Understanding crocodile behaviour

by S.R.B. Dissanayake, S. Wijeyamohan and Charles Santiapillai

Relationships between mankind and reptiles had been rather tenuous over the period of time turbulent and tentative. Many people abhor reptiles such as snakes and are absolutely frightened and terrified of crocodiles.

This is understandable given that crocodiles are dangerous reptiles designed for a predatory lifestyle. Hence an understanding of the biology and behaviour of crocodiles can not only help reduce the conflict between man and crocodile, but also highlight their ecological and economic importance.

Of the 13 species of true crocodiles found globally, two, namely the Freshwater crocodile (Crocodylus palustris) and the Saltwater crocodile (C. porosus) occur locally. Crocodiles belong to an ancient group of reptiles that appeared on the face of the earth over 240 million years ago.

They are great survivors who witnessed the rise and fall of the dinosaurs and outlived them by 65 million years. Their basic body plan was so efficient that it had remained virtually unchanged over millions of years. Hence crocodiles are referred to as 'living fossils'. They are among the most resilient of species, able to bounce back, provided their habitats remain intact. Although a few population have declined in size, none of the species has become extinct.All crocodiles are characterised by long jaws, a protective armour of scales, a streamlined body and a long tail. The saltwater crocodile is the world's largest reptile; and also the deadliest. Crocodiles have an excellent immune system that prevents life-threatening infections following vicious territorial fights in waters teeming with bacteria and other microbes. Thanks to the immune system injuries heal quickly while crocodiles sulk. Thus they can survive even under the most appalling conditions.

Reptiles

Crocodiles being reptiles, are cold-blooded. This means unlike birds and mammals, they lack a thermostat and so cannot generate internal body heat to maintain a constant body temperature. Their body temperature is determined by that of their environment. They perform well between 30 and 33 degrees centigrade. Thus heating and cooling are important for crocodiles. They move in and out of water depending on whether it is hot or cold outside.

Crocodiles are opportunistic feeders and feed on a variety of prey that range in size from shrimp to buffalo calves. Thus, they are key predators at the top of the food chain in aquatic habitats and help keep the balance in the complex web of life in wetland ecosystems. Crocodiles are ambush predators that sit and wait for any unsuspecting prey to come to the water's edge so that they can lunge and bring it down.

The movement of prey is monitored through the sensory pits along the sides of the jaws. As the crocodile struggles with its bulky prey underwater, it is prevented from drowning by the closure of the palatal valve at the back of the throat. Once underwater, the flaps on the nostrils remain closed to prevent water entering the lungs. Being cold-blooded, crocodiles have a low metabolic rate and hence they do not have to feed constantly instead they can fast for long periods. Since their stomach is relatively small, crocodiles cannot eat a huge meal in one go.

This is why they stash their bulky prey among roots in the aquatic habitats so that they can return to eat a portion of it at a time.

While they bite into their bulky food, crocodile eyes can sometimes froth and bubble, giving the appearance of shedding tears.

As Kent Vliet from the University of Florida observed, crocodiles do "bawl while banqueting", but for physiological reasons and not due to any reptilian remorse. Men do not cry in public, unless they are of the crocodile variety. The teeth are all sharp and conical in shape designed to stab, impale and kill the prey and not to chew or masticate it. They are also constantly replaced throughout life.

The tongue is immovable and food is simply swallowed. Stones in the stomach (called gastroliths) may help grind the large pieces of ingested food. They may also function as ballast.

Hunt

Crocodiles usually hunt in the late evenings or nights but digest their food during the day when ambient temperatures are high. For digestion to take place enzymes are needed and enzymes can act only at high temperatures. In the absence of enzyme action, food instead of being digested, will rot and so the animal can die of food poison. Hence crocodiles have to increase their body temperature.

This they do by lining themselves in such a way as to receive the maximum heat from the sun. If the body is over-heated, there is a real danger that the brain becomes either dysfunctional or dead. This is prevented by basking during which crocodiles can be seen facing the sun directly with their mouths wide open. The warm blood that comes to the buccal cavity exchanges its heat with the outside in the process cools before reaching the brain.

