Fig. 1 From black (hedgehog) and white (hyena) through green (goose) and blue-grey (snake), or smelling of violets (otter), dung packages come in all shapes and sizes.
What’s brown and sounds like a bell?
Dung!
Monty Python’s Flying Circus, December 1969
IN ITS MOST familiar sense, either from our own personal first-hand experience, or from our close historical proximity to stock animals, dung is brown. This is the default colour of the scatological cartoon, or the plastic doggy poop bought in a joke shop. But anyone who has trodden the minefield of dog mess along an urban street knows that the droppings left behind by lazy and careless owners can be anything from pale yellow to red to black. And in widening the boundaries to cover all animal excrement, even this colour palette soon broadens out. Hyena dung is white, bird splashes are piebald, white and grey; reptile waste is anything from pale grey to inky blue-black (my pet garter snake Bella shunts out vaguely greenish goo twice a month); aphid honeydew is clear or, as its name suggests, slightly honey-coloured; caterpillar frass can be black, green or even turquoise verging on blue. Like many things in nature, colour is not a useful guide.
Instead, a better start can be made by trying to understand dung in terms of a simple schoolbook equation of its basic biochemistry:
Food – nutrition + waste = dung
Fig. 1 From black (hedgehog) and white (hyena) through green (goose) and blue-grey (snake), or smelling of violets (otter), dung packages come in all shapes and sizes.
Anyone who can remember back to biology lessons may recall vague snippets about salivary amylase, gastric acid or the pyloric sphincter. Whatever the complex chemistry going on in the intestinal tract, the process that takes food and makes excrement begins with digestion, and to fully appreciate exactly what dung is, it is as well to start with a brief look at this process.
Human digestion is fairly well studied, and since humans are omnivores, eating a huge range of different foodstuffs around the world, our understanding of how it all works is useful when looking at other animals, in particular those mammals with whose similarly brown faeces we are intimately familiar.
It all starts with chewing, the obvious mechanical breaking down of big bits into smaller bits using the teeth. This makes swallowing a lot easier, but it also helps get the digestive juices working quickly on the ground-up mush, rather than having to cope with rough, tough chunks. Not all animals chew quite as politely as would-be humans at the dinner table, grinding the requisite 40 chomps before swallowing. Birds have no teeth, and apart from a cursory crunching, usually to stop it wriggling, they gollop down their food whole. Instead, they have a gizzard to do the chewing. This muscular portion of the upper stomach often contains grit or small stones and the rhythmic squeezing movement of the gizzard walls helps crush and pulverise the food into a more manageable substrate for digestion. Some reptiles and fish also have gizzards. Whatever the mechanisms for chewing, the result is a more-readily digestible raw material for the stomach – it’s all about increasing the surface area of the food particles so that the chemical processes of digestion can get to work more easily.
In humans, chewing is not just about cutting and crushing; digestion of some foods actually begins in the mouth, with digestive enzymes in the saliva. Starch (the primary carbohydrate in foods such as bread, potatoes, pasta and the like) is attacked by salivary amylase to produce various sugars. A simple home experiment involves over-chewing a piece of bread (without swallowing), which becomes noticeably sweeter after only a couple of minutes. My old biology teacher Mr McCausland introduced me to that one during O-levels (the old name for GCSEs, for any younger readers), ostensibly to show us a practical demonstration of hydrolysing starch catalysis, but I suspect also to fill our incessantly noisy mouths with slightly stale loaf to shut us up for a bit. The mouth is a neutral or slightly alkaline environment, but the swallow takes boluses of chewed food down the oesophagus (gullet), into the highly acid stomach.
Although the hydrochloric acid in the stomach is strong enough to dissolve iron, its purpose is not just to attack the food, but rather to create the right chemical environment in which highly complex food-digesting enzymes can get to work. Serious protein digestion gets going now as these immensely complicated molecules are snipped into smaller units. The acid also kills most bacteria should any have been on the food when it was eaten. Eventually a sloppy ‘chyme’ is produced – this is the smelly pale-yellowish acrid liquid full of chopped carrots which is revealed if you are unfortunate enough to vomit a couple of hours after eating. Chyme is slowly released in gentle squirts through the pyloric sphincter, the muscle-ring one-way valve at the far end of the stomach, down into the small intestine.
