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includes crocodiles, alligators and gharials. Of the 14 species of crocodiles extant, two occur in Sri Lanka: (a) freshwater (‘freshy’) or marsh or mugger crocodile Crocodylus palustris, and (b) saltwater (‘salty’) or estuarine crocodile C. porosus. Both species are listed on Appendix I of CITES.
Distribution
The freshwater crocodile or the mugger is found in India, Pakistan and Sri Lanka, where it occurs in the numerous man-made lakes or tanks in the low country dry zone. As there are no natural lakes in Sri Lanka, the construction of thousands of tanks by the ancient kings must have enhanced the spread and survival of the ‘freshy’. About a hundred years ago, it was so numerous and widespread that Sir Emerson Tennent referred to the still waters and tanks of the northern provinces literally “teeming with crocodiles”. One of the legacies of the 18-year old armed conflict in Sri Lanka was the return of the crocodiles to the Jaffna Peninsula, from where they were thought to be extinct several decades ago. Crocodiles have also been ‘discovered’ recently from the island of Mannar. Today the ‘freshy’ is confined to the first peneplain below an altitude of 100 m. The ‘salty’ is more widely distributed globally, from Sri Lanka and the east coast of India in the west to the Caroline Islands in the east. In the past, the ‘salty’ was very common along the western and southern coastal areas of Sri Lanka. It is estimated that there could be at least 1,500 freshwater crocodiles and perhaps no more than 300 estuarine crocodiles in Sri Lanka.
Ecology
Crocodiles occur in a variety of wetlands. The ‘salty’ inhabits the coastal flood plains, where the waters become progressively saline during the dry season. It can tolerate salinities up to 5.6%, which is about 1.8 times the salinity of sea water. However, freshwater swamps provide ideal nesting areas. The smaller ‘freshy’ inhabits tanks, lakes and rivers. The ‘salty’ being more solitary, seldom basks in groups and so may be recognized.
Although crocodiles are excellent predators, they are often indiscriminate and opportunistic feeders. They hunt at night, adopting a ‘minimum exposure’ position in which only the eyes, ears, nostrils and the cranial platform are above water. Thus they wait half-submerged for land-bound prey such as spotted deer, wild boar, or even a buffalo calf to disturb the water surface. They have specialized sensory organs known as dome-pressure-receptors (DPRs) on their faces that can detect tiny disruptions in the surface of water. As these are found even on fossils, DPRs are a unique, conserved feature of semi-aquatic crocodilians. At the back of the mouth is the palatal valve which is another unique adaptation to seal off the throat from both air and water. With the palatal valve shut, a croc can grasp large prey underwater, without water flooding down the throat. While diving, blood flow is shunted away from the lungs into the systemic circulation by utilizing a bypass (the foramen of Panizza) between the divided ventricle of the four-chambered heart. Prey varies from crabs, prawns or shrimps in the case of hatchlings, to fish, aquatic birds and large terrestrial herbivores in the case of adults. Before a croc can attack a large herbivore in water, it needs to rotate its head sideways by 45 degrees. This is achieved by making the rest of the body rigid through the contraction of the longitudinal muscles along the back and tail. This results in ‘tail-arching’ which allows the head to be swung efficiently. The muscles that help close the jaws are strong and extremely powerful, while those that are involved in opening them are weak. (A 2-m long croc’s jaws can be kept from opening with a mere rubber band around its snout!). Crocs can hear over a wide range of frequencies, and are among the few reptiles that have a voice. They have also good eyesight and their vision is greatly enhanced by a layer of guanine crystals behind the retina, known as the retinal tapetum, which reflects any light that reaches it. (If a torch is shone on a croc, its eyes would reflect red – the eyeshine). It thus intensifies the image, making it easy for crocs to see even under low light intensity. Furthermore, in a croc that is 5 m long, the two eyes are separated by only 7 cm. Such close placement of the eyes makes it possible for crocs to have binocular vision – a feature that helps estimate depth – for predation. Crocs have also very good sense of smell. They have no salivary glands and their fleshy tongue is immobile since it is attached along its length between the lower jaws. Despite its size, a croc usually cannot eat larger prey whole given its small stomach. This is why it often stashes its prey underwater or in a swampy area, and returns at intervals to feed. Prey is swallowed in large pieces. An unusual feature of the stomach is the presence of stones or gastroliths, which help in digestion by tearing up the parts of prey swallowed. The digestive enzymes are so strong (low pH levels) in the stomach that they can digest even bones. But their activity depends on ambient temperature. If the outside temperature is too low, crocs may stop feeding completely. At low temperatures, food will rot before it can be digested and assimilated. Crocs are efficient in converting what they eat – about 22% on a fish diet – and can go for long periods without feeding, during which they rely on the fat stored in the tail. They can also feed on carrion. Teeth may be lost but they are replaced throughout life.
Being reptiles, crocodiles cannot maintain a constant internal body temperature through physiological means. They are referred to as poikilotherms (variable body temperature) or ectotherms since their body temperature is determined by the outside heat source available. Crocs prefer a temperature range of 30-330C. To remain within this range, they would often move in and out of water. In hot weather, they would bask by orienting themselves in such a way that the maximum body is exposed to the sun. Once the body is heated, crocs would face the sun and open their mouths to allow the brain to cool through evaporative heat loss of the blood in the buccal cavity.
