Excrement is one of the most universally produced and universally avoided substances on Earth, yet it plays a surprisingly central role in medicine, ecology, agriculture, archaeology, and even wildlife conservation. Every animal with a digestive tract generates it, and far from being mere waste, feces carry a dense record of an organism’s diet, health, and microbial community. Human stool is roughly three-quarters water, with the remaining solid fraction dominated by bacteria, undigested plant matter, proteins, and fats, all in proportions that shift with what you eat.1PubMed Central. The Characterization of Feces and Urine: A Review of the Literature to Inform Advanced Treatment Technology That composition makes excrement far more informative, ecologically powerful, and medically useful than its reputation suggests.
What Excrement Is Actually Made Of
If you were to dry a typical human stool sample, the solid portion would be between 84 and 93 percent organic material. Of that organic fraction, bacterial biomass accounts for roughly a quarter to over half. The rest is a mix of protein, undigested carbohydrates and plant fiber, and a smaller share of fats.2PubMed Central. The Characterization of Feces and Urine: A Review of the Literature to Inform Advanced Treatment Technology The high bacterial content is worth pausing on: your stool is not mostly leftover food. It is mostly dead and living microbes that populated your gut, mixed with whatever your body and those microbes could not break down further.
The water content of feces varies quite a bit, from about 63 to 86 percent across studies, largely because dietary fiber absorbs water in the colon. A person eating a high-fiber diet tends to produce softer, wetter stools, while someone on a low-fiber diet passes drier, more compact ones. This is the biological reality behind the Bristol Stool Form Scale, a clinical tool developed in the 1990s that classifies stool into seven types based on shape and consistency. The scale correlates well with how quickly material moves through the gut: hard, lumpy stools indicate slow transit, while loose or watery ones suggest fast transit.3PubMed. Stool form scale as a useful guide to intestinal transit time Doctors still use the Bristol scale as a rough, noninvasive way to gauge gut transit time without imaging or more invasive tests.4PubMed Central. Prediction of Delayed Colonic Transit Using Bristol Stool Form and Stool Frequency in Eastern Constipated Patients: A Difference From the West
Why It Smells the Way It Does
The distinctive odor of feces comes from a cocktail of volatile compounds produced by gut bacteria as they ferment undigested food. Several sulfur-containing gases contribute, but one of the signature molecules is skatole, an organic compound produced when bacteria break down the amino acid tryptophan. Research on fermented manure has confirmed a strong correlation between tryptophan availability and skatole production.5PubMed Central. Effects of L-tryptophan, Fructan, and Casein on Reducing Ammonia, Hydrogen Sulfide, and Skatole in Fermented Swine Manure Hydrogen sulfide, indole, and various short-chain fatty acids round out the bouquet. Because the microbial community and the diet vary from person to person, the smell of excrement is genuinely individual. Changes in odor can even signal shifts in gut health, though no one has turned that into a reliable diagnostic yet.
The revulsion you feel toward that smell is not arbitrary. Disgust researchers describe it as part of a “behavioral immune system,” an evolved psychological response that steers organisms away from sources of infection. Feces are a primary transmission route for parasites, bacteria, and viruses, and the disgust response appears across a wide range of species, not just humans.6PubMed Central. Disgust as an adaptive system for disease avoidance behaviour In other words, the fact that excrement repels you is a feature, not a bug. It kept your ancestors from drinking contaminated water and eating off fouled ground.
