Chimpanzees that have had positive experiences with humans appear to trust people more than they do baboons and unfamiliar chimps, a new study suggests.

The findings, published in the latest Proceedings of the Royal Society B, indicate that chimpanzees can learn to bond and exhibit empathy for members of another species, such that trust develops even at the subconscious level.

As for what chimps think of kind and caring humans, lead author Matthew Campbell told Discovery News, "I have no doubt that we are different in their minds, but an okay kind of different."

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Campbell, a researcher at the Yerkes National Primate Research Center, said that an older female chimp named Tai is so pleased to see co-author Frans de Waal, whom she's known for 20 years, that she excitedly pants, bobs her head and stretches out her hand. All of these are behaviors chimps use when greeting each other.

For the study, Campbell and de Waal used contagious yawning to measure "involuntary empathy" among 19 adult chimps at Yerkes that were all raised by other chimps in captivity.

"We think that the mechanism for copying the yawns of others is the same for copying other facial expressions, like happiness, sadness or fear," he explained. "For our purposes, yawning is simply a contagious expression we can easily see and count. Contagious smiles, frowns and fearful expressions may be tiny twitches of muscles that cannot be seen, but yawns can't be missed."

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He added, "We catch all of these expressions more the closer we feel to someone, and that's why we think that empathy is involved."

The behavior is further thought to occur at the subconscious level, suggesting that the trust between the individuals happens this deeply as well.

The chimpanzees yawned in sync with humans, as well as trusted family members and chimp friends. They did not exhibit such involuntary empathy for unfamiliar chimpanzees and Gelada baboons, however.

Chimpanzees yawned in sync with humans, research finds.Flickr

The researchers conclude that their responses were based on life experiences.

"I think that they may have been conditioned to think that humans are generally okay," Campbell explained. "Therefore, meting a new human may be an opportunity for a new positive interaction, since that has been their experience."

Chimpanzees are territorial in the wild and exclude strangers, so unfamiliar chimps could have evoked an innate hostile response. Baboons, on the other hand, are "basically meaningless" to these captive chimps, so the chimps were indifferent to them.

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Chimpanzees, therefore, are not completely hard-wired to feel a certain way about any given primate, including humans. They instead show flexibility in forming trusted, empathic connections with different species, including unknown members of that species.

Elainie Madsen of Lund University has done earlier work on yawn contagion among young chimpanzees, so she was interested to see that even older chimpanzees showed flexibility in forming relationships. This is important because it suggests that human relationships -- both with other people and with other species -- can change for the better at any time.

"Is there some experience that would lead chimpanzees to engage more positively with strange chimpanzees?" Campbell asked. "If so, the method for changing this response could be useful for increasing empathy in humans as well. This is the topic I want to study next."