Nile crocodile

• 1. Introduction

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  The Nile crocodile, Crocodylus niloticus, produced a male shot near Mwanza, Tanzania, that measured 6.45 metres and weighed about 1043 to 1089 kilograms. He is the largest accurately measured male of the species on record. Animals of that scale live in 26 African countries, and the species holds the title of largest predator on the continent.

  Several puzzles sit behind those numbers. How does a reptile that heavy stay hidden until the instant it strikes? What does a crocodile parent actually do once the eggs are laid? How did a species pushed toward extinction come to be listed as common today? And why do people who live beside African rivers keep meeting it at the water's edge?

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• 2. Built for Ambush

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  A Nile crocodile carries 64 to 68 cone-shaped teeth, each pointed and made to sink into flesh and hold on. For most of the animal's life, a broken tooth is simply replaced. The enlarged fourth tooth in the lower jaw stays visible when the mouth is shut, as it does in every true crocodile.

  Brady Barr measured the bite of an adult at 5000 pounds of force. Yet the muscles that open the mouth are remarkably weak. A person can easily hold the jaws closed, and duct tape can bind even large crocodiles. Alligators and caimans have broader skulls, but their bite is proportionally about the same. The strongest bite ever tested in a laboratory belonged to a male saltwater crocodile of roughly 4.59 metres.

  Two hours is how long a Nile crocodile can sit motionless underwater. The reason is an extraordinary level of lactic acid in its blood, a level that would kill most vertebrates. The same chemistry is a weakness. A long struggle can push lactic acid to a lethal level and cause the organs to fail. Wild cases are rarely recorded, and the known ones usually follow humans mishandling a crocodile and wearing it out. The heart has four chambers and an elongated septum, and it resembles a bird's heart in how efficiently it oxygenates blood. The nostrils, eyes and ears all sit on top of the head, so the rest of the body can stay concealed beneath the surface.

  In 1961, Cott studied breeding adults from Uganda and Zambia and found an average of 3.16 metres and 137.5 kilograms. That sits well below the 4.5 to 5.5 metres sometimes quoted as typical, which most studies treat as the ceiling for the largest animals. Graham's large sample from Lake Turkana, then called Lake Rudolf, averaged 3.66 metres and 201.6 kilograms. Females are usually about 30% smaller than males. Even so, a male averaging 4 metres and about 280 kilograms outweighs the lion, whose males average 188 kilograms and top out at a known 313.

  Captive giants feed the legend. Big Daddy, at the Mamba Village Centre in Mombasa, Kenya, measures 15 feet and weighs 2200 pounds. In Burundi, a crocodile called Gustave is estimated at 20 feet and close to 2000 pounds, and he is believed to have killed hundreds of people. Among museum skulls in Arba Minch, Ethiopia, taken from Lake Chamo, the largest measures 68.6 centimetres, with a lower jaw of 87.

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• 3. Patience and Ambush

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  A stomach not much bigger than a basketball sits inside an average adult Nile crocodile, so this is far from a voracious eater. Cott found that 20.2% of adults between 3 and 4 metres long had empty stomachs. At Lake Turkana in Kenya, 48.4% of the crocodiles examined were empty. Brooding females always have empty stomachs and can go several months without eating. A cold-blooded metabolism makes that possible, and ambush lets the crocodile wait hours, days and even weeks for the right moment.

  Most land hunting happens at night, from ambush near forest trails or roadsides up to 50 metres from the water. Speed on land is no match for most terrestrial prey, so the crocodile depends on cover. In one recorded case, an adult charged up a bank to kill a bushbuck. It then dragged the kill into the bush instead of the water. Two subadults were once seen carrying the carcass of a nyala across land together. In South Africa, a game warden in savannah scrub far from any water reported a crocodile leaping up to seize a donkey by the neck.

  In the water, subadults and smaller adults use their bodies and tails to herd fish toward a bank. They then eat them with quick sideways jerks of the head. Groups also form a semicircle across a river to block migrating fish. Some crocodiles sweep their tails to knock land animals off balance and into the water, where drowning is easier.

  Sometimes three or four dozen crocodiles feed together on a dead hippopotamus, tolerating one another. Groups may travel hundreds of metres from the water to reach a carcass. They also steal kills from lions and leopards, and lions will do the same to them when food is scarce. Those disputes can become a tug-of-war over a carcass, though serious bloodshed is seldom seen. Much of what fills crocodile stomachs may be carrion, a role resembling that of vultures or hyenas on land. To tear off chunks, crocodiles sharing a kill brace against each other, bite down and twist in a death roll. A lone crocodile may first wedge its prey under branches or stones.

  Large, old males sit at the top of a strict hierarchy of size, with first access to food and the best basking spots. Breaking that order is often violent and sometimes fatal.

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• 4. Stones, Burrows and Tales

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  Between 9:00 a.m. and 4:00 p.m. on sunny days, more than half the crocodiles Cott observed lay basking with their jaws open, if left undisturbed. Gaping helps regulate temperature, and a crocodile with its jaws bound in extreme midday heat can die from overheating. Gaping can also be a threat display, because some crocodiles have been seen doing it at night, when overheating is no risk. At Lake Turkana, crocodiles rarely bask at all, for reasons nobody knows.

