Crocodiles- The price of power

by Matthew Norton

Energy is a major currency in the natural world. Every living thing gets it by eating food, using sunlight to make sugars, or by exploiting some other natural resource. But to do anything productive, and that includes just being alive, one has to spend energy and a lot of it. This is especially true in the top predators, whose powerful hunting techniques usually incur such a high energy cost that it makes sense to maintain a budget and only unleash their full potential at the opportune moment. 

Crocodiles, such as the particularly impressive saltwater crocodile and Nile crocodile, are no exception. Though thoroughly lazy when bathing in the sun, as many reptiles are compelled to due to being cold blooded, they are not to be underestimated when they are on the hunt. As well as their formidable size and teeth, these crocs are also patient, often stalking their prey in murky water.  Once in range, the crocodile will lunge forward with incredible speed and strength. If done right, their target will have no idea what hit them until it’s too late.

The saltwater crocodile Crocodylus porosus  (left) and Nile crocodile Crocodylus niloticus  (right) are two large and powerful crocodile species who are not to be underestimated, especially when they’re in their element. This quote from the Australian Museum website about the Saltwater crocodile (also known as the estuarine crocodile) says it best.
“does not suffer foolish humans that enter its watery domain”
A Nile crocodile ambushing wildebeest as they cross the Mara river in Tanzania.

But as I said before, these bursts of power come at a cost, not just in the energy required for these strikes, but also in the speed at which this energy is needed. In turn, this requires a change in how that energy is released in the cells that make up the crocodile’s muscles.

Under normal conditions, energy is stored in an animal’s body in the form of glucose sugars. The energy is then released by a process called aerobic respiration (with oxygen), which consists of a long series of chemical reactions that includes oxygen and a great deal of cellular trickery to squeeze every last piece of energy out of each molecule of glucose.

But when that same animal needs a lot of energy very quickly, which tends to happen when you are hunting down your food, or running for your life, the aerobic method is too slow to keep up with demand. In this situation, anaerobic respiration (without oxygen) is the preferable alternative as it releases energy far quicker, though at the cost of efficiency. This version of respiration also leads to the build up of a waste product called lactic acid.

A summary diagram of the reactions of aerobic respiration.
Each glucose molecule is twisted, turned and changed to release the energy contained within. The ultimate goal is charge a bunch of ADP molecules into ATP, which for all intensive purposes are tiny flying batteries that fly off and deliver the energy where its needed. After which, they go back to being ADP until they get another recharge.
Aerobic respiration is a slow and complicated process (for a cell),  but it can squeeze over 30 ATP recharges out of each glucose molecule. Two ATPs are spent during the glycolysis stage, but there is still a sizeable net profit to be made from this process. Meanwhile, anaerobic respiration produces a pitiful 4 ATP recharges per glucose and still has to endure the two ATP investment.
 
A summary diagram of the reactions in anaerobic respiration, which is pretty much the first stage of aerobic respiration modified into its own self contained loop. In this version of the energy release process, the pyruvate molecules that come out are converted into lactic acid in order to reset the glycolysis cycle.  As the body continues to rely on anaerobic respiration this lactic acid builds up, which is what causes the aches and pains we get from intense exercise.
Once aerobic respiration has the chance to take over again, additional oxygen is brought to covert the excess lactic acid into less troublesome chemicals. This is often referred to as repaying an ‘oxygen debt’.

The body of any animal can only tolerate so much excess lactic acid, but it’s in this inconvenient side effect that crocodiles have a surprising advantage. For they can tolerate incredibly high levels of the acid that would kill most other animals. Perhaps this allows them to throw extra power into their attacks, or perhaps this extra resistance is linked to other behaviours that require anaerobic respiration in some capacity.

Crocodiles would have little oxygen to spare while they are submerged in water and preparing for an attack. During this time, they can minimise their energy consumption to some extent, but there are still basic life functions that cannot be switched off. To keep their body ticking over, they have to resort to anaerobic respiration and then launch at their prey with a body already brimming with lactic acid. Depending on the size of their catch, they may also have to drag it back into the water and tenderise their meal into manageable chunks. With barely a chance to catch their breath. 

This double whammy of a high energy demand during their attacks and a low oxygen supply during their underwater stalking may go some way to explain why the lactic acid tolerance of crocodiles is so high. And with the toll it would take on their existing energy reserves, it’s no wonder they have such voracious appetites. 

