Our closest animal relatives have stomachs of steel when it comes to some things that would make us gag—yet researchers have finally discovered their disgust triggers.
The Disgusting Truth About Our Primate Cousins
If you’ve ever watched a nature documentary showing chimpanzees casually picking through their own excrement to find undigested seeds and popping them into their mouths, you might have wondered if these animals experience disgust at all.
The answer, it turns out, is a resounding yes—but not in the way you’d expect.
In groundbreaking research conducted at the International Centre for Medical Research in Franceville (CIRMF) in Gabon, scientists have uncovered that chimps do indeed have disgust responses, but their triggers differ dramatically from our own.
While humans might retch at the thought of handling feces, chimps show their strongest aversion to something else entirely: the unseen and unknown.
When presented with food placed on visible faux excrement, chimps hesitated but ultimately ate it. However, when asked to reach into an opaque box containing a mysterious soft, moist substance to retrieve food, many refused outright.
In this study, an astonishing 76 percent of chimps rejected food rather than reach through the unknown squishy material—a finding that challenges our understanding of primate psychology and evolutionary responses to contamination.
This discovery doesn’t just reshape our understanding of chimp behavior—it offers profound insights into the evolutionary underpinnings of our own disgust responses and how they protect us from disease.
The Disgust Experiment That Pushed Chimps to Their Limits
The research team from Kyoto University, led by primatologist Cecile Sarabian, designed a series of increasingly stomach-turning experiments to test the boundaries of chimpanzee tolerance.
Their 41 test subjects—20 males, 21 females—all live at the research center in Gabon, where they enjoy a vegetarian diet of fruits, vegetables, and baked soybeans.
These unsuspecting chimps soon found themselves facing food challenges that would make even the contestants on Fear Factor think twice.
The experimental design was elegant in its simplicity yet comprehensive in its approach. The researchers created three distinct scenarios to test different sensory pathways of disgust:
Visual disgust: Food was placed directly on top of either a realistic feces replica or brown foam that resembled excrement.
Olfactory disgust: Fruit was infused with the smell of bodily fluids including blood, semen, and feces.
Tactile disgust: Using an opaque box, chimps had to reach through a soft, moist substrate (a specially formulated dough with poop-like consistency) to retrieve their food reward.
Each test was carefully documented, with researchers recording the chimps’ reactions, hesitations, and whether they ultimately consumed the food.
The results revealed fascinatingly nuanced responses. When it came to visually contaminated food, the chimps showed clear hesitation but ultimately overcame their reluctance. Similarly, with food that smelled like bodily fluids, they displayed initial aversion but eventually ate it anyway.
But the tactile test proved to be their breaking point. When faced with reaching into the unknown to touch something soft and wet—even though they couldn’t see it—many chimps flat-out refused to proceed, forgoing food rather than risking contact with the mysterious substance.
Sarabian observed that “the chimpanzees’ reluctance to touch soft and moist substrates, even at the cost of losing food, might be an adaptation to avoid pathogen risk from fecal contamination specifically, or biological contaminants more generally.”
The Evolutionary Puzzle of Disgust
For decades, the prevailing theory among evolutionary biologists and psychologists has been that disgust evolved as a protective mechanism—a “behavioral immune system” that helps organisms avoid potentially contaminated substances and the pathogens they might contain.
This makes intuitive sense. Humans who instinctively avoided feces, rotting meat, and suspicious-smelling foods would have had better odds of avoiding infectious diseases. Those survival advantages would have been passed down genetically, gradually hardwiring disgust responses into our psychology.
But if disgust serves such a clear adaptive function, why do chimps—who share approximately 98.6% of our DNA—seem so comfortable with behaviors that would make most humans sick to their stomachs?
The Pattern Interrupt: It’s Not About Disgust, It’s About Information
Here’s where we need to fundamentally rethink our understanding: Disgust may not primarily be about avoiding specific substances, but rather about navigating informational uncertainty.
The conventional wisdom suggests that evolution should have programmed chimps to avoid feces because of its disease risk. Yet they regularly interact with it, sometimes even consuming it. Meanwhile, they show extreme aversion to touching invisible substances that merely feel unpleasant.
