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Three stills from a computer animation of an apelike head yawning: chimpanzees that view the animation often yawn too. Such contagious yawning is based on motor mimicry and empathy. In contrast, chimpanzees do not yawn in response to still images or to animations that depict simple mouth movements (such as opening and closing). Animation produced by J. Devyn Carter using LightWave 3D from NewTek, Inc. Click here for 3-D animations.

Empathic synchronization of bodies assumes many forms: moving when others move, laughing when others laugh, crying when others cry, yawning when others yawn. Most of us have reached the advanced stage at which we yawn even at the mere mention of yawning—as you may be doing right now!—but that is only after lots of face-to-face experience.

Virtually all vertebrates, including fish, show the peculiar “paroxystic respiratory cycle characterized by a standard cascade of movements over a five- to ten-second period” that defines the yawn. Chimpanzees yawn when they see others of their species yawn—as do some monkeys, as well as dogs. Yawn contagion, like laughter, also works across species. I once attended a lecture on involuntary pandiculation (the medical term for stretching and yawning) with slides of horses, lions, and monkeys—and soon the entire audience was pandiculating. Since it so easily triggers a chain reaction, the yawn reflex opens a window onto mood transmission, an essential part of empathy. Notably, children with autism spectrum disorders are immune to the yawns of others, thus highlighting the social disconnect that defines their condition.

Synchrony may be expressed in the copying of small body movements, such as a yawn, but it also occurs on a larger scale, such as group travel. It is not hard to see its survival value. You’re in a flock of birds and one bird suddenly takes off. You have no time to figure out what’s going on: you take off at the same instant. Otherwise, you may be lunch. Or your group becomes sleepy and settles down, so you too become sleepy and rest.

Mood contagion serves to coordinate activities, which is crucial for any traveling species (which most primates are). If my companions are feeding, I had better do the same, because once they move off, my chance to forage will be gone. The individual that doesn’t stay in tune with what all the others are doing will lose out, like the traveler who doesn’t go to the restroom when the bus has stopped.

The power of synchrony can be exploited for good purposes. On one occasion in the Netherlands, a herd of some 120 horses got trapped on a patch of dry pasture in the middle of a flooded area. With twenty horses already drowned, people were attempting to save the others. One of the more radical proposals was for the army to erect a pontoon bridge, but the local riding club came up with a far simpler solution. Four brave women on horseback mixed in with the stranded herd, then splashed through a shallow area and, like pied pipers, drew the rest with them in single file. The horses had to swim a few stretches, but all made it safely to terra firma.

Movement coordination both reflects and strengthens bonds. Two horses hitched to a cart, for example, may at first jostle each other and pull at cross-purposes, each following its own rhythm. But after years of working together, the two end up acting like one, fearlessly pulling the cart at breakneck speed through water obstacles during cross-country marathons. They become so attached to each other that they object to even the briefest separation.

Synchrony, in turn, builds upon the ability to map one’s own body onto that of another, and make the other’s movements one’s own, which is exactly why someone else’s laugh or yawn makes us laugh or yawn. Body mapping starts early in life. A human newborn will stick out its tongue in response to an adult doing so, and the same neonatal imitation applies to monkeys and apes. How does the baby know that its own tongue, which it can’t even see, is equivalent to the pink, muscular organ that it sees slipping out from between an adult’s lips? In fact, the word “know” is misleading, because it all happens unconsciously. Scientists may bring up neural resonance or mirror neurons [see “Mental Mirrors,” May 2008], but that hardly makes it less miraculous.

The automatic nature of empathy is revealed through imitation—an anthropoid forte, as reflected in the verb “to ape.” Give a zoo ape a broom, and he’ll move it across the floor the way the caretaker does every day. Give her a rag and she’ll soak it and wring it out before applying it to a window. Hand him a key, and you’re in trouble!

Previous studies of ape mimicry tested whether apes imitated human experimenters in white coats. The results cast doubt on apes’ proclivity for imitation. But my chimps at the Yerkes research center obviously haven’t read the scientific literature: imitating is an integral part of their daily life, and they do so spontaneously, often without any reward in mind. I set up an ambitious research project together with a British colleague, Andrew Whiten of the University of St. Andrews, to find out just how well chimps learn food-obtaining tasks from one another. From an evolutionary viewpoint, it doesn’t really matter what they learn from us humans—all that matters is how they deal with their own kind.