Stool as a Medical Diagnostic Tool
Modern medicine has learned to read feces for clues the body cannot communicate any other way. One of the most practical advances is the use of fecal calprotectin, a protein released by white blood cells called neutrophils when they accumulate in inflamed sections of the bowel. Measuring calprotectin levels in a stool sample helps doctors distinguish inflammatory bowel diseases like Crohn’s disease and ulcerative colitis from irritable bowel syndrome, which produces similar symptoms but without the same kind of tissue inflammation.7PubMed Central. Faecal calprotectin testing for differentiating amongst inflammatory and non-inflammatory bowel diseases: systematic review and economic evaluation A normal calprotectin level reliably rules out active inflammatory bowel disease, saving patients from unnecessary colonoscopies. Beyond initial diagnosis, calprotectin tracking helps monitor disease activity, predict relapses, and assess whether a treatment is working.8PubMed Central. From bench to bedside: Fecal calprotectin in inflammatory bowel diseases clinical setting
On a population level, excrement has become a surveillance tool. Wastewater-based epidemiology, which gained widespread attention during the COVID-19 pandemic, detects viral genetic material in sewage to track outbreaks before clinical testing catches up. The approach works because infected people shed viral particles in their stool, often before they develop symptoms or seek medical care. Wastewater monitoring can provide early warnings and real-time trend data, especially in communities where clinical testing is limited.9Heliyon. Wastewater surveillance for viral pathogens: A tool for public health
Fecal Transplants and the Therapeutic Power of Microbes
Perhaps the most counterintuitive medical use of excrement is fecal microbiota transplantation, in which processed stool from a healthy donor is introduced into a patient’s gut. The primary application is recurrent Clostridioides difficile infection, a debilitating and sometimes life-threatening condition where a toxin-producing bacterium takes over the gut after antibiotics have wiped out normal flora. Fecal transplants resolve about 90 percent of recurrent cases.10PubMed Central. Immunological mechanisms of fecal microbiota transplantation in recurrent Clostridioides difficile infection
The treatment works through several overlapping mechanisms. Donor microbes directly compete with C. difficile for space and nutrients. They also restore the production of metabolites like secondary bile acids and short-chain fatty acids, which inhibit C. difficile’s ability to grow and produce spores.11PubMed Central. Understanding the mechanisms of efficacy of fecal microbiota transplant in treating recurrent Clostridioides difficile infection and beyond On top of that, the transplant appears to calm the intense inflammatory response that C. difficile triggers, partly by boosting regulatory immune cells that dial down inflammation in the gut lining.12PubMed Central. Immunological mechanisms of fecal microbiota transplantation in recurrent Clostridioides difficile infection Researchers are investigating whether fecal transplants could help with other conditions tied to disrupted gut microbiomes, though the evidence outside C. difficile is still early-stage.
Excrement as Ecological Engine
In natural ecosystems, excrement is one of the most important vehicles for moving nutrients from one place to another. The “whale pump” is a vivid example. Whales feed at depth and defecate near the ocean surface, releasing plumes rich in nitrogen, iron, and other nutrients that phytoplankton need to grow.13PLoS ONE. The Whale Pump: Marine Mammals Enhance Primary Productivity in a Coastal Basin Research on baleen whale excrement has found that it supplies iron in a form that is immediately usable by marine organisms and remains stable over time, supporting the idea that whales provide a genuine ecosystem service by recycling trace nutrients within the sunlit upper ocean.14Communications Earth & Environment. Organic ligands in whale excrement support iron availability and reduce copper toxicity to the surface ocean
Seabirds perform a similar function on land. A meta-analysis found that soils at seabird colonies have dramatically higher nutrient concentrations than nearby control soils: ammonium levels were on average about 41 times higher, nitrate about 5 times higher, and total phosphorus about 4 times higher.15PubMed Central. The influence of seabirds on their breeding, roosting and nesting grounds: A systematic review and meta‐analysis These nutrient deposits shape the plant communities and soil ecosystems around nesting sites, sometimes for centuries after the birds have moved on.
Dung beetles are another key player. By burying and aerating cattle dung, they alter the conditions inside dung pats in ways that reduce greenhouse gas emissions. One study found that beetle activity cut methane emissions from individual dung pats by about 15 percent and reduced total greenhouse gas output in carbon-dioxide equivalents by 7 percent over a pat’s roughly two-month lifetime.16Scientific Reports. The role of dung beetles in reducing greenhouse gas emissions from cattle farming Other experiments have found even larger effects: one trial using introduced beetle species reported an 85 percent reduction in cumulative methane from colonized pats compared to controls, with total greenhouse gas flux nearly 18 percent lower.17Ecological Entomology. Introduced dung beetles suppress methane emissions from cattle dung and alter the temporal dynamics of greenhouse gas flux The magnitude depends on beetle species: a study testing different assemblages found that certain large tunneling species actually increased methane output because of how they construct brood balls, while a mix of the three most common species delivered the biggest reduction, about 32 percent in carbon-dioxide equivalents.18PLOS ONE. Greenhouse gas emissions from dung pats vary with dung beetle species and with assemblage composition In the context of a full meat or dairy lifecycle, the beetle effect on total emissions is tiny, less than 0.2 percent, but for pasture-level methane budgets, the contribution is meaningful.19Scientific Reports. The role of dung beetles in reducing greenhouse gas emissions from cattle farming
Seed Dispersal and the Limits of Gut Passage
Many plants rely on animals to eat their fruit and deposit seeds elsewhere in a pile of natural fertilizer. This process, called endozoochory, is widespread and genuinely important for plant distribution. A review of studies across dozens of animal species found that gut passage alters germination in about half of all plant species tested, with enhancement occurring roughly twice as often as inhibition.20Perspectives in Plant Ecology, Evolution and Systematics. Effect of seed passage through vertebrate frugivores’ guts on germination: a review Digestive acids can soften tough seed coats, giving the embryo an easier exit.
