Following a deadly viral outbreak among captive Japanese macaques in South Texas, surviving monkeys displayed a striking increase in caution toward contaminated items. Researchers found the animals avoided potentially dirty surfaces in 53 percent fewer trials than unexposed peers.
A natural experiment at a Primate sanctuary in southern Texas has given researchers a rare window into how disease outbreaks reshape animal behavior long after the acute threat has passed. The facility, which houses Japanese macaques descended from animals brought from Japan in 1972, experienced an outbreak of simian retrovirus Type II in 2020. The infection, which causes an AIDS-like immunodeficiency, spread primarily through bites and scratches, forcing facility staff to implement strict quarantine and testing protocols across the separate enclosures. Some macaques passed away quickly from severe hemorrhaging, while others became ill gradually.
Sanctuary Experiment Tests Contamination Avoidance in Macaques
Once the outbreak subsided, researchers tested 55 macaques to see how they would choose between food placed on a clean wooden surface and food placed on a board covered in macaque feces. To eliminate any actual risk of infection or odor, the feces were sealed beneath a layer of clear epoxy that preserved visual cues. Unexposed control animals typically pay little mind to such surfaces when hungry. In the trials, however, monkeys from groups that had experienced the outbreak ate from the contaminated surface in only 47 percent of trials, compared with 79 percent for monkeys from the unexposed group. Living through the disease outbreak appeared to make those monkey groups more cautious around food that looked contaminated.
Groups have histories, and those histories show up in behavior.
Stephan Kaufhold, doctoral researcher in Cognitive Science, via Ucsd
The findings point toward what scientists call a behavioral immune system, suggesting that primates learn pathogen avoidance socially through lived experience. Interestingly, the viral pathogen at the center of the outbreak was transmitted via blood and saliva rather than foodborne contamination. Yet witnessing illness among their neighbors heightened the animals’ sensitivity across the board, altering their reactions to visual cues of dirtiness regardless of the actual transmission route. They were somehow adapting to a potential risk that they were perceiving.
Infection Exposure Versus Social Proximity in Primate Groups
A notable aspect of the study was that individual infection status did not alter the behavioral outcome. Within the outbreak-exposed enclosures, some macaques contracted the virus and survived, while others lived alongside sick peers without ever falling ill themselves. When tested afterward, monkeys from outbreak-exposed groups behaved similarly whether or not they themselves had been infected.
The ones who used to live with the ones who were sick behaved exactly the same way.
Federico Rossano, Comparative Cognition Lab director at the University of California, San Diego, via Nytimes
Researchers also noted that background matters significantly for captive primates. The sanctuary population included individuals born in zoos, laboratories, or private homes as pets, alongside those born at the facility. Macaques that had been reared by humans proved more likely to consume the food on the contaminated surface, likely because they missed out on early opportunities to learn avoidance cues from mothers or peers.
Many primate studies still fail to factor in the life history of primate participants, as if background or experience doesn’t play a critical role in exhibited behaviors. This paper does the opposite.
Elodie Freymann, primatologist at Brown University, via Nytimes
Broader Implications for Animal Behavior and Disease Avoidance
Across the animal kingdom, behavioral modifications designed to sidestep illness are well-documented. Wild mandrills, for instance, inspect feces for parasites by sniffing them before grooming conspecifics, and various species exhibit self-isolation when infected.
The monkeys see disease and sick individuals and increase their overall sensitivity, even for a transmission route that is completely unrelated to how the disease actually spreads.
Stephan Kaufhold, cognitive science researcher, via Nytimes
This heightened state of caution suggests that the primates are employing a general defensive strategy to avoid potential infection, regardless of the specific biological mechanism of transmission.