Monkeys Grew Cautious of Germs After an Outbreak, Study Finds

When a new disease sweeps through, humans start wiping down groceries and flinching at sneezes. New research suggests monkeys do something strikingly similar. Japanese macaques that lived through a deadly viral outbreak in 2020 grew noticeably more cautious of germs, avoiding food placed on surfaces that merely looked contaminated, according to a new study from the University of California San Diego, as reported by phys.org. The finding adds to growing evidence that Cautious of Germs behavior is not hardwired alone — it is also shaped by what a group has lived through.

The experiment was elegantly simple. Researchers offered the macaques food on two surfaces: a clean one and one that visually suggested contamination, and in some trials swapped the clean surface for a brightly colored kitchen sponge to check whether the animals were simply avoiding unfamiliar objects. Monkeys from groups that had experienced the outbreak ate from the seemingly contaminated surface in only 47 percent of trials, while monkeys from a group that never experienced it ate in 79 percent of trials. That gap held whether or not the individual monkey had itself been infected — living through the outbreak, not getting sick, appeared to drive the change.

Caution without knowledge

What makes the result especially intriguing is that the macaques had no way of knowing how the disease spread. The outbreak of simian retrovirus Type II, a virus that can cause AIDS-like immunodeficiency, struck a primate sanctuary in southern Texas in 2020 and was transmitted through bites and scratches, not through contaminated food or feces. The monkeys were, in a sense, being wrong about the threat — and yet they became wary of it anyway.

"Even with no knowledge of how disease is transmitted, being exposed to others getting sick and dying changes the likelihood of engaging in behaviors that potentially could lead to some form of sickness," said Federico Rossano, a cognitive psychologist at the school and a senior author of the study. "Groups have histories, and those histories show up in behavior," added study co-author Stephan Kaufhold, who led the work as a doctoral researcher. "Living through the disease outbreak appears to have made those monkey groups more cautious around food that looks contaminated."

The team calls this pattern part of the behavioral immune system: a collection of behaviors that help animals avoid infection before the body's biological defenses ever engage. Cautious of Germs behavior, in this framing, is a first line of defense — crude, overgeneralized, and sometimes aimed at the wrong vector, but better than nothing when a new pathogen arrives.

A second experiment, and a striking twist

The researchers did not stop there. In a second experiment, they ran the same test on macaques with very different upbringings. Pet monkeys who had grown up living with humans were more likely to eat from the seemingly contaminated surfaces than monkeys born in sanctuaries and raised by macaque mothers. "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 said. Even after living with other monkeys for several years, that early experience still shaped how they responded to potential sources of disease.

That twist matters because earlier research had shown that macaques are not always germ-averse. Given a favorite treat like a peanut, wild Japanese macaques would still eat food off a feces-covered surface, skipping the caution for less tempting food like wheat grains, according to Popular Science's coverage of the findings. In other words, the baseline is not pure disgust — it is a cost-benefit calculation that an outbreak can permanently retune. The behavioral immune system, it seems, learns.

What it means for humans

The study lands as a mirror held up to our own pandemic behavior. In the early days of COVID-19, households wiped down packages and hoarded sanitizer even before scientists confirmed how the virus spread — a generalized caution not unlike the macaques avoiding visually contaminated surfaces for a virus that traveled through bites, as Nautilus reported. The researchers say the monkey findings cannot directly answer whether humans retain such changes long after the threat passes, but they show that experience-driven Cautious of Germs behavior is not uniquely human.

There is a broader evolutionary claim at stake, too. Disgust is one of our most primal emotions, guarding us against pathogens by making us recoil from contaminated things. The new work suggests that while the capacity for disgust is ancient and shared, its calibration is personal — written by the outbreaks a group survives and even by the childhood it had. The study was published in Royal Society Open Science. For more science with a personal angle, see how gene therapy gave blind patients their sight back, how plants speak in electricity, and how scientists turned locusts into living chemical sensors.