The human small intestine is another world altogether. A convoluted 6 metre stretch of narrow, finger-thick tubing, it is the major site of digestion and food absorption. Several profound chemical changes start within the first few centimetres. Sodium bicarbonate is released by the pancreas, a large gland sitting under the stomach; this neutralises the stomach acid and creates a slightly alkaline background. The pancreas also releases alkali-controlled enzymes – important components are proteases and peptidases to continue the digestion of proteins, and more amylase to break down starches. A thick yellowish-brown liquor called bile is also dribbled into the intestine from the gall bladder. It helps break down clods of insoluble fat into an emulsion of microscopic globules. The bile also contains a yellow waste substance called bilirubin, which is made from haemoglobin (the red oxygen-carrying molecule in the blood) as damaged red blood cells are broken down and destroyed in the liver. As it traverses the digestive tract, bilirubin changes to another strongly coloured chemical called stercobilin, it is this dark brown pigment that gives mammalian dung its characteristic brown colour.
Most of the bewilderingly sophisticated biochemical substances that make up living organisms, and therefore the food that we eat, are based upon long chains of repeating chemical units, like the beads on a series of pearl necklaces. The chains fold and twist, and are cross-linked like knitting to form everything from the proteins which build the bulk of muscles, and the meat that we eat, to starch (the energy source in foods such as spaghetti and doughnuts) and polyunsaturated fats (our consumption of which we try to reduce by choosing low-fat margarines). The enzymes from the pancreas, and others secreted by the small intestine itself, work on digesting our food like chemical scissors, trimming, then snipping off individual beads from these intricate long-chain necklaces. Truly vast substances containing many thousands (sometimes millions) of atoms are reduced to the basic tiny molecules of the individual units from which they are made up.
Proteins are reduced to their constituent amino acids, starches are reduced to simple sugars, and fats are reduced to short-chain oils. Each of these fundamental building block types is just a few atoms (maybe 10–100) in size – and small enough to pass through the semi-permeable membranes of the gut wall, to be whisked off around the body in the blood and lymphatic transport systems. To facilitate this removal of useful chemical nutrients from the chyme, the interior surface of the small intestine is wrinkled and minutely convoluted, covered all over with tiny finger-like extensions (villi) that give it the appearance under the microscope of a thick shag-pile carpet. This has the effect of hugely increasing the surface area across which nutrient absorption into the blood takes place. Choose your own incredible statistic here, the usual one is that the surface area of the human small intestine, if it were to be flattened out, is the size of a tennis court (260 square metres). If memory serves, that same Mr McCausland (in A-level biology now) had us calculating this based on a section of sheep intestine he’d got from his neighbourhood abattoir. After poring over the microscope and calibrating the graticule eyepiece measurement scale, we probably spent the entire lesson counting sheep villi and extrapolating up from microscopic cylindrical tendrils to so-many hundred square metres of laboratory floor carpet.
The final leg of the digestive conveyor belt is the large intestine, the colon, about a metre long and wrist-thick. Some last-minute nutrient absorption occurs here, but its most important function is to remove water from the digestion remains.
By now the semi-liquid chyme has become a stiff semi-solid. After, perhaps, several days of slow onward movement through the digestive tract, much of what the human body can use in the way of nutrients has been removed from the food. What remains is the key constituent of plant and vegetable foodstuff that we cannot digest – fibre (sometimes called roughage in older textbooks). Fibre is made up of undigestible chemical chains such as cellulose and lignin. These are the substances that give plants their incredible toughness and strength, and which in non-food species provide us with fibres for other uses – cotton jeans, linen sheets, wood-pulp paper.
