Outbreak experience alters food safety behaviors in macaque monkeys

Macaque monkeys that survived a lethal disease outbreak demonstrated markedly heightened caution toward food appearing contaminated, according to new research from the University of California San Diego. The study indicates that pathogen-avoidance behavior is shaped by experience rather than biological immunity alone, offering insights into the evolutionary origins of disgust.

The behavioral immune system in primates

Published in Royal Society Open Science, the findings illuminate what scientists term the "behavioral immune system," a set of actions animals use to prevent infection before their biological defenses activate. Federico Rossano, associate professor at UC San Diego's Department of Cognitive Science and senior author, stated that non-human primates exhibit this same response. He noted that even without knowledge of transmission methods, witnessing others fall ill and die alters the likelihood of engaging in risky behaviors.

Natural experiment at Texas sanctuary

The research originated from an unexpected event at a primate sanctuary in southern Texas, home to groups of Japanese macaques descended from animals brought from Japan in 1972. In 2020, staff confirmed simian retrovirus Type II, which causes AIDS-like immunodeficiency, in several groups. Transmitted primarily through bites and scratches, the virus led to severe illness and death, prompting strict quarantine measures. This created a rare natural experiment: three groups experienced the outbreak, while one did not.

Visual cues drive avoidance decisions

Researchers tested 55 macaques by offering a choice between food on a clean wooden surface and food on a surface covered with real macaque feces sealed under clear epoxy. This setup preserved visual contamination cues while eliminating smell and infection risk. To rule out fear of unfamiliar objects, some trials replaced the clean surface with a brightly colored kitchen sponge. Previous studies showed wild Japanese macaques would eat from feces-covered surfaces if offered high-value treats like peanuts, but avoided them for lower-value items like wheat grains.

Group history shapes individual caution

Macaques from outbreak-exposed groups ate from the visually contaminated surface in only 47% of trials, compared to 79% for the unexposed group. Notably, individuals within exposed groups behaved similarly regardless of whether they had been infected themselves. Stephan Kaufhold, who led the study as a doctoral researcher, explained that living through the outbreak made the groups more cautious around food that looked contaminated, suggesting the behavioral change stemmed from shared group history rather than personal illness.

Transmission mode irrelevant to avoidance

The virus was not spread through contaminated food; it transmitted via blood and saliva through bites and scratches. Despite this, macaques that witnessed fellow primates falling sick became reluctant to consume food placed on surfaces suggesting contamination. While the study cannot directly answer whether human behavior changes long after immediate threats pass, it provides evidence that experience-dependent shifts in pathogen avoidance are not unique to humans.

Early life rearing impacts behavior

An additional component of the paper examined 54 macaques from seven species with varied rearing backgrounds, including sanctuary-born, former laboratory, zoo-born, and former pet animals. Former pets, raised by humans rather than other macaques, were significantly more likely to eat contaminated food than sanctuary-born peers raised by mothers. Kaufhold suggested these monkeys may have missed opportunities to learn typical avoidance behaviors from conspecifics early in life, indicating that early social environments shape responses to potential disease sources.

Implications for understanding disgust

The results highlight the role of social learning in maintaining heightened pathogen avoidance over time. Although researchers did not measure disgust directly, the macaques' ability to modify avoidance based on experience mirrors human patterns where sensitivity to cues like feces or spoiled food shifts with context. Future studies could investigate whether newly introduced animals adopt the cautious behaviors of groups that have previously endured outbreaks.