Flatulence, scientifically known as flatus, is a byproduct of digestion across a vast array of animal life. It represents the final step in the chemical process where food is broken down to release energy. The production and expulsion of internal gases indicate an animal’s diet, the efficiency of its digestive system, and the composition of its internal microbial community. Examining which animals fart and which do not offers a window into the diverse evolutionary adaptations animals use to process nutrients.
The Biological Basis of Gas Production
The primary source of flatulence is not the animal’s own digestive enzymes but the metabolic activity of its gut microbes. These microscopic organisms, primarily bacteria and archaea, live in a symbiotic relationship with their host. They specialize in breaking down complex carbohydrates, such as cellulose and other plant fibers, that the host animal’s stomach and small intestine cannot digest.
This process, known as anaerobic fermentation, occurs in the oxygen-deprived environment of the large intestine or specialized stomach chambers. As the microbes break down the undigested food residue, they produce gaseous byproducts. The main components of this internal gas are odorless, including Carbon Dioxide (CO2), Hydrogen (H2), and Methane (CH4).
The characteristic odor associated with flatus comes from trace gases that constitute less than one percent of the total volume. These odorous compounds, such as hydrogen sulfide and volatile fatty acids, are also products of microbial metabolism. The specific gases produced, and therefore the resultant smell, are heavily influenced by the animal’s diet and the particular species of bacteria and archaea present in its gut.
High-Volume Producers and Their Diets
The volume of gas an animal produces is directly linked to the amount of difficult-to-digest fiber in its diet and the structure of its digestive tract. Ruminant animals, such as cows and sheep, are among the most prolific gas producers due to their specialized multi-chambered stomachs. Their rumen acts as a fermentation vat where methanogenic archaea convert CO2 and H2 into large quantities of methane.
A single dairy cow can produce an estimated 250 to 500 liters of methane each day, though the vast majority of this gas is expelled through belching rather than flatulence. Hindgut fermenters, like horses, also produce significant gas as fermentation occurs in their large intestine and cecum. In horses, the sudden introduction of rapidly fermentable feeds, like grains, can lead to an overproduction of gas, sometimes causing painful digestive issues.
Another high-volume producer is the termite, which requires symbiotic gut protozoa and bacteria to break down the cellulose in wood. Collectively, termites are estimated to contribute between one and five percent of global methane emissions, equivalent to up to 20 million tonnes of methane annually.
The Science Behind Animals That Don’t Fart
Several animal groups are confirmed or strongly suspected non-farters because their anatomy or digestive process prevents gas accumulation and expulsion. This absence of flatus is primarily due to either a lack of gas-producing microbes, a specialized digestive system, or rapid transit time.
Many simple marine invertebrates, such as jellyfish, sponges, and corals, do not fart because they lack a complex digestive system and an anus. Sponges rely on intracellular digestion, and jellyfish possess a gastrovascular cavity with a single opening that serves as both mouth and anus. Since they lack the necessary closed, anaerobic environment and microbial communities, the conditions for fermentation are not met.
Birds represent another major group that does not produce flatus, a phenomenon explained by two biological factors. First, the avian digestive tract is short and highly efficient, allowing food to pass through quickly, often in under an hour for small species. This rapid transit time provides insufficient opportunity for extensive bacterial fermentation and gas buildup.
Furthermore, the microbial communities in the guts of most birds lack the same population of methanogens and other gas-producing bacteria found in mammals. If a bird’s gastrointestinal tract shows excessive gas on a radiograph, it is considered a sign of a pathological condition rather than a normal biological function.
Mammalian exceptions also exist, such as the sloth, which has a multi-chambered stomach and an extremely slow digestive process that can take up to 30 days. While their foregut fermentation produces a significant amount of methane, the gas is not expelled rectally. Instead, the gas is absorbed into the bloodstream and then exhaled through respiration, a mechanism that helps them conserve energy by avoiding the muscular effort of passing gas. Bats bypass gas production due to having less intestinal tissue and an accelerated nutrient absorption rate, an evolutionary adaptation to minimize body mass for flight.