The best-known butt-breather is a turtle, but it is far from the only animal that pulls oxygen in through its rear end. Sea cucumbers, dragonfly nymphs, and some small fish do it too, each having arrived at the trick separately. Breathing through the rear is less a single oddity than a recurring solution for animals that spend long stretches underwater or in low-oxygen water. The idea has even reached hospitals, where researchers are testing whether oxygen given through the rectum could help people whose lungs are failing.1PubMed. Mammalian enteral ventilation ameliorates respiratory failure
Which Turtles Breathe Through Their Butts
Turtles, like birds and reptiles generally, have a single rear opening called the cloaca, which handles waste, urine, and reproduction. Many freshwater turtles also have a pair of thin-walled pouches branching off it, called cloacal bursae. These can take oxygen out of water that is pumped in and out.2Transactions of the Kansas Academy of Science. The Taxonomic Significance of Cloacal Bursae in Turtles
Back in 1799 a naturalist saw thirsty freshwater turtles take in water through the rear, and water moving in and out fit several explanations at once.3Biological Reviews. Role of urinary and cloacal bladders in chelonian water economy: historical and comparative perspectives For two centuries scientists argued over whether the sacs were for taking up water, controlling buoyancy, or breathing, and most recent work favors breathing.4Biological Reviews. Role of urinary and cloacal bladders in chelonian water economy: historical and comparative perspectives Breathing won out because experiments measured oxygen actually leaving the water and entering the animal.
In a living turtle, this looks like rhythmic pumping of water in and out of the rear opening while the animal rests underwater. The sacs work a little like gills turned inside out, with blood vessels running close to the water. A resting turtle can stay submerged far longer than holding its breath alone would allow. It is a supplement to the lungs, not a replacement, and these turtles still surface to breathe air. Some turtles also absorb oxygen through the lining of the mouth and throat, and skin takes in some too.
Australia’s Fitzroy River turtle is the champion, relying heavily on rear breathing through its cloacal bursae.5SpringerLink / PubMed Central. Response of heart rate and cloacal ventilation in the bimodally respiring freshwater turtle, Rheodytes leukops, to experimental changes in aquatic PO2 In well-oxygenated water its heart rate stayed steady even during dives of up to 3 days, meaning it wasn’t rationing oxygen but getting it from the water.6SpringerLink / PubMed Central. Response of heart rate and cloacal ventilation in the bimodally respiring freshwater turtle, Rheodytes leukops, to experimental changes in aquatic PO2 When the water itself was low in oxygen, the turtle switched from extracting oxygen to conserving it so it could still stay down longer.7SpringerLink / PubMed Central. Response of heart rate and cloacal ventilation in the bimodally respiring freshwater turtle, Rheodytes leukops, to experimental changes in aquatic PO2
Not every turtle does this. Only some freshwater species have well-developed bursae, and having them doesn’t mean a species depends on them; how much they contribute varies. Sea turtles and land tortoises rely on their lungs.
Why Turtles Evolved Rear Breathing
The likely reason the bursae evolved is breathing during underwater hibernation or summer dormancy.8Transactions of the Kansas Academy of Science. The Taxonomic Significance of Cloacal Bursae in Turtles A cold, inactive turtle needs so little oxygen that slow rear breathing can cover it.9Transactions of the Kansas Academy of Science. The Taxonomic Significance of Cloacal Bursae in Turtles Turtles are cold-blooded, so as the water chills their bodies burn energy much more slowly, and a thin-walled sac can supply the resulting trickle of oxygen. That matters not only near freezing but at sluggish temperatures of about 40 to 60°F, common in winters even in subtropical places.10Transactions of the Kansas Academy of Science. The Taxonomic Significance of Cloacal Bursae in Turtles Cold water also holds more dissolved oxygen than warm water, which helps. In cold, well-aerated water, freshwater turtles can meet all or most of their oxygen needs by breathing from the water.11PubMed Central. Physiology of hibernation under the ice by turtles and frogs In summer, when the turtle is active and the water is warmer, the same sacs couldn’t keep up, so it still needs its lungs.
