stylistic:tone, structural:scholar_edition
Non-human primates

Non-human primates display signs of disgust and aversion to biological contaminants. Exposure to bodily excrements that usually elicit disgust reactions in humans, such as feces, semen, or blood, have an impact on primates' feeding preferences. Chimpanzees generally avoid the smells of biological contaminants, but only show a weak tendency to move away from these odors, possibly because olfactory stimuli are not enough to give chimps a high enough threat level to move away. Chimpanzees physically recoil when presented with food items on soft, moist substrates, possibly because in nature, moisture, softness, and warmth are characteristics needed to grow pathogens. These responses are functionally similar to what humans' responses would be to the same kinds of stimuli, indicating that the underlying mechanism for this behavior is similar to ours.

Chimpanzees generally avoid food contaminated with dirt or feces, but most individuals still consume these kinds of contaminated foods. While chimps do show a preference for food items with lower contamination risk, they do not avoid risk altogether, as most humans would. This may be due to a trade-off between the nutritional value of the food items and the risk of infection from the biological contaminants, with the chimps weighing the benefit of the food more heavily than the risk of contamination. In contrast to chimpanzees, Japanese macaques are more sensitive to visual cues of contaminants when there is no accompanying odor. Bonobos are most sensitive to fecal odors and rotten food odors. Overall, primates incorporate various senses in their feeding decisions, with disgust being an adaptive trait that helps them avoid potential parasites and other threats from contaminants.

The most frequently reported disgust-like behavior in non-human primates is expelling bad-tasting food items, but even this behavior is not very common. This might be because primates effectively avoid potentially bad-tasting food items, and food that is avoided cannot be expelled, hence the low observation rate of this behavior. Primates, notably gorillas and chimpanzees, occasionally make facial expressions such as grimacing and tongue protrusions after having bad-tasting food. Individual primate preferences vary widely, some tolerating extremely bitter food, while others are more particular. Taste preferences are more often noticed in high ranking individuals, likely because lower ranked individuals may have to tolerate less-desired foods.

While in humans there is a strong difference in disgust reactions between the two sexes, this difference has not been documented in non-human primates. In humans, women generally report greater disgust than men. In bonobos and chimps, females are not any more avoidant than males of contamination risk. There is some evidence suggesting that juveniles are less contamination-risk avoidant than adults, which is in line with research on the development of the disgust response in humans.

Coprophagy is commonly observed in chimpanzees, possibly suggesting that chimps do not really have a disgust mechanism the way humans do. Coprophagy is usually only done to re-ingest seeds from one's own feces, which is less risky than ingesting others' feces in terms of exposure to new parasites. Additionally, chimps often use leaves and twigs to wipe themselves when they stepped in others' feces instead of removing it with their bare hands. Great apes almost always remove feces from their bodies after accidentally stepping in it, even in instances where it would be beneficial to wait. For example, when grapes are being passed out to chimps and they accidentally step in feces, they almost always take the time to stop and wipe it off even if it means missing out on food.

Unlike in humans, the avoidance of social contamination (ex: staying away from sick conspecifics) is rare in great apes. Instead, great apes often groom sick conspecifics or just treat them with indifference. Additionally, great apes treat products of a sick conspecific such as mucus or blood with interest or indifference. This is in contrast with human disease avoidance, where avoiding those who appear sick is a key feature.

Taken together, studies on the disgust reaction in primates show that disgust is adaptive in primates and that the avoidance of potential sources of pathogens is triggered by the same contaminants as for humans. The adaptive problems that primates faced did not align to the degree that they did for early humans, which is why disgust manifests differently in humans and non-human primates. Differences in disgust responses between humans and non-human primates likely reflects their unique ecological standpoints. Rather than disgust being a unique human emotion, disgust is a continuation of the parasite and infection avoidance behavior found in all animals. One theory explaining the difference is that since primates are largely foragers and never shifted to the hunter-scavenger lifestyle with a diet high in meat, they were never exposed to the new wave of pathogens that humans were exposed to, as well as the selection pressures that would come with this diet. Therefore, the disgust mechanisms in primates remained muted, only strong enough to address the distinct problems primates faced in their evolutionary history. Additionally, disgust-like behavior in great apes should be lower than in humans because they live in less hygienic conditions. Humans' clean habits over generations has reduced how frequently we are exposed to disgust elicitors and has likely expanded the stimuli that would elicit disgust reactions in us. Great apes on the other hand are constantly exposed to disgust elicitors, leading to habituation and a muted form of disgust compared to modern humans.