Crocodiles and Turtles of Borneo

ESTUARINE CROCODILE

The crocodiles, together with its relatives - the alligators, the Tropical American caimans and the Old World gharials - are the only surviving members of the great group of reptiles, the Archosauria, that included the well-known dinosaurs and the flying pterosaurs. Over the past 200 million years, the order Crocodilia, the largest of the living reptiles, had diversified considerably spreading to all parts of the world and adapting to a wide variety of ecological niches.

The distribution of the estuarine crocodiles, Crocodylus porosus stretches from the Western coast of India to Northern Australia. Its tolerance of salt water has allowed it to occupy the coastal waters and has been known to colonize some of Sabah's offshore islands. The adaptations that account for the success of the crocodilian species are structural, physiological and behavioural. Because of their great size, heavy armour and powerful biting power, mature crocodiles are immune to attack by any predator in their habitat except man. The eight-inch babies, on the others hand, are relished by many other creatures. Otters, large birds and even adult crocodiles are among the avid eaters of the baby crocodiles.

Crocodiles can be distinguished from alligators or caimans by its distinctive feature of its jaws when closed. Their elongated jaws bear a single row of teeth that do not meet when the jaws are closed. The teeth of the upper jaw generally lie slightly to the outside of the teeth of the lower jaw; so that they are visible when the jaws close. Consequently, in crocodiles, but not in alligators or caimans, an enlarged fourth lower tooth fits into a notch in the upper jaw rather than into a socket; it therefore remain visible.

Another adaptive feature of the crocodile is that it possesses a tissue that can rise from the floor of the mouth to overlap a bony fold on the roof of the mouth; completely isolating its mouth from the pharynx. This enables the crocodile to keep its mouth open in the water without flooding its lungs and its stomach. By this means, it can breathe through its valved nostrils even when its mouth is filled with water. The valves of the nostrils prevent flooding of the nasal tube when the animal is submerged.

The crocodiles in general have superior sound perception. Its sense of smell is also advanced and its brain is more complex than that of any other reptile. In the capture of prey, crocodiles are not sit-and-wait hunters as often reported but highly active and versatile ones. They move slowly in the water with only its nostrils and the top of its head are visible above the surface. With a sudden sideways snap of its jaws, it can fish effectively. The crocodile snaps at its prey with a sideways movement of the jaws because their eyes are at the side of its head. They can also attack by jumping entirely out of the water and onto the land. Prey that is caught on land is often killed by drowning and then dismembered in the water. Their teeth are not suited to chewing, so a crocodile either eats the prey whole, or shake the body vigorously until some of the flesh breaks away. Despite voracious appetites, mature crocodiles can go without food for a long time.

Crocodiles have low metabolic rate and thus need to raise body temperature by basking in the sun.
For the first year of the lives, young crocodiles feed on small animals, frogs, dragon flies, crabs and even mosquito larvae. They have been seen cornering the larvae by curving their bodies and tails around them. As it grows, the amount of insect in its diet falls and turns to eating snails and fish. The adult crocodiles are said to feed on anything it can outswim or ambush and overpower. Odd objects like chunks of wood, pebbles and even rocks are found in crocodiles stomach. These are however not proof of foolish feeding by the crocodiles. The hard objects are taken in as "gizzard stones" to help grind the coarse food. The large chunks of food are reduced to digestible forms by the milling or churning action of the "gizzard stones" contained in the crocodiles stomach that are set in motion by regular contractions of the strong stomach muscles. These "gizzard stones" also serve to stabilize the crocodiles body. The "stones" lie in the stomach, below the center of gravity and work as a counterpoise to the bouyant lungs. This is particularly useful when the crocodiles are young, at which age, they are too heavy and cannot float easily at the surface.

Another behavioural adaptation that contributes to the success of the crocodiles is basking. Reptiles have low metabolic rate and thus need to conserve energy. Crocodiles do this by raising its body temperature during basking. They come out of the water at sunrise and lie on the banks in the sun. When their bodies have warmed up, they would move into the shade or back into the water. By staying underwater at night, they conserve heat because water holds its heat better than air. This direct use of solar energy enables large crocodiles to survive even when food is scarce.

