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Nose Dive: A Field Guide to the World's Smells

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When I was growing up in suburban Chicago in the 1950s, after Sunday afternoon dinners that often centered on rare roast beef, my parents would take all four children on car rides into the countryside. We were grossed out by the smells of the dairy farms and incredulous that, as my father was happy to insist, our delicious dinner could have come from those animals. We had a mantra for the moment we got the first whiff: "Red meat from a cow? Pee-YOO!

I later learned that we were echoing an ancient exclamation at something rotten and stinking, the Indo-European root pu, from which stem 'putrid' and 'putrefy.' Animal excrement is generally disgusting to us. But this is apparently a reaction that we learn, not an automatic biological reflex. Young children aren't repelled by excrement, and many mammals practice coprophagy, or excrement eating, some of our primate relatives included. In her 1983 book Gorillas in the Mist, Dian Fossey noted that gorillas of all ages have been observed eating dung, their own and others, fresh from the source: the animals catch the dung lobe in one hand before it contacts the earth. They then bite into the lobe and while chewing smack their lips with apparent relish. Rabbits and some other plant-eating mammals get the vitamin B12 they need by routinely eating food twice, the second time after its residues have been enriched by their gut microbes. Studies of rabbits and mice have found that the presence of their excreted pellets in the cage tends to lessen aggressive behavior, lower heart rates, and offer positive, comforting effects, perhaps because they suggest familiarity and therefore safety.

Few primates of the human tribe ever smack their lips at the smells of animal excrement, and many find them overwhelmingly disgusting at close range in an enclosed space. The scientists who actually choose to study animal excrement and the volatile compounds it emits do so largely to figure out how to reduce the offensiveness of feedlots and hog farms. But there are also contexts in which these smells are less offensive, appealing in their own way, maybe even comforting when we can associate them with the open-air countryside and stables and small farms.

THE SMELL OF excrement from an animal's digestive system comes largely from the anaerobic microbes thriving in its oxygen-scant lower reaches. The microbes feast on food residues that haven't been digested and absorbed by the animal body, and also residues from the animal body itself, mainly the cells lining the digestive tract that are constantly being sloughed off and replaced, and the protein-rich mucus that lubricates it.

The physical mass of excrement can be up to half microbial cells, and the density of microbes in the lower digestive tract of animals is among the highest of any known niche on the planet. So there's a lot going on in there, and all that activity generates volatiles - molecules that are small and light enough to fly out of their source and into the air. Sulfurous hydrogen sulfide and methanethiol are almost always prominent because there's usually plenty of oxidized sulfur in the animal gut - all green plants carry sulfur-containing lipids, and intestinal mucus contains sulfated carbohydrates - and anaerobes use it as an electron donor to generate energy. Two protein by-products are especially emblematic of excrement: barnyardy cresol and fecal skatole. The generic excremental mix runs a close second in breath-stopping power to the smell of animal death.

Some animal excrements have a distinctive makeup and smell that can be traced to a particular diet or metabolism. Horse excrement is less offensive than many, and was even described as sweet by the 18th-century physician and natural philosopher George Cheyne. The horse and its microbes digest its plant foods quickly and only partly, so much of its excrement is relatively odorless fiber. The volatiles are dominated by the carbon rings cresol and phenol, which we also encounter in asphalt and disinfectants, and which can therefore seem less specifically fecal. By contrast, cattle are endowed with several stomachs, including the microbe-packed rumen, and they have the habit of regurgitating the rumen contents for another chew to get the most out of their plant feed. The excrement of beef and dairy cattle is therefore rich in the full range of metabolic volatiles. Omnivorous pigs get some of their nourishment from high-protein animal materials, and they produce excrement especially rich in branched acids, sulfides, and carbon rings. For some reason the pig gut and its microbiome are notably prolific of fecal-smelling skatole, some of which is transported from the intestine and stored in fat tissues all over the body, where it can contribute to the special 'pigginess' of pork.