But the picture is not entirely rosy. Research on temperate grassland species found that seeds passed through ungulate and rabbit guts had much lower germination rates, between 0 and 26 percent, compared to seeds sown directly onto dung or bare soil without gut passage.21Journal of Ecology. Germination success of temperate grassland species after passage through ungulate and rabbit guts The implication is that while the dung itself provides a rich nutrient bed for germination, the mechanical and chemical damage of digestion can destroy many seeds in the process. The net benefit of being eaten and excreted depends heavily on the specific plant and the specific animal doing the eating.
Excrement as Armor, Signal, and Territory Marker
Animals have found remarkably creative uses for their own waste beyond simple elimination. Several species of tortoise beetle larvae construct shields or thatches from their fecal matter and carry them as mobile armor. In one species, Hemisphaerota cyanea, the larva builds a thatch from long filamentous fecal strands that completely conceals it, and experiments showed that both a ladybug larva and a predatory bug were unable to get through the barrier.22PubMed. Defensive use of a fecal thatch by a beetle larva (Hemisphaerota cyanea) Another leaf beetle larva uses its fecal shield not just as a physical barrier but as a chemical one. In lab trials, ants strongly preferred larvae that had their shields removed, and chemical extracts from the fecal material alone were enough to repel ants from an unrelated prey item, confirming that the shield works through chemistry, not just bulk.23PubMed Central. The fecal shield is a double-edged sword for larvae of a leaf beetle
Mammals use excrement for communication. Territorial male antelopes defecate strategically, depositing smaller volumes more frequently than females or juveniles and placing their dung on established middens along territorial borders or directly on top of a female’s waste.24Animal Behaviour. Scent marking in a territorial African antelope: II. The economics of marking with faeces Lemurs maintain communal latrine sites that serve as multimodal signal stations, combining fecal deposits with glandular scent marks. Males do most of the scent marking at these sites, and the pattern shifts with the breeding season: overmarking of female scent marks appears to function as a mate-guarding strategy aimed at deterring rival males.25Animal Behaviour. Latrine behaviour as a multimodal communicatory signal station in wild lemurs: the case of Hapalemur meridionalis
Rabbits and the Strategy of Eating It Twice
Some animals take nutrient recovery a step further by eating their own feces. Rabbits are the best-known practitioners of coprophagy: they produce two types of droppings, and the soft, mucus-coated cecotropes are consumed directly from the anus, usually at night. This is not a pathology or a sign of nutritional desperation. Cecotropes are rich in B vitamins, volatile fatty acids, and microbial protein that were synthesized by bacteria in the cecum but could not be absorbed there because absorption happens higher in the digestive tract. Eating them gives the rabbit a second pass at extracting those nutrients. Research on rabbits prevented from engaging in coprophagy found that their growth performance declined, an effect attributed to the loss of nutrient reabsorption from soft feces.26PubMed Central. Impact of coprophagy prevention on the growth performance, serum biochemistry, and intestinal microbiome of rabbits
From Manure to Megawatts
Livestock excrement represents a significant untapped energy resource. Anaerobic digestion, a process in which bacteria break down manure in the absence of oxygen, produces methane-rich biogas that can be burned for heat or converted to electricity. One estimate calculated that the roughly 95 million animal units in the United States could produce about 1 percent of the country’s total energy consumption, or around 2.4 percent of its electricity, if their manure were fully digested and the biogas converted using standard microturbines.27Environmental Research Letters. Cow power: the energy and emissions benefits of converting manure to biogas China, which produces enormous volumes of livestock manure, has estimated that manure-derived biogas could supply roughly 4 to 5 percent of the country’s total energy demand.28PubMed. Biogas energy generated from livestock manure in China: Current situation and future trends
The climate benefit is double: capturing methane from manure prevents it from entering the atmosphere as a potent greenhouse gas, and burning that biogas displaces fossil fuels.29Environmental Research Letters. Cow power: the energy and emissions benefits of converting manure to biogas Treated manure solids, known as biosolids, can also be applied to agricultural land as fertilizer, though long-term use requires monitoring. A study of Spanish soils amended with sewage sludge–derived biosolids found that concentrations of lead and mercury increased over time, though they stayed within legal limits, and silver showed up at elevated levels, likely traced to the growing use of silver nanoparticles in consumer products that end up in wastewater.30PubMed. Long-term use of biosolids as organic fertilizers in agricultural soils: potentially toxic elements occurrence and mobility The mobility of these metals in the soil was low, suggesting limited risk of them leaching into groundwater, but it is a reminder that modern waste streams carry traces of whatever we manufacture and consume.
