A number of questions could still be answered to make a stronger case that nonhuman primates really do feel gratitude, or some evolutionary building block of gratitude.
First, a great deal of evidence in humans suggests that gratitude is important for forming new relationships, but not so much for maintaining existing ones. If the opportunity arose to study a newly formed group of primates over the long term, we would expect to see reciprocity diminish over time.
Along the same lines, we should also expect to see more reciprocity among non-kin than kin. Since helping your family also increases your genetic contribution to the next generation, reciprocity is not really necessary for motivating that kind of helping behavior.
Finally, all of the research to date uses repayment of favors as the purported way to acknowledge a past favor done. While this is likely the best, most observable measure we have, it is quite different from a verbal thank you given from one human to another. There may be a subtle expression, gesture, or some other communication that other species use to acknowledge kind acts. The trick will be for us to develop a deep enough understanding of other species’ communication systems to discover it.
We’ve come a long way since Darwin first proposed that gratitude may be a universally experienced emotion. Although we are not yet at the point where we can speak chimp well enough to understand their expressions of gratitude, the behavior of our closest relatives certainly suggests that we humans are not alone in the importance we place on gratitude. The research suggests that, in all likelihood, our propensity for gratitude really does have deep evolutionary roots, and it will be up to us to find out how deep they go.
We often blame our worst tendencies, like aggression and competition, on our evolutionary history. It’s important to remember that some of our most positive qualities like empathy and gratitude are also a part of this history. When we discover these traits in our closest relatives, it’s a powerful reminder that the good in human nature is deeply rooted, as well.