The monkeys' responses, it turns out, depended on the sex of photographed macaques. If a male monkey saw photos of two other males, he would spend less time looking at his half-brother and more time staring at the complete stranger. The scientists believe that this behavior -- a glance at a relative but a glare at an outsider -- underlines a sense of recognition. To a macaque, an unfamiliar monkey is, by default, a rival to be kept under a watchful eye. Siblings of the same-sex, however, get a pass.
In tests when the photos were of the opposite sex as the monkey (a female macaque looking at a half-brother and a strange male, for instance), the macaques made no such distinction about where they directed their gaze. Breeding instincts, the scientists say, may have distracted the male monkeys; for the females, any male staring them down, related or not, might seem threatening.
Overall, however, the photos piqued the monkeys' curiosity. "They were very keen in participating in these visual experiments," Pfefferle says. "One female kept following us just to have another glance at the pictures."