Rhesus macaques are able to spot an unfamiliar sibling just by looking at photos, according to a new study in the journal of Current Biology. These strangers (brothers and sisters who are paternal half-siblings) aren't exactly long-lost relatives: Macaques, raised by their mothers, are rarely acquainted with their fathers or their dads' other kids. But an international team of primatologists has found that adult macaques can sense if another monkey is a relative simply by looking at facial features.
Duke University researcher Dana Pfefferle tells The Conversation that, during her work on the island of Cayo Santiago near Costa Rica, she spied the occasional macaque who would approach a separated-by-birth brother or sister. To see if that sense of recognition held true, the scientists tested close to 90 wild monkeys on their ability to spot siblings.
Out on Cayo Santiago (also known as Monkey Island, home to roughly 1,000 rhesus macaques and not much else) a primatologist held up two photos -- one of a relative, the other of a stranger -- to see how the macaques reacted.
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."
It's not the first time macaques have shown a fondness for photographs, either. As part of a 2011 study of wild Barbary macaques, primatologist Julia Fischer found that monkeys (particularly the young ones) would attempt to greet animals in photos with a smack of the lips.