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.
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."