This pattern reverses our understanding of how disgust operates. What the research suggests is that visible contamination—even something as objectively risky as feces—is less threatening than an invisible, unknown contaminant.
Why? Because with visible contamination, the animal has complete information. It can see the potential threat, assess it, and make a conscious decision about whether the reward outweighs the risk. With invisible contamination, the animal lacks critical information about what it’s touching and the potential dangers it presents.
This information-based theory of disgust explains why the chimps were more willing to eat food placed on fake feces (where they could see and evaluate the contaminant) than to reach into a box with an unknown substance (where they couldn’t assess the risk).
It also explains why humans—who have developed cultural and technological means of dealing with potential contaminants—might maintain stronger aversions to visible contamination than chimps do. Our expanded cognitive capacity allows us to understand invisible threats at a conceptual level, making us wary of even visibly “clean” items we suspect might be contaminated.
As Andrew MacIntosh, senior researcher on the study, explained: “What’s great about these experiments is that the observed responses are functionally similar to what ours would be, providing evidence that the mechanism underlying their behavior could be similar to ours.”
This insight doesn’t just reframe how we understand chimp psychology—it challenges foundational assumptions about the evolutionary purpose of disgust itself.
The Hygiene Paradox: Clean in Some Ways, Gross in Others
Complicating matters further is the observation that chimps demonstrate surprisingly sophisticated hygiene behaviors in some contexts while exhibiting seemingly reckless disregard in others.
Previous research has documented chimpanzees meticulously washing sandy food before eating it, using leaves to wipe their bodies clean, and even developing food preparation techniques that remove potentially hazardous substances. These behaviors suggest a sophisticated understanding of contamination.
Yet these same fastidious animals will sometimes consume their own excrement or that of other group members—a behavior called coprophagy that seems to directly contradict their apparent concern for cleanliness.
This apparent contradiction makes more sense through the lens of cost-benefit analysis. Washing sand off an apple requires minimal effort and prevents the unpleasant experience of gritty food. The benefit clearly outweighs the cost. Similarly, reaching into an unknown substance presents an ambiguous level of risk that may not justify the reward of food.
But eating feces-contaminated food—particularly when that food contains valuable undigested nutrients like seeds—presents a different calculation. Here, the nutritional benefit may outweigh the relatively known and manageable risk of disease, especially for an animal with a robust digestive system adapted to processing a wide range of foods.
Dr. Frans de Waal, a renowned primatologist not involved in the study, has previously noted that “chimps have a remarkably robust digestive system capable of handling bacterial loads that would make humans seriously ill. This doesn’t mean they’re immune to disgust—it simply means their disgust triggers have evolved to focus on the threats most relevant to their physiology and environment.”
The Universal Language of “Yuck”
While the specific triggers may differ, the facial expressions and behavioral responses associated with disgust appear remarkably similar across primate species.
When encountering something disgusting, chimps display many of the same reactions humans do: nose wrinkling, mouth gaping, head retracting, and abrupt withdrawal movements. These similarities suggest that the basic architecture of disgust has deep evolutionary roots predating the human-chimp split approximately 7 million years ago.
Sarabian’s team noted these characteristic disgust behaviors throughout their experiments. “Although we cannot directly measure the subjective emotional experience of the chimpanzees,” Sarabian explains, “their behavioral responses mirror those we see in humans encountering disgusting stimuli.”
These shared expressions serve as a form of biological communication, warning other group members about potential threats. A chimp that recoils dramatically from a food source signals to others that something may be wrong with it, potentially protecting the entire group.
This social dimension of disgust represents another adaptive advantage. By communicating aversion through facial expressions and body language, disgust becomes a powerful tool for cultural transmission of danger avoidance—teaching group members, particularly offspring, which substances and behaviors to avoid.
The Disease Avoidance Hypothesis
The study strongly supports what scientists call the “disease avoidance hypothesis” of disgust—the idea that disgust evolved primarily to help organisms avoid substances that might make them sick.
“If chimpanzees and other primates can discern contamination risk via different cues, individuals with higher sensitivities to faeces and other bodily fluids may be less infected, which could have important health benefits,” says Sarabian.
In environments teeming with potential pathogens, any behavioral adaptation that reduces infection risk, even marginally, would confer significant evolutionary advantages. Those advantages would accumulate over generations, gradually shaping the disgust responses we observe today.
