Estuarine Crocodile

Introduction

The largest of all living reptiles and an iconic Australian animal, the Estuarine Crocodile does not suffer foolish humans that enter its watery domain.

Identification

Typical crocodilian appearance – long well-toothed snout, eyes and nostrils set atop the head, thick skin with embedded bony plates (osteoderms), long muscular tail, streamlined body and short limbs with clawed webbed feet. The Estuarine Crocodile has a broad snout that is less than twice as long (from tip to midpoint between the eyes) as the width of the head. Adults typically have a heavy build and range in colour from golden tan to grey to black, with irregular darker mottling. Juveniles are brightly patterned with dark spots/bands on the body and tail; these tend to fade with age. Belly is cream-coloured.

Average total length for adults is 3-5m, with males typically growing much larger than females.The largest Estuarine crocodile reliably measured was caught in the Mary River in the Northern Territory in 1974. The headless carcass measured 548±8cm and the skull (midline length) measured 66.6cm, giving a total length of at least 615cm. However, the largest skull known measured 100cm, suggesting that it may have come from an even larger animal. The largest female reliably measured had a total length of 4m.

Habitat

The species can be found in a wide range of habitats, including rivers, estuaries, creeks, swamps, lagoons and billabongs. They can tolerate salinities ranging from 0% (freshwater) to 35% in full strength sea water, and have even been recorded in water twice as saline (70%) as sea water. Estuarine Crocodiles seem to be limited in their upstream movement primarily by physical barriers such as escarpments and other types of rapidly rising ground. In northeastern Queensland, they do not occur above 250m.

Distribution

Historically, Estuarine Crocodiles occurred from southwestern India and Sri Lanka east through southeastern Asia, the Philippines, the Indonesia Archipelago and northern Australia to the Solomon Islands. They are able to cross large stretches of open ocean, with records of individuals being found on remote islands in the Indian Ocean and the western Pacific Ocean, e.g. Fiji.

In Australia, Estuarine Crocodiles are found in northern coastal areas and drainages, from Broome in northwestern Western Australia to the Gladstone area in southeastern Queensland. They also occur on a number of islands off the Northern Territory and Queensland coasts which may be as far as 96km from the nearest point on the mainland.

Seasonality

Estuarine Crocodiles are active throughout the year.

During the cooler months in the Northern Territory (June-July) large crocodiles are often seen basking on the open mud banks, however in the warmer months (October-December) they seem to noticeably avoid the sun, and use the shade of mangroves when out of the water. Studies suggest that Estuarine Crocodiles rarely, if ever, voluntarily let their body temperatures rise much above 35° C before moving to a cooler location in the water or the shade.

Feeding and diet

Estuarine Crocodiles are mostly nocturnal but will also hunt during the day should an opportunity arise. They will eat just about any animal that they can catch and overpower. Small crocodiles feed frequently on small prey, such as insects and crustaceans, whereas larger individuals feed less often and on bigger prey, including water birds, sea turtles, and mammals up to the size of water buffalo. They are also known to be cannibalistic. Juvenile crocodiles tend to ‘sit and wait’ in shallow water for suitable prey to come within striking distance, although they may also chase small animals and can leap from the water to snare overhanging prey. Larger crocodiles actively hunt and are attracted to any movement that may represent a potential meal. When prey is detected the crocodile makes a stealthy approach under water, keeping any exposure to a minimum. Once in range the crocodile lunges rapidly and slams its jaws shut on the victim – the force of this alone may be enough to kill it. Small prey is simply crushed and swallowed, however larger prey may be dragged to deeper water before being dismembered and eaten. If the meal is too big to be swallowed whole, the crocodile will grab hold with its jaws and shake violently or roll to tear off a manageable piece. Because the tongue and skull bones of a crocodile are not very movable, food is tossed around in the mouth to manipulate it into a position for swallowing. After eating its fill (a crocodile’s stomach is relatively small), the crocodile may store the remains in mangroves or underwater to feed on again at a later time.

Estuarine crocodiles are also scavengers and will come on to land to feed on carrion or unattended catch.

Other behaviours and adaptations

With their strong homing ability, Estuarine Crocodiles are able to find their way back to their territory after being displaced, be it in the same river system or a different river system. In one Northern Territory study of “problem” crocodiles captured in one river system and released into another, recaptured individuals were able to find their way back to the site of their original capture at a mean distance of 25.6km (straight line return) and a mean distance of 65.0 km (assuming a coastal return). Some crocodiles returned in as short a time as 10 days and as long as 3.7 years. These data suggest that, from a management point of view, capturing problem crocodiles and releasing them in other areas is not particularly effective.

Communication

Estuarine Crocodiles communicate with each other using sound, visual and chemical signals. Hatchlings ‘chirp’ to gain the attention of their mother and to keep members of the creche together. Adults and juveniles may emit a low rumbling growl in response to a predator, and males will also growl to advertise their presence during the breeding season. Body posture is used as a visual form of communication, e.g. raising the snout to signal submission, tail arching as a threat display. Estuarine crocodiles may also communicate via chemical means, although to what extent is not clear. Glands underneath the chin and within the cloaca exude a ‘musk’ which may play a role in courtship or territorial marking.

