Crocodiles are quite fascinating animals and the more that people know about them, the better they can understand them. Hopefully by understanding them, people may develop an appreciation of crocodiles which in turn may lead to some rational decisions being made about their long-term conservation and management here in Australia.
Crocodilians are the worlds largest and perhaps most exciting reptiles. They are also great survivors and their prehistoric ancestors, the Archosaurs, date back over 240 million years to the Triassic period. They have survived major upheavals such as the break up of the continents and the ice ages. They have witnessed the rise and fall of the dinosaurs and have seen the evolution of mammals and birds. Since man colonised the world, no species of crocodile has become extinct; however 17 out of the 23 species of crocodilians around the world are endangered.
They have distinctive features such as long jaws, protective armour, streamlined body and long tail. These, together with various anatomical and physiological adaptations, make the crocodile perfectly suited to an aquatic and predatory lifestyle. Their features have changed very little from those of their prehistoric ancestors, proving that their body form has been highly successful in nature. Some experts believe that the crocodile, in its present form, has not changed for the last 100 million years. Maybe a perfect design?
Crocodiles are cold blooded and have a body temperature similar to the surrounding air, land or water. Since they lack a reptilian thermostat, they seek a habitat with warm water and air temperature all year round. Though much maligned, crocodiles play an important role in wetland environments. They help keep the balance in the complex web of life in freshwater and estuarine ecosystems. They are key predators at the top of the food chain and eat a wide range of prey. They are also prey, when smaller, to other animals such as feral pigs, goannas, turtles, barramundi, sea eagles and even other crocodiles. Being predator and prey, the crocodile plays an important role in keeping a wetland ecosystem healthy and when a wetland habitat is healthy, the fishery is considered to be healthy too.
The Freshwater crocodile (Crocodylus johnstoni) is endemic to Australia. It is found nowhere else in the world. “Freshies” occur only in inland waters of Northern Australia, and in Queensland; they are found mainly in the rivers and swamps of Cape York Peninsula, areas bordering the Gulf of Carpentaria and the northwest. They also live in the tidal reaches of some rivers and therefore co-exist at times with estuarine crocodiles. They have a narrow snout and reasonably straight jaw line with even sized teeth. Males can grow to 3 metres but animals larger than 2.5 metres are rare. Females rarely exceed 2.5 metres. They are not “man-eaters” and feed mainly on insects, frogs, lizards, turtles, with bats, birds and small mammals being taken occasionally at the water’s edge. Even larger “freshies” tend to eat very small food items.
The Estuarine (Saltwater) crocodile (Crocodylus porosus) is found in the warm climate from Sri Lanka and India in the west to the Caroline Islands in the east, to the north from Burma and South-East Asia, to Australia, Vanuatu and the Solomon Islands in the south. In Australia, they are restricted to northern parts of the continent, from about Rockhampton on the east coast of Queensland to Broome on the west coast of Western Australia. Estuarine crocodiles are the largest of living crocodilians and quite probably the largest living reptiles on earth. They have broader snouts than “freshies” with an uneven “up and down” jaw line and irregular sized teeth. They vary in colour from grey, olive-brown to almost black, with ragged dark mottling. Males can grow to 7 metres (23 feet) but most are less than 5 metres. Females are usually less than 4 metres in length and may begin nesting at about 12 years of age. Maximum lifespan is unknown however it is estimated that they can live for at least 70 to 100 years.
The skin of the saltwater crocodile, especially from the belly surfaces, is the most prized of all crocodilian skins for fashion leather. Demand for skins (for use in shoes, belts, suitcases, briefcases and handbags) supported a significant crocodile harvesting industry in northern Australia between the end of World War II and the 1960s. Two factors led to the industry’s decline and final cessation. Firstly, saltwater crocodiles progressively became more difficult to find. Their populations declined due to relentless and skilled hunting. Secondly, by the late 1960’s early 1970’s, conservationists and governments were concerned that the species might become extinct in Australia. At a time when the industry had almost literally ‘killed the goose that laid the golden egg’, federal and state governments protected crocodiles by legislation. “Freshies” have oesteoderms (boney buttons) in their belly scales, are much smaller and have slower growth rates. This greatly reduces the value of their hides and makes them less attractive for leather.
“Salties” breed in the wet season (Nov-Mar) and build a nest consisting of a large mound of vegetation and soil. The nest is usually located in grass or fringing forest along the banks of a watercourse or freshwater swamp. About 50 eggs are laid inside the nest mound and incubation takes between 65 and 110 days. The female usually guards the nest vigorously as she hides in a nearby wallow. The incubation temperature determines the gender of the hatchlings with very high or low temperatures producing females, and temperatures of 31º to 32ºC producing males. Pigs and goannas eat crocodile eggs and floods destroy many nests. These factors contribute to the what sounds like an alarming statistic that about 25% of eggs laid will hatch. From those hatchlings that emerge, less than 1% survives to adulthood. This is about average for most species in nature.
Young crocodiles are born with a horn or egg tooth on the tip of their snout, which later drops off. This enables them to break out of the egg. When hatching, young crocodiles squawk to attract the mother, who may in turn dig them out and carry them gently to the water in her mouth. Unhatched eggs are sometimes rolled around on the mother’s tongue to help the young emerge. Hatchlings are about 70 grams and about 25-30 centimetres long and may be protected as a crèche for several months in the water by the mother. More than half the hatchlings die in their first year of life because young crocodiles become prey for other animals. A high proportion of juveniles are displaced from rivers by larger crocodiles and many die during their exile. They are also known to have a homing instinct with some specimens having travelled more than 60km to return to their capture site.