Even when lurking just above the water, crocodiles can be difficult to spot from a distance (most animals don’t have access to cameras with zoom functionality). Underwater, they would be practically invisible.
During which time they can minimise their energy consumption such as staying very still and slowing their heart rate right down. The heart of a crocodile even has a built in option for diverting blood away the lungs.
Normally, the right side of heart would send blood to the lungs through the pulmonary artery to pick up oxygen. That blood then comes back to the left side of the heart. The left side then pumps the oxygen rich blood through a second artery (called the aorta) to the rest of the body to deliver the oxygen.
But during a dive, sending blood to pick up oxygen that isn’t there would be pointless . So the crocodile’s heart has a third artery, the left aorta, that picks up the low oxygen blood in the right side of the heart and sends it back round the body. Skipping the lungs entirely.
The result of these extreme measures is that crocodiles can stay underwater for hours, though their dives usually only last around 10-15 minutes.

The lives of animals are full of choices that are influenced by how best to spend their energy reserves and this can lead to behaviours that would otherwise seem odd. Predators can ignore food sources that are too difficult to crack and prey can choose to ignore the first whispers of danger rather than lose valuable feeding time over a false alarm. Sometimes, parents will even abandon, or consume their current batch of young in hope that their future offspring will prove to be a better investment. Just like any human entrepreneur, animals often have to take risks by making, or not making, an energy investment for a chance to reap the best rewards in life.

From a human perspective 

Crocodiles are formidable hunters and they have been known to attack humans from time to time. Saltwater crocodiles and Nile crocodiles have racked up the highest number of attacks, both fatal and non-fatal, worldwide with a total of 1,350 and 1,005 attacks over the last ten years respectively. This sounds horrific, but these numbers average out at just over 230 attacks per year for both species combined, for a planet with billions of people. Like with sharks, the chances of being attacked by a crocodile are extremely remote and can be reduced further with little more than common sense.  

Swimmers, and anyone else in the area, would be wise to take heed of warning signs like these.
There are also other precautions you can take, such as not dangling limbs over the sides of boats, exercising caution at night, camping at least 50m away from the shore and not washing dishes, or preparing food by the water’s edge.
 

But let us not forget that humans exploit crocodiles for their meat and skin, often in dedicated farms. From time to time, we also capture and relocate crocodiles to zoos, for research purposes and to move problematic individuals away from a populated area. 

Meat from a Saltwater Crocodile served with rice and a creamy sauce (left).  Wallets made from crocodile skin (right).
 
A crocodile farm in Australia (left) and in Israel (right). The latter looks much more spacious and comfortable for the animals at face value.
But with admittedly little in the way of context  to these two photos I’d advise caution in making any quick judgements about these two farms. But I think we can all agree on the general principle that if we are to farm and eventually slaughter animals, they should be made as healthy and content with their lives as is possible.

Since it’s rare for crocodiles to be taken away without a fight, the risk to the humans involved is obvious. But as the croc itself struggles against the ropes and nets, they build up a lot of lactic acid in their blood and muscles. It attempts to restrain the animal drag on too long, this build up can surpass even their astronomical tolerance with the crocodile eventually struggling itself to death.  Sometimes, they may be captured alive only to succumb to the ordeal a short while later due to the oxygen not coming in fast enough to deal with the adverse effects on their body.

Thankfully, there have been attempts to develop new methods of capture that would make the whole experience less stressful for the crocodile. And probably less tense for their human captors as well.

A study published in 2003 trialed the use of ‘electrostunning’ on saltwater crocodiles (effectively applying a taser to the back of the neck). Compared to manual restraint alone (in this case noosing the animals with rope), the electrostunned crocodiles didn’t experience as much of a build up in lactic acid, or in other telltale chemicals in the blood. There was still a build up regardless of what method was used, as there would have been some maneuvering involved to get the crocs from the electrostunned group into position.

A later study applied a similar test, but on Nile crocodiles, stating outright that different crocodile species might react differently to electrostunning. In the end, they also concluded that electrostunning was better than manual restraint (albeit for different reasons) for welfare of the animals and the safety of their human handlers. In a dramatic twist, someone was actually bitten while grappling with one of these crocodiles from the manual restraint group.

Pitted against each other in a straightforward fight, a human being would not last long against a fully grown crocodile. But in the real world, our ingenuity has firmly shifted the balance in our favour thanks to a few millennia of technological advancements. So bolstered are we by boats, chemical formulas and electrical appliances that we don’t always need to kill everything in sight, even when they have powerful muscles and sharp teeth. A concept that would have seemed impossible to our caveman ancestors. 

Thanks for reading

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2 thoughts on “Crocodiles- The price of power”

  1. Great post enabling us humans to understand the crocodiles world and the dangers its powerful body faces plus how humans have tried to dictate it’s survival. A lot to think about, Thanks Matthew

    Sent from my iPhone

    >

    Like

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