Koala mothers feed their babies two very different things at two very different life stages: milk for the first several months, and then a specialized form of their own feces, called pap, to prepare the joey for a lifetime of eating eucalyptus leaves. The pap stage is what surprises most people, and it turns out to be one of the more remarkable feeding strategies in the mammal world. The whole sequence, from pouch milk to pap to independent leaf-chewing, reflects a problem that koalas had to solve evolutionarily: eucalyptus is tough, toxic, and nutritionally poor, and a joey cannot just start eating it without help.
Months of Milk Inside the Pouch
A koala joey is born after roughly 35 days of gestation, and at birth it is tiny, blind, hairless, and smaller than a jellybean. It crawls into the mother’s backward-facing pouch and latches onto one of two teats, where it will stay attached almost continuously for several months. During this early phase, milk is the joey’s only food source, and the composition of that milk is not static. Proteomic analysis has identified hundreds of distinct proteins in koala milk, and the mix shifts dramatically as the joey grows.
In early lactation, the milk is loaded with immune-related proteins that support the joey’s essentially nonexistent immune system. By mid-lactation, the protein profile shifts toward neuroimmune protection and energy metabolism, matching the joey’s rapid brain and body development. In late lactation, immune competence proteins ramp up as the joey prepares to face the outside world. Researchers identified 874 proteins in early-lactation milk, 1,220 in mid-lactation, and 498 in late lactation, with each stage’s protein clusters aligning with what the joey’s body needs at that moment.1International Journal of Biological Macromolecules. Lactation-stage specific protein shifts in koala milk mirror the joey’s growth needs This isn’t just milk getting richer or thinner over time; it’s a fundamentally different nutritional package at each stage.
The joey stays in the pouch for about six months total, though it begins poking its head out and exploring earlier than that. Milk remains an important part of its diet even after it starts emerging, but the transition to solid food begins with something unexpected.
What Pap Actually Is
Pap is a soft, semi-liquid substance that the mother produces from her own digestive tract. It looks nothing like regular koala droppings. Normal koala feces are dry, compact pellets with a water content around 55% and a pH of about 5.5. Pap, by contrast, is about 82% water with a near-neutral pH of 7.0, which strongly suggests it comes directly from the cecum rather than passing through the full length of the large intestine where water is normally reabsorbed.2Australian Journal of Zoology. Microbiological Studies of the Intestinal Microflora of the Koala, Phascolarctos-Cinereus .2. Pap, a Special Maternal Feces Consumed by Juvenile Koalas The cecum is the part of the koala gut where the real work of eucalyptus fermentation happens, a large pouch at the junction of the small and large intestines where specialized bacteria break down plant fiber and detoxify eucalyptus compounds.
So pap isn’t just diluted feces. It’s more like a concentrated delivery of the mother’s cecal contents, packaged in a form soft enough for a joey to consume. The joey eats it directly from the mother’s cloaca, typically around the time it first begins emerging from the pouch. This behavior, technically a form of coprophagy, happens multiple times and was observed in every mother studied in the key research on the topic.2Australian Journal of Zoology. Microbiological Studies of the Intestinal Microflora of the Koala, Phascolarctos-Cinereus .2. Pap, a Special Maternal Feces Consumed by Juvenile Koalas
Why Pap Matters for Digesting Eucalyptus
Eucalyptus leaves are a terrible food source by almost any standard. They are fibrous, low in calories, and packed with tannins and other toxic compounds that most mammals cannot handle. Adult koalas manage this diet because their enormous cecum houses a community of microorganisms that ferment the plant fiber and break down the toxins. But a joey is born with a sterile gut. It has none of these microbes. Without them, it would be unable to extract any nutrition from eucalyptus and could be poisoned by the tannins.
Pap solves this problem. It contains dramatically higher concentrations of the gut bacteria the joey needs compared to ordinary koala feces. Specifically, pap carries 23 to 41 times the viable counts of tannin-protein-complex-degrading enterobacteria found in the mother’s regular droppings. In some of the mothers studied, these tannin-degrading bacteria were found in pap but were completely undetectable in their normal feces.2Australian Journal of Zoology. Microbiological Studies of the Intestinal Microflora of the Koala, Phascolarctos-Cinereus .2. Pap, a Special Maternal Feces Consumed by Juvenile Koalas The evidence points to pap being specifically produced from a part of the gut where these specialized bacteria are most concentrated, making it an efficient microbial starter culture for the joey.