The muscles that open the jaws are very weak while those that close them are extremely powerful. The pressure from a croc bite can be more than 5,000 pounds per sq. inch compared to the pressure of 350 pounds per sq. inch from a Rottweiler bite!

One of the interesting features of crocodilian physiology is their ability to maintain strenuous activity only for a short time. Such extreme exertion of energy takes place anaerobically (without oxygen) while fighting with other crocodiles for territory or mates, and during feeding after which they become exhausted and need to rest to recover and repay the "oxygen debt". During such anaerobic exertions, lactic acid builds up in the blood.

At high concentrations, the blood will become too acidic and it can prove fatal for crocodiles. This is one reason for mortality of crocodiles during capture operations.

Vision

Crocodiles have excellent vision and can see well even in dim light thanks to the retinal tapetum made of a layer of guanine crystals at the back of the eye that acts as a reflector. Light that enters the eye through the pupil is reflected back for enhanced vision.

When a light is shone on crocs in the night, they can be easily detected by their "eye shine" which is the reflection from the retinal tapetum. Furthermore, being efficient aquatic predators, crocodiles enjoy binocular vision which enables them to judge the distance to their target prey accurately before leaping out of water.

This is possible mainly because of the placement of eyes close together in front of the head so that the fields of vision from right and left eyes overlap. Even on a crocodile that is 5m long, the eyes are situated only 7cm apart.

Crocodiles are also among the few reptiles that have a 4-chambered heart as in birds and mammals to separate the blood that goes to the lungs (pulmonary circulation) from that goes to the rest of the body (systemic circulation). Thus, there is no mixing of oxygenated blood from the lungs and deoxygenated blood from the body.

It is this feature that enabled crocodiles to grow big and become the largest reptiles on earth. Large animals need high pressure from the systemic left ventricle to pump blood around the body and such high pressure would rupture the delicate alveolar tissues in the lungs.In the 4-chambered heart, the smaller right ventricle provides the correct pressure to pump the blood to the lungs.

Furthermore, while diving, the valve on the "foramen of Panizza" can shut off the pulmonary circulation completely since it is redundant until the animal surfaces to gulp air. Thus during diving, blood is diverted from the pulmonary circulation to the rest of the body.

Crocodiles are also unique in their ability to control their heartbeat. The high levels of haemoglobin in the blood enable crocodiles to carry more oxygen for release to the oxygen-starved tissues while remaining submerged - an excellent adaptation for hunting in water.

Breeding

Crocodiles, especially the "salties" are highly territorial during the breeding season. Males being polygynous acquire harems and defend their territories aggressively. Mating is preceded by elaborate courtship.

Females lay between 40-60 eggs either in a hole (by freshwater crocs) or on a mound (by saltwater crocs). Despite the parental care, 80% of the eggs do not develop during incubation. Of a cohort of 1000 eggs, only eight crocs may survive to reach five years of age! Thus, every living adult crocodile is a wonder of nature that has survived against all odds. Crocodiles are efficient colonisers of freshwater ecosystems.

A combination of amphibious way of life and cryptic behaviour enables crocodiles to survive even in densely populated areas.

Saltwater crocodiles are potentially dangerous animals that can kill man and livestock in areas where their natural prey base has eroded.

While it may be possible to constrain the movement of such large terrestrial animals as elephants by erecting electric fences or digging trenches, no such methods will work for crocodiles.

Economically, crocodiles are extremely valuable for their skin and meat while ecologically they, as top aquatic predators, are important in maintaining the structure and functioning of aquatic habitats.

Conserving crocodiles in Sri Lanka is not easy as they are not cute, cuddly or charming.

They are rightly perceived by the public as dangerous animals to be avoided at all costs. Besides, crocodiles in Sri Lanka have had a bad press.

They are portrayed as villains in villages.Crocodiles kill much fewer people in Sri Lanka annually than poisonous snakes, rabid dogs, mosquitoes, marauding elephants and speeding maniacs on the motorways. Conservation programs should not be developed without reference and respect to local attitudes.

As long as people perceive crocodiles simply as dangerous pests and refuse to acknowledge them as potentially important economic and ecological assets, they are unlikely to appreciate the value of crocodiles and thus become committed to conserving them.