Monkeys That Lived Through A Viral Outbreak Avoid Tainted Food

Deadly Outbreak Linked to Greater Germ Caution in Monkeys

Monkeys from groups hit by a deadly virus outbreak ate food off a feces-coated surface in 47% of trials, compared with 79% for monkeys from a group the outbreak never reached. That gap, found in a new study of macaques in Texas, hints that living through a frightening disease event may leave animals far more cautious about what goes in their mouths.

Researchers at the University of California San Diego wanted to know whether monkeys adjust their germ-avoidance habits with experience, as people do. Scientists call this the behavioral immune system, and in people it includes disgust, the recoil from spoiled food, waste, or sick people. It remains unclear whether disgust is uniquely human or has older roots in other primates.

Published in Royal Society Open Science, the work included two experiments. One tested how upbringing shapes caution around dirty food, and the other took advantage of an unplanned outbreak at a sanctuary. Neither measured emotions, only behavior.

A Deadly Virus Outbreak Gave Researchers a Natural Experiment

In January 2020, the Born Free USA Primate Sanctuary in Texas confirmed that Simian Retrovirus Type II had reached its Japanese macaque population. The virus spreads through blood and saliva, usually from bites and scratches during fights, and shares some similarities with AIDS in humans. Some infected monkeys died quickly from severe bleeding, while others developed long-term immune problems.

Three of the four groups tested were exposed. The fourth, called Kyoto, never was, giving the team a comparison group without anyone being infected on purpose. Researchers set two grape halves on the ground about a meter apart. One sat on a plain wood block or a brightly colored sponge, and the other sat on a surface topped with real monkey feces sealed under clear plastic, so it looked dirty but carried no smell or infection risk. The team recorded which grape each monkey took first and whether it ever ate the one on the dirty surface.

Monkeys approached on their own, so researchers could not choose who showed up. From October 2022 to March 2024, the team collected 281 trials across 55 Japanese macaques, and 197 produced usable results.

Outbreak-Exposed Monkeys Refused Contaminated Food More Often

Across all groups, monkeys favored the clean grape, choosing the dirty one first in only 29% of trials. The real split came afterward, when monkeys could take both grapes. In the 80 trials from the unexposed group, monkeys typically ate both. In the 117 trials from exposed groups, they were far more likely to leave the dirty grape behind. Passing up free food costs a primate calories, which makes the extra caution a real trade-off.

Did actual infection explain the caution? Monkeys in exposed groups behaved alike whether or not they had been infected, and both were more careful than the unexposed group. In other words, catching the virus seemed to matter less than living through the outbreak. The authors called this comparison exploratory, since only nine monkeys in exposed groups were confirmed uninfected.

Nobody measured the monkeys’ caution before the outbreak, so the groups may have differed from the start. The study shows a link, not proof that the outbreak changed behavior.

Monkeys Raised by Mothers Avoided Germs More Than Former Pets

In a separate experiment, the team tested 54 macaques from seven species at the sanctuary, offering each a grape on a clean surface and one on a feces-coated surface up to eight times. Fourteen monkeys had been born at the sanctuary and raised by their mothers, 15 were former pets raised by humans, 19 were former lab animals, and 6 were zoo-born.

Overall, monkeys picked the dirty option in 32% of trials. Sanctuary-born monkeys were less likely to pick it than former pets, though the gap was modest. Zoo-born and lab-raised monkeys did not differ clearly from the sanctuary group. Sex and social rank made no difference in either experiment. Young monkeys may learn by watching elders forage, a possible explanation the study did not test.

Monkeys also grew more willing to eat the dirty food over repeated trials, a sign of getting used to the sight. And fear of new objects was not the whole story, because in both experiments the dirty food put monkeys off more than the unfamiliar sponge did.

Monkey Germ Avoidance May Predate Human Disgust

Caution that shifts with upbringing and recent disease resembles the way human disgust varies, and studies cited in the paper found people grew more sensitive to disgust during the COVID-19 pandemic. Humans and macaques last shared an ancestor roughly 25 to 30 million years ago, so flexible germ avoidance in macaques points to an old survival system that human disgust may have built on.

Raising young primates with their mothers or other monkeys, rather than humans, could also help them build healthy avoidance, which matters for animals meant to return to the wild.

Whatever drove the caution in those Texas monkeys, a germ-avoidance habit that bends with experience looks less like a human invention and more like an old primate survival tool.

---