Of course, there's a second major animal excretion. Urine is a fluid that animals excrete separately from the semisolid remains of digestion, though they often end up mixed with each other on the ground. It carries primarily the waste products of the animal's own metabolism, and in particular the nonvolatile nitrogen-containing urea and uric acid, along with small quantities of amines. Urine is practically microbe-free until it leaves the body, but once it does, microbes feed on its urea and uric acid and boost the emissions of ammonia and amines: hence the 'urinous' character we ascribe to these volatiles.

Bird excrement has a distinctive and especially pungent smell because it combines the smells of digestive excrement and nitrogen disposal. Probably to help conserve water, most birds put their excess nitrogen exclusively into uric acid, which is much less soluble than urea and so can be excreted as a semisolid paste along with wastes from the digestive tract. Bird excrement is essentially dung and urine rolled into one. It's usually dominated by ammonia and amines, along with vinegary and cheesy acids.

IMMEDIATELY ANNOYING AS they are, the smells of animal excrement are also tokens of an otherwise largely insensible but existential crisis for much of life on Earth. They're a reminder of the inescapable reshuffling of matter and energy that keeps the great game of complexity going. The stench of the modern feedlot signals the fateful move by which Hero Carbon, the most gregarious and constructive of chemical elements, has managed to vault to new levels of invention, but at the price of devastating much of its achievement to date. That move was the arrangement of carbon chains into Homo sapiens, animals capable of mobilizing matter and energy on an unprecedented scale, and thereby damaging intricate ecosystems across the planet.

We have various terms for excrement, polite and impolite, one of them being 'waste,' from a root meaning 'empty or 'desolate.' In fact the strong smell of animal excrement signals its richness in diverse carbon and sulfur and nitrogen molecules, and so its value as nourishment for other living things. When early farmers discovered that value thousands of years ago, they made possible the long-term success of agriculture, and with it the development of civilization.

Our Stone Age ancestors must have been very familiar with the smells of excrements. Hunters would have used those smells as other carnivores do, to locate their prey. Dogs joined human communities something like 30,000 years ago. Around 10,000 years ago, the first settled agricultural communities domesticated goats, sheep, cattle, and horses to take advantage of their milk and meat, hair and hides, and their plow-pulling muscle power. Archaeological remains indicate that their dung was used as a building material and fuel for fire, uses that live on today in less industrialized parts of the world, along with fumigation to eliminate insect infestations and even recreational cow 'chip' throwing. Excremental smells must have permeated the life of the early farmers.

At some point early farmers also observed that when excrement happened to be trodden into the soil, it improved the growth and productivity of food crops - as we now know, because it replaces nutrients taken from the soil by the crops, and has beneficial effects on the soil's physical structure and biological diversity. Archaeologists have found that the organized application of excrement to crop fields goes back at least 7,000 years in Greece and central Europe.

Because this practice improves crop productivity slowly, and so represents a long-term investment of labor and resources in the soil, it may well have helped inspire the earliest ideas of land management, ownership, and property. The word manure itself has its origins in nothing at all to do with excrement: the roots are Latin words for 'hand' and 'work,' and the compound of the two originally meant 'to cultivate' or 'to hold property.'

So the smell of manure - excrement prepared for the soil and worked into it - is an ancient sign of the most fundamental forms of care and cultivation, labor and value, the practice of feeding the soil so that it will continue to feed us. Manure is milder, tamer, less aggressive than fresh excrement, generally made by mixing excrement with stable straw and feed residues and stockpiling it for some time to make it less concentrated and to eliminate disease microbes. The addition of low-nitrogen material, exposure to the air, and the metabolism of aerobic microbes all combine to reduce the levels of ammonia and amines and sulfides - though high-nitrogen chicken manure is noticeably more redolent of ammonia than cow and steer manure. These are the smells that we can still encounter in farm country, in plant nurseries and hardware stores, in our own backyards. Not exactly pleasant, but positive.

Highly unpleasant and negative are the raw, uncomposted, intense smells that emanate from concentrated animal feeding operations, or CAFOs, which confine and raise large numbers of animals - hundreds, thousands, hundreds of thousands - in a small area, and have come to dominate modern meat and dairy production over the last few decades. They accumulate huge quantities of excrement that can be smelled from miles away. I live in central California and pass by the Harris cattle ranch on Interstate 5 near Coalinga whenever I drive between San Francisco and Los Angeles. Even with the car windows closed, I can smell it long before I see it. Tens of thousands of beef cattle are confined there, each animal generating some 65 pounds of urine and excrement a day. Today's formulated feeds usually supply more nitrogen than the animals would obtain from their natural diet of plants, so their excrement is especially rich in the most offensive volatiles, the branched acids, cresol, skatole, ammonia, and amines.

