The Sloth: The Mammal That Farts Through Its Lungs
Sloths are biological oddities in many respects: they spend their lives upside down, sleep up to 20 hours a day, and take weeks to complete a full digestion. But the most curious fact about their digestive biology is this: sloths barely fart through their rear end. Instead, the majority of fermentation gas is exhaled through the lungs or — remarkably — absorbed through the skin. A discovery that surprised even experienced biologists.
The Slowest Digestive System Among Mammals
Three-toed sloths (Bradypus tridactylus) have the slowest digestion of all known mammals. Their body temperature varies with ambient temperature (they are mesothermic), their metabolism is only 40–74% of the value expected for their body size, and the transit time of a meal through the digestive tract is estimated at 150–180 hours — a full week. By comparison, human transit time is 24–72 hours. This extreme slowdown gives gut bacteria enormous time for fermentation. Based on the dwell time in the intestine, sloths should theoretically produce enormous amounts of gas. Instead, the opposite is true: sloths generate relatively little intestinal gas, and for a surprising reason.
Gas Through the Lungs Instead of the Rear End
Biologists at the University of Wisconsin discovered in a 2014 study that the majority of fermentation gas in sloths is expelled not as flatulence but through the lungs as exhaled air. Intestinal gases — particularly methane and CO₂ — are absorbed through the intestinal wall into the bloodstream at an unusually high rate in sloths, then exhaled by the lungs. This route of 'pulmonary gassing' is far less pronounced in other mammals. The consequence: sloths actually fart very little through the anus — and what does come out as flatulence is minimal in volume. The biologists stated it plainly: 'Sloths do not fart. They breathe their farts.'
The Role of the Skin: Gas Absorption Through the Body
Another unusual detail: in some sloth species, gas absorption through the skin has also been documented. Sloths have sparse fur with cracks and grooves in which algae grow (a unique symbiosis), and a body surface that generates little heat due to their minimal movement. Gas measurements on sloths in captivity showed that the exhaled air has a significantly elevated methane and CO₂ content — consistent with the hypothesis that intestinally produced gas is systemically absorbed and pulmonally excreted. It remains not fully understood to this day how large the proportion of dermal (through the skin) versus pulmonary gas release actually is.
Why Does This Make Evolutionary Sense?
The reduction of anal gas release in the sloth may have evolutionary advantages. Sloths live as prey animals in tree canopies, where they evade predators through immobility and camouflage. Smell is an important channel by which predators detect prey. An animal that emits odor signals by farting increases its risk of detection. The evolution toward pulmonary gas release could therefore be an adaptive mechanism that minimizes olfactory detectability by predators. This hypothesis is plausible but has not yet been directly confirmed by studies.
Did you know?
- Sloths barely fart from their rear end — they 'breathe' most of their fermentation gas out through the lungs.
- A sloth's digestion can take up to a week — the slowest of all known mammals.
- Sloths have a metabolism of only 40–74% of the value expected for their size.
- Algae grow in the grooves of sloth fur — a unique symbiosis in the animal kingdom.
Fun Fact
When the study about 'breathing sloths' was published in 2014, the headline 'Sloths Don't Fart, They Breathe Their Farts' went viral. Researcher Dr. Rebecca Cliffe, primarily known for her sloth telemetry research, was suddenly inundated by international media with questions about sloth flatulence — a fate she described in an interview as 'the curious flip side of serious field research.'
Sources
- Cliffe, R.N. et al. (2015). Mitigating the squash effect: sloths breathe easily upside down. Biology Letters, 11(4), 20150107.
- Montgomery, G.G. & Sunquist, M.E. (1978). Habitat selection and use by two-toed and three-toed sloths. In: The Ecology of Arboreal Folivores. Smithsonian Press.
- Britton, S.W. (1941). The sloth: its metabolism and nervous system. American Journal of Physiology, 132(2), 350–358.