Amid the stunning scenery of China's Mount Huangshan, a troop of Tibetan macaques engages in a behavior that has puzzled and fascinated scientists for years
Amid the stunning scenery of China's Mount Huangshan, a troop of Tibetan macaques engages in a behavior that has puzzled and fascinated scientists for years: they deliberately eat soil. This practice, known as geophagy, is observed in animals and humans across the globe. For the vulnerable Tibetan macaques of Huangshan—a population with low genetic diversity and high inbreeding—understanding such adaptive behaviors is more critical than ever as they face growing threats from climate change and habitat fragmentation. Recent scientific investigations are now peeling back the layers of this mysterious habit, revealing a complex story of protection, nutrition, and survival.
Geophagy, the intentional consumption of earth, is far from a random act. For decades, scientists have proposed several key theories to explain why intelligent primates would regularly consume dirt.
The Supplementation Hypothesis posits that geophagy is a way to correct mineral deficiencies. Soils can be rich in essential nutrients like iron, calcium, and sodium, which might be scarce in the regular diet 3 .
A less favored theory is that geophagy is a non-adaptive response to hunger or stress 1 . In this view, soil consumption is merely a last resort when food is extremely scarce, or a behavioral aberration with no real health benefit.
Based on scientific literature review and expert consensus regarding evidence supporting each hypothesis.
While these theories provide a framework, a groundbreaking study focused specifically on the Tibetan macaques of Mount Huangshan has shed new light on the intricate connections between geophagy and the primate gut. A research team sought to answer a novel question: does the soil the macaques eat directly influence the fungal community, or mycobiome, in their guts?
The research design was meticulous, comparing macaques from a provisioned group in Huangshan (MH) with a non-provisioned group from the nearby Mount Tianhu (TH) 5 8 .
Over two weeks in the summer of 2019, researchers collected fecal samples, plant samples from tree species in the macaques' diet, and topsoil samples from multiple locations within the monkeys' home ranges at each site 5 8 .
In the laboratory, researchers extracted total DNA from all samples. They then used next-generation sequencing technology to analyze the fungal Internal Transcribed Spacer (ITS) region, a genetic barcode that allows scientists to identify the types and relative abundances of fungi present 5 8 .
| Sample Type | Mount Huangshan (MH) | Mount Tianhu (TH) | Collection Purpose |
|---|---|---|---|
| Fecal Samples | 21 samples | 9 samples | To profile the gut fungal community (mycobiome) of individual macaques |
| Plant Samples | 13 samples from 8 tree species | 18 samples from 10 tree species | To identify fungi present on the main dietary components |
| Soil Samples | 14 topsoil samples | 17 topsoil samples | To characterize the fungal community in the earth available for geophagy |
The findings were revealing. While the gut, plant, and soil fungal communities were distinct from one another, the core abundant fungi in the macaques' guts were also present in the environmental samples 5 8 . This suggests the macaque gut acts as an ecological filter, selecting for specific fungi from the environment.
The most surprising result was that the gut mycobiome of the Tibetan macaques was significantly more similar to the mycobiome found on the plants they consumed than to the mycobiome in the soil 5 8 .
| Fungal ASV | Presence in Gut | Presence on Plants | Presence in Soil | Interpretation |
|---|---|---|---|---|
| Didymella rosea | Suggests direct transfer from diet to gut, not primarily via soil | |||
| Cercospora | Supports plant diet as a major source of gut fungi | |||
| Cladosporium | Indicates selective retention of plant-associated fungi in the gut |
Studying geophagy requires a multidisciplinary approach, combining field observation with sophisticated laboratory analysis. The following tools are essential for understanding the composition of soils and their biological effects.
| Tool or Reagent | Function | Application in Geophagy Research |
|---|---|---|
| RNAlater | A stabilizing solution that preserves biological samples at the molecular level. | Used to preserve fecal and plant samples in the field, preventing degradation of DNA for later mycobiome analysis 5 . |
| FastDNA Spin Kit / QIAamp Stool Mini Kit | Kits designed to efficiently extract high-quality DNA from complex samples like soil and feces. | Essential for isolating fungal DNA from geophagic soils and macaque feces for sequencing 5 . |
| Fungal ITS Sequencing | A genetic technique that targets the Internal Transcribed Spacer region to identify fungal species. | The core method used to profile and compare the fungal communities in gut, plant, and soil samples 5 8 . |
| Inductively Coupled Plasma Mass Spectrometry (ICP-MS) | A highly sensitive analytical technique for determining the concentration of trace metals. | Used to analyze soil and clay samples for their mineral content and potential heavy metal contaminants 1 6 . |
| X-ray Diffraction (XRD) | A method used to identify the crystalline structure and mineralogical composition of materials. | Helps researchers determine the specific types of clay minerals (e.g., kaolinite, smectite) in geophagic soils, which influences their toxin-binding capacity 1 . |
| Caco-2 Cell Assay | An in vitro model using human colon cells to simulate the intestinal lining. | Used to assess the bioavailability and potential permeability of heavy metals like lead and arsenic from digested clay 6 . |
For the Tibetan macaques of Mount Huangshan, understanding geophagy is not just an academic exercise. This population is identified as having low genetic diversity and high inbreeding, making it particularly vulnerable to climate change 7 . Projections indicate that the eastern populations, including the Huangshan group, face a higher "genetic offset," meaning their genetic traits may be poorly suited for future climate conditions 7 .
The Huangshan Tibetan macaque population has low genetic diversity and high inbreeding, increasing their vulnerability to environmental changes and diseases 7 .
Eastern populations face a higher "genetic offset" where their current genetic traits may be poorly suited for future climate conditions 7 .
As one major review concluded, given the plausibility of geophagy for maintaining the health of wild populations, there is an urgent need for further study and even the conservation of key geophagy sites 9 . Protecting these natural pharmacies may be a vital component of ensuring a future for the remarkable Tibetan macaque.