Skatole is also found in cigarette smoke and is known to damage lung DNA. Methane is combustible - it's the main component of natural gas - and has caused explosions in CAFOs. Some workers have been fatally overcome by excremental fumes. Because CAFO operators frequently dispose of excrement as cheaply as possible, dumping it into open storage lagoons or spraying it directly onto fields, even their neighbors can suffer health effects from the volatiles, and nearby soils and waterways can become badly polluted.

It's exactly because CAFOs are offensive and harmful that the volatiles of animal excrement have been so well studied. Crazily but appropriately, chemists borrow the terminology of top, middle, and base notes from the perfume world to describe the smells of CAFOs. The top notes, very volatile and quickly dispersed, are ammonia and hydrogen sulfide. The more persistent middle notes include amines, thiols and sulfides, aldehydes and alcohols and ketones. The constantly present base notes are the short-chain straight and branched acids, cresol and other phenolics, and skatole. In a 2006 study of swine and beef cattle operations, barnyard cresol was identified as the primary offensive odor, and could be detected as far as 10 miles downwind. It's probably the first long-distance hint I get of that I-5 Eau de Coalinga.

So the smells of CAFOs are smells of modern industrial agriculture, different from manure in both quality and significance. They're still organic, manifestations of the basic workings of living things, but they're the smells of a rupture in the system that in nature and traditional agriculture returned matter and energy from the soil to the soil. They're the smells of organic matter and energy isolated and withheld from the broad cycle of life on Earth.

Why don't CAFOs compost their excrement into manure? Because it doesn't pay. Chemists began to unravel which components of manure are essential plant nutrients in the 19th century; early in the 20th, the Germans Fritz Haber and Carl Bosch figured out how to manufacture salts of ammonia in factories directly from nitrogen gas in the air. The key: using tremendous amounts of chemical energy stored in the remains of ancient plants - coal or petroleum or natural gas—to generate very high temperatures and pressures. So began the era of concentrated chemical fertilizers, which contributed to steep increases in agricultural productivity, and in turn to steep increases in the human population. Across the world today, the use of synthetic fertilizers outweighs manure by something like five to one.

The invention and triumph of synthetic fertilizers has been a very mixed blessing. They helped propel the rapid development of civilization and new technologies for manipulating matter. But they also lie at the root of profound damage to the complex biological world that made human life possible in the first place. Overworking agricultural soils, eliminating more and more wild habitat to feed and house our billions, treating animals inhumanely, polluting soil and air and waters, altering the energy balance between Earth and sun and so causing global climate change: The stench of CAFOs signifies all this. It's a smell to be open to and pondered by anyone who wants to know how our world works, and how it doesn't.

There's one particularly odorous form of animal excrement that links the manure millennia with the synthetics centuries. Guano is a term now popularly applied to bird and bat excrement, but the word comes from the language of the Quechua peoples of ancient Peru, where it named the extraordinarily nitrogen- and phosphate-rich deposits of excrement from fish-eating seabirds - cormorants, pelicans, and boobies - on islands off the Peruvian coast. The deposits are as concentrated as they are due to the birds' protein-rich animal diet and the dry climate, which dehydrates the excrement while minimizing the conversion of uric acid to ammonia and amines that otherwise evaporate. By roosting on the islands over the course of centuries, the birds built up deposits of guano as much as 100 feet deep, which were mined by Quechua peoples for some 1,500 years.

Trade in guano boomed in the 19th century. Historians suggest that it helped stimulate the general adoption of soil nutrient concentrates in European and American agriculture and the decline in productive use of farm animal excrements. Guano is still mined in a number of places in the world. I bought a bag once to smell the ammonia and fishy amines that may have begun as muscle molecules propelling silvery fish through the oceans years, maybe centuries, ago. Then I returned their matter and energy to the game of complexity playing out in my vegetable garden, and in me.

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