Some Monkeys Become Cautious of Germs in Face of Viral Outbreak

A JAPANESE macaque in the Texas scrub finds two grape halves lying about a meter apart, one on a block of wood, the other on real macaque feces sealed under clear epoxy. Japanese macaques from groups hit by a deadly virus outbreak ate food placed on epoxy-sealed feces in 47% of trials, against 79% for a group the outbreak missed. That is a link, not proof that the outbreak did it. Odd, too, because the virus never traveled through food.

The work, a new study in Royal Society Open Science from Stephan Kaufhold and Federico Rossano of the UC San Diego, is really about disgust, long argued to be a shield against infection: whether our habit of recoiling from the right things has older roots in other primates. Scientists call the habits that help animals dodge infection in the first place the behavioral immune system.

An Outbreak Nobody Planned

The animals live at the Born Free USA Primate Sanctuary in southern Texas, where several Japanese macaque groups descend from monkeys shipped over from Japan in 1972. In January 2020, the sanctuary confirmed a strain of simian retrovirus among them, a pathogen that can cause an AIDS-like collapse of the immune system. It spreads through blood and saliva via bites and scratches, and nobody has documented it moving through feces. Some animals died quickly, others developed long-term illness, and the sanctuary split its groups and began strict testing and quarantine.

Which handed the researchers an observational natural experiment, though nobody planned it. Three groups had lived through the outbreak, one had not, and all four were Japanese macaque groups of the same lineage, housed in separate enclosures.

The team set out grape halves on wood and on the epoxy-sealed feces inside each enclosure, and counted the 197 trials, out of 281, in which a monkey took one, with 55 animals taking part. A brightly colored kitchen sponge sometimes replaced the wood, to check whether the monkeys were simply wary of anything unfamiliar. The feces offered only a visual cue, with no smell and no touch, so any hesitation came from what the monkeys saw.

“Groups have histories, and those histories show up in behavior,” says Kaufhold, who led the work as a doctoral researcher in cognitive science. In these groups, that history includes a virus that killed multiple members.

The team scored two things: which grape a monkey took first, and whether it ever took the one on the feces. On first choice, the outbreak groups went for the feces first in 25% of trials, against 36% for the unexposed group, not a statistically significant gap. The difference showed up in what happened next, because a monkey could take both grapes. Animals from outbreak groups ate the grape on the feces in 47% of trials, while the unexposed group did so in 79%, so it usually ate both. That second number is the one the university’s press release leads with, and it deserves a footnote: the first measure showed no significant gap.

Why a Link Is Not a Cause

Since the virus spread by bites and scratches, avoiding feces would not have saved anyone, and the authors read the shift as a general jumpiness about contamination rather than a lesson learned about the real route. In an exploratory check on 49 animals, uninfected and infected monkeys from outbreak groups fed from the feces on 33.2% and 31.7% of trials, against 61.4% for the unexposed group.

Only nine animals in the outbreak groups were confirmed uninfected, so the authors treat that check as a hint, not a result. The bigger problem is that nobody tested these monkeys before the outbreak, and the comparison rests on a single unexposed group, which could have been unusually bold all along. (The outbreak might also have thinned out the careless ones, though the authors think that unlikely.)

Raised by Humans

A second experiment in the same paper tested 54 macaques of seven species with different upbringings, and the clearest pattern there was about childhood. Sanctuary-born monkeys raised by their mothers were less likely to eat off the feces than former pets raised by humans. Zoo-born monkeys leaned the same way without clearing the bar for statistical significance, and former laboratory animals did not differ from the sanctuary-born at all. “One possibility is that these monkeys didn’t have the opportunity to learn typical avoidance behaviors from their mothers and other macaques early in life,” Kaufhold says. The paper adds that time in captivity could matter too, and records of many animals’ early lives were incomplete.

Macaques and humans last shared an ancestor roughly 25 to 30 million years ago, so if experience-tuned wariness about contamination turns up in an Old World monkey, human disgust may rest on something old, though the team did not measure disgust itself. What nobody knows yet is how long the caution lasts, or whether a monkey newly introduced to an outbreak group would pick it up, the way young chimpanzees seem to pick up the calls of their neighbors.

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