Nile Crocodile

The Nile crocodile hunts by waiting. It lies motionless underwater, often near a water hole, with only its eyes and nostrils showing, until prey comes to drink. Then it lunges forward in one sudden attack, seizes the animal and drags it in. Large prey such as wildebeest and zebra are drowned or crushed, often with a spinning death roll. Adults eat mostly fish, up to 70 percent of the diet, but the biggest crocodiles also take buffalo and young hippos. One animal can eat up to half its body weight in a single feeding, then spend a long time digesting it. On land, it reaches only about 14 km/h (9 mph), so it depends on surprise, not on chasing. Patience, not speed, wins the meal. It also scavenges carrion, so no free meal is wasted.

A Nile crocodile carries hunting equipment in its skin. Small bumps called integumentary sensory organs cover its face and body, and they detect tiny movements and changes in water chemistry and temperature, even in murky water. Special valves in the throat and nostrils close during an attack, so no water enters while it grips prey. An adult can hold its breath for up to 15 minutes, which lets it wait unseen below the surface. Hatchlings begin with a smaller menu of insects, small fish, crustaceans and amphibians, then move to bigger prey as they grow. Crocodiles also scavenge carrion. Adults have few natural enemies apart from humans, although hippos are considered the greatest danger, and they cool down by retreating to the water after basking. Together, these tools make each ambush far more precise.

Survival begins in a nest that many animals want to rob. Each egg is about 7.5 cm (3 in) long and weighs about 110 g (3.9 oz). Mongooses, baboons, hyenas, warthogs, bush pigs, vultures, marabou storks, and Nile monitor lizards all raid nests, so the mother guards hers for about 10 to 14 weeks and eats little or nothing. Hatchlings measure only about 28 cm (11 in) and weigh about 80 g (2.8 oz), yet their distress calls carry up to 20 m (66 ft). Young crocodiles grow about 30 cm (12 in) a year at first, but they take 12 to 19 years in the wild to mature. Males breed at over 2.7 m (9 ft) and females at about 2.4 m (8 ft). Every one of these steps protects the next generation.

Nile crocodiles are more social than their reputation suggests. Both sexes gather at basking sites, and groups can range from just 2 animals to as many as 200. A group resting on land is called a bask, and a group in the water is a float. Males defend territories along the shoreline, and large animals guard the best hunting spots, while smaller ones are tolerated unless they threaten the dominant crocodile. Size also seems to set the feeding order, with the largest eating first when a group herds fish into shallow water. They communicate with calls and hisses. Females nest close to one another, and many spend the hot hours sunning with their mouths open, which helps them lose heat before they slip back into the river. It is an orderly society for a reptile.

Mating takes place in the dry season, and the males put on a show. A courting male arches his tail, raises his head and neck, lashes the water and blows bubbles, usually in shallow water. Rival and partner alike take part in jaw rubbing and gaping, which are done without biting. Larger females mate earlier and with the largest males. Both sexes mate with several partners, so a single nest can hold hatchlings from different fathers. Afterward the female digs a hole nest in sandy soil near water, about 40 by 60 cm (16 by 24 in), and buries her eggs 30 to 45 cm (12 to 18 in) deep. She fills it in flush with the ground and often returns to the same nesting site year after year. It is careful, slow work.

Inside the nest, temperature decides who hatches. Eggs held at 28 to 31 °C (82 to 88 °F) produce only females, while at 34 °C (93 °F) about 81 percent of hatchlings are male. Eggs that stay below 27 °C (81 °F) never hatch. Too much shade can cool a nest and skew a clutch toward females. When the young are ready, they call from inside the eggs, and the mother digs open the nest. She then picks up hatchlings in her mouth, which is gentle despite those jaws, and carries them to the water. Males have even been seen helping to crack eggs. The young stay near adults for up to 3 months, and that early protection helps explain how this ancient reptile has lasted so long. The nest shapes the whole family.