Feces

Feces (also called faeces, fæces, or poop) are the solid or semi-solid leftovers of food that the small intestine couldn't digest, later broken down by bacteria in the large intestine. They contain small amounts of metabolic waste, including bacterially altered bilirubin and dead cells shed from the gut lining. These materials exit the body through the anus or cloaca during defecation. In agriculture, feces can serve as fertilizer or soil conditioner; they can also be burned for fuel, dried for construction, or used medicinally—for instance, in fecal transplants.

Characteristics

Together, urine and feces are known as excreta. The characteristic smell of feces comes from skatole (produced from tryptophan via indoleacetic acid and decarboxylation), thiols (sulfur-containing compounds), amines, and carboxylic acids. This unpleasant odor is thought to deter humans from touching or consuming feces, which could cause sickness or infection. Defecation involves pressures that can reach 100 mmHg (13.3 kPa) in humans and 450 mmHg (60 kPa) in penguins.

Ecology

Muscular contractions and gas buildup in the gut generate the force needed to expel feces, prompting the sphincter to release the pressure. After digestion, an animal’s waste retains a significant amount of energy—often about 50% of the original food’s energy. This means a large portion of consumed energy remains available for decomposers in ecosystems. Many organisms feed on feces, including bacteria, fungi, and insects like dung beetles, which can detect odors from far away.

Some species specialize in feces, while others use it as a dietary supplement. This behavior, called coprophagia, occurs in young elephants (to gain gut flora from their mothers), as well as in dogs, rabbits, and monkeys. Feces and urine reflect ultraviolet light, which helps raptors like kestrels—able to see near-ultraviolet—locate prey by their middens and territorial markers. Seeds are also commonly found in feces.

Frugivores (fruit-eating animals) disperse seeds when they eat fruit and later excrete them. This method of seed dispersal is highly successful: seeds far from the parent plant face less predation and often come with their own fertilizer, provided they survive the digestive tract. Organisms that consume dead organic matter or detritus are called detritivores.

Fecal resistome

For soil nutrients to be maintained, feces should ideally return to the area where the food originated—something that often doesn’t happen in human societies, where food is transported from rural areas to cities and feces are disposed of into rivers or seas. The "fecal resistome" is the total gene pool in human and animal feces that confers antimicrobial resistance. Antibiotics taken by humans and animals mostly leave the body through feces and urine, which contain antibiotic resistance genes (ARGs). These genes act as environmental pollutants, spreading through agricultural practices like manure application and through wastewater systems.

When contaminated feces enter new ecosystems, their microbes promote horizontal gene transfer, spreading ARGs to native microbes. ARGs persist longer in soil when manure is used, and they can remain in the environment due to co-resistance with metals, which continue to exert selective pressure. Even without antibiotics, fecal matter alters soil composition and encourages the growth of pre-existing resistant bacteria. Human defecation frequency varies: some people go several times a day, others once every two or three days.

Human feces

Extended hardening of feces that disrupts this routine for days or more is called constipation. The appearance of human feces depends on diet and health. Normally, it is semisolid with a mucus coating. Bile and bilirubin from dead red blood cells give it a typical brown color.

After the first stool (meconium), a newborn’s feces contain only bile, appearing dark yellow-green. Breastfed babies produce soft, pale yellowish, and relatively mild-smelling matter; once the baby starts eating solid food and bilirubin from dead red blood cells appears, the stool turns brown. Throughout life, stool color and texture vary: rapid passage through the intestines makes it greenish, while a lack of bilirubin gives it a clay-like appearance. Feces are a key indicator of the gut microbiome, which is an important marker of health and disease.