INTRODUCTION – WHAT IS DUNG?

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

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.

PAYING LIP SERVICE TO FOOD

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.

YOU ARE WHAT YOU EAT

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...