Outbreak-Exposed Macaques More Wary of Grapes on Simulated Feces, Study Finds
Macaques that survived an outbreak were less likely than naive peers to take a grape placed on simulated feces, a study found.
The sanctuary housed more than 100 macaques. In 2020, some began falling ill with SRV, an HIV-like virus. Once infected through bodily fluids, a macaque can deteriorate quickly and die within three to four weeks, said Stephan Kaufhold, a cognitive scientist at UC San Diego. Many died; others developed chronic immunodeficiency. Staff separated the monkeys, creating groups that had either experienced the outbreak, by illness or by seeing others sick, and those that had not.
To test whether that history changed their caution, Kaufhold and colleagues offered individual monkeys two red grapes. One sat on a plain, clean-looking piece of wood. The other sat atop macaque feces encased in epoxy, so the animals were not eating from actual waste. Kaufhold said the glossy specimen looked gross and also triggered disgust in humans.
From 2022 to 2024, the team ran nearly 200 trials. Among monkeys that had not lived through the outbreak, 79% ate the grape on the feces. Among outbreak veterans, 47% did. Kaufhold said he was surprised and saw the gap as clear evidence that something had changed. He suspects a heightened sensitivity to disgust, an emotion whose evolutionary roots may lie in protecting the body from contamination.
The researchers cannot know whether macaques actually feel disgust. Andrew MacIntosh, a biologist at the Wilder Institute who was not involved in the study but has run similar experiments with macaques, said it is plausible that past exposure to outbreaks could make animals more cautious or risk-averse, but he wants more evidence. The experiment took place several years after the outbreak, he said, leaving time for other factors to shape the behavior.
MacIntosh also said the fecal-avoidance test bears little relation to how SRV spread, through bodily fluids transmitted by scratches or bites. Kaufhold responded that the study does not claim macaques understand viral mechanisms, only that they showed increased caution. People did not initially know how SARS-CoV-2 spread either, he said, yet many took extreme steps to avoid infection.
Scientists have documented disease avoidance across animals, from monkeys to nematodes, which can sense and steer clear of feces that may harbor harmful bacteria. Kaufhold's question was how flexible that avoidance is: whether an outbreak can make individual animals warier of pathogens.