What Ancient Excrement Tells Archaeologists
Preserved feces, called coprolites, are among the most information-dense artifacts an archaeologist can find. A single coprolite can reveal what a person or animal ate, what parasites they carried, what plants grew nearby (through trapped pollen), and even fragments of ancient DNA. Researchers use microscopy, immunological assays, and genetic sequencing to extract this information.31PubMed Central. Deciphering Diets and Lifestyles of Prehistoric Humans through Paleoparasitology: A Review Coprolite analysis has helped reconstruct migration patterns, dietary transitions from foraging to agriculture, and the spread of infectious diseases through ancient populations. Despite this potential, coprolites remain underutilized compared to tools like pottery and bone analysis, partly because they require specialized processing and partly because, well, they are ancient feces and researchers have historically been drawn to more glamorous materials.32Earth-Science Reviews. The what, how and why of archaeological coprolite analysis
Sanitation and the Civilization-Shaping Problem of Human Waste
The management of human excrement has been one of the most consequential public health challenges in history. Before modern sanitation infrastructure, fecal contamination of drinking water was the primary route for diseases like typhoid fever, cholera, and dysentery. The relationship between sanitation investment and disease decline is well documented: an analysis of 16 U.S. cities between 1889 and 1931 found that each dollar per capita invested in water supply was associated with about a 5 percent decrease in typhoid transmission, and each dollar invested in sewer systems was associated with about a 6 percent decrease.33PubMed Central. Changes in historical typhoid transmission across 16 U.S. cities, 1889-1931: Quantifying the impact of investments in water and sewer infrastructures The pattern was consistent across cities despite wide variation in the specific technologies and timing of implementation.34PubMed Central. Making a Difference? The past, present, and future of typhoid control
Separating human waste from drinking water remains one of the highest-impact interventions in global health. In places where that separation is incomplete, the same fecal-oral diseases that ravaged 19th-century cities still circulate today. The lesson is stark: excrement is dangerous not because of what it is but because of where it ends up.
Tracking Wildlife Without Catching Them
For conservation biologists, one of the most valuable things about excrement is that animals leave it behind voluntarily. Collecting fecal samples lets researchers study wild populations without trapping, sedating, or even seeing the animals. DNA extracted from feces can identify species, distinguish individual animals, and reveal population structure. Researchers monitoring coyotes in the Appalachian mountains used DNA from scat to estimate population density, survival rates, and growth across multiple sampling sessions, demonstrating that the method works even for species that live at low densities over large areas.35The Journal of Wildlife Management. Monitoring coyote population dynamics with fecal DNA and spatial capture–recapture
The technique has been pushed even further with elephants. By combining specialized DNA extraction methods, researchers have reconstructed complete mitochondrial genomes from free-ranging elephant feces and generated partial nuclear genome data, despite the fact that only about 12 percent of the DNA in the samples was actually from the elephant. The rest belonged to the animal’s gut microbes.36PubMed Central. Combining methods for non-invasive fecal DNA enables whole genome and metagenomic analyses in wildlife biology That means a single pile of dung can simultaneously tell you about the elephant’s genetics and the composition of its microbial community. For endangered species that are difficult to observe directly, fecal genomics is becoming one of the most powerful and least intrusive monitoring tools available.