Neither dragons nor dinosaurs, crocodiles are the paradigm of an evolutionary recipe that proved successful – little about them has changed in the last 100 or so million years. They are perfectly designed apex predators with potentially massive bodies powered by robust muscles, covered in armored scales, and driven by clinical, calculating instinct. Crocodilians are also the ultimate masters of the ambush approach, drawing on their innate reptilian capacity for absolute stillness until launching an assault.
Of the five recognized crocodile species in Africa, the Nile crocodile (Crocodylus niloticus) is by far the most common and one of the largest crocodilian species in the world, second only in size to the saltwater crocodiles of Asia, Micronesia, and Australia. They have a life expectancy of up to 80 years and range in size from 30cm hatchlings to adults weighing over 500kg, with the largest, ever recorded individual measuring 6.45m from snout to tail and weighing 1,089kg. Somewhat unexpectedly, they are more closely related to birds than they are to other reptiles.
It is common to hear crocodiles described as “living fossils”, but this is not entirely accurate. Their ancestors successfully survived a mass extinction event around 250 million years ago. From there, the surviving evolutionary line branched into the Archosaurs (“the ruling lizards”) – with one earlier branch leading to the crocodilians and the other later branch leading to the dinosaurs (and, ultimately, birds). Unlike their dinosaur cousins, however, the crocodilian ancestors were destined to survive another mass extinction event at the end of the Cretaceous.
Fossils of these crocodilian ancestors indicate a far greater variety in terms of size, shape, and hunting styles than seen today. Some were terrestrial and predatory and bounded across land to chase down their prey, whilst others were predominantly herbivorous or omnivorous. For those that took to the water, all weight restrictions lifted and, for a time, mega-crocs the sizes of buses haunted the oceans and deep rivers of early Earth. From the Late Cretaceous and with the extinction of the dinosaurs, through ice ages, and the subsequent rise of the mammals, the surviving crocodilians have barely altered.
The lack of evolutionary action can be at least partly explained by a successful foundation for exceptional hunting abilities. While younger Nile crocodiles are primarily reliant on regular meals of invertebrates, amphibians, and fish, those that survive to adulthood will attain the status of apex predator, capable of ambushing large prey twice their size including buffalo, giraffe, humans and even young elephants.
Unique among reptiles, crocodiles have a four-chambered heart, improving the efficiency of the transportation of oxygenated blood around the body which, combined with extremely high lactic acid levels, allows them to stay submerged for up to 2 hours at a time (provided they remain inactive underwater). Specialised muscles attached to the lungs, liver and pelvis can contract to pull the lungs backwards into the body cavity, changing the crocodile’s buoyancy and allowing it to submerge without creating ripples and alerting potential prey to their presence.
While their approach to hunting is silent and deadly, crocodiles are surprisingly vocal animals and never more so than during the breeding season (the timing of which varies depending on the area). Males show off by “roaring”, slapping their snouts on the water and exhaling sharply, intimidating rivals, and attracting females.
Two months after mating, the female selects a suitable nesting site on the shore or in a dry riverbed and digs a hole in which to lay her eggs. The clutch size will vary depending on the size of the female but is usually between 25-80 eggs. Unlike other crocodilian species, the female Nile crocodile does not use moribund vegetation to incubate the eggs so she will be selective in ensuring that the nest receives adequate sunlight to maintain the temperature. Like several other reptile species, crocodiles have temperature-dependent sex determination – if the temperature is between 31.7˚ and 34.5 ˚C, the offspring will hatch as males, anything above or below that range will hatch as females.
MANEATERS AND MONSTERS
The young crocodiles approximately double their length during the first two years, spending almost as much of that time on land hunting invertebrates as they do in the water. However, as they grow, their hunting habits become almost entirely aquatic, and they become more dangerous to people.