Beyond just the tannin-degrading specialists, pap provides a broader microbial community. Compared to regular feces, pap has higher overall microbial density, greater microbial evenness, and a higher proportion of rare bacterial taxa.3PubMed. Maternal inheritance of the koala gut microbiome and its compositional and functional maturation during juvenile development That last point matters more than it might sound. Many of the rare taxa in pap are species that would struggle to colonize the joey’s gut if delivered in tiny quantities. By being present in higher proportions in pap than in regular feces, these rarer microbes have a better shot at establishing themselves in the joey’s developing gut ecosystem. The mother is essentially handing her offspring a curated microbial library rather than a random sample.
The Timeline From Pouch to Leaves
The full feeding arc goes roughly like this. For the first five to six months, the joey is exclusively milk-fed inside the pouch. Around month six, the joey begins emerging from the pouch and starts consuming pap, though it continues to drink milk as well. Pap feeding episodes happen repeatedly over several weeks. Gradually, the joey begins nibbling on eucalyptus leaves, at first likely softened or partially pre-chewed, while still returning to the pouch and still nursing. By about 12 months of age, the joey is fully weaned and eating eucalyptus independently, though it may ride on the mother’s back for a while longer before dispersing.
The overlap between milk, pap, and early leaf consumption is important. The joey doesn’t abruptly switch from one food to the next. There’s a transitional window where all three are happening. The pap inoculates the gut with the right microbes, and then the joey begins testing eucalyptus while it still has the safety net of mother’s milk for calories and hydration. This gradual transition gives the gut microbiome time to establish and mature before the joey is fully dependent on its ability to ferment leaves.
What This Costs the Mother
Producing both milk and pap is energetically expensive. Koalas are already operating on thin nutritional margins because eucalyptus is so calorie-poor. During peak lactation, female koalas increase their food intake by about 35% to meet the demands of reproduction.4Functional Ecology. A flexible digestive strategy accommodates the nutritional demands of reproduction in a free‐living folivore, the Koala (Phascolarctos cinereus) For an animal that sleeps up to 20 hours a day partly to conserve energy on its low-quality diet, eating a third more food represents a significant shift in daily activity and energy allocation.
Pap production adds another layer of cost. The mother’s cecum is diverting its microbial contents into a special excretion for the joey rather than completing its own digestion normally. The exact metabolic cost of pap production hasn’t been precisely quantified the way milk production has, but given that pap is essentially liquefied cecal material, the mother is giving up some of her own digestive capacity during the pap-feeding period. This is one reason why female koalas typically produce only one joey per year and invest heavily in each offspring rather than spreading their energy across multiple young.
How Pap Compares to Other Mammal Feeding Strategies
Pap feeding strikes most people as bizarre, but the underlying principle is less unusual than it seems. Many herbivorous mammals need to acquire gut microbes from their mothers or the environment to digest their adult diet. Rabbits produce cecotropes, soft droppings from their cecum that they eat themselves to re-digest nutrients and recirculate gut bacteria. Elephant calves eat their mother’s feces to seed their own gut flora. Ruminants like cattle acquire their rumen microbiome partly through contact with their mother and herd-mates.
What makes koala pap distinctive is how specialized it is. It isn’t just the mother’s regular droppings being consumed by the young. It’s a physically and chemically distinct substance with a different consistency, different pH, and a dramatically different microbial profile. The mother’s body appears to actively produce pap as a separate output from normal defecation. That level of specialization, where the parent creates a bespoke microbial delivery system for the offspring, is rare among mammals.
Why Pap Feeding Matters for Conservation
Understanding pap has practical consequences for koala conservation. Orphaned joeys raised in wildlife rehabilitation centers don’t have access to their mother’s pap. Without it, they may fail to develop the gut microbiome necessary to thrive on eucalyptus. Rehabilitation programs have experimented with providing orphaned joeys with fecal material from healthy adult koalas as a substitute, and the science behind this approach is gaining traction.