Painted and snapping turtles can survive up to 4 to 5 months with no oxygen at all.12PubMed Central. Physiology of hibernation under the ice by turtles and frogs They slow their body’s energy use sharply and use their shell and bones to soak up the acid that builds up.13PubMed Central. Physiology of hibernation under the ice by turtles and frogs
Sea Cucumbers Breathe Through Their Butts Too
Sea cucumbers have a respiratory tree, a branching set of breathing tubes attached to the cloaca.14PubMed Central. Plasticity of Respiratory Function Accommodates High Oxygen Demand in Breeding Sea Cucumbers So you can count a sea cucumber’s breathing rate by watching its rear opening move.15PubMed Central. Plasticity of Respiratory Function Accommodates High Oxygen Demand in Breeding Sea Cucumbers The animal is soft and slow-moving with no lungs or true gills, and its rear opening leads straight to the tubes, which makes the rear the natural doorway for water. Seawater pumped through the branches passes close to the body fluid, letting oxygen cross thin walls.
The tubes are important but not the only route. In ordinary seawater, about 60% of the oxygen some sea cucumbers take in comes through the respiratory tree.16The Biological Bulletin. BIMODAL GAS EXCHANGE AND THE REGULATION OF OXYGEN UPTAKE IN HOLOTHURIANS Larger individuals of some species depend more on it, while other species can make up the loss by absorbing more through their skin.17The Biological Bulletin. BIMODAL GAS EXCHANGE AND THE REGULATION OF OXYGEN UPTAKE IN HOLOTHURIANS
A breath starts when the animal opens its anus and widens the cloaca to suck in seawater.18Aquaculture Science. Ventilation in Respiratory Tree of the Japanese Common Sea Cucumber, Apostichopus japonicus It then closes the anus and squeezes the water up into the branching breathing tubes, repeating this several times.19Aquaculture Science. Ventilation in Respiratory Tree of the Japanese Common Sea Cucumber, Apostichopus japonicus After a short pause it opens everything and expels the water in one rush.20Aquaculture Science. Ventilation in Respiratory Tree of the Japanese Common Sea Cucumber, Apostichopus japonicus
The same doorway attracts lodgers. Eel-like pearlfish often live inside sea cucumbers’ breathing tubes or body cavity.21Marine Biology. Further insight on carapid—holothuroid relationships Some only use the host as shelter, while others are parasites that eat its reproductive organs.22Marine Biology. Further insight on carapid—holothuroid relationships A pearlfish typically enters tail-first through the rear opening, and the host can’t easily keep it out without closing off its own breathing.
Dragonfly Nymphs Breathe and Swim With Their Rear
Dragonfly nymphs live in water and breathe it through a rectal gill, a gill inside the rear gut.23PubMed. The bimodal gas exchange strategies of dragonfly nymphs across development Only the larger dragonflies do this; damselfly nymphs have three leaf-like tail gills instead. It is made of many thin folds lining the last part of the gut, which greatly enlarges the surface for oxygen to cross. In one common European species, a nymph weighing about a quarter of a gram has about 12 square centimeters of gill surface.24PubMed. Morphometric analysis of the larval branchial chamber in the dragonfly Aeshna cyanea Müller (Insecta, Odonata, Anisoptera)
The nymph draws water into this chamber, holds it briefly while oxygen passes into the blood, and then pushes it out. Nymphs pull about 40% of the oxygen out of each breath of water and keep that rate even when oxygen gets low.25PubMed Central. Oxygen extraction efficiency of the tidally-ventilated rectal gills of dragonfly nymphs Because the folds are inside the body, they are protected from damage and from clogging with pond debris. It’s also why the nymph’s abdomen visibly moves in and out as it breathes.
The pump does double duty. Nymphs breathe and move using repeated water jets through the anal opening.26PubMed. Asymmetry in the jet opening: underwater jet vectoring mechanism by dragonfly larvae A forceful exhale shoots water out and pushes the nymph forward in jerky bursts.27Advanced Odonatology. The rectum of larval dragonflies as jet-engine, respirator, fuel depot and ion pump Some nymphs also gulp air into the rectal gill, skimming the surface or sucking in air bubbles.28PubMed. The bimodal gas exchange strategies of dragonfly nymphs across development
Things change when the nymph becomes an adult. The adult lives in air and breathes through tiny openings along its body, so the rectal gill is no longer used for water breathing.
Fish That Breathe Air Through Their Gut
The Asian weatherloach, a common aquarium and pond fish, uses its hindgut as an extra air-breathing organ.29PubMed. Is there a compromise between nutrient uptake and gas exchange in the gut of Misgurnus anguillicaudatus, an intestinal air-breathing fish? The hindgut is packed with tiny blood vessels right in the lining, ideal for absorbing oxygen.30PubMed. Is there a compromise between nutrient uptake and gas exchange in the gut of Misgurnus anguillicaudatus, an intestinal air-breathing fish? This ability starts forming about 10 days after hatching.31PubMed Central. Developmental transcriptome analysis and identification of genes involved in formation of intestinal air-breathing function of Dojo loach, Misgurnus anguillicaudatus Gut-breathing fish tend to have a gut that digests less and has a thin, heavily blood-supplied lining, with muscle rings walling off the breathing part.32Fish Physiology. The GI Tract in Air Breathing
The loach rises to the surface, takes a mouthful of air, and swallows it down into its gut. As the air moves along, oxygen passes through the lining into the blood vessels there. The remaining gas, now with less oxygen, leaves through the vent as a small stream of bubbles, and the fish returns to the bottom to repeat the cycle as needed. The fish gulps more often in warm or stale water, so frequent gulping can hint that the water is low in oxygen.