Adult crocodiles have a strong sense of territory. The first year or two of their lives, the young crocodiles spend their lives in or near the mothers territorial den pool. Once they are grown, they leave home and start travelling, making dens of their own and presumably defend their territory from encroachment by fellow crocodiles of either sex. It is this habit of controlling its environment which has allowed all crocodilian species to live through the ages. To a crocodile, home is a nest, a pool, a den and also a system of trails. They live a long time in one place which may pass from one generation to the next.

The crocodiles, like most reptiles, lay eggs. They lay their eggs some 5 months after fertilization. The clutch size varies from 16 eggs to more than 80. When egg-laying time approaches, the female selects a nesting site, deposits her eggs and buries them under a mound of leaves. She now remains on the nest or in its immediate vicinity; defending a fairly large territory around the nest. The incubation period lasts between 80-90 days. The life span of any crocodilian is at least 25 years and probably exceeds 50 years.

Little is known concerning the ecology and conservation status of C. porosus in Borneo. As a result of several factors, including crocodile attacks on man in Sarawak and the general lack of knowledge of crocodile ecology and behaviour, an extensive survey to obtain reliable estimates of crocodile populations and their habitat was initiated at Klias River, Sabah.

Crocodiles move slowly in the water with only it's nostrils and the top of its head are visible above the surface.
Crocodiles are said to have been common in the Klias River previously, though the last attack in the Kota Klias was reported to have taken place prior to World War II. Thus they are unlike the crocodiles in Sarawak which are reputed as man-eaters. The man-eating habit varies and it may be that only certain individuals will attack man. Generally it seems that crocodiles are likely to be more aggressive when their stream and pools dry up so they cannot escape or when they are guarding their eggs and youngs. Despite this generally safe situation, local residents apparently encourage hunting of crocodiles. Illegal hunting has been a common practice, though not usually by the residents themselves. The last observed illegal harvest took place in March, 1988, when 48 animals were captured or killed from the immediate area of the Klias. Two independent surveys of the area have been done previously in 1984 and 1985 and results from both studies indicated very low densities of C. porosus.

In 1988, a total of 12 surveys were carried out by zoologists of Universiti Kebangsaan Malaysia Sabah Campus (UKMS) over a period of approximately 13 months along an approximately 37 km section of the Klias River. These intensive surveys reported 182 crocodile sightings during the 13 months study period, with a range of 6-33 individuals sighted per survey. Population estimates gave a density of 1.12 - 3.60 per km. The existing crocodile population structure in the Klias River reflects a population of maturity.

Adult crocodiles have a strong sense of territory especially when guarding their eggs in the nest.
Small crocodiles showed consistent preferences for the upper mangrove section of the river, a location undoubtedly related to the adjacent and extensive areas of grassland and scrub well-suited to nesting. They also demonstrated a significant preference for brackish water. It has been reported that C. porosus favours brackish water areas, due to the presence of prawns. Many small crocodiles were seen snapping at, and apparently feeding on juvenile prawns during surveys. Both prawns and hatchling crocodiles are known to grow faster in brackish water. Small juvenile C. porosus clearly showed preferences for warmer water that tends to select temperatures of 31-35^ºC while feeding, and 25-30^ºC while fasting. Large crocodiles do however seem to be more wide ranging and may be more inclined to enter freshwater swamps than smaller C. porosus. Sightings of this group also appeared to be independent of water temperature between 28-32^ºC.

Studies done by the UKMS scientists so far indicate that generally the populations of crocodiles inhabiting the Klias River and surrounding tributaries do not appear to be threatened. Illegal hunting is sporadic though not yet intensive in the Klias area. A factor that has resulted in increased illegal hunting is the fantastic rise in the price of crocodile hides that are turned into expensive fashionable products. Therefore, if the crocodile is to be saved, its reprieve must come from a change in fashion. The vogue for crocodile bags, belts and shoes must first pass before poaching are considered non-profitable and would stop. Crocodiles, in fact, seem well adapted to cope with most human activities in Sabah, with the exception of habitat destruction. The crocodile's swampland habitat has been rapidly disappearing in Sabah due to drainage and reclamation for development projects. The newest threat to the Klias crocodiles is the possible development of the area for aquaculture.