Research has shown that faecal inoculations can meaningfully alter a koala’s gastrointestinal microbiome. In one study, wild-caught koalas that preferred one eucalyptus species were given encapsulated fecal material from koalas that fed on a different species. After inoculation, the treated koalas’ gut microbiomes shifted to resemble those of koalas eating the donor’s preferred tree species, and the greater the microbiome shift, the more of the new eucalyptus species the koalas consumed.5Animal Microbiome. Faecal inoculations alter the gastrointestinal microbiome and allow dietary expansion in a wild specialist herbivore, the koala This suggests that the microbial transfer doesn’t just survive the process; it functionally changes what the koala can eat.
For conservation, this opens several possibilities. Orphaned joeys could receive standardized microbial supplements to replace the pap they missed. Koalas being relocated to areas with different eucalyptus species could be pre-inoculated with microbes suited to the local trees, easing the dietary transition that often causes relocated koalas to struggle or starve. And the protein profiling work on koala milk could inform the design of better milk replacers for joeys in care, matching the immune and nutritional support the joey would normally receive from its mother at each stage of development.1International Journal of Biological Macromolecules. Lactation-stage specific protein shifts in koala milk mirror the joey’s growth needs
Common Misconceptions About Koala Feeding
One widely repeated claim is that joeys eat pap because it contains pre-digested eucalyptus that the joey gradually learns to tolerate, as though the mother is feeding them a diluted version of their future diet. This misses the point. The primary function of pap is microbial transfer, not nutritional content. The joey isn’t getting eucalyptus-flavored baby food; it’s getting a gut microbiome transplant. The bacteria and other microorganisms in pap are what allow the joey to eventually digest eucalyptus on its own. Without the microbes, the actual leaf matter wouldn’t help.
Another misconception is that pap is identical to regular feces, just softer. As the research makes clear, pap is chemically and biologically distinct from normal koala droppings. Its much higher water content, neutral pH, and dramatically elevated counts of specialized bacteria all point to it being a separate production from the cecum, not simply loose stool. Describing it as “the mother’s poop” is technically not wrong, since it does exit via the same route, but it understates how different this substance is from what a koala normally leaves on a tree branch.
A third misunderstanding involves timing. Some accounts suggest the joey eats pap once and then moves on to leaves. In reality, pap feeding occurs multiple times over a period of weeks, giving the microbial community repeated opportunities to establish in the joey’s gut. A single dose would be unlikely to achieve the kind of stable colonization that a joey needs to become a functional eucalyptus-processing machine.
The Koala Cecum and Its Outsized Role
Much of what makes koala feeding biology work comes down to one organ: the cecum. In most mammals, the cecum is a small, mostly vestigial pouch. In koalas, it is proportionally one of the largest in the animal kingdom, stretching to roughly two meters in length. This enormous fermentation chamber is where the real digestive work happens. Fibrous eucalyptus cell walls that the stomach and small intestine can’t break down get fermented here by the resident microbial community, extracting the modest calories available and neutralizing some of the toxic tannins and phenolic compounds.
The cecum is so central to koala survival that the entire reproductive feeding strategy revolves around it. The milk keeps the joey alive and developing. The pap transfers the cecal microbiome. And the eucalyptus diet that the joey graduates to only works because the cecum provides the fermentation chamber where those transplanted microbes can do their job. Every stage of joey feeding is, in a sense, in service of getting that cecum populated and operational.
This is also why habitat loss that forces koalas onto unfamiliar eucalyptus species can be so damaging. A koala’s gut microbiome is tuned to the specific eucalyptus species it eats. If a koala is displaced to an area with different trees, its existing microbiome may not efficiently process the new leaves, leading to malnutrition even when surrounded by apparently edible foliage. The faecal inoculation research mentioned earlier directly addresses this problem, and it owes its logic to the same principle behind pap: if you can change the gut microbiome, you can change what a koala is able to eat.5Animal Microbiome. Faecal inoculations alter the gastrointestinal microbiome and allow dietary expansion in a wild specialist herbivore, the koala