Macaques That Lived Through a Virus Outbreak Got Pickier About Dirty Food
After a virus swept through a Texas primate sanctuary, the monkeys there became far less willing to eat food that looked contaminated, even though the virus spread by bites.
In 2020, a virus swept through a primate sanctuary in Texas. Afterwards, the monkeys that had lived through it became noticeably pickier about eating food that looked dirty — even though that virus did not spread through food at all.
That is the finding of a study published in Royal Society Open Science, and it hints at where our own feelings of disgust may come from.
Fruit next to feces, and who would still eat it
Cognitive scientists from the University of California San Diego tested 55 Japanese macaques at the sanctuary. Each monkey was offered a choice: fruit sitting on plain wood or a kitchen sponge, or fruit sitting on a surface covered with real macaque feces sealed under a clear layer of epoxy. The feces were visible but sealed away, so the fruit was not actually contaminated. It only looked that way.
For a monkey, this is not always an obvious choice. Earlier research has shown that macaques will eat food that appears to be contaminated by feces if the reward is good enough.
In the new tests, the gap between groups was large. Macaques from the group that had not been exposed to the virus ate the contaminated-looking fruit in 79 percent of trials. Macaques from groups that had lived through the outbreak picked it in only 47 percent of trials.
The outbreak mattered, not the infection
Three of the four macaque groups at the sanctuary were hit by simian retrovirus Type II, a virus that causes an immune deficiency similar to AIDS in primates. The fourth group was not.
The cautious monkeys included animals that had never been infected themselves. That points to the experience of the outbreak — the illness spreading around them — rather than contact with the virus, as the thing that changed their behavior.
There is a further twist. Simian retrovirus Type II spreads through bites and scratches, not through feces or food. So the monkeys were avoiding a kind of contamination that had nothing to do with how the disease actually moved between them. They did not know how the virus spread, much as people did not know in the early days of COVID-19.
"Groups have histories, and those histories show up in behavior," study author Stephan Kaufhold of UC San Diego said in a statement. "Living through the disease outbreak appears to have made those monkey groups more cautious around food that looks contaminated."
What a monkey learns early in life
A second experiment by the same team used the same setup with macaques that had different upbringings. Monkeys kept as pets and raised among humans were more likely to eat the contaminated-looking food 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 may still shape how they respond to potential sources of disease."
The researchers say the work offers clues about the evolutionary origins of disgust — the instinct that makes people recoil from feces, vomit and rotting food. Disgust is powerful and automatic, but it is also shaped by experience. The macaques suggest that this shaping is not unique to humans.