〔 There are 27 to 29 upper and 25 or 26 lower teeth on each side. These teeth are not received into interdental pits; the first, second, and third mandibular teeth fit into notches in the upper jaw. The front teeth are the largest. The snout is narrow and long, with a dilation at the end and its nasal bones are comparatively short and are widely separated from the pre-maxillaries. The nasal opening of a gharial is smaller than the supratemporal fossae. The lower anterior margin of the orbit (jugal) is raised and its mandibular symphysis is extremely long, extending to the 23rd or 24th tooth. A dorsal plate is formed from four longitudinal series of juxtaposed, keeled, and bony scutes. The length of the snout is 3.5 (in adults) to 5.5 times (in young) the breadth of the snout's base. The nuchal and dorsal scutes form a single continuous plate composed of 21 or 22 transverse series. Gharials have an outer row of soft, smooth, or feebly keeled scutes in addition to the bony dorsal scutes. They also have two small post-occipital scutes. The outer toes are two-thirds webbed, while the middle toe is only one-third webbed. They have a crest on the outer edge of the forearm, leg, and foot. Typically, adult gharials have a dark olive colour tone, while the young are pale olive, with dark brown spots or cross-bands.〔Boulenger, G. A. (1890). ''The Fauna of British India, including Ceylon and Burma. Reptilia and Batrachia''. London: Taylor and Francis.〕 The well-developed, laterally flattened tail and webbed rear feet provide tremendous manoeuvrability in deepwater habitat. On land, however, an adult gharial can only push itself forward and slide on its belly. The laterally compressed tail serves both to propel the animal and as a base from which to strike at prey.〔 The three largest specimens reported were a 6.5-m gharial killed in the Gogra River of Faizabad in August 1920, a 6.3-m individual shot in the Cheko River of Jalpaiguri in 1934, and a giant of 7-m animal, which was shot in the Kosi River of northern Bihar in January 1924. Though specimens of over were not uncommon in the past, such large individuals are not known to exist today.〔Wood, G. L. (ed.) (1983). ''The Guinness Book of Animal Facts and Feats''. Sterling Pub. Co Inc. ISBN 978-0-85112-235-9〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「gharial」の詳細全文を読む スポンサード リンク
|