Watching from the waters

The Nile crocodile, Crocodylus niloticus, is one of the most feared animals in the world and is undisputedly the most dangerous species of crocodilian to humans - being estimated to kill around 200 people each year. This is mainly due to its close proximity to our settlements and its indiscriminate diet - with it hunting anything that moves! Growing as long as 16 feet (5 metres) and weighing up to 225kg (500lb), the Nile crocodile can be found through central and south-east Africa and on the western shores of Madagascar. 

This is believed to be one of the largest specimens of Nile crocodile ever caught! Its huge size makes it no wonder why many Ancient Egyptians incorporated the beast into their belief system and worshipped Sobek, the crocodile god that lived in Crocodilopolis - a great city of crocodiles!

Probably one of the main reasons that the crocodile is such a proficient predator and is so lethal to humans is because it is a silent ambush predator - its flat body allows it to remain completely submerged, even in the shallow waters by river banks; and it has slightly raised nostrils and eyes that can be poked above the surface while keeping everything else hidden underwater, allowing it to see and breath virtually unnoticed. This allows the crocodile to get within yards of its prey (including humans), completely unnoticed and once it is ready, it lunges out of the water. Once the crocodile has lunged it is normally too late for its victim due to the speed of the attack and the animal is then crushed in the crocodile's vice-like bite and dragged into the water. After it has bitten an animal, Nile crocodiles (as with most crocodilians), have an overwhelming instinct to role - this is commonly known as 'death rolling' and, by spinning like a corkscrew, the crocodiles tear huge chunks of flesh out of their prey. This helps to compensate for their ineffective teeth, which are unusually blunt for a predator.

A Nile crocodile that has caught a gazelle. Nile crocodiles can eat up to half their body weight at a time and their bite force has been measured to be as high as 5, 000 lbf (22N), which is like being hit by a very fast-moving truck! Like most crocodiles, N. niloticus has a primary and secondary bite where an individual bites its prey once and then re-bites it without relaxing their first bite, helping to drive its blunt teeth even further into the flesh of the animal.

Ambushing their prey in this manner means that they can remain motionless in the water for very long periods of time and so, can conserve enormous amounts of energy. Crocodilians can also reduce their energy expenditure even further by slowing down their breathing and reducing their metabolism using a sophisticated cardiac shunting system that diverts blood away from their heart. Although cardiac shunting mechanisms are found in many species of reptile none have one as advanced as crocodiles. This, along with the fact that they have the most advance heart physiology in the world, has led many scientists to believe that crocodiles were once endothermic (warm blooded) like mammals and birds! Such scientists believe that their warm blooded ancestors slowly evolved back into being ectotherms (cold blooded) when they began to ambush their prey from water since a cold blooded body-plan would be much more beneficial for such a strategy; mainly because it wastes much less energy than trying to maintain a body temperature that is hotter than the surrounding water - just think about how cold you get if you spend too long in a cool bath or pool!

Thus, crocodiles can remain even in fairly cool water for long periods of time and wait for a potential meal to come to them! This is usually when an animal ventures too close to the waters edge for a drink... Nile crocodiles don't even have to use their muscles to hold their head up so can save even more energy while they wait! Their lungs act as ballast tanks and when they are inflated with air, actually push the crocodiles snout and eyes up above the water level! In fact, crocodiles can slow down their energy expenditure so much that they can hold their breath underwater for up to 2 hours when they inactive; often, lying in little 'dens' that they've dug out of the riverbed with their arms.

Certain species of bird, such as the spur-winged plover (Vanelus spinosus), have been seen picking scraps of meat from between crocodile teeth. It is not known for certain whether the birds are merely 'running the gauntlet' to obtain food at great personal risk or whether crocodiles are allowing them to this; forming a mutualistic relationship where the birds are fed while keeping the crocodile's mouth and teeth clean.