Fibre was a major preoccupation for my parents’ generation, and whereas I was fascinated by the numerous helpful vitamins thoughtfully provided and carefully listed on the packets by the manufacturers of sugary breakfast cereals, my Mum’s shopping choices were more often influenced by the roughage content of the bran-based stodgier end of the edibility spectrum. It was during the 1950s and 1960s, when processed foods, cheaper meat and sliced white-pap bread started to appear on the supermarket shelves, that the connection between healthy food intake and healthy stool output was first used (albeit very tastefully and subtly) as a marketing tool. Being ‘regular’ was considered a feminine virtue and a sign of manly rectitude. There was, at this time, a growing realisation that the human taste for sweet, easily digestible titbits was replacing a more rounded diet of mixed fibrous fruit and vegetables, and that this was playing havoc with a digestive tract inherited from our long-distant ancestors, one more suited to the foraged nuts, fruit and roots on which proto-humans first fed.
Lack of fibre in the diet didn’t just mean reduced faeces; this was not just a simple equation of less in one end, less out the other. It meant less regular throughput, intestinal stagnation, backing up of waste, and rectal compaction to the point of discomfort and risk to health. Constipation is a singularly human obsession, and we’ll be straining to understand its implications regularly throughout this book.
By lucky happenstance, I was able to do some personal research into constipation early on in the writing of this book, when I was hospitalised with extreme abdominal pain and excruciating muscle cramps. Fearing it might be gall or kidney stones, hernia or diverticulitis, I trekked up to my local accident and emergency unit late one Saturday evening. I was poked and prodded, drained of numerous blood samples and eventually X-rayed, but the tests were negative and the diagrams showed it was, to give it the medical term, simply faecal loading. I was a bit bunged up in there. Several days of laxative oils and glycerine suppositories got things moving again, much to everyone’s relief.
There is another key ingredient in the final quasi-excrement as it passes through the large intestine, and by the time it is ready to be ejected from the body, it makes up over half the dry weight of the faeces – bacteria. This is where our knowledge of human digestion starts to wear a bit thin. There may be 100 trillion bacteria in an average human intestine, that’s 100 million million, or 1 with 14 zeros after it – a mind-boggling number, and more than ten times the number of body cells in the average human’s entire body. There are thought to be 300–1,000 different bacterial species in there; it’s difficult to quantify exactly, because they are difficult to identify and difficult to grow and study in laboratory cultures. What are they all doing?
Most people’s idea of ‘bacteria’ may be of horrible germs that cause sickness, disease or death, but the gut flora, to give it its usual, slightly more passive, euphemistic name, is a perfectly normal, healthy, indeed necessary part of being a human. Unlike the bacteria that cause, say, tuberculosis, cholera or salmonella poisoning, these natural gut-dwelling microbes are not attacking or parasitising the human body, nor are they accidental inhabitants (sometimes called commensal, meaning non-harmful coexistence), they are better described as being mutualistic – host and occupier each benefiting from the presence of the other.
The bacteria benefit because they are supplied daily with a fresh input of partly digested food passing through the guts, on which they and their descendants can feed; and in return they further digest the remaining substances, their own very different enzymes snipping away at the slowly fermenting chemical dross that would otherwise be unavailable to our own bodies’ somewhat limited digestive machinery. These last-minute digestive products are absorbed, along with some of the water, before the final bodily waste product is ready to be voided.
This usual, normal, natural process goes on in the human body every day, and for the most part we are completely oblivious of it, but when things change, we can get a fascinating insight into the digestive processes, and we can understand parallels in other animals.
Human stool is roughly 75% water. Anyone with a regular balanced diet should be familiar enough with their own bowel movements to know when things are ‘fine down there’, but when things go wrong, either way, we notice immediately. Water content in human faeces can actually range from about 50 to over 90%; this translates to an ease-of-passage range, from reluctant constipation to explosive diarrhoea. Helpfully, there is a simple medical scale, with pictures – the Bristol stool chart – which classifies this range into seven discrete categories by outward visual appearance. No messy weight and density tests are necessary.
At the one extreme, diarrhoea can be life-threatening; the International Centre for Diarrhoeal Disease Research in Dhaka, Bangladesh, was set up because, after malaria, diarrhoea is the world’s single biggest killer of children under 5. The usual cause is viral or bacterial infection of the gut by inappropriate micro-organisms, and the body’s response is to flush out the system as quickly and effectively as possible to get rid of the offending invaders. Shutting down or reversing the intestines’ water-absorption pathways keeps the gut contents highly liquid, and the rhythmic waves of muscular contraction (peristalsis) that usually gently squeeze the digesting food through the digestive tract now force it out under pressure. Fast ejection (like vomiting) gets rid of the offending microbes and helps prevent dangerous, possibly life-threatening bacterial toxins building up; it may even stop these micro-organisms invading the interior of the body.
