“Two chimps had been shut out of their shelter by mistake during a cold rain storm. They were standing dejected, water streaming down their shivering bodies, when Professor Köhler chanced to pass." Upon opening the door for the two chimps, explained Dr. James Leuba in 1928 in Harper's Magazine, “instead of scampering in without more ado, as many a child would have done, each of them delayed entering the warm shelter long enough to throw its arms around his benefactor in a frenzy of satisfaction.”

“Chimpanzees,” primatologist Frans de Waal points out, “do not normally hug their caretakers for no reason.” It’s a compelling image, isn’t it? The idea that at least some animals might be capable of feeling and communicating their thankfulness? As we dig into our Thanksgiving turkeys this holiday, it is worth stopping to consider whether the idea of gratitude is the sole domain of our species.

But just what sort of behaviour counts as gratitude?

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For impalas, tick removal is so much easier when there's someone around to help. Image: William Warby, Flickr

Here's one possible example. Impala are small African antelopes that groom each other. It's a means of keeping clean and healthy. They can't remove the ticks from their own bodies, so each impala relies on others to help. Grooming exchanges among African impala are usually unsolicited: one individual grooms the neck of a second individual, and then the second individual returns the favour, grooming the first individual for an equivalent amount of time. 

Vampire bats have a similar deal. They need blood to survive and they need to feed at least once every three days. So what happens if a vampire bat misses a meal? The other bats will come to her aid, regurgitating blood to feed her. It's not just between mothers and their offspring, but rather between unrelated adult bats.

Does the impalas' 'you scratch my back; I'll scratch your back' contract suggest something akin to human gratitude? While these are fascinating examples of animal behaviour, they're best explained by something simpler than gratitude: symmetry-based reciprocity.

In other words, the mutual back-scratching of the impala and blood-vomiting of the vampire bat could simply be a result of close quarters: individuals who hang out together will tend to engage in reciprocal interactions, but only because they tend to hang out together. It's more 'pay it forward' and less 'pay it back'. These sorts of interactions do not require any sophisticated mental computation for directing repayment only at certain individuals or for keeping track of services received and rendered over time – abilities that underlie gratitude.

So what about dogs? Surely the dog owners among us have witnessed something resembling gratitude in our furry companions. "Even though we have all heard of pets adopted from a miserable stray existence into the comfort of modern homes," write Bonnie and de Waal, "it is impossible to tell if their greater-than-average appreciation (e.g. tail wagging, purring) of our care and food has anything to do with gratitude." In the science of animal behaviour, it is necessary to interpret things in the simplest manner possible. "The simpler alternative is that, after prolonged deprivation, there is a contrast effect that lasts a lifetime, making these animals show greater-than-average expressions of pleasure at receiving a full bowl of food."

In other words, are dogs simply expressing pleasure, rather than gratitude? It turns out it may not be quite that simple after all. If that pleasure is expressed towards a particular individual – such as the one providing the food – then that may indeed be closer to what we think of as true gratitude.

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Studies show that adult chimps are more likely to share food with individuals who help them with grooming. Image: adamisher, Flickr

De Waal observed the common exchange of food for grooming among chimpanzees in order to determine if the trade of food for grooming is simply the result of proximity (as in the impala or vampire bat), or good feelings (as in the adopted domestic dog), or if it is somehow more cognitively intensive, requiring the ability to keep track of who owes whom and for what.

The researchers found that adult chimps were more likely to share food with individuals who had groomed them earlier that same day. Since the chimps shared their food only with their former grooming partners instead of with just anyone, de Waal reasoned, chimps must keep track of favours given and received, and they must be able to distinguish among different social partners. This form of reciprocity, then, is driven by more than just a good mood. It's something more like gratitude. 

In another experiment, primatologists Seyfarth and Cheney played recordings of vervet monkey calls and measured the reaction of recently groomed individuals. The type of vocalisation they used was a call vervets rely on to threaten enemies and to solicit the support of friends, in anticipation of a conflict. When vervet monkeys heard the calls of a previous grooming partner, they paid more attention than when the calls were recorded from other individuals.

It may not be exactly the same as human gratitude, but some animals do seem capable of expressing something that comes quite close to thanksgiving. Just don't expect your turkey, golden brown and delicious, to send any thanksgiving in your direction.

An earlier version of this post appeared in 2010 on The Thoughtful Animal at Scientific American.

Top header image: John Wright, Flickr