Smaller crocodiles appear to feed throughout the year, reducing their intake during cooler periods. Larger crocodiles are affected more by cool weather and their food intake is greatly reduced or can stop altogether. They can live for months at a time without feeding as they carry extensive energy supplies in the form of fat. The wet season seems to be the period when growth and feeding are maximised in crocodiles of all sizes. Young crocodiles eat small animals such as crabs, prawns, fish, frogs and insects. Larger crocodiles take bigger prey including pigs, birds, reptiles, turtles, wallabies and even other crocodiles. This cannibalistic behaviour is believed to be an important population controlling mechanism.
Crocodiles have a tendency to retain hard, indigestible objects in their stomach and most crocodiles over two metres long have been found to have stones in their stomach. It is believed that the stones are used as gastroliths (to aid in grinding up food) and that they also may function as ballast (just as in a ship’s hull). This tendency for retention of hard objects can also confound coroners’ inquests where people have been killed by crocodiles. Along with the human remains, bullets are often recovered from the stomachs of large crocodiles. The bullets usually come from other animals that have been shot and have died, and are later eaten by crocodiles. The animals are digested but the bullets remain.
A unique feature of crocodilian physiology is their ability to maintain strenuous activity for only short periods of time, after which they become totally exhausted. This can occur during the capture of prey, being captured or even fighting other crocodiles. This extreme exertion is carried out anaerobically (without oxygen), and must be followed by a period of rest so that the “oxygen debt” can be repaid. The result of anaerobic activity is a build up of lactic acid in the blood, making it acidic. Although crocodiles can withstand higher levels of blood acidity than other animals, sometimes it can prove fatal. This is why larger crocodiles, over 5m, often die during capture operations, if they are allowed to struggle excessively.
Crocodiles use a combination of active hunting and the more passive “sit and wait” strategy. Juveniles tend to position themselves in shallow water with all four feet on the bottom and wait until potential prey comes within striking distance of the jaws. The movement of prey is detected by the enlarged sensory pits along the sides of the jaw. The most common strategy of larger crocodiles involves an underwater approach to potential prey on the bank, in the water at the bank or in overhanging vegetation. Once a crocodile is attracted by the movement, sound and perhaps smell of potential prey, it will orient its head towards the prey, submerge (usually without a ripple), and swim underwater until it reaches the immediate vicinity of the prey. Then as the head silently emerges, if the prey is within striking distance, it will lunge with the jaws opening then slamming shut. A crocodile can lunge more than half its body length into the air or out on the bank. Once caught, small prey is usually crushed and swallowed. Larger prey is squeezed tightly until all movement stops. The largest prey evokes the full attack sequence. Once the crocodile has a grip, it will roll to throw the prey off balance so it can be dragged into deeper water and drowned. Because the stomach of the crocodile is small relative to the size of some prey taken, head shaking, thrashing and rolling is used to dismember large prey into smaller pieces for eating.
Estuarine crocodiles are unique in the reptile world and use their blood system to remove salt from the body. Lingual glands at the back part of their fleshy tongue excrete excess salt when the animal is living in a highly saline environment. They also are one of the few reptiles to have a four chambered heart (just like us) and have the ability to slow their heart rate down to one beat every thirty seconds or so. They have been observed in captivity holding their breath for up to four to six hours underwater. In the wild in Queensland, one crocodile was observed making a voluntary dive and holding its breath for three hours and ten minutes. Salties have a protective translucent third eyelid called the nictitating membrane, which closes sideways across the eye. This allows them to see and swim at the same time. It is just like swimmers’ goggles, “crocodile style”. At night, crocodiles are easily detected by their “eye shine” which is the red reflector look that they assume when illuminated in the dark. The tail of the estuarine crocodile is 49.5% of its total body length, the longest of any crocodile. The tail is used to propel the animal through the water and scutes (spikes) along the top of the tail are an important part of the tail. Not only do they increase the surface area and therefore thrust for the tail, they are made of cartilage, have a good blood supply and are an important device used for temperature regulation.
Although crocodilians have a rich repertoire of behaviours, there is little information available on those of saltwater crocodiles. In the “daily life” category of behaviours, regulating body temperature is perhaps the most obvious. With a preferred body temperature of 30° to 33° C, crocodiles use the water, sun and shade to regulate their body temperature and move between these warm and cool parts of their environment to adjust it. They are regularly seen basking in the sun with a mouth gaping posture. One theory is that this is done to cool the brain through evaporation from the palate while the body is heated by the air and sun. Other theories include strengthening jaw muscles or maybe a display to other crocodiles. One of the most common displays seen during interactions between crocodiles is “snout lifting”. This signals, “I give in”. It is very common when large crocodiles approach smaller ones. Salties adopt an inflated posture that seems to be a threat display. This is often accompanied by “tail arching”, a display in its own right but also a means of making the body rigid to enable the head to be swung with power. The same posture can be adopted before “head slapping”, which signals a crocodile’s presence to all around. Saltwater crocodiles are rated out of all crocodilians as the most intolerant of other members of the same species. One behavioural trait that is well documented is that saltwater crocodiles are highly territorial. Little is known about the territory sizes or the frequency with which successful challenges to territories are made.
The Environmental Protection Agency, Queensland’s nature conservation authority, are attempting to meet the challenge of protecting crocodiles and preventing their extinction, while trying to ensure that people can safely co-exist with these animals. However, despite wide community awareness of and response to warning signs, and educational information provided by fauna authorities, occasionally crocodiles continue to pose a threat to people.