Although Nile crocodiles often hunt via ambushing their prey, they can also hunt with a much more proactive and energetic approach - chasing after fish and eating them underwater. It is believed that Nile crocodiles prefer to eat larger terrestrial game that they hunt with surprise attacks preferentially and switch to smaller fish when they aren't available or during then night when their mammalian prey is less active. Nile crocodiles are thought to prefer to ambush mammals for two main reasons: firstly, since it is less energetically expensive as mentioned above; and secondly, because crocodiles are far-sighted underwater due to the closure of their protective nictitating, or 'third', eyelids. Closing their third eyelid helps to protect their eye from underwater debris but also bends the light that passes through it so that the lenses in their eye can't refract the light enough to focus on close-up objects. Obviously, this will severely reduce their ability to hunt underwater and hunting in such a manner would probably be impossible if they didn't have many sensory papillae around edge of their lower jaw. These papillae act a bit like whiskers in dogs and cats and are able to detect even very minute vibrations in the water, allowing the crocs to sense fish that are close to their mouth! Crocodiles use much more energy when hunting fish in such a manner and as a result, can only hold their breath underwater for about 15 minutes since their muscles require more oxygen.

Their ruthless ability to surprise their prey and the fact that they eat humans has undoubtedly earned the Nile crocodile a fearsome reputation as a heartless monster. Oddly however, this isn't true and Nile crocodiles are one of the few crocodilians that care for their young - with mothers watching over their nests from when they lay their eggs to when they hatch! Most crocodilians abandon their nests as soon as they have lain their eggs and leave their offspring to fend for themselves. Furthermore, many crocodilians are cannibalistic and will eat juvenile crocodiles so that they may actually kill their own young unknowingly...

The gender of crocodiles is determined by the temperature that the eggs are at during their incubation, with temperatures of 32 - 33C producing males. Anything warmer or cooler than this narrow temperature range produces females, which is of great concern to conservationist biologists since global warming may result in more females being born and less males! The small size of the baby crocodiles makes them very vulnerable to predation early in life, but as they grow bigger and, if all goes well, the crocodiles can live for up to 100 years!

The Nile crocodile then, is a brutal and ruthless predator that preferentially hunts by ambushing unsuspecting animals from water. It accomplishes this using a range of physiological and behavioural adaptations and techniques which allow them to get within yards of their prey, gliding through the water causing barely a ripple... Despite their danger to man and the polluting effects that our lifestyle has on the waters of their habitats, the Nile crocodile is thriving and is classified as being of 'Least Concern' by the IUCN. This is not surprising really, since crocodilians are ancient animals that even trod the world with the dinosaurs! Their famed resilience allowed them to survive through the catastrophe that killed off the once mighty dinosaurs; and maybe, will even mean that they will survive for long after man has disappeared...

Humans: the world's only super predator?

Everyone is familiar with how predators and prey interact with each other - predators try to eat prey species and prey species, which do not want to be eaten, fight back or run away. These interactions have led to an 'evolutionary arms race' between predators and prey: where predators evolve adaptations, such as weapons, to help them kill their prey; and their prey evolve defences to counteract these weapons.

A good example of this is the cheetah, which has evolved to be able to run extremely quickly in order to catch gazelle, their main source of prey. Gazelle have worked out that they cannot outrun cheetahs, which are the fastest land-animal, so instead have evolved to be able to abruptly change direction whilst running. This allows them to be able to twist and turn even when sprinting and helps to protect them from cheetahs, which can only run in near enough straight lines. Furthermore, cheetahs are under a great deal of strain whilst running and their core body temperature rises extremely quickly so they cannot run for long periods of time. Consequently, gazelle have also evolved to be good long-distance runners, with most of their muscles being made from fast oxidatve fibres, which allows them to run for much longer than cheetahs can. As the next step in the evolutionary arms race it may be predicted that cheetahs will evolve to be able to run for longer or develop greater manoeuvrability whilst running, allowing them to be able to hunt gazelle more efficiently.



