![]() 2009 certain seed-harvesting birds: Heinrich et al. 2008 Visalberghi and Néel 2003 Visalberghi et al. Kinesthetic perception of weight has been investigated in a handful of non-human animals (great apes: McCulloch 1941 Schrauf and Call 2011 Schrauf et al. ![]() An object’s weight can be directly assessed by kinesthetic feedback, in which the muscular effort required to move and lift the object is processed by the nervous system (Robinson 1964). Weight has paramount importance in determining how animals establish efficient interactions with physical objects, as it determines the grip and lifting force required (Johansson and Flanagan 2009). The pattern and complexity of object manipulation is thought to reflect the level of cognitive development as well as that of manual control (Byrne 2001 Hayashi 2015). Animals use different grasping organs (e.g., trunk, tongue, mouth, hands) to select, pick, transport and process food items, with a few species (especially among primates, but also elephants and birds) notable for their manipulative skills (e.g., Hayashi 2015 Martin and Niemitz 2003 Parker 1974 Rutz et al. Many animal species rely on object manipulation to accomplish tasks crucial for their survival, particularly in the context of food retrieval and processing. Our results show that chimpanzees anticipate the weight of an object using long-term force profiles and suggest that, similarly to humans, they use internal representations of weight to plan their lifting movements. After using a hammer to crack open one nut, chimpanzees tuned down the lifting acceleration for the hollowed hammers, but continued lifting natural hammers with the same acceleration. We found that, when lacking previous experience, chimpanzees lifted hollowed hammers with a higher acceleration than natural hammers (overshoot effect). We used calibrated videos from camera traps to extract kinematic parameters of lifting movements. To this end, we conducted a field experiment presenting chimpanzees with natural wooden hammers and artificially hollowed, lighter hammers of the same size and external appearance. In this study, we tested whether wild chimpanzees use long-term force profiles to anticipate the weight of a nut-cracking hammer from its size. However, despite enduring interest in tool use in non-human animals, there has been very little investigation of their ability to form an expectation about an object’s weight. ![]() Skillful object manipulation is crucial for many animals, particularly those that rely on tools for foraging. Such a long-term force profile, formed through repeated experiences with similar objects, has been proposed to improve manipulative performance. When humans are about to manipulate an object, our brains use visual cues to recall an internal representation to predict its weight and scale the lifting force accordingly. ![]()
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