We all have our own diarrhoeal anecdotes, and in any other circumstances I’d keep mine diplomatically quiet. But since this is actually a book about my own exploration of excrement I cannot pass without at least commenting obliquely on the accident in the Temple of the Buddha’s Tooth in Kandy, Sri Lanka, in 1992, where it was forcefully brought home to me that not all of the world’s drinking water is safely potable. I was saved major embarrassment by the hasty hailing of a friendly tuk-tuk driver who quickly returned me to the safe and private confines of the family-run guest house in which we were lodging, where for several days I closeted myself away in my room and recuperated on thin vegetable gruel and bottled water.
Diarrhoea can also be caused by allergic reactions, physical or chemical damage to the gut linings, poisoning, alcohol abuse or age-related blood-vessel damage. As well as any underlying cause, the primary health risk, especially in the very young or the very old, is dehydration because of the body’s continued and copious water loss. The usual medical response is treatment with rehydrating fluids (thin soup is ideal) carefully balanced to replace sugars and salts that also pass from the body during diarrhoeal attack.
Fig. 2 Charming rural vignette from Bewick’s A General History of Quadrupeds (1790) with obligatory pat.
Any visitor to a dairy farm may wonder if cows suffer constant diarrhoea, since their excrement is very runny and forms splashed circular pats when it pours messily out. For cows, however, this liquid dung is entirely normal, because rich green grass is a natural laxative (it has a very high fibre and water content) and bovine nutrition is based on the bucket digestion technique rather than the long narrow tube system used by humans.
The cow’s digestive system works on a much larger scale than a human’s, not just because the cow is physically larger, but because it is specially adapted to cope with processing enormous quantities of tough grass cellulose. Initial grazing is little more than cutting and swallowing; the chewing is done later, at the cow’s convenience. A large and multiple-compartment stomach allows the cow to regurgitate mouthfuls of ‘cud’ from the first portion of its stomach, the reticulum, which acts as a large storage pouch. Grinding of the grass occurs as the cow ruminates (chews the cud), and often looks like the cow is insolently chewing gum as it nonchalantly lounges about in a sheltered portion of the field. The second time the food is swallowed it passes into the largest section of the stomach, the rumen, often vibrantly described in agricultural information leaflets as being the size of a garden dustbin; here bacterial-led bucket-style fermentation really gets to grip with digesting the tough grass cellulose. Eventually the fibrous soup passes through small, then large intestines, with their concomitant nutrient and water absorption.
Fig. 3 A cow pat is roughly 75% water, a semi-liquid which pools into neat round field ornaments. Horse dung is only slightly less water (72%), but is firmer, with longer strands of fibre, so maintains its shape as decorative road apples.
The water content of cow dung is roughly 75%, remarkably similar to that of normal human faeces. The liquid nature of cow dung is down to the fact that the fibre (which is mostly undigested in humans, and so gives our stools their firm texture) is much more broken down by the cow’s stomach bin of bacterial enzymes, and it exits the animal in a more fluid, rather than a semi-solid state.
Despite the fact that horses also graze grass, their droppings are not at all liquid, indeed their dry, rather pleasantly aromatic dung sometimes seems little more than the partially decomposed grass cuttings on the compost heap, making it the manure of choice for gardeners and smallholders. Horses also digest grass via bacterial fermentation, but instead of this taking place early on in the gastrointestinal tract, in the stomach, it takes place much further down the digestion pathway, in the caecum, a pouch at the junction of small and large intestines. The caecum is reduced to the appendix in non-grass-eating humans, but in horses it fulfils its original digestive purpose, and is a metre-long sack.
Horse dung is also about 72% water, just a little drier than cow or human dung, but perhaps the absence of cud chewing and late digestion allows more complex fibre to pass through into their droppings. If their digestion is slightly less efficient than that of cows, horses make up for it by spending more time actually grazing, and less time chewing the cud.