There is a trade-off, because the same gut must digest food and absorb air, and a thin lining good for oxygen is less suited to digesting. So these fish keep the breathing zone near the end of the gut, after most digestion is done.
Breathing even seems to help digestion. When loaches were kept from gulping air, food moved through their gut more slowly.33PubMed Central. The Structure of Digestive Tract Coordinating Digestion and Respiration in an Air-Breathing Weatherloach, Misgurnus anguillicaudatus Air breathing seems to help push food along the intestine.34PubMed Central. The Structure of Digestive Tract Coordinating Digestion and Respiration in an Air-Breathing Weatherloach, Misgurnus anguillicaudatus Normally a meal clears the loach gut in about 8 hours.35PubMed Central. The Structure of Digestive Tract Coordinating Digestion and Respiration in an Air-Breathing Weatherloach, Misgurnus anguillicaudatus
Can Humans Breathe Through the Butt
Not naturally. People have no breathing sacs or gill-like tissue in the rear. Still, researchers gave mice and pigs with failing breathing either oxygen gas or an oxygen-carrying liquid through the rectum.36PubMed. Mammalian enteral ventilation ameliorates respiratory failure Both methods raised blood oxygen and improved survival.37PubMed. Mammalian enteral ventilation ameliorates respiratory failure In pigs, the liquid raised arterial oxygen from about 55 to 61 mmHg within 30 minutes.38PubMed Central. Enteral liquid ventilation oxygenates a hypoxic pig model That is a modest but real boost, mainly entering through the large intestine’s blood veins.39PubMed Central. Enteral liquid ventilation oxygenates a hypoxic pig model The large intestine can do this because its lining is thin and rich in blood vessels, since it absorbs water and some nutrients. The work won a 2024 Ig Nobel Prize, the humorous award for science that makes people laugh and then think.40Research Horizons. Ig Nobel Prize Awarded to Takanori Takebe for “Butt-Breathing” Study – Research Horizons
The method relies on a special liquid that carries lots of oxygen, given like an enema. It’s meant as a backup alongside ventilators, not a replacement for lungs.
In a first trial, 27 healthy men held the liquid, without oxygen added, in the rectum for 60 minutes.41News. First Human Trial Demonstrates Safety of Rectal Oxygen Delivery for Respiratory Support Side effects were mild gut symptoms with nothing serious.42News. First Human Trial Demonstrates Safety of Rectal Oxygen Delivery for Respiratory Support But the test showed safety only, not whether it helps patients breathe.43PubMed. Safety and tolerability of intrarectal perfluorodecalin for enteral ventilation in a first-in-human trial It is not available as a treatment.44Research Horizons. IgNobel ‘Butt Breathing’ Idea from 2024 Moves Closer to Real Treatment
This is a hospital technique under study, not something to try. Putting gases or liquids in the rectum at home can injure the bowel, and anyone short of breath needs emergency care, not experiments.
Why So Many Animals Ended Up Breathing This Way
Turtles, sea cucumbers, dragonfly nymphs, and loaches are only distantly related, so each evolved rear breathing separately. This is convergent evolution: when unrelated animals face similar problems, natural selection can produce similar solutions in each one. Wings in birds and bats are a familiar example. Here, the common ancestor of these four groups didn’t have the trick, so seeing it repeatedly suggests it is an easy one to arrive at given the right starting equipment.
The rear end offers that equipment. It already opens to the outside and can pump water or air in and out, and its lining can become thin and rich in blood vessels. It also sits away from the mouth, so the animal can breathe while feeding or hiding. Like frogs and salamanders breathing through skin and throat, rear sacs give a slow backup oxygen supply.45Transactions of the Kansas Academy of Science. The Taxonomic Significance of Cloacal Bursae in Turtles The payoff is greatest for animals that are slow, cold, or in still water. Rear breathing is slow, so none of these animals rely on it during hard activity, and every case pairs it with another breathing method.