Many animal species rely on object manipulation to accomplish tasks crucial for their survival, particularly in the context of food retrieval and processing. 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. 2016). 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).

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). 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). 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. 2012; capuchin monkeys: Schrauf et al. 2008; Visalberghi and Néel 2003; Visalberghi et al. 2009; certain seed-harvesting birds: Heinrich et al. 1997; Langen 1999). It has been shown that, at least in humans and chimpanzees, kinesthetic feedback perceived during movement is stored in short-term motor memory and can be recalled during repeated manipulations of the same object (Kent 2006; Povinelli 2012). In addition, humans are known to anticipate the weight of an object prior to any kinesthetic interaction with it: by using a cluster of visual stimuli (e.g., size, texture, color), our brains can recall from memory an anticipatory representation of the weight of an object we have never interacted with before (Buckingham et al. 2009; Gallivan et al. 2014; Gordon et al. 1993; van Polanen and Davare 2015). This representation (described as a ‘long-term force profile’ by Povinelli 2012 and as an ‘internal model’ by Krakauer and Shadmehr 2006) is formed through a generalization of repeated previous experiences with similar objects and is rapidly updated using kinesthetic information acquired during the movement itself if the predicted weight does not match the subsequent perception (Flanagan and Beltzner 2000; van Polanen and Davare 2015). Long-term force profiles have been proposed to be necessary for dexterous object manipulation by ensuring a stable grasp, determining speed and precision in execution and producing a smooth lift, as kinesthetic feedback mechanisms alone are generally too slow (Johansson and Flanagan 2009; Johansson and Westling 1984, 1988; van Polanen and Davare 2015).

The use of tools is regarded as one of the most sophisticated forms of object manipulation, as tool users must establish a dynamic spatial relation between at least two objects (Fragaszy et al. 2004; Hayashi 2015; Matsuzawa 1996). Tool use allows for a more efficient exploitation of available resources (Boesch and Boesch 1993; Möbius et al. 2008; Shumaker et al. 2011; Tebbich et al. 2002), with the selection of a suitable tool and its precise manipulation drastically influencing the overall outcome (Fragaszy et al. 2010; Luncz et al. in press). It is thus not surprising that the ability to form internal representations for object weight is well documented in humans, the species with the most outstanding technological achievements, and it seems likely that the same ability is found in animals that routinely engage in object selection and skillful object manipulation. Chimpanzees, one of our closest living relatives, are indicated by some authors to share with humans the cognitive machinery for dealing with their physical world (e.g., Boesch and Boesch 1990; Goodall 1970; Tomasello and Herrmann 2010). In the wild, all known chimpanzee populations have been observed to use and manufacture tools, showing a diverse and highly complex repertoire of tool use (McGrew 2010). Surprisingly, despite enduring interest in tool use in chimpanzees, as well as other non-human animals (Sanz et al. 2013), there has been very little investigation of their ability to form long-term force profiles to derive expectations about an object’s weight from visual cues.

Hanus and Call (2008) have shown that captive chimpanzees can infer the location of a food item based on the effect it exerts upon a balance and thus use a dynamic visual stimulus to identify the heavier of two objects. However, these authors did not directly investigate whether chimpanzees, like humans, form long-term force profiles and apply them when interacting with objects. Typically, studies on the anticipation of weight in humans address the appreciation of a relationship between size and weight by asking experimental subjects to lift objects of different sizes but the same weight, and measuring lifting (or grip) forces and/or lifting accelerations (e.g. Gordon et al. 1991; Flanagan and Beltzner 2000; Rabe et al. 2009). In this type of experiment, anticipation of weight based on size is revealed by an excess of force/acceleration in grasping/lifting the larger object compared to the smaller object (‘overshoot’), which tends to disappear after a few trials as the subjects become familiar with the actual weights. To our knowledge, the only study of this kind in non-human animals was conducted by Povinelli (2012) and did not provide conclusive results. In that study, laboratory chimpanzees were trained to displace two boxes of different sizes but the same weight and the maximum height of each displacement was measured. As predicted by the weight anticipation hypothesis, chimpanzees lifted the larger box higher than the smaller box. However, this difference persisted in subsequent trials, after chimpanzees had obtained kinesthetic information about the actual weights of the boxes. The latter result suggests that the difference in box size acted as a confounding variable in this experiment. While human subjects can be asked to lift objects according to the experimenter’s requirements, Povinelli’s (2012) chimpanzees were performing a non-goal-oriented task, and they might have been motivated to lift the larger box higher regardless of their anticipation of its weight. Even more importantly, the power of this study might have been limited by the choice of height (i.e., the final result of the movement) as the index of a subject’s expectations. In fact, in humans, expectations about object weight become visible in their motor output during the initial phase of lifting, in the form of early peaks in the lifting force (Flanagan and Beltzner 2000). Finally, Povinelli’s (2012) experiments involved captive chimpanzees which had to familiarize themselves with a limited set of unusual objects before using them in a non-goal-oriented task, but the ability to form ‘long-term force profiles’ via generalization of past experiences is probably better investigated by observing animals interacting with objects used in daily life that belong to a well-defined functional category, including thousands of individual objects that are, or could be, used in a goal-oriented and ecologically relevant routine activity.

