Gharials (Gavialidae)

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Gharials

(Gavialidae)

Class Reptilia

Order Crocodylia

Family Gavialidae


Thumbnail description
A large crocodilian with extremely narrow jaws; adult males have a distinctive knob over the nostrils

Size
13.1–16.4 ft (4–5 m)

Number of genera, species
1 genus; 1 species

Habitat
Rivers

Conservation status
Endangered

Distribution
Northern part of the Indian subcontinent

Evolution and systematics

In the geologic past the family Gavialidae was widespread. Fossils of some 12 species, all from the Cenozoic era (65 million years ago till the present), have been recovered from India, South America, Africa, and Europe. Paleontologists have argued that the Indian gharial separated from the rest of the crocodilians in the Mesozoic era (251–65 million years ago). Recent molecular studies, however, indicate a divergence between Gavialis gangeticus and its nearest relative, the Malayan gharial (Tomistoma schlegelii) as recently as 20 million years ago. Buccal cavity (mouth) morphology of the gharial suggests that the species is able to excrete salt, hence transoceanic dispersal might have aided speciation.

The taxonomy for this species is Indian gharial Gavialis gangeticus Gmelin, 1768, Ganges River.

Other common names include: French: Gavial du Gange; German: Schnabelkrokodil; Spanish: Gavial del Ganges.

Physical characteristics

Gharials have the longest snout of all crocodilians and are among the largest. Males average 13–15 ft (4–4.5 m) with a maximum recorded length of over 19.7 ft (6 m). Females average 11.5–13 ft (3.5–4 m). Adults are dark or brownish olive; hatchlings are grayish brown with five irregular transverse bands on the body and nine on the tail. They are white or yellowish

white on the underside. The body form is sleek, the scales smooth, and the head is well differentiated from the body. Gavialis is unique among crocodilians in that it is the only species to display visible sexual dimorphism: mature males develop a cartilaginous knob, or narial excresence, on the tip of the snout, females do not. This knob creates a buzzing sound during exhalation, a social signal during courtship. It has no other known function. The most aquatic of all crocodilians, Gavialis do not migrate long distances overland, and adults are unable to lift their bodies off the ground.

Distribution

Current known distribution of the gharial is noncontinuous, up to 1,640 ft (500 m) above sea level in India, Nepal, and Pakistan. A few isolated individuals have been reported from Bangladesh and Bhutan.

Habitat

Gharials are found in deep, fast-flowing rivers, and prefer to occupy junctions and bends in these rivers where pools are deep and current is reduced. Exposed midriver banks are used as basking sites, particularly in the cooler winter months, and steep sand banks are used as nesting sites.

Behavior

Gharials are social crocodilians and groups of adults congregate at suitable sites. They rarely venture far from the safety of the aquatic environment into which they can dive when alarmed. Males become territorial in the courtship/mating season, and spar for territory in shallow water. Lying parallel to each other, males raise heads well out of the water, and attempt to push each other off balance with their snouts. Violent "snout bashing" and biting has been observed. Female Gavialis become territorial in the nesting season, and will also defend nests from predators.

Feeding ecology and diet

Gharials are the only known crocodilians to subsist almost entirely on a diet of fish. In the juvenile stage, the species may also consume tadpoles, shrimp, and water insects. Prey is sought by either the "sit and wait" technique, or by actively foraging, and fish are captured with lightning fast lateral swipes of the jaw. Fish are impaled on the sharp teeth and the gharial throws them to the back of the throat by jerking the head upward. The extremely narrow snout of Gavialis reduces friction in the water as compared to broad-snouted crocodilians, another factor contributing to its efficiency as a fish catcher.

Reproductive biology

Gavialis males mature at about 11.5 ft (3.5 m) at the age of 15, when the narial excresence becomes large and rounded. Females mature at 10 ft (3 m), as young as eight years old. The breeding season is markedly seasonal throughout the

range of the species; courtship and mating occur in December to January and nesting from March to May. The duration of the nesting season is shorter at higher latitudes (22 days on the Narayani River in Nepal) and longer at southerly latitudes (57 days at the Madras Crocodile Bank). Evidence from captive-breeding studies indicates that larger females nest first, inferring that these females are mated with first. Females select nest sites at least 5 ft (1.5 m) above water level, and may dig trial nests before nesting. Clutch size closely correlates with the size of the female, ranging from 12 to nearly 100 eggs in very large females. The first clutches laid by females typically contain a high proportion of infertile eggs. Eggs measure 2.2 by 3.4 in (55 by 86 mm), and weigh 5.5 oz (156 g). Embryonic development is rapid, particularly in the later stages of development. Incubation temperature determines the incubation period and sex of hatchlings, as well as hatchling size. Incubation period is shorter at higher temperatures (an average of 53 days at 91°F/33°C) and longer at lower temperatures (93 days at 82°F/28°C). Females have been observed to guard nests in the wild, dig out hatchlings from nests, and display prolonged protection of their crèche of hatchlings. Males may also protect the young. Eggs are

preyed upon by jackals, pigs, hyenas, mongooses, and monitor lizards. Hatchling mortality is high, largely due to floods in the monsoon period (when the eggs hatch), and predation by large wading birds and soft-shelled turtles.

