Feces

Feces are the solid or semi-solid remains of food that a body cannot digest. This material is left over after the small intestine has finished its work. In the large intestine, bacteria break these remains down even further. Feces also contain metabolic waste products, such as bilirubin that has been changed by bacteria. They also contain dead epithelial cells, which are cells from the lining of the gut. Together with urine, these materials are known as excreta.

The process of removing feces is called defecation. This happens when material is discharged through the anus or a cloaca. To move the material out, the body must create pressure. This pressure comes from muscular contractions and a build-up of gases inside the gut. When the pressure is high enough, it prompts the sphincter to release the feces. In humans, this pressure can reach 13.3 kPa. In penguins, the pressure is much higher, reaching 60 kPa.

Feces have a very distinctive smell. This odor is caused by several different compounds. One is skatole, which is produced from tryptophan via indoleacetic acid. Other contributors include thiols, which are sulfur-containing compounds, as well as amines and carboxylic acids. One specific molecule involved is hydrogen sulfide.

Scientists have hypothesized that this bad odor acts as a deterrent. It may help prevent humans from touching or consuming it, which could cause sickness or infection.

In nature, feces play a vital role in the biogeochemical cycle. This is the process of recycling organic matter back into simpler forms. Many organisms, known as detritivores, survive by eating this dead organic matter. While feces contain less energy than the original food, they often still hold about 50% of that energy. This provides a massive energy source for decomposers like fungi and bacteria. Some animals even practice coprophagia, which is eating feces to gain nutrients. For example, young elephants eat their mothers' feces to get essential gut flora.

Feces also help plants spread across the world. Frugivores are animals that eat fruit. When these animals eat fruit, they may swallow the seeds. If the seeds can survive the journey through the digestive system, they are eventually released in feces. This is a successful way for plants to disperse seeds far away from the parent plant. Being far away helps the seeds avoid heavy predation. Additionally, the feces provide a built-in fertilizer for the new plant to grow.

Humans have used feces for many different purposes throughout history. In agriculture, animal feces like manure or guano are used as fertilizer. In some countries, dried dung from animals like bison or cattle is burned as fuel. Even more unusual uses exist, such as making coffee. Kopi luwak is made from beans eaten and excreted by Asian palm civets. Black Ivory coffee is made using coffee beans digested by elephants. In Thailand, even paper is manufactured from elephant dung.

Scientists also study ancient feces to learn about the past. A coprolite is a piece of fossilized feces. These were first described by William Buckland in 1829. Before that, people called them "fossil fir cones." Coprolites are important because they provide direct evidence of what extinct animals ate. They can be as small as a few millimeters or over 60 centimeters long. Researchers also study palaeofeces, which are ancient feces found in archaeological sites. By analyzing seeds, bones, or even ancient DNA, they can learn about the diet and health of ancient people.