In the present study, we proposed a new test of whether chimpanzees use internal representations (long-term force profiles) to anticipate the weight of novel objects of a known category based on their size. We tackled the main weaknesses of previous studies by performing a field experiment, involving wild chimpanzees dealing with objects of a highly familiar functional category that are lifted to fulfill a well-determined goal, and using camera-trap video recordings to measure lifting accelerations.

Chimpanzees (Pan troglodytes verus) of the Taï forest, Côte d’Ivoire, habitually crack open several species of hard-shelled nuts by placing them on anvils (hard roots, stones or branches within a tree) and pounding them with wooden or stone hammers (Boesch and Boesch 1982; Visalberghi et al. 2015; weight range 0.2–15 kg). Nut-cracking has been described as one of the most complex forms of tool use in non-human animals, as it implies the establishment of two spatial relationships among three objects (hammer, nut and anvil), requires bimanual (and sometimes foot) coordination and a high level of motor control (Boesch and Boesch 1993; Bril et al. 2009; Matsuzawa 1996). The nut-intake rate is influenced both by the number of strikes needed to open a nut and by the precision of hammering (Luncz et al. in press; Sirianni et al. 2015). Hammer weight clearly influences the efficiency of a nut-cracking session (Boesch et al. 2017; Fragaszy et al. 2010; Luncz et al. in press), and chimpanzees have been shown to be sensitive to the functionality of this physical property by selecting for hammer weight (laboratory experiments: Schrauf et al. 2012; field observations: Boesch and Boesch 1982, 1984; Luncz et al. 2012), and by doing so in a sophisticated, conditional way (assessed in wild chimpanzees by Sirianni et al. 2015). Nut-cracking movements by wild chimpanzees thus appear to be an ideal model for investigating the anticipation of weight in non-human animals.

We performed our field experiment in the Taï forest, providing wild chimpanzees with nut-cracking hammers of a familiar material commonly available in the forest litter (Coula wood). All hammers were of the same size. Some hammers, however, were artificially hollowed so as to be lighter than natural, solid hammers.

We compared lifting accelerations for the modified (hollowed) and natural (solid) hammers, predicting that, if chimpanzees anticipate the weight of hammers based on their size, they should initially (when they lift each hammer for the first time) apply a similar force to the solid and the hollowed hammers, which would result in a higher acceleration for the latter type (‘acceleration overshoot’). Moreover, we predicted that, as chimpanzees learn about the actual weights of the two hammer types (e.g. after cracking one nut open), they would continue to apply the same force (and hence obtain the same acceleration) to natural hammers, but would instead ‘tune down’ the force used for hollowed hammers (lighter than expected), so that the acceleration overshoot would tend to fade away. Indeed, observing such a ‘tuning down’ for hollowed hammers, but not for natural hammers, would rule out the only possible alternative explanation for the initial overshoot (i.e., that chimpanzees apply a fixed ‘standard’ force when they first lift a hammer, regardless of its appearance and size), thus conclusively demonstrating anticipation of weight and long-term force profiles in chimpanzees.