Abstract
New Caledonian crows were presented with Bird and Emery's (2009a) Aesop's fable paradigm, which requires stones to be dropped into a water-filled tube to bring floating food within reach. The crows did not spontaneously use stones as tools, but quickly learned to do so, and to choose objects and materials with functional properties. Some crows discarded both inefficient and non-functional objects before observing their effects on the water level. Interestingly, the crows did not learn to discriminate between functional and non-functional objects and materials when there was an arbitrary, rather than causal, link between object and reward. This finding suggests that the crows' performances were not based on associative learning alone. That is, learning was not guided solely by the covariation rate between stimuli and outcomes or the conditioned reinforcement properties acquired by functional objects. Our results, therefore, show that New Caledonian crows can process causal information not only when it is linked to sticks and stick-like tools but also when it concerns the functional properties of novel types of tool.
Highlights
Chimpanzees use tools in the wild flexibly
New Caledonian crows did not spontaneously use stones as tools by dropping them into the water-filled tube to bring floating food within reach. This indicates that the crows did not have a priori knowledge that dropping stones into the tube would raise the water level
After observing how stones falling into the water affected the water level and position of the floating food, the crows solved the task
Summary
Chimpanzees use tools in the wild flexibly (for review see [1]). That is, tools are used in different contexts, such as foraging, grooming, and social interactions, and many different types of natural material, including stone, are involved in tool use and manufacture. New Caledonian crows (Corvus moneduloides) show sophisticated tool behaviours both in the wild [2,3,4,5] and in captivity [6,7,8,9,10], their natural tools are manufactured from plant material and are only used during foraging. Their tool behaviour is, much more context-specific than that of chimpanzees. Two otherwise non-tool using species, rooks and Eurasian jays, use stones to obtain out-of-reach food and can learn to discriminate between functional and non-functional objects and substrates [13,14,15]
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