Sheep dung is drier still, 65% water. Sheep, like cows, are ruminants, and cellulose is attacked in the large rumen portion of the stomach. Modern farmed sheep are thought to be descended from the wild sheep of the Middle East, traditionally a hot and arid area, where water retention would have been important for survival before they were domesticated. Passing copious liquid dung would never have been an evolutionarily successful strategy. It would also have clogged their woolly hindquarters, attracting flies; in well-run modern farms, a sick animal passing loose stools can still easily be the victim of ‘fly-strike’, where fly maggots infest the dirtied wool on the rump, and even start to attack the animal’s underlying flesh.
An added bonus of dry sheep dung is that it makes it much more convenient to handle when looking for dung beetles. There was that occasion, I remember it well, crossing a sheep-grazed meadow on the South Wales coast, where I insisted on picking up (with my bare hands) and breaking open the hard semi-dry sheep nodules, to find various western and montane dung beetles I had never seen before. It was all very exciting, but it wasn’t long before I was told by my girlfriend, in no uncertain terms, that if I did not stop forthwith, our relationship would be at an end. I had to be a bit more discreet after that.
Rabbits are also grass-grazers and have evolved a third mechanism for releasing the nutrients tied up in the tough cellulose fibres. Like the sheep, they have a caecum at the junction of small and large intestines, where bacterial fermentation takes place, but, perhaps because of their small size, this does not always extract enough nutrition for them. Instead, they recycle the food, by eating their own dung.
As they feed, rabbits release hard, dry, round pellets (crottels), pale brown like compacted hay, the size of children’s small marbles; these are the final waste droppings and the familiar, almost odour-free residue left on their grazing grounds, on the ant hills from which they scan for danger, or in the domestic hutch. But as the day’s new intake of grass reaches the end of the digestive tract on its first pass through, rabbits also produce dark, smooth, soft, mucus-covered pellets called caecotropes. These are re-eaten (caecotrophy) as they are produced, and are taken directly into the mouth, from the rabbit’s anal vent, usually in the safety of the burrow; they are sometimes also called ‘night faeces’, so the behaviour is seldom directly observed. Some friends of mine once kept a rabbit as a house-pet, but they were less than impressed to observe this behaviour close-up, at night, on the pillow, as the friendly rabbit visited them whilst they tried to sleep.
The mucus covering allows the caecotropes to pass through the acid of the stomach, so that nutrients can be extracted from the food by continued bacterial fermentation during its second pass through the gut.
Most other mammal grazers, whether of grass, herbs or tree leaves, use variations on these digestive mechanisms to extract the sometimes meagre nutritive value from readily available but poor-quality plant foodstuffs; and they produce dungs of relatively similar chemical and fibre make-up, although differing in their water content and size according to each species’ evolutionary history and individual ecology.
As a general rule carnivores have much shorter intestines, relative to their outward body size, than do herbivores. They do not need the lengthy processes that herbivores require to digest those chemically tough plant fibres such as cellulose. Digesting a meat meal takes much less time and energy than digesting vegetables.
Typical hunting carnivores – such as cats, dogs, foxes, hyenas, wolves – have very short gastrointestinal tracts. Partly this is because proteins are quickly and efficiently metabolised by the well-oiled digestive enzymes in the stomach and intestines, but partly this is a response to the quality of the meat meals that these animals actually get to eat.
There runs an old adage, fondly regurgitated by biology lecturers:
Q: What does a herbivore eat?
A: Herbs.
Q: What does a carnivore eat?
A: Whatever it can get.
Despite graphic images from wildlife documentaries showing dramatic hunting kills by big cats or wolf packs, being a carnivore is fraught with difficulties. Hunting other animals is a dangerous business, where prey is likely to fight back as if its life depended on it – with the very real potential of serious injury to the predator. Chasing large mobile prey can be dangerous and exhausting. There is always a balance between hunting easy small prey, sometimes barely a mouthful, versus bringing down a prize trophy to feed the victor to satiation, and perhaps all of its pack too. This leads to some less than savoury food choices being made by the hungry fox or the desperate lioness. Scavenging from the kills of others, from the victims of disease, starvation or thirst, or making do with nutritionally dubious worms, maggots, caterpillars and other insects, means that the digestive systems of many top-end predators are subject to the sort of chemical or bacterial insult that would cause serious, even fatal, food poisoning in humans.
