Walking through Harvard Yard, you see it every day - one person stops to look up at a tree, perhaps trying to catch a glimpse of hawks that call the area home - and soon most passers-by are stopping to look in the same direction.
It's a phenomenon known as "gaze following" - and although it's been demonstrated in dozens of species, researchers have theorized that it may develop in a unique way in humans, because it plays a critical role in learning and socialization.
A new study, however, shows that gaze following in monkeys develops in a way that's nearly identical to humans, suggesting that the behavior has deep evolutionary roots. The study is described in a May 11 paper published in the Proceedings of the Royal Society B.
"Even though it seems like it's a very simple thing, this is a foundational social and cognitive skill that humans have. And there has been little research on how this skill develops in other species," said Alexandra Rosati, Assistant Professor of Human Evolutionary Biology and the first author of the study. "This is the largest study ever looking at gaze following in monkeys. We followed how this skill developed through their whole lifespan and examined the psychological mechanisms they were using to exhibit this behavior."
By studying more than 480 monkeys ranging from two weeks to 28 years old, Rosati and colleagues from Yale University and the University of Pennsylvania found that gaze following in macaques first appears a few months after birth, peaks among juveniles and then slowly declines into old age. The study also revealed - just as in humans - that female monkeys were more sensitive to gaze cues than males.
"We found that monkeys are very similar to humans in the developmental pattern across their lifespan," she said. "That we were able to find this pattern in species with very different life histories than our own suggests that this might be a very evolutionarily conserved pattern of social development."
To get those results, Rosati and colleagues conducted an unusual experiment - they travelled to Cayo Santiago, a small island off the eastern coast of Puerto Rico, inhabited by a colony of about 1,500 macaques that live in free-ranging groups.
"We would approach a monkey when it was sitting calmly and try to attract their attention" she said. "As soon as the monkey looked at the experimenter, the experimenter would just look straight up. A second person would be filming the monkey, so we could see if the monkey also looked up."
Importantly, Rosati said, the team decided to conduct the study with macaques not because they were similar to humans, but because they were very different.
"Compared to other primates, humans have a much longer juvenile period, we have a very long life span, and there are characteristics of human aging - like menopause - that are not shared with other primates," Rosati said. "These monkeys are quite dissimilar from us in a lot of these life history characteristics, and we thought this is a great test of whether those human life history characteristics are tightly intertwined with this cognitive development pattern. If we could show that the monkeys' social cognitive trajectory is very similar to ours, that lets us make inferences about what is driving this pattern in our species."
Going forward, Rosati said the hope is to correlate the differences in behavior among monkeys with variations in their social behavior and even to their genetic differences.
"We want to integrate this data with what's going on in real life," Rosati said. "We want to see what's happening with the 30 percent of juvenile monkeys that don't follow gaze. Can we find genes that are associated with that behavioral difference? Similarly, is it the case that monkeys that gaze follow a lot as juveniles are more socially competent?"
Ultimately, Rosati said, the study reveals that gaze following - while not unique to humans - likely serves as the foundation for a host of more advanced social skills humans rely on.
"This is a critical skill for humans - it's important for the theory for mind, communication, it's how you learn about the culture you're growing up in," Rosati said. "And the fact that gaze following can be disrupted in individuals with autism suggests that early disruptions in how you respond to social cues can develop into a much more pervasive problem. The fact that monkeys show this sensitivity...suggests that humans are building upon this biologically shared propensity to respond to these cues. It's not just something different in our species alone."