Crocodiles have been around for 200 million years, but they’re certainly not primitive. Karen Partridge reveals the survival strategies that keep these regal reptiles on top.
Crocodiles and alligators are supreme survivors. In their tenure on Earth, they’ve endured the impacts of meteors, planetary refrigeration, extreme upheavals of the Earth’s tectonic surface and profound climate change. They were around for the rise and fall of the dinosaurs, and even 65 million years of supposed mammalian dominance has failed to loosen their grip on the environments they inhabit.
Biologist Dr Chris Lavers calls crocodilians “the closest approximation to an unsurpassable ecological design in the entire history of tetrapod life.”
The first crocodile-like ancestors appeared about 230 million years ago, with many of the features that make crocs such successful stealth hunters already in place: streamlined body, long tail, protective armour and long jaws.
Crocodilians have no lips. When submerged in their classic ‘sit and wait’ position, their mouths fill with water. But they are saved from death by drowning by the palatal valve at the back of the tongue, which closes to stop water pouring into the lungs.
The nostrils on the tip of the elongated snout lead into canals that run through bone to open behind the valve – allowing the crocodilian to breathe through its nostrils even though its mouth is under water. When the animal is totally submerged, another valve seals the nostrils, so the crocodilian can open its mouth to catch prey with no fear of drowning.
What they lack in lips crocodilians make up for in teeth. Pointed and sharp, they’re designed to pierce and hold rather than cut and chew. Upper and lower teeth interlock when the mouth closes, driving into prey’s flesh from both directions to create a vice-like grip.
Crocodilian jaws are also designed for grabbing and holding. The muscles that close them generate such power – a large saltwater crocodile can easily crush a pig’s skull – that it would be difficult for anything to force the mouth open.
Gripping is key to crocodilian hunting strategy: victims are often held under water until they drown. Specialised interlocking processes in the crocodile’s neck vertebrae increase their strength and rigidity, helping the predator resist the efforts of victims to break free.
The thin skin on the crocodilian head and face is covered with tiny, pigmented domes, forming a network of neural pressure receptors that can detect barely perceptible vibrations in the water. This enables a crocodile lying in silent darkness to suddenly throw its head sideways and grasp with deadly accuracy small prey moving close by.
These modifications go deeper. Even the animal’s internal organs have been customised to fit the crocodilian lifestyle. Other reptiles have a three-chambered heart, but crocs, like birds and mammals, have a four-chambered heart.
The crocodilian heart is unique in having an actively controlled valve that can redirect, at will, blood flow away from the lungs and recirculate it around the body, taking oxygen to where it’s needed most.
In addition, a quirk in the structure of their haemoglobin means crocodilians can make the most efficient use of the oxygen available to them and stay submerged for an hour or more.
Compared to mammals and birds, crocodilians have slow metabolisms that burn much less fuel, and are ideally suited to relatively unstable environments that would defeat mammals with their high food demands.
Like all reptiles, crocodilians rely on external heat sources to keep them functioning. By moving between water and land, sun and shade, they warm up or cool down as necessary. This means crocs only require small amounts of food to survive: a large crocodilian can easily survive on one good meal a month.
In many places inhabited by crocodilians, the hot season brings drought that dries up their hunting grounds and takes away the means to regulate their body temperature. Many crocs protect themselves from this by digging burrows and entombing themselves in mud, waiting for months without access to food or water, until the rains arrive. To do this, they sink into a quiescent state called aestivation.
At least nine species of crocodilian are thought to aestivate during dry periods. Kennett and Christian’s six-year study of Australian freshwater crocodiles Crocodylus johnstoni which spend up to four months underground, revealed that during aestivation, the crocodiles’ metabolic engines tick over, producing waste and using up water and fat reserves. Waste products are stored in the urine, which gets increasingly concentrated as the months pass.
However, the concentration of waste products in the blood changes very little, allowing the crocodiles to function normally. Furthermore, though the animals lost water and body mass (just over one-tenth of their initial mass) while underground, the losses were proportional: on emergence, the aestivating crocodiles were not dehydrated and exhibited no other detrimental effects.
This ability of individuals to sit out the bad times and endure long periods of enforced starvation must surely be key to the survival of the crocodilian line through time.