Gabriella’s study, “Strategic exploration in a problem-solving task by capuchin monkeys (Sapajus apella),” has now finished! This blog post serves to document the phases of the experiment and how the monkeys performed.
With regards to background literature, several studies have found that animals use information seeking to locate food. For example, Visalberghi and Néel (2003) found capuchin subjects use sound and/or weight of a nut to infer its contents (full or empty) by tapping or dropping the nuts.
Similarly, Hanus and Call (2011) found chimpanzees identify a target food item (juice) faster when the discriminative variable was causal (weight), rather than arbitrary (color), by lifting the bottle stimuli to find out which of five was rewarded.
Figure 1a (above). Matoury, Sol, and Anita investigate the wrapped PVC pipes in their enclosure.
Figure 1b (left). 100 prepped PVC pipes for one monkey group, for one session!
The question therefore remains whether nonhuman primates will strategically explore stimuli to determine the location of food. This study thus aimed to test if capuchin monkeys, after experience with stimuli containing food that rattles, exhibit strategic exploration (i.e., shake stimuli) to locate food, and critically, how this compares to exploration in conspecifics exposed to similar stimuli that do not rattle.
Phase 1 of the experiment ran from May-June 2024. Twice a week, Gabriella prepared both groups of monkeys 100 PVC pipes wrapped four times in paper and connected to straps for the keepers to attach to the platforms in their enclosures (Figure 1a; 1b; video 1). In both monkey groups, a fifth of the pipes contained food inside (Figure 2), and in the East group, the food rattled when shaken, but in the West, the food made no noise (video 2). The idea behind this design was to determine whether the East (and not the West) monkeys would come to explore the pipes strategically (e.g., shake the pipes to hear the rattle and identify the food).
During each hour-long session, Gabriella stood outside with multiple cameras and live-coded the behavior of each monkey (Figure 3). This required identifying all individuals outside on each platform as they were all behaving simultaneously. Not an easy task!
After Phase 1, Phase 2 ran from July-September 2024. Here, individual monkeys were given their own bundle of five wrapped PVC pipes in the indoor cubicles. Similarly to Phase 1, one of the five pipes contained food, but in this phase, monkeys from both groups experienced food that rattled when shaken. The idea of this phase was to investigate whether the East monkeys (those with experience with the sound condition) could identify the food pipe quicker than West monkeys, and if so, if they had developed strategies to do so. Excitingly, some of the East may have developed strategies to identify the food pipe, such as Cayenne’s wobble method (video 3).
Video 3. To locate the pipe with food, Cayenne seems to wobble the pipes one at a time.
Critically, however, at this time we cannot say for certain if the East (and not West) developed strategic curiosity techniques to find the food. But watch this space for when Gabriella publishes the study towards her PhD to find out how the monkey groups compared!