  Herodotus, who reported seeing the species in Lake Moeris in Egypt, also described a bird called the Trochilus that entered a crocodile's mouth to pick off leeches. Guggisberg, in 1972, saw birds take scraps of meat from the teeth of basking crocodiles without going inside the mouth. MacFarland and Reeder reviewed the evidence in 1974 and found only a few reports of sandpipers removing leeches from the mouth and throat scales and snapping at insects along the body.

  One crocodile measuring 3.84 metres and weighing 239 kilograms held 5.1 kilograms of stones in its stomach, perhaps a record. These swallowed stones, called gastroliths, are absent in hatchlings. They become common in crocodiles of 2 to 3.1 metres and then grow rare again in very large animals, so some may eventually be expelled. Specimens shot near Mpondwe on the Semliki River carried stones even though no stony ground lay nearby. The same was true on the Kafue Flats, the Upper Zambezi and the Bangweulu Swamp. Cott judged the stones most likely act as ballast for stability and sinking. Alderton, in 1998, reported a radiology study in which stones helped grind food inside a small crocodile. The purpose is still not settled.

  In Royal Natal National Park, Pooley found that young crocodiles of 60 to 90 centimetres dug tunnels in dry, cool weather. Most tunnels ran 1.2 to 1.8 metres deep, and the longest reached 3.65 metres. Inside, the crocodiles became lethargic in a state of aestivation, similar to hibernation. Only the largest ventured out to sun on the warmest days. This has been recorded from May to August.

  Farther north, on Madagascar, the remnant population has gone further still and adapted to living inside caves.

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• 5. Nests and Survival

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  Males reach sexual maturity at about 3.3 metres and 155 kilograms, usually between the ages of 12 and 16. To attract a female, a male bellows, slaps his snout on the water and blows water from his nose. Males near the same size may fight, and the clashes can be brutal and occasionally fatal. More often the loser withdraws into deep water. A courting pair warble and rub the undersides of their jaws together. In most regions, the pairing lasts for a single year.

  One to two months after mating, the female digs a hole a few metres from the bank, up to 0.5 metres deep. She lays on average between 25 and 80 eggs. A female of 2.75 metres reportedly never lays more than 35, while one of 3.64 metres can lay up to 95. Unlike most crocodilians, she buries the eggs in sand or soil rather than rotting vegetation, so the nest needs sunlight for warmth. In South Africa, the invasive plant Chromolaena odorata has spread along nesting banks and caused nests to fail by blocking that sun.

  The incubation period is about 90 days, shorter than the 110 to 120 days of the American alligator. A guarding mother sometimes falls into a trance-like state. Some have shown no reaction while being pelted with stones. Others attack fiercely, sometimes joined by another crocodile that may be the father. Until 1952, communal nesting sites survived at Ntoroko peninsula in Uganda. In one, 17 craters lay within an area of 25 by 22 yards.

  Temperature decides the sex of the young. If the nest stays below 31.7 degrees Celsius or rises above 34.5, every hatchling is female. Males appear only inside that narrow band. Before hatching, the young chirp in high-pitched voices, which signals the mother to open the nest. She may roll eggs in her mouth to crack the shells and may carry the hatchlings to the water.

  Hatchlings are 280 to 300 millimetres long and weigh around 70 grams. A mother protects her young for up to two years, and neighbouring mothers may form a crèche. She may carry the young in her mouth, in her throat pouch or on her back. By the end of two years the young are about 1.2 metres long and drift away from older crocodiles' territories.

  An estimated 10% of eggs survive to hatch, and only 1% of hatchlings reach adulthood. The Nile monitor alone can destroy about 50% of the eggs in studied nests. The Australian perentie, by comparison, takes about 90% of freshwater crocodile eggs. Marabou storks, mongooses, baboons and even the red flour beetle also take their share. African fish eagles take hatchlings of a few months, and honey badgers prey on yearlings. Perhaps no predator is deadlier than larger crocodiles of their own species, which are cannibalistic.

  Those who make it can live a very long time. Potential maximum lifespan is put at 70 to 100 years, although no crocodilian commonly exceeds 50 to 60 years in captivity.

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• 6. Family Tree and Range

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  The scientific name joins the Greek kroke, meaning pebble, with drilos, meaning worm, a reference to the rough skin. The word niloticus means from the Nile River. Local names are many. Swahili speakers say mamba, Sotho and Tswana speakers say kwena, and in Arabic it is timsah al-nil.

  As many as seven subspecies have been proposed, yet none is formally recognized. One of them, the West African crocodile, Crocodylus suchus, is now widely treated as a separate species. Crocodiles from Mali and the Sahara reach only 2 to 3 metres. The IUCN does not recognize the split, because its last evaluations came in 2008 and 2009, before the key studies appeared. Variation within the group is greater than in any other recognized crocodile species. Lake Turkana crocodiles have more osteoderms on the belly and so are worth less in the leather trade. That may explain an exceptionally large local population in the late 20th century.