Breeding behaviours

The buildup to the wet season in northern Australia stimulates courtship and mating behavior. As the reproductive season approaches, males advertise themselves in conspicuous displays intended to drive off rival males and stimulate receptive females. Male interaction includes chasing, head-slapping and growling, and can escalate to full-blown combat leading to serious injury and death. Females also become intolerant of other females and will jostle for dominance. Courtship observed in captive animals involved snout contact and rubbing, body-riding, vocalizing, bubbling and circling, with both partners constantly submerging and resurfacing. Copulation lasts up to 15min and may take place while totally submerged. In Arnhem Land, females are estimated to reach sexual maturity at a snout-vent length of about 110 cm and males at about 160 cm, however at that size they may be prevented from reproducing by larger dominant animals.

Nesting takes place throughout the wet season (from late October to May-June). The female selects a secluded area typically close to permanent water (usually within 20m), and constructs the nest by first making a clearing and then scraping piles of vegetation and earth together to form an elliptical mound up to 2.5m long and 80cm high. Some nests are built on floating mats of vegetation that extend out from the river bank. Once the mound is constructed the female digs an egg chamber and lays up to 71 hard-shelled eggs (average of around 50). The eggs are covered over and are incubated both by heat generated from the rotting vegetation and by solar radiation. In the Northern Territory, most nests have an adjacent “wallow”, a muddy depression in which the female stays to guard the nest.

Despite fluctuating external conditions, the temperature in the egg chamber stays relatively stable, with the preferred temperature being around 31-32°C. Like other crocodilians, the sex of the developing embryos is determined by the incubation temperature - cooler temperatures tend to produce females and warmer temperatures produce males. If the temperature in the central part of the nest rises above about 33-34°C or goes below 26-28°C for very long, the embryos can be killed or experience deformities such as misalignment of the jaws, deformities of the spine, curling of the tail and an inability to absorb the yolk once it is enclosed within the body cavity.

The young hatch after 2-3 months, depending on the incubation temperature, and begin to ‘chirp’ to attract the attending female. After helping to dig out the hatchlings the female assists in carrying them in her mouth down to the water where she continues to protect them for some time.

Conservation status

• Vulnerable (Queensland)
• Specially protected (Western Australia)

Predators

Eggs are subject to predation by goannas and feral pigs, however unlike Freshwater Crocodiles this doesn’t appear to be a major cause of eggs mortality, presumably due to the presence of the guarding female (Freshwater Crocodiles generally don’t guard their nests). Most Estuarine Crocodiles eggs do not survive to hatching for other reasons, including infertility, flooding, overheating, poor gas exchange, and desiccation. It is estimated that up to 75% of eggs laid in a season will not hatch.

Young hatchlings may fall victim to birds of prey, large fish, freshwater turtles, and other crocodiles, and very few will make it to adulthood. Once they do they have little to fear besides larger crocodiles and humans. Prior to the 1970’s Estuarine Crocodiles were hunted extensively for their skins and their numbers dwindled considerably. Since being protected they have made a spectacular comeback, reclaiming much of their original territory. Many though are still killed accidentally in fishing nets every year.

The species’ recorded endoparasites include nematodes (round worms), pentastomids (tongue worms) and trematodes (flukes).

Management

Today, attempts are made to trap problem crocodiles and relocate them to crocodile farms rather than release them back in the wild (due to their homing ability they would otherwise return to their capture site). Education programs and warning signs along waterways aim to protect both people and crocodiles.

Limited numbers of crocodile eggs, juveniles and adults are taken from the wild every year to supply crocodile farms with breeding stock and animals for commercial harvesting (meat and skins).

Danger to humans

The Estuarine Crocodile is a top predator in its environment, and a large specimen is likely to consider humans as potential prey. Since receiving protection in the 1970’s, Estuarine Crocodiles have steadily increased in number, and encounters with humans have become more frequent. Despite numerous crocodile warning signs around popular waterways, people choose to ignore the risks and many attacks and near-misses have resulted. Taking certain precautions in crocodile-inhabited areas greatly reduces the risk of encounter:

• Obey all crocodile warning signs – they are there for a reason
• Do not assume that a shallow pool, drainage canal or even a ditch is safe, especially if the water is muddy
• Do not allow pet dogs to roam near the water. Supervise children at all times and explain the dangers
• Do not walk around at night without a torch if fishing or camping near water
• When camping, choose a site well away from the water, preferably on high ground. If camping on a beach, be aware that Estuarine Crocodiles sometimes come ashore at night
• When fishing stand at least 3m back from the water’s edge, and cut the line if it becomes entangled rather than wade in
• Never leave animal carcasses, fish guts, etc. near where people swim, fish or moor boats
• If boating do not allow arms or legs to dangle over the sides

Most attacks have occurred on swimmers or on people canoeing or bending down at the water’s edge. This low profile seems to elicit a greater predatory response than from a person standing upright, even if in shallow water.

A person seized in the water by a Estuarine Crocodile has little chance of escaping without serious injury, if at all. Resulting wounds are usually horrific and likely to become infected.

References

Cogger, H. (2000) “Reptiles and Amphibians of Australia”, Reed New Holland

Greer, A.E. (2006) “Encyclopedia of Australian Reptiles : Crocodylidae”, Australian Museum

Ehmann, H. (1992) “Encyclopedia of Australian Animals : Reptiles”, Australian Museum, Angus & Robertson

Webb, G. & Manolis, C. (1989) “Crocodiles of Australia”, Reed Books