Fig. 4 Another vignette from Bewick’s A General History of Quadrupeds (1790), which was intended as a children’s book.
The shortened digestive tract means that such rancid or infected food passes swiftly through the gut, allowing at least some nutritive goodness to be removed, but ejecting the remains before mortally dangerous bacterial toxins can build up. Finding a small pool of strong-smelling liquid fox diarrhoea on the patio by my back door is sometimes a sign that the animal has been scavenging rotting food from the bins again.
Mammals might produce the dung with which humans are most familiar, but they are in the minority when it comes to the general mechanisms of expelling bodily waste. Mammals produce solid excrement from the anus, but they also produce a separate liquid in the form of urine. Urine is produced by filtration of the blood passing through the kidneys. It is mostly water (90–98%), but also contains waste products from the body’s daily activities, notably a substance called urea – CO(NH2)2. When the body assembles or disassembles amino acids (those digested chemicals broken down from proteins in food) into its own complex biochemicals, there is often an excess of nitrogen (N) compounds; the simplest chemical reaction would produce ammonia (NH3), but this is highly toxic to most organisms, hence its use in powerful domestic cleaning products. Urea combines two ammonium units into a substance that is neither acid nor alkali, is highly soluble in water and relatively non-toxic. Although urine is not pleasantly appetizing, it is also at least not revoltingly undrinkable; otherwise all those dreadful survival tales would never have had survivors to tell them. Urine can be safely stored in the bladder until it is convenient to expel it.
In mammals, urine and faeces are discharged separately, but in almost all other living creatures, from albatrosses to zebra spiders, they are combined in a common storage cavity at the end of the digestive tract called the cloaca (from the Latin cluere, to purge or drain, see page 23), before being released together. This accounts for the multicoloured and multitextured splash of the bird dropping, and why, for example, guano (long-term accumulations of bird excrement) is so rich in nitrogen and so highly valued as agricultural fertiliser.
Dung, droppings, faeces, excrement, whatever you decide to call it (there’s a long list in chapter 13), varies considerably from species to species across the animal kingdom, and its final appearance is completely dictated by the animal’s foodstuff, and the metabolic chemical processes used to cope with extracting nutrients from it. The nature of an animal’s dung will also change if its diet changes.
Arguably, human diets have changed more in the last 50 years than in the previous 50,000. Sugar- or at least carbohydrate-rich foods have proliferated, and processed food has become the norm for many in the western developed world. Where previously ‘roughage’ was seen as the unimportant nutritional dregs of a diet-plan still rooted in the Stone Age, its importance to human health is only recently becoming clear. Although there are not too many scholarly studies on human excrement, there are suggestions that the vast majority of supposedly healthy westerners are actually chronically constipated. Nutritionist John Cummings wrote a seminal paper on constipation (1984), bemoaning the inadequate intake of dietary fibre, and to lighten the mood introduced it with this superb quote:
I have known … the happiness of a whole household to hang daily on the regularity of an old man’s bowels. The gates of Cloacina open, the heavens smile and all goes smoothly. ‘Master’s bowels have not acted today’ from the lips of the faithful butler and the house is shrouded in gloom. Goodhart (1902)
This is not to be confused with the acute, sudden-onset, constipation that sent me crawling to my local hospital recently. The Bristol stool chart, that handy identification guide to stool consistency, is a neat tool for a broad assessment of our abdominal health. A diet high in fibre, it turns out, protects us from heart disease, fatty build-up in the arteries, diabetes and bowel cancer, as well as averting the painful strains of passing the hard pellets of virtually dry faecal material when we become constipated. Drives to increase vegetable and fruit intake have recently focused on getting people to eat their ‘five-a-day’ portions of these richly fibrous foods, with moves to up this to seven-a-day, or even ten.
