This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
Two years ago, Sarah Brosnan and Frans de Waal at the Yerkes National Primate Research Center found that brown capuchin monkeys also react badly to receiving raw deals. Forget bananas – capuchins love the taste of grapes and far prefer them over cucumber. If monkeys were rewarded for completing a task with cucumber while their peers were given succulent grapes, they were more likely to shun both task and reward.
That suggested that the human ability to compare own efforts and rewards with those of our peers evolved much earlier in our history than we previously thought. Of course, animal behaviour researchers always need to be careful that they’re not reading too much into the actions of the animals they study.
It’s easy to suggest that the monkeys were motivated by envy, fuelled by directly weighing up their rewards with those of others. But they could equally be driven by greed of frustration. They could simply have coveted the better reward regardless of the fact that it was given to their partner. Alternatively, they could have been frustrated at being given grapes in previous trials and having to contend with cucumbers.
To rule out these alternative explanations, de Waal and Brosnan tasked graduate student, Megan van Wolkenten with repeating their earlier study with subtle tweaks. Their new results firmly show that monkeys can indeed spot unjust deals and respond with envy and apathy.
The trio worked with 13 capuchins who were asked to hand over a small granite rock in exchange for a cucumber or grape reward. They tested the monkeys in pairs, sat in adjacent wire cages so that each individual could see what its partner was getting.
If both partners were rewarded equally, they completed the task about 90% of the time, regardless of whether they were given grapes or cucumbers. Even if they were shown their future rewards before the experimenters reached for the rock tokens, they didn’t make any special efforts to earn the grapes.
That suggests that they’re not being greedy after all and are more than happy to work for a cucumber reward if their peers are rewarded equally. However, if monkeys were given cucumbers while their partners received grapes, they only cooperated 80% of the time and as the trials continued, they were more and more likely to refuse.
The researchers also found that monkeys were just as likely to hand over the tokens, regardless of whether they received a grape or a cucumber in the previous round. That effectively discounts the frustration angle, which suggests that cucumbers fail to meet the lofty expectations set by grapes.
The trio of researchers also found that the monkeys weren’t just fussed about rewards. They also compared their efforts to those of their partners and were less likely to cooperate if they had gone to more trouble to get their rewards.
If one monkey exchanged tokens for cucumbers while their partner got one for free, it was still happy to complete the task 90% of the time. But if it had to hand over three rocks for the same reward, it only complied 75% of the time. The monkeys became even more indignant if their slacker partners were given grapes for slacking. Now, they were making more effort and getting poorer rewards and their tendency to hand over rocks fell to new lows.
However, if both partners were given grapes, they were willing to do whatever it took to get them, inequity be damned. It seems that capuchins aren’t willing to act disdainfully in the face of really good rewards.
Together, these new results show that capuchins react negatively to unequal rewards and are motivated neither by greed nor frustration. Capuchins hunt squirrels as a team and once food is found, they willingly share it out among the group. Their intolerance for unequal handouts would foster greater cooperation among monkey troupes by preventing any individuals from monopolising the spoils.
In other studies, pairs of capuchins who cooperate for unequal rewards do better in the long run if they swap who gets the lion’s share. De Waal speculates that this need to share the spoils of a hunt could be the origin of our own disdain for inequality.
Even so, de Waal notes that the monkeys’ aversion to injustice isn’t on a par with humans. They don’t like getting less than their peers, but they don’t react to getting more. If anything, this worsens any inequality since monkeys that do badly end up shunning the task and its reward altogether, while the one that’s better off continues to be rewarded.
It may be that in a more realistic situation, monkeys that were ripped off could just leave and find other social partners, but only further research would tell.
Reference: van Wolkenten, M., Brosnan, S., de Waal, F. (2007). Inequity responses of monkeys modified by effort.Proceedings of the National Academy of Sciences, 104(47), 18854-18859.
To me, and I suspect many readers, the quest for information can be an intensely rewarding experience. Discovering a previously elusive fact or soaking up a finely crafted argument can be as pleasurable as eating a fine meal when hungry or dousing a thirst with drink. This isn’t just a fanciful analogy – a new study suggests that the same neurons that process the primitive physical rewards of food and water also signal the more abstract mental rewards of information.
Humans generally don’t like being held in suspense when a big prize is on the horizon. If we get wind of a raise or a new job, we like to get advance information about what’s in store. It turns out that monkeys feel the same way and like us, they find that information about a reward is rewarding in itself.
Ethan Bromberg-Martin and Okihide Hikosaka trained two thirsty rhesus monkeys to choose between two targets on a screen with a flick of their eyes; in return, they randomly received either a large drink or a small one after a few seconds. Their choice of target didn’t affect which drink they received, but it did affect whether they got prior information about the size of their reward. One target brought up another symbol that told them how much water they would get, while the other brought up a random symbol.
After a few days of training, the monkeys almost always looked at the target that would give them advance intel, even though it never actually affected how much water they were given. They wanted knowledge for its own sake. What’s more, even though the gap between picking a target and sipping some water was very small, the monkeys still wanted to know what was in store for them mere seconds later. To them, ignorance is far from bliss.
In a classic episode of the Simpsons, Homer’s brain explains to him that “money can be exchanged for goods and services”. That’s obviously true for humans (even cartoon ones) but monkeys use an altogether different form of payment – grooming. It’s as close to a currency as monkeys have and it can be redeemed against a wide range of goods and services including more grooming, a free pass from aggression, permission to handle babies, back-up in fights and even sex.
