Bonobos, our closest relatives alongside chimps, seem to have a similar disgust system to humans’. Credit: University of Kyoto.

It’s not just humans that get grossed out by putrid stenches and moist textures — bonobos do it too. According to a new study, for these gentle primates, having access to clean food sources is important and they will generally stay away from grub in close proximity to feces, soil, or bad smells. We share 98.7% of our DNA with bonobos, more than we do with any other creature on Earth, which leads to many similarities between bonobos and humans. Perhaps, the origin of disgust might be traced back as a result of findings like these.

Yuck!

Isn’t it funny how we all share the same bodily and emotional reactions? That’s no accident as most can be traced back to some evolutionary adaptation. Scientists think that disgust is an adaptive system that evolved to protect animals from parasites and various pathogens, as a counter-strategy meant to mitigate infections. These threatening organisms often congregate in rotten food or bodily fluids, which we humans find repellent. The same reaction seems to exist in other animals. For instance, grazing ungulates prefer to feed away from areas contaminated by feces, and our primate cousins seem to employ similar strategies as well.

In 2017, researchers at the Primate Research Institute, Kyoto University, reported the first experimental evidence that potential exposure to biological contaminants (feces, blood, semen) can influence feeding decisions in chimpanzees (Pan troglodytes troglodytes). Now, the same team, led by primatologist Cecile Sarabian, has come to similar conclusions regarding bonobos (Pan paniscus).

The researchers carried out a series of experiments in which captive bonobos were presented with various food choices: food contaminated with feces or soil, chains of food items linked to a contaminant, previously contaminated food, or only the odors of feces or rotting food.

The bonobos were happy to eat the clean, uncontaminated food but stayed away from any contaminated food like it was the plague. The closer a foodstuff was to a contaminant, the stronger the repulsion. Conversely, when food was farther away from a contaminant, the primates’ sensitivity waned. “Some individuals just refused to take any food rewards when the latter involved contamination,” according to Sarabian.

In another experiment, the bonobos were found to be less likely to touch or taste substrates, or even to use tools to achieve such a goal, when confronted with foul smells. These are precisely the reactions that you would expect to see in an animal with a system of disgust set in place.

“These findings build up on the growing literature in primates and other animals, helping us to see convergences and differences in the disgust response across taxa at a behavioral and physiological level, as well as what may have driven such differences through evolutionary times,” Sarabian told ZME Science.

As a side note, it wasn’t quite easy setting up the experiments. Due to new sanctuary policies, the researchers were no longer allowed to perform the experiments on isolated subjects, which led to some unexpected outcomes.

“When I first started by presenting feces replica instead of real conspecific feces, bonobos gathered around and some bold individuals started touching it and sniffing their fingers just after. Then, it became a “thing”, they all wanted to touch and sniff the fake poop — not showing much interest for the piece of brown sponge used as the control. Eventually, after repetitive investigations, bonobos may have realized the trick and ended up stealing the fake poop… playing with it — Fieldwork FAIL!” Sarabian said.

Bonobos stayed away from foods in close proximity to feces. Credit: Kyoto University / Cecile Sarabian.

Interestingly, infants and juveniles showed much less precaution when handling food close to contaminants, much like human infants behave. Although the infants might get sick as a result of this kind of behavior, the researchers hypothesize that there are some advantages like building their immune system during a critical time in their development.

Previously, Sarabian and colleagues also studied Japanese macaques, which also seem to elicit disgust responses.

“The most intense disgust response I ever witnessed across different field sites and species goes to a Japanese macaque from Koshima island, who accidentally stepped on a poop, and subsequently crossed all the beach jumping on her two hands and remaining foot to find a dead tree trunk where she meticulously rubbed her soiled foot,” Sarabian recounted.

Despite the similarities between human and bonobo disgust responses, there seem to be some key differences. We are pretty reluctant to ingest novel food that looks different from what we’re used to — that’s weird, basically. This kind of behavior — the inclination to stay away from or be cautious around new foods — is called neophobia but Bonobos don’t seem to share it, not at the same intensity at least. During experiments, bonobos stuffed themselves with fruits they had never seen before with no apparent sign of hesitation.

VIDEO: A plum, apple, and papaya were arranged in front of bonobos. Since apples are rare in this environment avoidance of it would indicate food neophobia. In fact the bonobos had not trouble eating each fruit, exhibiting food neophilia. At least for novel fruit. Credit: Kyoto University / Cecile Sarabian. 

As an important caveat, the researchers have not proven that bonobos express disgust, or not in a way that we can recognize, at least.

“Our results look as though the bonobos’ behaviors are driven by disgust, only because we know human behavior in similar situations is driven by disgust, but this is only a functional resemblance, and we cannot know what the bonobos are thinking or feeling at the time they are responding in our experiments,” co-author Andrew MacIntosh, Associate Professor at the University of Kyoto, wrote in an email.

That being said, the researchers plan on continuing to study bonobos and other primates with the intention of investigating the origins of disgust in humans. Sarabian is currently conducting fieldwork in the equatorial forest of the Democratic Republic of Congo, where she is investigating the link between mouthing behaviors and parasite infection in wild bonobos.

“The first step is to test whether the behavioral responses to things that humans find disgusting are similar in other species, like the bonobos tested here. Substances like feces and rotten foods are more or less universal disgust elicitors in humans, so we expect to see similar responses in nonhuman primates. If our responses to these things is seen in other species, it means that they likely have a shared evolutionary history that predates the appearance of humans. Furthermore, humans are pretty visual, so we expect that seeing disgusting things will elicit a ‘disgust response’. But we also know that we exhibit similar responses/feelings when we smell feces or rotten foods, so perception of ‘disgusting things’ is multi-modal,” MacIntosh said.

“Observing similar responses to disgust elicitors in other species, including through different sensory modalities like sight, smell, and touch, can assess whether our own responses are shared with other species,” he added.

The findings appeared in the journal Philosophical Transactions of the Royal Society B.