Monkeys Unconsciously Synchronize Their Behavior, Just Like Humans

Society, as many an anthropologist has argued, is imitation. We learn to talk, walk and behave by studying how others talk, walk and behave. But we also imitate each other unconsciously—aping each other’s moods, movements and speech. We synchronize with one another without realizing it, whether it’s the way we fall in step on the sidewalk, or smile back at someone without thinking.

But humans aren’t the only ones. Animals do it, too. Schools of fish suddenly turn together on a dime--as do murmurations of sarlings. Fireflies blink in unison. Cicadas sing with one voice.

Scientists never knew why, exactly, animals do this—whether such behavior is intentional or unconscious. But researchers at the RIKEN Brain Science Institute in Japan have, for the first time, created synchronization among macaque monkeys in a lab setting, suggesting such forms of imitation may be partly unconscious in monkeys, just like it is with us.

For a study published today in the journal Scientific Reports, scientists separately trained several monkeys to push two light-up buttons, alternating between the buttons in a relatively smooth, rhythmic sequence, encouraging them with sounds, button-lighting, and treats. When two such monkeys were placed in front of one another and instructed to begin their button-pushing sequence, their movements naturally fell in line with one another’s.

Pushing buttons in a lab in unison wasn’t something they were naturally inclined to do. They had synchronized each other’s newly learned actions spontaneously.

To test the conclusion a little further, the researchers conducted a second experiment in which those trained monkeys were placed in front of a video depiction of a monkey pushing the buttons in sequence, this time manipulated at varying speeds. Again, the real-life monkeys came into lockstep and sped-up or slowed down in keeping with the videos. (A third experiment determined that the visual component was most important in the videos, not the audio.)

Decades of research suggests many animal brains are deeply hardwired for imitation. In 1963, a Danish psychologist named Torsten Ingemann Nielsen devised a clever experiment wherein human subjects were asked to watch their gloved hands through a viewing box as they traced a preexisting, straight vertical line with a pen. They didn’t know the box contained mirrors that fooled them into believing someone else’s gloved hand was actually theirs.

As the hand they believed was their own traced the line, it began to veer off-course to the right. Believing their hands were going astray, most participants compensated by moving their actual hands to the left as they “traced” the line. Seeing someone else’s hand movements had tricked their brains into believing those movements were their own.

Research suggests the reason could have something to do with so-called “mirror neurons,” which fire exactly the same way when we perform an action as when we watch that action being performed by someone else.

In the 1990s, a group of Italian scientists led by Giacomo Rizzolatti discovered mirror neurons by accident while monitoring the synaptic activity of a specific neuron in a monkey that fired each time it reached for a peanut. One day, they noticed that the neuron fired the same way when the monkey saw an experimenter grab the peanut instead. Repeat tests confirmed that, for that neuron, there was seemingly no difference between doing and watching.

In recent years, regions of mirror neurons have been isolated on the cellular level in the human brain—what Daniel Glaser, a neuroscientist at University College London who has worked on such research, called “the mechanism which is absolutely fundamental to the way that we see other people in the world.” Glaser appears in this 14-minute segment from NOVA, which does a nice job of explaining the mirror neuron system.

The Japanese researchers for the current study believe the mirror system is probably at play in their button-pushing experiments, as well. Macaque monkeys have been shown in repeated experiments to possesses mirror neuron circuitry. They were the ones used in Rizzolatti’s peanut studies. And a 2006 study led by Pier Francesco Ferrari showed that baby macaques could, during a specific period of their development, mimic human facial movements, like sticking out one’s tongue.

Such hardwired, unconscious actions suggest an evolutionary advantage where it appears. For humans, the researchers note, “a number of studies have further shown that spontaneous imitation or spontaneous synchronization apparently ‘smoothed’ conversation and facilitated the building of social rapport.” When you smile, I smile. If your brow furrows, mine does, too. We empathize, and the movements of our faces reflect that empathy in conversation.

Something similar could be going on with monkeys, which could in turn teach us more about ourselves. The current study's researchers admit they don’t have a full picture yet of the reasons for monkey synchronization, but note that experiments like this could be helpful in studying human autism.

“The simple (button-pressing) task could be utilized for brief assessments of autistic children,” they write, “who show difficulty in responding to social cues and in maintaining social relationships.”