Abstract
The extent to which tool-using animals take into account relevant task parameters is poorly understood. Nut cracking is one of the most complex forms of tool use, the choice of an adequate hammer being a critical aspect in success. Several properties make a hammer suitable for nut cracking, with weight being a key factor in determining the impact of a strike; in general, the greater the weight the fewer strikes required. This study experimentally investigated whether chimpanzees are able to encode the relevance of weight as a property of hammers to crack open nuts. By presenting chimpanzees with three hammers that differed solely in weight, we assessed their ability to relate the weight of the different tools with their effectiveness and thus select the most effective one(s). Our results show that chimpanzees use weight alone in selecting tools to crack open nuts and that experience clearly affects the subjects’ attentiveness to the tool properties that are relevant for the task at hand. Chimpanzees can encode the requirements that a nut-cracking tool should meet (in terms of weight) to be effective.
Highlights
Wild chimpanzees display a variety of tool-using behaviors
Nut-cracking behavior is found in several communities of wild chimpanzees (Guinea: [4], [8], [9]; Ivory Coast: [10], [11], [12]; Liberia: [13], [14]; Sierra Leone: [15])
Loi and Zamba, differentiated between three visually identical hammers differing only in weight to crack open a nut. These two subjects showed a preference for a certain hammer weight in their overall tool choice, and this selectivity emerged from the chimpanzees experiencing the differences in tool effectiveness
Summary
Wild chimpanzees display a variety of tool-using behaviors. Among these, nut cracking has been considered as one of the most complex forms [1,2,3]. In East and Central Africa, nut cracking is completely absent [16], [17], apart from a single population of chimpanzees that live east from the Ivory Coast [18]. This is intriguing given that all the necessary elements (nut species, supply of stones, sticks and roots for anvils) are available [19] ecological factors alone cannot explain the absence of this behavior [20]. Even within nutcracking communities, some individuals never acquire the skill [5]
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