What’s particularly fascinating is how these responses are calibrated to each species’ specific vulnerabilities and environments. Humans, with our relatively vulnerable digestive systems and cultural practices of food sharing, have developed strong aversions to visible contamination. Chimps, with more robust digestion and different feeding patterns, show more variable responses.
This tailoring of disgust responses to specific ecological niches helps explain why disgust triggers vary not just between species but also between human cultures. People living in environments with higher pathogen loads typically display stronger disgust responses than those in environments with fewer disease threats.
Beyond Bodily Fluids: The Expanding Scope of Disgust
In humans, disgust has expanded far beyond its original disease-avoidance function to encompass moral and social domains. We speak of being “disgusted” by corrupt behavior, “repulsed” by ethical violations, and “sickened” by injustice—using the language of physical contamination to describe completely abstract transgressions.
This raises an intriguing question: Do chimps experience similar extensions of disgust beyond physical contamination?
While the current research doesn’t directly address this question, some primatologists believe chimps may indeed experience rudimentary forms of “moral disgust.” Chimps have been observed ostracizing group members who violate social norms, showing aversion to individuals who hoard food, and punishing those who don’t reciprocate grooming or food sharing.
These behaviors suggest that the cognitive architecture supporting moral disgust may have roots predating human evolution. Just as physical disgust helps us avoid biological contaminants, moral disgust may help social species avoid “social contaminants”—individuals whose behavior threatens group cohesion and cooperation.
Applications Beyond Primatology
Understanding chimpanzee disgust responses has applications far beyond academic primatology. The insights gained could inform multiple fields:
Public health: By understanding the evolutionary roots of disgust, we might develop more effective interventions to promote hygiene behaviors in human populations.
Conservation: Knowledge of what disgusts chimps could help create more effective barriers to keep wild chimps away from human settlements or dangerous areas without using physical barriers.
Animal welfare: Understanding disgust triggers helps create more comfortable environments for captive chimps and other primates.
Comparative psychology: These findings provide a framework for investigating disgust across other species, potentially revealing evolutionary patterns.
The research also highlights the ethical importance of providing retirement sanctuaries for research animals. The team notes that the chimps involved in this study are headed for a retirement sanctuary currently being built in Gabon—recognition that these complex, sensitive animals deserve consideration beyond their scientific value.
The Lingering Questions
While this research offers fascinating insights into chimp psychology, it also raises numerous questions for future investigation.
Do juvenile chimps show different disgust responses than adults, suggesting a learned component to these behaviors? How do disgust responses vary between wild and captive populations? Do other great apes—gorillas, orangutans, and bonobos—show similar patterns of disgust?
Perhaps most intriguingly, how might disgust responses among non-human primates inform our understanding of human behavioral immune systems, particularly in contexts like pandemic response?
Sarabian and her team plan to expand their disgust research to other non-human primates, potentially creating a comparative framework that reveals how disgust evolved across the primate lineage.
The Evolutionary Takeaway
What started as a simple question—can chimps be grossed out?—has opened a window into the complex evolutionary history of an emotion most of us take for granted.
The answer is yes, chimps can absolutely be disgusted—but not always by the things that disgust us, and sometimes by things we might never consider. This research reveals disgust not as a fixed response but as a flexible adaptation shaped by each species’ unique ecological challenges and physiological vulnerabilities.
For humans reading about chimps picking seeds from feces, the reaction might be one of revulsion. For the chimp, it’s simply efficient nutritional recycling with manageable risks. Meanwhile, that same chimp might refuse perfectly good food rather than reach into an ambiguous, squishy substance—a caution many humans would consider excessive.
These differences don’t diminish our connection to our primate cousins; they enrich our understanding of how evolution shapes behavior in nuanced, context-specific ways. In the subtle expressions of disgust we share with chimps, we glimpse not just our evolutionary past but the sophisticated ways our shared ancestors navigated a world filled with invisible threats.
As we continue to unravel these evolutionary connections, we gain not just scientific knowledge but a deeper appreciation for the biological wisdom embedded in our most visceral reactions—the instinctive “yuck” that has helped keep primates safe for millions of years.
The study was published in Royal Society Open Science.
