If a colleague of yours was rewarded for their work while you received nothing for your (similarly sized) efforts, you would probably be quite peeved. Now it seems that man's best friend also shares our disdain for unfairness.
Humans are notorious for our dislike of injustice. It rankles us to see others being rewarded or penalised unfairly. We not only have the capacity to recognise when someone else is being rewarded beyond their efforts, but the inclination to punish them for it, often at personal expense. But other species behave in the same way - recent studies have found that capuchin monkeys and chimpanzees also have the ability to compare their payoffs and efforts to those of their peers, and they too frown on unequal rewards.
Now, Friederike Range from the University of Vienna has found that dogs may also share the distaste for inequity that primates seem to have, although to a much simpler degree. Together with colleagues from Austria, Range compared the reactions of pairs of dogs as they were rewarded with food for giving their paws to a human partner.
When both dogs were rewarded with a piece of dark bread, both offered their paws for the vast majority of 30 trials. But if one was rewarded and the other not, the spurned animal stopped cooperating at some point and on average, they only offered their paw in two-thirds of the trials. They were also visibly more distressed, scratching themselves, yawning and avoiding the gazes of their partners. And when they did play along, they did so more hesitantly, making Range ask for their paws more often before they gave in.
Their behaviour didn't change simply because they weren't being remunerated. If the dogs were tested alone, they took much longer to stop cooperating when the rewards stopped coming than they did in the presence of another dog, and they showed fewer signs of stress.
In some ways, these results aren't surprising. The majority of dog species are sociable and cooperative animals. Domestic dogs, in particular, form elaborate and intimate relationships with their human owners and show a high level of empathy for their masters' behaviour, even catching their yawns.
So they clearly value equality, but only to a certain extent. Range found that they weren't sensitive to the quality of their reward or to the efforts they went through to receive it. If one dog was given dark bread while the other was given a much more delectable piece of sausage, neither refused their paws. Likewise, both animals continued to cooperate as long as they both received food, even if one didn't have to stick its paw out for it.
Monkeys react differently. They care about the quality of the food they are offered rather than just its presence and they will react negatively if a peer is given a much tastier morsel, like a grape instead of a slice of cucumber. Do dogs think in a fundamentally different way? Range believes that the question is too difficult for these results to answer fully. Certainly, the owners of all the dogs in the study confirmed that their pets would normally work harder for a sausage than a piece of bread.
Perhaps even bread was a powerful enough motivator to ensure the dogs' cooperation. Perhaps their training had conditioned them to continue working as long as they received some form of compensation (all the dogs had been heavily trained by humans and worked in fields such as police-work or guiding blind people). It's even possible that the process of domestication is responsible, making dogs more likely to accept a wider range of rewards compared to their peers.
The other alternative, of course, is that dogs behave differently to primates because they lack the same calibre of intelligence. To behave like primates, the dogs would have to compare the effort they've made to the reward they've been given and do the same for their partner. They then have to weigh these against each other, making a comparison of comparisons. That's not as easy as it sounds and so far, the only species that have shown such mental agility are humans, chimps and, to a lesser extent, baboons. Perhaps dogs only have the smarts to reject inequality at a basic level.
Range also found that the dogs were only sensitive to unfair situations where they lost out. Those that were given food never rejected it, even if their partners got nothing. Monkeys, on the other hand, will sometimes reject food if other monkeys receive none, paying a cost to maintain an even standing with their neighbours. Humans, of course, do the same.
It's also unclear how the actions of domestic dogs would generalise to other canines, to other domestic species or indeed, to other social animals. If future studies show that other social species (say, dolphins or meerkats) demonstrate the same preferences, does that mean that several lineages evolved an aversion to unfairness independently, or is this a very old behaviour that has become more sophisticated among the primates? Only more experiments will tell.
Reference: F. Range, L. Horn, Z. Viranyi, L. Huber (2008). The absence of reward induces inequity aversion in dogs Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0810957105
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