There are anecdotal reports of short-term dietary changes having knock-on effects when it comes to defecation, particularly in the details of aroma and texture, but many of these turn out to be the false bravado associated with drinking too much beer and eating too much curry.
There are, however, several food items which can significantly affect faecal output. In late summer the fox droppings of southern England change from smooth coils of oily grey slime to rough crumbly black and red cylinders, as foxes gorge on the plentiful blackberries ripening in the bramble bushes. In a bizarre study of pig nutrition, Edward Farnworth and his colleagues found they could alter the smell of pig dung by feeding the animals ground Jerusalem artichokes mixed in with their usual swill (Farnworth et al. 1995). The dung was lighter in colour, more brown and green, but less yellow, and it was judged to be ‘sweeter, less sharp and pungent’. This was partly because it had less smell of skatole, a powerfully aromatic chemical that gives dung its offensive odour – just the thing for assuaging the sensibilities of the pig-farm’s close neighbours.
Perhaps the greatest switch in manure output comes with weaning. Anyone familiar with cattle or dairy farming will know that for the first few weeks, when the calf is taking only its mother’s milk, its dung is yellow and, if anything, even runnier than normal cow manure – little more than slightly fermented yogurt with a dusting of colour from the bile. When I first saw pats of this at my uncle’s North Kent farm many years ago, I thought it looked more like pools of the stuff they use to mark double yellow no-parking lines down the edge of the road. But as the animal starts eating its own food, the gradual change to normal dung occurs. It is at this time that the calf must acquire its own bacterial gut flora. Exactly how cows (or humans, for that matter) obtain their private internal colonies of gut bacteria is still being studied. It is not as simple as accidentally eating food contaminated with faeces; that is a recipe for disaster. But faeces-dwelling bacteria, which then become soil-dwelling, are somehow ingested and soon become established. There probably is something in the old adage that you have to eat a peck of dirt before you die. But you do have to be careful.
As with any animal, part of the trick to survival is learning what you can eat, and what you can’t. Contrary to popular rural myth, cows do not just graze everything in their path. Strangely, for those bemoaning the loss of the clover-filled hay meadow, cows should not eat lots of fresh clover. The natural fermentation process in the rumen produces gases which the cow normally releases through belching, but there are substances (as yet not completely identified) in legumes – plants such as clovers, lucern or alfalfa – that upset the bacterial digestion processes. As well as gas, the bacterial digestion of these plants also produces a sticky slime, which creates a thick foam of many small bubbles, rather than a large belchable single bubble. Unable to burp up the gaseous foam, the bubbles remain trapped in the cow’s rumen, which threatens to bloat into a balloon to the point of tearing or haemorrhaging, with life-threatening consequences. The various treatments, devised over centuries (some less sensible sounding than others) include burning feathers under the cow’s nose, giving it a pint of gin, taking it for a vigorous run, or placing a stick or rope through the animal’s mouth to encourage salivation to break down the foam. As a last resort for a prostrate animal unable to move, the swollen belly is stabbed with a trocar, a large hollow dagger that releases the gaseous pressure build-up in the rumen.
Gases continue to be produced throughout the small, then large intestine, and it is some of these, together with volatile chemicals such as skatole, that give excrement its distinctive smell. There are too many baked bean jokes to make this a surprise to anyone. Two of the most obvious gases are hydrogen sulphide (H2S), the smell of rotten eggs, and methane (CH4), the same odourless natural gas piped from oil wells to the gas cooker. The one may offend the nose, but the other is a greenhouse gas reckoned to be 70 times more potent, in terms of its potential to raise global temperatures in the next 20 years, than carbon dioxide (CO2). Worldwide, livestock is calculated to release over 100 million tonnes of methane a year, and these figures are frequently cited when analysts try to explain climate change. Contrary to comic-book jokes, and quite a few mistaken newspaper articles, most of this gas is released through belching, at the mouth end, rather than with the dung at the anal end, but the chemical processes that give rise to it are a direct consequence of the grass forage being eaten in the first place, and the cellulose fermentation mechanisms that have evolved to digest it. Suddenly, what goes into one end of an animal, and what comes out of the other, turns from being crude schoolboy humour to a serious environmental issue.