A simple sleight of hand

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December 10, 2013 by comparativelypsyched


Every once in a while an article about a commonplace primate behaviour can be more interesting than loftier reports of monkey cultures or ape empathy. For example, a recently published study asked a very simple question: How do New World monkeys grasp a stemmed glass. While the question is straightforward, the findings demonstrate how small differences in a primate’s morphology can influence behaviour in a way that may confound our interpretations of cognition and intelligence.

To examine how primates plan actions, Stacey Zander and her colleagues at Bucknell University presented squirrel monkeys (Saimiri sciureus) and capuchin monkeys (Sapajus apella) with overturned champagne glasses (see the picture below), each glass containing a small marshmallow reward. The treat was attached to the inside of the glass (where the stem meets the bowl) and to obtain the reward a monkey needed to turn the glass over. Each monkey was given numerous opportunities to interact with the overturned glass and the experimenters observed how they would grasp the stem of the glass at each interaction. 


An example of the overturned glasses used in study.

The most natural way to hold a cylindrical object is with a power grip, wrapping your hand around the object and holding it between your fingers and palm. When the cylindrical object is orientated vertically, you can use two types of power grip, your thumb can be on the top or the bottom of the hold (see the examples of power grips in the picture below). It is more intuitive to grasp with the thumbs-up method (think of holding a baseball bat, or a vertical railing on public transport), however for Zander’s study the more intuitive action is not the most clever.


The two methods of grasping a cylindrical object with a power grip.

When faced with the champagne task it is more efficient to grasp the cylindrical stem with your thumb facing downwards. As the goal is to turn the glass over, a thumb-down hold facilitates an easy rotation of the hand bringing the reward quickly to the monkey’s mouth. When grasping the stem in this manner you are using second-order action planning; i.e. you are inhibiting a more intuitive action and planning one step ahead, simplifying the overall process. Did capuchin and squirrel monkeys use this form of advanced planning? Interestingly, different patterns of behaviour were observed in each species. Nearly all squirrel monkeys use second-order planning, while few capuchins did. The graphs below show the number of thumbs-up and thumbs-down grasps used by both the squirrel monkeys (top bars) and the capuchins (bottom bars).


Number of grasps by squirrel monkeys (top) and capuchins (bottom); thumbs-up (light bars) and thumbs-down (dark bars)

The squirrel monkeys in this study were highly biased towards the more efficient thumbs-down technique, while the capuchin results were more mixed. Uncertain about how to interpret the differences found in the capuchin monkeys, the researchers considered a number of possible influencing factors. Did age influence second-order planning; did the older, wiser capuchins plan ahead while their younger group-mates rushed in with an inefficient thumbs-up approach? This was not the case; older capuchins performed similarly to their younger counterparts, and the forward-planning squirrel monkeys were not significantly older than the capuchins. Maybe some capuchins were more experienced with experimental protocols and had been inadvertently cued to interact with a piece of apparatus in a certain way (i.e. had some capuchins been trained to use the thumb-up grasp?). Again, there was no difference in second-order action planning between experimentally experienced and naive monkeys. The authors conclude that it is likely that the results of this study indeed represent a real species difference in second-order planning. Squirrel monkeys seem to be the savvier primate in this regard.

So, are these squirrel monkeys more intelligent than their close primate cousins? If so, this is an interesting case. Capuchin monkeys have long been considered the intellectual outliers of New World primates. They have large brains for their body size (Stephan, Barbon, & Grahm, 1988), and are well known for their extraordinary ability to use tools (see this wonderful example of nut cracking behaviour). In fact, capuchin monkeys, when compared with other closely related species, are masters of manipulating their environment in intelligent ways. But why does this intelligence not transfer to flipping champagne glasses? The researchers think that this difference may be due to one simple difference between the two species, something unrelated to intelligence. A capuchin monkey can easily execute a type of grasp called the precision grip, while a squirrel monkey cannot.

A precision grip is (not surprisingly) more precise than a power grip, involving a more delicate manipulation of an object between a finger and thumb, or between two fingers. When a precision grip is a possibility it may not be necessary to get the action right on the first attempt. If capuchins first execute a clumsy power grip, they can later correct for this mistake by using the more precise alternative. As squirrel monkeys are restricted to grasping objects between their fingers and palm, an initially clumsy grip could be difficult to correct and may result in the object falling from the monkey’s grasp (the object may become irretrievable if the monkey is in their natural, arboreal environment). The lack of flexibility in the hands of the squirrel monkey may have inadvertently selected for greater flexibility in their cognitive approach to this task. On the other hand (pun intended), the nimbler digits of the capuchin monkeys may make serious consideration of the “champagne-conundrum” unnecessary.

If this interpretation is correct, we are seeing a beautiful example of how the bodies of animals affect cognition. When we consider differences between a fish and a bird it is clear that their bodies facilitate a very different set of behaviours. However, we are less inclined to think like this when morphological differences are less striking. Non-human primates often possess a very similar body plan to us, and we may be especially tempted to interpret differences in behaviour as differences between evolved brains, not evolved bodies. There is no denying that the brain is an incredibly important organ, but it is not the only important organ. The take-home message from this lovely study is that when examining behaviour it is important to not get caught up in the obvious cognitive interpretations. The body and brain are intrinsically connected and they are terribly good at communicating with each other. When we consider the capabilities of the hand, or the foot, or the wing, we may find that the capabilities of the brain are not worth considering at all.


Stephan, H., Barbon, G., & Frahm, H. D. (1988). Comparative size of brains and brain components. In Steklis, H. D., & Erwin, J. (Eds.). Comparative Primate Biology. New York, NY: Wiley-Liss.

Zander, S. L., Weiss, D. J., & Judge, P. G. (2013). The interface between morphology and action planning: a comparison of two species of New World monkeys. Animal Behaviour, 86, 1251–1258.


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