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Habitat
Saltwater crocodiles (Crocodylus porosus SCHNEIDER 1801) are distributed throughout the coastal regions of northern Australia, from Broome in Western Australia to Rockhampton in Queensland.
It is found primarily along mangrove-lined tidal rivers (in brackish water) up to 200 km from the coast, and flood plain billabong's, creeks and freshwater swamps up to 100 km from the coast.
Adults can venture out to sea and swim around coastlines or between islands. Individuals have been found to travel over 1000 km by sea.
They can be found basking on the exposed mud of tidal riverbanks during the cooler months, but they spend more time in water and shade during the hotter summer months, coming out to hunt in the evening and night.
Saltwater crocodiles are found in higher densities in areas with a good food supply and abundant nesting areas, where recruitment into the population is higher.
Feeding
Hatchlings contain a good yolk supply, on which they can survive for days and even weeks if necessary, although they have sharp teeth and will begin feeding almost immediately.
Juveniles take a wide variety of small prey items such as insects, other arthropods, crustaceans and small fish. Larger animals take bigger versions of these prey items, plus amphibians, reptiles, birds and small mammals.
Very large animals will take almost anything including dingoes, wallabies, shore birds, other crocodiles, large reptiles, domestic animals, cattle and even people. They will also eat carrion, being attracted from some distance to reach it (even out of water). Smaller animals tend not eat carrion, however, which can rot in their guts before they have chance to digest it.
Teeth are designed for holding rather than cutting, but they help to penetrate and crush the prey within the incredibly powerful jaws. Larger prey are broken into smaller pieces either by a violent flick of the head, or a twisting / rolling action of the body (if the prey is secured or held down by its own weight).
Swallowing must occur above the water surface, or water will flood the lungs and the crocodile may drown. Normally, a fleshy "palatal" valve at the back of the throat prevents this from happening when the head is submerged.
The typical crocodilian feeding strategy is to wait close to the water's edge and pounce upon prey which ventures too close. Larger animals will actively follow and move towards potential prey items, but in most cases the animal doesn't reveal itself until just before the attack.
Stones and pebbles are often ingested to aid digestion - crushing food by a grinding action within the gizzard of the stomach. Stones also act as a ballast, which is important in maintaining buoyancy.
Sub-adult Saltwater Crocodile gaping - serves to cool a hot head, and also to threaten
Behavior
Most of their time is spent thermoregulating, to maintain a body temperature between 30 and 32 degrees Celsius (the optimum temperate for digestion and other activities), and in maintaining their territories. Territorial behavior relies upon a number of subtle cues, such as changes in body posture and low-frequency vocalisation. Violence is generally avoided, as serious injuries and death can occur in larger animals.
Sub-adult Saltwater Crocodile photographed at Dubai Underground Aquarium
Nesting
Saltwater crocodiles are mound nesters, using a variety of grasses and other materials. By creating a mound, the eggs are raised about the ground water table, and the risk of flooding is slightly diminished. Yet, many nests are lost to flooding each year. When the eggs are submerged, oxygen cannot reach the embryos and they die within a few hours. Other major causes of death are overheating (temperatures rising above 34 degrees C) and predation by goannas / monitors and feral pigs.
Nesting sites near water are selected, so the females can remain on guard, and so the hatchlings do not have far to travel. Courtship occurs in September and October, and nesting takes place throughout the wet season, generally between November and March.
Between 40 and 60 eggs are normally laid (mean of 50), depending on factors such as the size of the female and age. Laying takes an hour. Eggs weigh around 113 grams and measure 80 mm x 50 mm. Within several hours of being laid, the embryo attaches to the eggshell and a white spot appears on the eggshell. Over the next couple of weeks, the band expands and covers the entire egg as the embryo grows.
The embryos do not have sex chromosomes, and sex is not determined genetically. Rather, sex is determined by temperature - mostly males are produced at around 31.6 degrees C, with more females produced at slightly lower and higher temperatures. Mean incubation period is around 80 days, and is dependent upon temperature. Hatchlings weigh around 72 grams and measure 29 cm total length.
The female remains close to the nest throughout incubation and defends it vigorously against any potential threat. Once the eggs start to hatch, the hatchlings produce characteristic calls which stimulate the female to begin digging the nest open. She carries hatchlings to the water in her mouth, gently breaking open unhatched eggs. Juveniles remain close to the female for several months after hatching in a creche, staying in touch with acoustic calls.
On average, at least 80% of all eggs die during incubation, and less than one percent of all hatchlings will reach maturity. Major predators of the hatchlings include birds and fish, but the major cause of death is cannibalism by other, adult crocodiles.
Growth
Males reach sexual maturity at around 3.3 metres (10.8 feet) at around 16 years old. Females reach sexual maturity at 2.3 metres (7.5 feet) at around 12 to 14 years old. Maturity is reached more rapidly in captive animals, which grow more quickly as their food intake is much higher.
Baby Saltwater Crocodile photographed at Animals Anonymous, South Australia
Maximum size in males has been documented at around 7 metres, and over 1000 kg in weight, but 5 to 6 metres is a more realistic maximum size. Maximum age is at least 70 years, perhaps over 100 years old. Growth generally continues throughout life, but is very slow in older animals. Large crocodiles can be aged by the number of growth rings in their bones.
Conservation
Unregulated hunting occurred over 25 years and brought the population to the brink of collapse. Since 1971 (and 1974 in Queensland), Saltwater Crocodiles have been protected. Populations have shown a massive recovery throughout Australia, particularly in the Northern Territory, yet public opinion about crocodiles is still mostly negative. Concern over increasing numbers makes it critical to gain public support about crocodiles. This can be achieved in several ways:
1. Education: myths about crocs do not help their image, and lack of knowledge about safety around crocs is dangerous.
2. Problem croc programs: so nuisance crocs can be safely removed
3. Utilisation programs: to give the crocs (and hence their habitat) some monetary value. This only works if proper controls are in place, otherwise illegal trade may be stimulated.
Safety
Crocs attract tourists to areas in which they occur, but they can also be dangerous. Safety measures include no swimming, no cleaning of fish on river banks, nor tethering dogs close to the water. Nesting areas should also be avoided.
Most attacks occur in the wet season, when salties do most of their feeding and growing. Despite notions to the contrary, they don't really bother small fishing vessels.
In Darwin, crocs outside the harbour area are protected, but those reported within it are either shot or moved to farms as part of the problem crocodile management program.
Fatalities are very uncommon - there have been only 7 deaths in 27 years of protection in the Northern Territory, and 14 throughout Australia. In most cases, the victim was swimming. In virtually all cases, death could have been avoided with correct education. The low incidence of attacks is due to the fact that most people in Australia are well educated about crocodiles.
Dr. Britton is from England, but has traveled widely in his studies of crocodilians. He is currently engaged in research in Australia.
Some of his work may be viewed at CROCODILIANS: Natural History and Conservation.
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