A cheetah chasing a juvenile gazelle. The cat can reach speeds of up to 75mph during the chase and will try to knock the gazelle over, allowing it to grab the underside of the gazelle's throat and suffocate it.

Arms races such as this example are common and exist in some form or another for every species of predator and prey. This is expected by scientists and obviously, makes evolutionary sense. What does not make sense however is that although humans have been around for 2 million years, in which time we have hunted other animals extensively, we do not appear to be engaged in this arms race. We have evolved no specialised weaponry to hunt our prey and none of the species that we prey upon have evolved abilities to defend themselves against us.

Many scientists believe that this is due to the unique way in which we hunt our prey. As I'm sure you know, humans are extremely intelligent and we have always used this intelligence to help us hunt other animals. Our brains have allowed us to create a large variety of tools to overcome our lack of physical weaponry and to develop innovative tactics to overcome our prey's defences.

One such ability of ability of humans is that we are able to throw much more efficiently than any other living animals - both in terms of power and accuracy. Therefore once our ancestors had invented spears, the once cutting-edge technology, we were able to kill our prey from a distance by throwing it - the ability to kill from a distance is unique to us, no other animal is able to do this! Our ability to kill animals from a distance meant that we have had no need to develop weapons such as poisons, claws and large teeth and our prey has not had the opportunity to develop resistance to such a hunting method. This is partly because once an animal had realised that we could kill it, it is already dead and cannot pass its genes on, which is the mechanism by which evolution works. Our intelligence has also meant that we could fashion dull clothing and refine our tactics to sneak up on our prey, so that we have not needed to evolve camouflaged skin either. In fact, the main physical adaptations that throwing would have selected for is strong shoulder and leg muscles and good binocular vision (predators have eyes that are mounted on the front of their head, which gives more precise vision and better depth perception), both of which humans have.

Tribesmen throwing spears. Note how the men are standing sideways, holding the spear in their right arms. When they throw their spear they twist their body around. This greatly increase the force of the throw by using the torque generated in their hips and shoulders.

Human intelligence and the fact that we hunted in packs has also meant that we were able to overcome the  'ten times larger' rule, which is where large terrestrial herbivores that grow ten times heavier than their largest predator become safe from predation, since their predators are unable to develop jaws strong enough to kill them. This explains the large size of elephants for example, and why the adult individuals of such species have no natural predators other than man - simply put, they are just too large for predators for hunt and the risk of injury to the predator is too high! Our ability to hunt animals much larger than ourselves helped the rapid spread of humans across the globe and allowed us to live in areas that would have otherwise been uninhabitable, such as the cold tundra of Northern Europe and Siberia. Human colonisation of tundra climates was possible due to the presence of mammoths, a species of megafauna, for two main reasons. Firstly, mammoths had no natural predators until we arrived, so that we did not need to complete for them with other predators; and secondly, because of the enormous amount of resources than we could harvest from each individual. These resources included meat for food, fur for clothing and vitreous humour from their eyes to make glue. Unfortunately mammoths, like most species of megafauna, had evolved with immunity to their natural predators and consequently, had few defences other than their size. This meant that we were able to kill them very easily and sadly, we drove them to extinction. The extinction of megafauna is a characteristic of human colonisation in new areas and most giant species became extinct around 40 million years ago.

An artist's impression of a group of early humans hunting a mammoth.

Our ingenuity has also meant that humans have been able to overcome most of the poisons that many animals use as defence mechanisms. The most common ways that we do this is by cooking the animal before we eat it, which causes the poisons to break down; or we can remove the poisonous tissues, which is possible because defensive poisons are often concentrated into certain organs, such as the liver. Thus, humans can kill and eat many animals that are avoided by other predators.

Therefore humans are remarkable hunters, being the only animals that can kill others from a distance, kill species much bigger than ourselves and can remove poisonous toxins from our food, which greatly increases the scope of our diet. Thus, it may be fair to say that humans are not merely predators, but are in fact super predators and that we are the most effective and efficient killers that have ever lived.