Trophic Level IV: Large Carnivores Crocodiles Page 1

  Nile Crocodile
Crocodylus niloticus

Length: 3 - 3.6m (10-12 feet), largest reach 6.3m (20 feet)

Weight: up to 1 000 kg (2,204 pounds)

Habitat: freshwater lakes, swamps, rivers

Food: Prey changes with the size of the crocodile. Young ones eat insects, crustaceans, turtles, birds and fish. Mature, fully grown crocodiles will feed on all of the above plus antelopes, zebra, buffalo and any other animal it can seize and drown. They have been known to take human beings.

Crocodiles, because they are cold-blooded, only need to eat half their body weight per year. A single wildebeest could provide more than enough food to see a crocodile through from one year’s migration to another. The annual migration draws large crocodiles to drinking and crossing spots along the rivers. The Grumeti and the Mara Rivers are both well known for their crocodiles.

At one such spot along the Grumeti, a stretch known as KiraWira, the herds come down to drink. Here, the crocodiles wait in ambush with their eyes just above the surface of the water. The herds are nervous as they come to the river pools to drink but their nervousness is not due to the crocodiles. Lions often lie in ambush too. Wildebeest know what to do if they meet a lion.

A New Enemy

A crocodile is another matter. Most have never seen a crocodile in their lives. Once confident that no lion is waiting, they wade into the water to drink. They pay little attention to the "floating logs" that drift closer to them. It is only when the crocodile lunges that the wildebeest flee. Often, the crocodile misses, but if it succeeds in getting a good grip on its prey, the wildebeest is doomed. The wildebeest will either be drowned or torn to pieces by other giant reptiles that rush in for their piece.

The wildebeest herd returns quickly. Since the crocodiles do not chase them, they do not seem to think of them as a real threat.

At the Mara, the wildebeest come, not so much for a drink, but to cross the river. Here, many die in the confusion that results when several thousand wildebeest crowd into a river. Soon, the river is lined with the bodies of hundreds of dead wildebeest, all ripening nicely for the crocodiles.

Crocodiles have 66 cone-shaped teeth which are ideal for grasping prey. Crocodiles cannot chew or bite. They must tear up a victim and swallow it in chunks. Rancid, decaying bodies are easier to manage. When it kills an animal the size of a wildebeest, the crocodile will sometimes drag it under the water and anchor it someplace where it will not drift away. Then it can feed at leisure as the corpse ripens.

At KiraWira, one film crew recorded large numbers of giant crocodiles gathering in a river pool to corral and feed on thousands of small fish that were trapped there.

In most areas, fish make up 80% to 90% of the crocodiles’ diet. Catfish are an important prey species.

Social System: Mature crocodile males form a dominance hierarchy in the mating season. They roar to announce their status. Females enter the male’s territory when they are ready to mate.

When it is time to lay her eggs, the female crocodile seeks a sandy beach in which to lay her 16 to 90 eggs. She scrapes a shallow depression and then covers the eggs with sand and debris. Several factors determine the location of the nest. There must be sufficient soil for her to dig. There must be nearby shade for her to cool off during the day. There must be permanent water. Some crocodiles have been known to go 400 meters (400 yards) from the water in order to find the conditions necessary to make her nest.

Once the eggs are laid, she will stay near the eggs for the 90 days it takes them to hatch. Sometimes, she waits in the water, while at other times she may be beside or even on the nest. Throughout this time she does not eat.

If she did not take these precautions, monitor lizards or mongooses would dig up the eggs and destroy her nest.

Sex Determination

When the young are ready to hatch, they make a yelp-squawking sound to which the female responds. She digs them out of their nest. The sound is audible for a distance of 20 meters (20 yards). She can help determine the sex of her babies by regulating the temperature of the nest. She does this through the selection of the nest site and by adding or removing soil. Cooler temperatures (26-30C or 79-86° F) will produce females. Hotter temperatures (31-34C or 88-93° F) produce males.

When her babies hatch, she grabs them in her massive jaws and gently flips them so that they are in her throat pouch. She carries them to the water, opens her mouth and the babies swim out. The babies will stay together in the water forming a creche for the first two months or so. If one is threatened, it issues a distress call that brings mother to its rescue. She will gather it in her mouth until the danger passes. Even so, only 1% survive their first year.

Young crocodiles are about 30 centimeters (1 foot) long at birth. After that, they will grow about 25 centimeters per year (10 inches) until they reach six years of age. Then their growth slows to a few centimeters (1 inch) per year. Males will not breed until they are over 2.7 meters long (9 feet) and females until they are 2.4 meters (8 feet). They reach these lengths when they are 17 years old.

Behavior: Cold-blooded animals regulate their body temperatures by moving from one location to another. A crocodile will bask in the sun during the cooler hours of the morning and evening but will seek relief from the heat by going into the water or by seeking shade. Crocodiles control their body temperatures and are able to maintain a level of 25.6 C (78° F) day and night without varying more than 2 or 3 degrees above or below.

Often, on hot days, crocodiles may be seen lying on the river banks with their mouths gaping. This posture allows moisture to evaporate from their mouths and helps cool them.

At times, when the river dries up, the Serengeti-Mara crocodiles cannot maintain their body temperatures in their normal way. Under these conditions, they may undertake a migration of sorts and head out over land to where there is permanent water. Failing that, they may find a burrow or den underground and retreat to it. Underground temperatures are fairly constant year round, no matter what the weather outside is. Here, the crocodile aestivates until rising water creates conditions necessary for its survival.

Swallowing Stones

An unusual facet of crocodile behavior is the habit of swallowing stones. These stomach stones (or gastroliths) are believed to serve several functions. First, they probably help the crocodile digest food by assisting in breaking down the chunks of meat swallowed when feeding. Second, they add extra weight, decreasing the crocodile’s buoyancy in water but improving its ability to swim. The stones also help increase the animal’s ability to pull large prey down beneath the water. The stones, because they lie in the center of the crocodile’s body, may also improve its stability in water by lowering its center of gravity. Studies have shown that crocodiles deliberately seek out such stones. Further, they are found in a constant ratio to the animal’s body. The standard load is equal to 1% of its body weight.

Predators: Nile monitor lizards frequently dig up and eat crocodile eggs. Spotted hyenas and mongooses will also eat them if they find an open, unprotected nest.

Young crocodiles are preyed upon by marabous and other storks, herons and egrets. Fish will also eat them.

Only adult crocodiles are relatively safe from predation. There is a documented case from Botswana where a leopard killed and fed on a crocodile for several days. The circumstances behind this were unusual. The crocodile was forced to look for water when its pool dried up.

Food Chains: Crocodile Eggs

Fig. # 1

Food Chains: Young Crocodiles

Fig: # 2

Food Chains: Adult Crocodiles

Fig. # 3

After: Cott, Hugh B., in Looking at Animals

Birds of Prey

This next section contains some of the ecosystem’s large predatory birds. Although they may sometime fall prey to leopards or lions, they are usually considered to be at the top of their food chain.

(see also: Appendix IX: Comparison chart of Serengeti-Mara’s Birds of Prey)

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Dave Taylor's African Safari - Book 5: Trophic Level IV: Large Carnivores (Standard Version)
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