Crocs are polygynous, with single males mating with a number of females. Dominant saltwater crocs occupy well-defined territories from which other males are excluded. Such territories may include the nesting sites of several females. Stimulus for ovulation is either increase in temperature (in the temperate regions) or the onset of the rainy season (in the tropics). Mating always takes place in water and is preceded by courtship. 4-6 weeks after mating, the female would start laying 10-50 hard-shelled eggs. Freshwater crocs are ‘hole nesters’ in that the females excavate a hole into which they would deposit their eggs, like turtles. Saltwater crocs are ‘mound nesters” since they construct a mound using earth and vegetation, in the center of which the female would deposit her eggs. Freshwater crocs are also ‘pulse nesters’ – all the females within a population would nest within a few weeks. Saltwater crocs are said to be ‘prolonged nesters’ since nesting may go on for 6 months. Since sex is determined by temperature in crocodiles, females select their nest sites with great care. Often, a number of ‘trial nests’ are made to reduce the risk of predation. Low temperatures (300C or less) mean slow development, leading to the production of exclusively females, while high temperatures (320C or 330C) produce mostly males. At intermediate temperature (310C), a mixture of males and females is produced. As hatching time approaches, the young crocs would produce a cheeping sound to alert the mother who would come to excavate the nests and carry the newly hatched young in her mouth to water. Most eggs fail to survive to produce hatchlings due to desiccation, infertility, flooding, predation or inadequate gas exchange. Out of every 1000 eggs that hatch, only 8 crocs would survive up to 5 years of age! But crocs, like elephants, continue to grow all their lives. When they reach a certain length, they expand in bulk. Although parental care is rare among reptiles, female crocodiles are very protective of their nests. Maternal behaviour includes nest defence, nest opening, manipulation of eggs to release hatchlings, and mouth transport of eggs and the young.
Conservation
Crocodiles are regarded as “keystone species” that maintain ecosystem structure and function through selective predation on fish species, recycling of nutrients and maintenance of wetlands in drought. But in Sri Lanka, they have a poor image and hence their long-term survival in the wild will depend on the attitude and tolerance of the local people. Rural people are often intolerant of large and dangerous predators in their backyard. Thus there is a need to provide incentives to the local people to maintain crocodiles and their habitats in the wild. Crocodiles have declined both in range and number since they were indiscriminately hunted in the past for skin and meat. Intensive inland fishing has also led to the decline of freshwater crocs in some areas, and crocs also get entangled in fishnets and drown. Contrary to the popular belief, crocs are beneficial to reservoir fishery. They do control a number of avian predators of fish such as storks, darters and pelicans. Conserving crocodiles needs relatively large, diverse and undisturbed wetlands, since crocodiles increase in size from hatchlings to adults through several orders of magnitude. Reclamation of swamps, draining of coastal wetlands, conversion of mangroves to prawn farms, and the removal of riverine forests are some of the threats faced by crocodiles in Sri Lanka. Mangroves are especially important for the protection of hatchlings, which seek shelter among the prop roots of Rhizophora spp, where they are safe from predators. Nevertheless, in Sri Lanka mangroves are being rapidly destroyed. One of the lesser known causes for the decline of crocodiles in Sri Lanka is increased soil erosion and consequent floods which destroy both crocodile nests as well as eggs. Crocodiles have a significant impact on the biodiversity, economic potential and ecosystem stability.
Discovery of crocodiles on Mannar Island, Sri Lanka
Serendipity is a word coined by Horace Walpole in 1754 to denote the faculty of making lucky and unexpected ‘finds’ by accident. Walpole in a letter to Sir Horace Mann wrote that he formed it on the title of a fairy story, The Three Princes of Serendip, because the princes “were always making discoveries, by accidents and sagacity, of things they were not in quest of.’ There is no better word than serendipity to describe how we stumbled on two adult crocodiles basking on a spit of land in one of the seasonal water holes in Mannar. This happened on Sunday 1 February 2004, when we traveled with students following Courses on Herpetology (ZL 212) and Avian Biology (ZL 213) from the University of Peradeniya on a field trip across the island of Mannar.
We were traveling down the A14 highway that goes all the way to Talaimannar on the western end of the island on Sunday when at 1055 h we stopped at a waterhole known as Kora kulam situated between the 89 and 90 km posts to observe the diversity of birds, especially the waders, when we spotted two “log-like” structures on a spit of land. Closer examination through binoculars revealed that these were two fairly large (between 2-3 m long) crocodiles. It is not known for certain to what species they belong, for as they were about to be photographed, a gunshot nearby drove them into water. It was an incredible find for there had been no account of crocodiles from Mannar, although according to the late P.E.P. Deraniyagala, the freshwater (‘freshy’) or marsh crocodile (Crocodylus palustris) “inhabits some of the islands to the north-west and north of Ceylon which it has reached either by crossing the sea during heavy rainy weather or by being isolated when these islands separated off the mainland”. However, Deraniyagala’s list of crocodile localities does not include the island of Mannar. Deraniyagala also referred to the fact that the salty is most common in the estuaries of large rivers and appears to prefer water that is only slightly brackish. It thus avoids the sea but is known to travel short distances by sea from one river mouth to another close by, when swept there by floods. The salty too has not been recorded from Mannar. It is possible that the two crocodiles observed in Mannar were salties (Crocodylus porosus), for it is unlikely that the freshy would survive in Mannar once the water holes become bone dry in the peak of the dry season. The salty, on the other hand may be able to return to the estuaries on the mainland by moving through the mangroves found along the causeway between the island of Mannar and the mainland, during periods of drought. But a positive identification of the species must be made before this issue can be resolved.