Conservation status

In Nepal, Gavialis populations are in decline, and fewer than 1% of all gharials hatched in nature reach a length of 6.6 ft (2 m). Egg loss by animal and human predators is high, and flooding causes significant loss of nests. A total of 55 wild gharial and 50 released gharials were observed in 1999 at Babai, Kali, Karnali, Koshi, Mahakali, Narayani, and Rapti River systems. The low number of males in the wild population (sex ratio one male to 10 females) may be adversely affecting the population. Reasons for decline are habitat destruction due to increasing human pressure on the environment from extensive agriculture, firewood collection, and cattle grazing.

In Bangladesh, Gavialis was believed to be extinct in the 1970s, but continued survival of the species was confirmed in the Padma (Ganges) and Jamuna (Brahmaputra) Rivers in 1981. A survey in 1985 recorded 18 individuals in the Padma River at Changhat, Charkhidipur, and Godagari localities. However, known nesting areas that produced up to 12 nests as recently as 1985 have seen no nests since 1990. Factors affecting declining gharial populations are fishing activities and habitat degradation.

In Bhutan, the species is near extinction in the wild, with isolated individuals reported from rivers near Bhutan's southern border with India.

In Pakistan, gharials have been extirpated from most of the country, with the main population of about 20 individuals occuring on the Nara Canal, part of the Indus River Dolphin Sanctuary. Small populations are believed to survive around Gudder Barrage (Sind) and Taunsa Barrage (Punjab). The Pakistan government is currently planning a restocking effort, perhaps using surplus captive stock from India.

In the Orissa state in India, Gavialis is now restricted to the Mahanadi River. Over 700 juvenile gharial have been released in the Satkoshia Gorge Sanctuary, but survival has been only 5% due to heavy human usage of the river for bamboo rafting and fishing. In the tristate National Chambal Sanctuary, encompassing the states of Rajasthan, Uttar Pradesh, and Madhya Pradesh, over 1,200 wild gharial were counted in recent surveys. Over 3,500 captive-reared gharial from wild eggs have been released in protected areas of these states, and several other small but vital breeding populations survive on the Girwa, Ken, and Son Rivers.

The gharial is cited as Endangered on the IUCN Red List, and is on Appendix I of CITES. There are an estimated 1,500–2,500 gharials in the wild.

Significance to humans

Gharial eggs are collected for food by indigenous people in some parts of their range, and the Hindu river goddess Ma Ganga is sometimes depicted riding on a gharial.


Resources

Books

Daniel, J. C. The Book of Indian Reptiles and Amphibians. New Delhi, India: Oxford University Press, 2002.

Ross, C. A., ed. Crocodiles and Alligators. New York: Facts on File Inc., 1989.

Periodicals

Negi, B., ed. "Indian Crocodilians." Envis 2, no 1 (June 1999).

Whitaker, N., and H. V. Andrews. "Reproductive Biology, Growth, and Incubation Temperature and Its Influence on Sex and Size of the Gharial (Gavialis gangeticus)." Gmelin at the Madras Crocodile Bank/Center for Herpetology (2002).

Whitaker, R., and D. Basu. "The Gharial (Gavialis gangeticus): A Review." Journal of the Bombay Natural History Society 79, no. 3 (1983).

Organizations

Crocodile Specialist Group, Florida Museum of Natural History. Box 117800, Gainesville, FL 32611-7800 USA. Phone: (352) 392-1721. Fax: (352) 392-9367. E-mail: [email protected] Web site: <http://www.flmnh.ufl.edu/natsci/herpetology/crocs.htm>

Madras Crocodile Bank/Center for Herpetology. P. O. Box 4, Tamil Nadu, 603 104 South India. E-mail: [email protected] vsnl.net

Other

Crocodiles: Status Survey and Conservation Action Plan, 1998 (Online). 1998 [cited January 2003]. <http://www.flmnh.ufl.edu/natsci/herpetology/act-plan/plan1998a.htm>

Ross, J. P., ed. Crocodiles. Status Survey and Conservation Action Plan, 2nd ed. Gland, Switzerland, and Cambridge, UK: IUCN/SSC Crocodile Specialist Group, 1998.

Florida Museum of Natural History: Herpetology. [cited January 2003]. <http://www.flmnh.ufl.edu/natsci/herpetology/herpetology.htm>

Romulus Earl Whitaker III, BSc

Nikhil Whitaker