The purposes of these exchanges go well beyond cleaning. Grooming, it seems, is also an enjoyable activity that releases brain signalling chemicals involved in pleasure and rewarding feelings. It’s a social bonding activity, the monkey equivalent of a human hug. Grooming does have costs though, despite its appearance as a leisurely activity. For a wild monkey, time spent cleaning a peer is time that’s not spent foraging yourself or watching out for predators. So it pays an individual to groom only as much as it needs to.
Cecile Fruteau from the University of Tilburg has been studying the exchange of grooming among wild vervet monkeys in South Africa’s Loskop Dam Nature Reserve. Through her experiments, she has shown that vervet grooming works like a biological market, governed by the laws of supply and demand. The amount that any individual is willing to give in exchange for a service depends on how rare or abundant it is.
In the forests of South America lives the unusual but aptly named owl monkey, or douroucouli. You could probably guess by looking at its large round eyes that it’s nocturnal, and indeed, it is the only monkey to be mostly active at night. But its eyes have many adaptations for such a lifestyle, beyond a large size.
The owl monkey’s retinas are 50% larger than those of a day-living monkey of similar size, like the brown capuchin. The proportions of different cells in their retina are also different. Owl monkeys have relatively few cone cells, which are responsible for colour vision and fewer ganglion cells, which process the signals from the cones. In contrast, they have many more rod cells, which are far more sensitive than cones and function best in low light, and rod bipolar cells, which transmit signals from the rods.
This is an eye that has sacrificed sharpness and colour for sensitivity. Nocturnal mammals the world over have developed a very similar suite of adaptations and according to Michael Dyer and Rodrigo Martins, these may be easier to evolve than you might think.
All of the cells in the retina are produced by a small group of stem cells called retinal progenitor cells (RPCs). As an embryo grows, its RPCs go through cycles of division, still maintaining their “stemness”. At some point, they leave this cycle and commit to becoming one of the various types of retinal cells. The fate they choose depends on when they leave the cycle. Those that are “born” early turn into cells that are important for daylight vision, such as cones and ganglions. Those that exit late become cells that play a greater role in night-vision, including rods and their bipolar cells.
This quirk of organisation means that the retina’s cells are always produced in a very specific order, with those that grant good night-vision cells appearing later. The upshot is that the owl monkey has been able to adapt its retina to see in the dark simply by tweaking the timing of its development. In its retinas, more RPCs commit to a particular fate later on in their cycle, producing fewer of the earlier types of cells and many more of the later ones. The result: an extra-sensitive retina with a complement of cells perfectly suited for nocturnal living, all triggered by a single change during development.
The eyes of the owl monkey hammer home an increasingly familiar message – you can get big results by very subtly tweaking the way that bodies develop, without any need for large-scale tinkering. Even the eye, an exceptionally complex organ, can be altered in a coordinated way, simply by shifting the timing of its development. It’s why the owl monkey, in a relatively short space of evolutionary time, has converted the daylight-loving eyes of its ancestor into a nocturnal model.
The blood that flows into our heads is obviously important for it provides nutrients and oxygen to that most energetically demanding of organs – the brain. But for neuroscientists, blood flow in the brain has a special significance; many have used it to measure brain activity using a technique called functional magnetic resonance imaging, or fMRI.
This scanning technology has become a common feature of modern neuroscience studies, where it’s used to follow firing neurons and to identify parts of the brain that are active during common mental tasks. Its use rests on the assumption that the flow of blood (“haemodynamics” to those in the know) is a decent enough stand-in for the firing of neurons – the latter creates a shortage of nutrients and oxygen that is corrected by the former.
But Yevgeniy Sirotin and Aniruddha Das from Columbia University have found that this assumption might not be entirely valid. They used a new technique to independently measure and compare nerve activity and blood flow in the brains of live monkeys. Sure enough, they found a blood flow pattern that reliably matched the activity of the animals’ neurons.
But they also spotted something that no one has seen before – a second haemodynamic signal, of equal strength to the first, that didn’t correspond to any local brain activity. This second signal was not a sign of parts of the brain that are active, but those that may need to be active in the near future. It seems that if the brain expects a task in the future, it can anticipate which of its regions will be needed and flush them with blood in preparation.
From the carpenter choosing the right strength of drill, or the artist selecting the right weight of pencil, humans have a natural talent for picking the right tool for the job. Now, it seems that monkeys are similarly selective about their tools. In the first study of its kind, Elisabetta Visalberghi from the National Research Council, Italy, found that capuchin monkeys are able to pick stones with the right properties for nutcracking.
Capuchins often use stones to crack otherwise impenetrable nuts upon hard, flat surfaces, turning innocuous forest objects into their own hammers and anvils. By examining their cracking sites, Visalberghi deduced that the animals were picky about their hammers, for the sites were littered with hard and heavy rocks that weighed as much as 40% of an adult. However, it was entirely possible that the monkeys used any old rocks and those that remained were simply the ones that hadn’t eroded yet.
Visalberghi decided to put eight wild capuchins to the test by allowing them to choose between sets of potential hammers. These monkeys regularly use stones to break into palm nuts and they need heavy, hard materials for the job. Visalberghi removed all the stones from the experimental arena and provided the capuchins with a choice of two – one of which would excel as a nutcracker and another which would not. The monkeys selected the right stone at least 90% of the time, correctly picking solid siltstone over crumbly sandstone, and big quartzile stones over small ones.
In both cases, the stones were natural ones of the type that the monkeys regularly encounter in their habitat. No surprise then that the animals immediately and confidently singled out the right tool. But Visalberghi found that they did just as well when choosing between artificial stones – they just spent more time examining the different options on offer.