The study involved male rhesus macaques—a species of Old World monkeys that is sensitive to red, green, and blue—ranging freely in Cayo Santiago, Puerto Rico. Two human experimenters, one male and one female, entered the monkeys’ colony and found isolated males to test. Both people knelt down, placed a Styrofoam tray in front of them, drew an apple slice from their backpacks, held the slice at chest level for the monkey to see, then placed the apple on the trays. Both stood up simultaneously and took two steps back.
The monkey typically went directly to the slice he wanted, ran off, and ate it.
The humans wore T-shirts and caps, whose colors—red, green, and blue—were changed in each of four conditions: red on female, green on male; then vice-versa; red versus blue; blue versus green.
The results were striking. The monkeys paid no mind to the sex of the experimenter. Green or blue made little difference to them either. But in the significant majority of cases, they steered clear of the red-clad humans and stole the food from the other tray.