One of the macaques at a Texas sanctuary, where researchers conducted tests to see if the monkeys had become germophobic after experiencing a viral outbreak.
Stéphane Kaufhold
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Stéphane Kaufhold
In the early days of the COVID-19 pandemic, millions of people who had rarely thought twice about avoiding germs became germophobes.
Some people wiped races before putting them in the refrigerator. Others leave mail quarantined outside for days, lest it harbor stray viruses. In hindsight, knowing that SARS-CoV-2 spread primarily through the air, these types of precautions seem extreme. But for now, it may have been a reasonable way to avoid an unknown pathogen.
Around the same time, another virus hit a macaque sanctuary in Texas. New search suggests that monkeys reacted the same way as humans to COVID-19 – by being extremely germophobic.
Humans are far from the only animals to avoid possible sources of disease. Scientists have discovered that creatures as diverse as monkeys and nematodes (aka roundworms) can detect and avoid feces, for example, which can harbor harmful bacteria.
“What has been less studied is the question of the flexibility of this avoidance,” says Stéphane Kaufholdcognitive science researcher at UC San Diego. He wanted to see if an epidemic could make monkeys even more wary of pathogens.
The outbreak at the Texas sanctuary, which is home to more than 100 macaques, offered Kaufhold and his colleagues an opportunity to put that question to the test.
Grape expectations
In 2020, macaques living in an enclosure at the sanctuary began contracting an HIV-like virus called SRV.
Once a monkey is infected through bodily fluids, the disease “can progress quickly, and about three to four weeks after being initially infected, you could die,” Kaufhold says. Many monkeys died, while others developed chronic immunodeficiency.
Sanctuary staff quickly began separating the monkeys, eventually creating two groups: those who had contracted the virus, either by getting sick themselves or seeing others get sick, and those who had not experienced the outbreak.
To see if these different stories made the monkeys more germophobic, the researchers presented the individuals with two tasty treats: grapes.
“They get a fantastic diet at the sanctuary, but no grapes every day, so the grapes were definitely a treat,” Kaufhold says.
A red grape was displayed on a nondescript, clean-looking piece of wood. The other was on macaque droppings, covered in epoxy. “Essentially, it’s not about making animals eat actual feces,” says Kaufhold. “It looked quite disgusting, feces with a wet, shiny appearance. It certainly worked to trigger disgust in humans as well.”
From 2022 to 2024, the researchers offered this choice to the monkeys in nearly 200 trials. Among the monkeys who were not affected by the epidemic, 79% ate grapes topped with droppings. But only 47% of epidemic veterans ate it, researchers report in the journal Royal Society Open Science.
Essentially, those who had not been aware of the outbreak were more likely to err on the side of caution and eat the grapes cooked in poop than those who had experienced it.
“I was surprised,” Kaufhold says. “I think that’s pretty clear evidence that something is going on.” Kaufhold thinks it might be an increased sensitivity to disgust.
“If we think about disgust, the evolutionary origins are most likely in trying to protect the body from contamination, because there are pathogens and parasites that we would ideally want to avoid,” he says. Becoming more easily disgusted after a devastating outbreak could pay off if it means avoiding infection, whatever the cause.
Of course, scientists can’t know whether macaques actually experience disgust. “But the possibility that things like disgust influence the behavior of other species, and particularly species like primates, is certainly a possibility worth considering,” says Andrew MacIntoshbiologist at Wilder Institute who did not participate in the study but carried out similar experiments with macaques.
“It seems plausible that past exposure to outbreaks could make individuals more cautious or more reluctant to take risks,” he says, but he thinks more evidence is needed to demonstrate this. For example, the experiment was carried out several years after the outbreak, enough time for other factors to influence these behaviors, he says.
MacIntosh also points out that the fecal avoidance experience has little to do with how the initial outbreak spread, through bodily fluids transmitted through scratches or bites. “The mode of transmission of this virus really has nothing to do with fecal contamination,” he says.
Kaufhold doesn’t think that’s a problem.
“We’re not claiming that the macaques had any understanding of the mechanism of viruses, it just seems to be a case of increased caution,” he says. Initially, people also didn’t know how SARS-CoV-2 spread, he says, but many went to great lengths to avoid infection. These impulses may not have been shaped by paranoia, but by evolution, as a way to ward off disease in an uncertain world.
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