A sloth can spend days processing a meal that another herbivore would handle much faster. The secret is not simply a slow stomach. Inside that digestive system lives a community of bacteria and fungi helping to tackle tough plant material.
A 2025 study compared the gut microbiota of two familiar sloths: the brown-throated three-fingered sloth, Bradypus variegatus, and Hoffmann's two-fingered sloth, Choloepus hoffmanni. The researchers found meaningful differences between their microbial communities and how those communities handle fibre.
The results add an important twist to the usual story about sloth digestion. Slow does not mean simple, and two animals that look similar in the canopy can run surprisingly different biological systems on the inside.
Why are leaves such a difficult meal?
Leaves are abundant in a rainforest, but they are not an easy source of energy. Their cell walls contain cellulose and other complex carbohydrates that mammals cannot efficiently break apart on their own. Leaves can also be low in calories and contain defensive chemicals made by the plant.
Sloths solve part of this problem by hosting microorganisms that help ferment plant material. The Sloth Conservation Foundation's digestion guide explains that gut microbes help break down leaves so their sloth host can obtain nutrients. This process takes time, fitting an animal whose whole lifestyle is built around careful energy use.
That partnership is called a microbiota: the community of microscopic organisms living in a particular place. A microbiome study can look at who is present, what genes they carry and what jobs they may be able to perform.
Two sloth species, two microbial communities
The researchers used metagenomics, metabarcoding and cellulose-degradation tests to compare samples from the two sloth species. Metagenomics reads genetic material from a whole microbial community, while metabarcoding uses selected genetic markers to identify groups of organisms. Together, these approaches offer a broader view than trying to grow a few microbes in a laboratory dish.
According to the study record in Europe PMC, bacteria dominated the microbiota in both species. Fungi made up only about 0.06 to 0.5 per cent of the metagenomic reads. That does not make fungi irrelevant, but it does show that they were a small part of the genetic material detected.
The bacterial balance differed too. Hoffmann's two-fingered sloths were dominated by groups called Bacillota and Bacteroidota, while brown-throated three-fingered sloths showed a higher presence of Actinomycetota. These names may be a mouthful, but the useful point is straightforward: the two hosts did not contain interchangeable gut communities.
The missing fibre specialists
The team expected to find plenty of microbes commonly associated with plant-fibre digestion in other herbivores. Instead, familiar bacterial groups such as Fibrobacter and Prevotella were scarce. Anaerobic fungi in the group Neocallimastigomycota, known for helping other herbivores break down fibre, were also present at low abundance, especially in the brown-throated sloths.
Functional analysis found relatively few carbohydrate-active enzymes needed to break large plant molecules into smaller parts. In cellulose tests, the study reported degradation of roughly 3 to 30 per cent of the plant material. That wide range matters. It is not a single universal score for every sloth, every meal or every species.
Nor does the result mean sloths are bad at being sloths. Efficiency is about the whole survival strategy. A slow digestive system can hold food for longer, while the animal limits energy use through measured movement and a low-output lifestyle. Evolution does not have to build the fastest machine. It builds one that works well enough in its environment.
What the study does not prove
Microbiome research often finds associations rather than a neat chain of cause and effect. This study shows that the two species differ in microbial composition and function, but it does not reduce their different metabolisms to one bacterium or fungus.
Diet, host genetics, digestive anatomy, age, health, location and care conditions can all influence a gut community. The researchers describe possible links running in several directions between microbes, metabolism and digestion. More sampling and experiments will be needed to untangle them.
It is also worth keeping the species straight. Two-fingered and three-fingered sloths belong to separate evolutionary families. Their similar upside-down lifestyle is a striking case of convergent evolution, not proof that their bodies work identically.
Why gut microbes matter for sloth care
Understanding a healthy sloth microbiota could eventually help wildlife veterinarians assess digestion, nutrition and recovery. That does not mean a simple probiotic treatment is ready. A microbial community is complex, and moving organisms between species without evidence could do more harm than good.
The immediate value is a better baseline. Rescue centres care for sloths affected by habitat loss, road collisions, dog attacks and power-line injuries. Feeding and rehabilitation decisions are safer when carers understand that each sloth species has its own biology, including differences hidden inside the gut.
For the rest of us, the lesson is wonderfully sloth-like: an apparently uneventful meal can contain a whole ecosystem of activity. The sloth may barely seem to move, but its microbial passengers are busy turning tough rainforest leaves into a life in the canopy.
