Monkeys That Survived an Outbreak Grew Warier of Contaminated Food, Study Finds

When a new virus spreads, people often become more cautious about germs, sometimes to extremes, as the early days of the Covid-19 pandemic showed. A new study suggests that monkeys may react in a similar way.

Researchers from the University of California, San Diego, found that Japanese macaques that had lived through a deadly viral outbreak at a Texas sanctuary were much less willing to eat food from a surface that appeared to be contaminated than macaques that had been spared. The study was published in the journal Royal Society Open Science.

The sanctuary in southern Texas is home to groups of Japanese macaques descended from monkeys brought from Japan in 1972. In 2020, staff confirmed that some groups had been infected with simian retrovirus type 2, which can cause an AIDS-like immune deficiency. The virus spreads mainly through bites and scratches, and it killed some animals and left others chronically ill. The sanctuary separated the groups and imposed testing and quarantine, leaving three groups that had experienced the outbreak and one that had not.

Between 2022 and 2024, the researchers offered the monkeys a choice between two grapes, a prized treat. One sat on a clean piece of wood. The other sat on real macaque feces sealed under clear epoxy, which kept the look of contamination while removing the smell and any risk of infection. Monkeys from the group that had escaped the outbreak took the grape from the dirty-looking surface in 79 percent of trials. Monkeys from the groups that had lived through it did so in only 47 percent.

Notably, the change did not depend on whether an individual monkey had been infected. Animals that had merely lived alongside sick and dying group members behaved much like those that had fallen ill. To rule out a simple fear of unfamiliar objects, the researchers in some trials replaced the clean wood with a brightly colored kitchen sponge.

Stephan Kaufhold, a cognitive scientist who led the study as a doctoral researcher at UC San Diego, told NPR he was surprised by the size of the effect and suspects a heightened sensitivity to disgust. The authors say the results suggest that pathogen avoidance, part of what scientists call the behavioral immune system, can be shaped by experience rather than being fixed.

Outside researchers urged caution. Andrew MacIntosh, a biologist at the Wilder Institute who has run similar experiments with macaques, told NPR the idea is plausible but needs more evidence. He noted that the tests took place years after the outbreak, leaving time for other factors to shape behavior, and that the virus spread through bites and scratches, not through fecal contamination. Scientists also cannot know whether the monkeys actually feel disgust.

The same paper reported a second experiment with 54 macaques of seven species raised in different settings. Former pets, which grew up among humans rather than with other monkeys, were significantly more likely to eat the contaminated-looking food than sanctuary-born monkeys raised by their mothers, suggesting that early life also shapes how wary an animal becomes.

The researchers say the work cannot show directly whether living through a pandemic changes human behavior over the long term, but it adds evidence that such experience-driven caution is not unique to people.