  DNA sequencing has overturned an old assumption. The Nile crocodile is more closely related to the crocodiles of the Americas than to its West African namesake. Those relatives include the American, Cuban, Morelet's and Orinoco crocodiles. The living species first appeared in Kenya during the Late Miocene-Early Pliocene. A fossil giant called Rimasuchus lloydi was once thought to be the closest relative, with a skull up to 97 centimetres. It turned out to be nearer to the dwarf crocodile. Crocodylus anthropophagus from Tanzania and Crocodylus thorbjarnarsoni from Kenya grew to about 7.5 metres. Their broad snouts suggest they specialized in large mammals and freshwater turtles much bigger than any in Africa today.

  The present range runs from Lake Nasser in Egypt to the Cunene River in Angola, the Okavango Delta of Botswana and the Olifants River in South Africa. Its historic range reached the Mediterranean coast and across the Red Sea into Syria. It is thought to have died out in the Seychelles in the early 19th century. On Madagascar, a 7,500-year-old skull found in 2022 suggests Nile crocodiles arrived before the endemic crocodile Voay went extinct. Salt glands let the species enter the sea. One specimen was recorded 11 kilometres off St. Lucia Bay in 1917.

  Nile crocodiles have also been captured in South Florida. Genetic tests show they are closely related to each other and most similar to South African animals, so they probably share a single source. It is unknown how many are loose there.

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• 7. Hunted and Protected

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  From the 1940s to the 1960s, hunters killed Nile crocodiles on a far larger scale than before, mainly for high-quality leather. Some also sought the meat for its supposed curative properties. Firearms made the killing easier. Between the 1950s and 1980s, an estimated 3 million were slaughtered, and the species came close to extinction.

  Today an estimated 250,000 to 500,000 live in the wild, and the IUCN classes the species as Least Concern. CITES lists it under Appendix I in most of its range and under Appendix II in the remainder. Ranching programs for skins have worked well. In 1993-80,000 Nile crocodile skins were produced, mostly on ranches in Zimbabwe and South Africa. Crocodile farming is one of the few burgeoning industries in Zimbabwe. The meat is another matter. Graham and Beard called its taste "indescribable" and its smell "repellent".

  New threats have replaced the old. At Lake St. Lucia, highly saline water pumped in through irrigation practices coincided with some crocodile deaths and a major population decline. In the Olifants River, which flows through Kruger National Park, many deaths are officially of unknown cause. Analysis points to pollutants from human activity, particularly the burgeoning coal industry. Crocodiles absorb them mostly by eating poisoned fish. At Lake Sibaya in South Africa, persecution is the direct cause of the failure to recover from the leather trade. In Egypt, the species appeared to have been wiped out as a breeding population at some point in the 20th century. It has since re-established itself locally, for example at the Aswan Dam.

  Crocodiles also keep fish in balance. By preying on mesopredators such as barbel catfish and lungfish, they stop those fish from overeating the stocks that other species depend on. When crocodile numbers fall, tilapia, an important commercial fish, declines through excessive predation. Humans are among the losers.

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• 8. People and Crocodiles

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  Researchers estimate that Nile crocodiles carry out 275 to 745 attacks a year, and 63% of them are fatal. The saltwater crocodile, by comparison, is estimated at about 30 attacks a year, half of them fatal. Together the two species kill hundreds, possibly thousands, of people annually, more than all other crocodilians combined. Before the leather trade thinned the population, a rough estimate put deaths at 1,000 a year, with about as many aborted attacks.

  Proximity explains much of the difference. Unlike the saltwater crocodile, the Nile crocodile lives beside human settlements through most of its range, so contact is frequent. Crocodiles as small as 2.1 metres can overpower small apes, which suggests children and smaller adults are at risk. Most fatal attacks come from crocodiles longer than 3 metres, and the average fatal attacker is estimated at 3.5 metres. From 1990 to 2006, lions caused about one-eighth as many fatal attacks in Africa. Crocodiles outnumber lions by more than a dozen to one, though probably fewer than a quarter are big enough to be dangerous.

  Korogwe District in Tanzania, the Niassa Reserve in Mozambique and the area around Lower Zambezi National Park in Zambia each see a dozen or more fatal attacks a year. Despite old claims, no trend shows that women and children are targeted more often. Most fatal attacks happen to people standing a few feet from the water on a gentle bank, wading in shallows, swimming, or letting limbs dangle from a boat or pier. Laundry workers, fishermen, game wardens and guides face the highest risk.

  Biologists who spent months or years in the field, among them Cott, Graham and Beard, and Guggisberg, found that with precautions their lives and their guides' lives were rarely at risk. Guggisberg also collected older writings that described an "appalling" lack of caution near large crocodiles. By contrast, bold lions caused proper panic. He suggested that regarding the crocodile as a lowly creature may explain why it kills more people than big mammal carnivores. Some of those 19th and 20th century accounts, he cautioned, should be read with a grain of salt.

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