Crocodiles have been poorly studied in Sri Lanka. During the British Colonial period, Mr. W.J.S. Boake published a Monograph on Mannar in 1888 in which he refers to the presence of both species of crocodile in the Mannar District (400 sq. miles) but not specifically in Mannar Island. According to Boake, the two species were known by their local names Chemmukkan (salty) and Chanakam (freshy) in Mannar District. After the pioneering work by Deraniyagala on crocodiles, the next remarkable piece of work was the survey carried out in 1977 by Rom and Zahida Whittaker of the Madras Snake Park Trust, on behalf of the Wildlife and Nature Protection Society of Sri Lanka. The Whitakers arrived by ferry from South India and landed at Talaimannar and travelled some 1,600 miles across much of the island of Sri Lanka on their Jawa Motorcycle examining ‘hundreds of ponds, reservoirs, streams and rivers’ and interviewing local residents and fishermen for information on crocodiles. They were able to gather detailed information on crocodiles from 40 representative tanks. Their survey was, and remains to date, the most comprehensive study of the distribution of crocodiles in Sri Lanka. This was followed by a re-assessment of the status of the two species of crocodiles in 2000. In both studies, there was no specific mention of crocodiles being present in the island of Mannar, and it was thought that they were either absent or extinct. Herein lies the significance of our recent observation in Mannar. Subsequent to our discovery of the crocodiles, we learnt that both species occur in ponds and seasonal water holes in Mannar, but this needs to be confirmed through more detailed surveys. We were also told that crocodiles were routinely killed as people are fearful of their presence in their neighborhood, especially in view of the fact that women and children use the waterholes for bathing. With an annual rainfall between 762 and 1,016 mm, Mannar is one of the driest places in Sri Lanka. The north-eastern monsoon brings rain during the short period from December to February. High daytime temperatures of over 300C have been recorded in the island between April and May. The fine clay soil becomes bone dry and stony hard during the drought and makes it extremely impervious to water. But during the brief rainy season, the island of Mannar undergoes a remarkable transformation and becomes a Mecca for the millions of migrant birds that arrive annually to escape the northern winter. It is during such time that crocodiles too benefit from the rich source of food available in the seasonal ponds and water holes. As Boake once remarked, every tank and swampy ground is full of fish, which in the dry season “bury themselves into the mud like the saurians”.
The discovery of crocodiles in the island of Mannar underlines the need for surveys across much of the north and east of Sri Lanka. There is a need to look for crocodiles in hitherto unsurveyed areas in the island. Long-term research on any particular species or ecosystem should be planned on the basis of a logical progression through a series of phases, involving the processes of discovery, assessment and prediction. In Sri Lanka, there is still much to be done in the discovery phase, given the island’s rich biodiversity. Crocodiles have been recognized as “keystone species” that maintain ecosystem structure and function through such activities as selective predation on fish species, recycling nutrients, and maintenance of wet refugia in droughts. It is remarkable that despite intensive hunting pressure in the past, crocodiles have managed to retain a toehold on the island of Mannar thereby proving that they are indeed tenacious survivors. Although many crocodile populations have been reduced in size, as a result of hunting for skin and meat, no species of crocodile has yet been driven to extinction. A potential threat for the future of the crocodiles may come from a rapid rise in the human population (through resettlement of refugees), the spread of settled agriculture, and the development of ill considered and over ambitious tourism facilities. Already, large tracts of land hitherto left in a state of wilderness have been cleared of vegetation to make way for the construction of buildings along the A14 highway in Mannar. The human footprint will soon be felt across much of the island of Mannar given its strategic importance as the gateway to India. The Kora kulam must be saved from the bulldozer, given its importance as a habitat for crocodiles and thousands of migrant birds. Conserving crocodiles will not be easy since they have a poor image in Sri Lanka. Many rural people loathe them as noxious animals and would not regret their disappearance from the neighbourhood. If people are to be persuaded to tolerate such potentially dangerous animals in their midst, then they need to gain tangible benefits from a carefully planned and controlled wildlife-based tourism. Reference
to the above material from:
©
Charles
Santiapillai1, S.
Wijeyamohan2, Rajnish
Vandercone2 and Ruchira
Somaweera1 (2004)
1
Department of Zoology, University of Peradeniya, Sri
Lanka
2
Department
of Biological Science, Vavuniya Campus of the University of
Jaffna,
Sri
Lanka
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