Stacking-block tasks have been applied to assess physical understanding in the context of combinatory manipulation [21,22]. Stacking-block tasks enable a direct comparison to be made between captive chimpanzees and human children in an identical test setting. Subjects received a set of four blocks (two cubic blocks and two blocks of a different shape) to be stacked up into a tower. To efficiently stack up the differently shaped blocks, the subjects were required to consider the shape and orientation of the blocks. In other words, subjects needed to selectively use appropriate orientation for stacking and to ensure that the top surface would support the next block. Both juvenile chimpanzees and human children of around 3 years of age were found to be capable of learning an efficient stacking strategy through their manipulation of blocks. Nut cracking is a rare example of behaviour that resembles stacking behaviour, as the nut should be appropriately placed on a surface that can support it while it is being hit by a hammer stone. Thus, a precise analysis of nut-cracking behaviour is essential to investigate the physical understanding of wild chimpanzees and to compare it with captive chimpanzees and/or humans.
Approximately 4 years after the first exposure to the nut-cracking behaviour, the same three individuals accompanied by their offspring participated in follow-up training sessions of nut cracking in a similar setting (figure 1). As shown in figure 2, two chimpanzees succeeded in cracking nuts in the follow-up training sessions. One of the chimpanzees had not succeeded in the first test session with human demonstration, but succeeded in cracking open nuts during the first follow-up session without any human assistance. The opportunity to perform nut-cracking behaviour had not been provided during the 4-year hiatus, thus revealing that the chimpanzees retained the procedural memory without receiving any direct reward for a substantially long time. The two successful subjects continued to crack open nuts, although they sometimes lost motivation, partly because of interference from their offspring.
M.objects Crack
A pair of captive chimpanzees: mother and offspring manipulating stones and nuts in a follow-up training session, in a nut-cracking situation originally reported in Hayashi et al. [23] (photo by Akihiro Hirata of Mainichi Newspaper). (Online version in colour.)
The number of nuts cracked open by the subjects in each session, including the first test session and the 10 sessions at the beginning of the follow-up training session that was originally reported in Hayashi et al. [23]. A chimpanzee, Pan, succeeded in cracking nuts from the first test session. Although a chimpanzee, Chloe, failed in the first test session, she succeeded in the first follow-up session conducted after a hiatus of approximately 4 years.
In the Bossou community, chimpanzee infants first succeed to crack open oil palm nuts from around 3.5 to 7 years old. Inoue-Nakamura & Matsuzawa [29] analysed precisely the manipulation of stones and nuts in infant chimpanzees before their first success. Chimpanzee infants show all of the fundamental actions necessary to perform nut cracking at the age of 2.5 years. However, they fail to combine fundamental actions in an appropriate sequence until they reach 3.5 years or more. The hitting action is performed in various combinations, such as hitting a nut on stone by hand, or hitting a nut on stone with a nut.
A comparison of chimpanzees and capuchins illuminates a marked difference in the pattern of development in the two nut-cracking species in terms of two types of behaviour. Chimpanzee infants of 1.5 years readily place nuts on an anvil stone and demonstrate the hitting action, but they rarely hit a nut with a hammer stone until they reach around 3.5 years. In the case of capuchin monkeys [30], they first demonstrate the hitting action before beginning to place nuts on an anvil stone. Thus, the order of acquisition of the two fundamental actions of placing and hitting with a hammer stone is completely reversed in chimpanzees and capuchins. This may indicate that capuchins have intrinsic tendencies to engage in hitting actions. By contrast, chimpanzees acquire hitting/percussive actions in line with other types of object manipulation during the course of cognitive development.
Hirata et al. [24] used anvil stone embedded in the ground of an outside enclosure. The hammers were attached to the anvils with chains. In this setting, one infant first succeeded when she was 1 year and 11 months of age [33], much earlier than has been recorded in wild chimpanzees. The setting of the objects may have helped the infant to focus on the task-related objects and to facilitate the exploratory manipulation of the anvils, hammers and nuts. Moreover, the infant had ample opportunity to observe the nut-cracking behaviour of experienced adults, including the mother. Abundant experiences in manipulative exploration in a physically- and socially enriched environment may have played an important role in the early acquisition of nut-cracking behaviour in the infant. Immediately after the first success, the infant was tested further, with detached stones in an experimental room. The infant readily transferred the knowledge gained from the embedded anvils in the outside enclosure to the use of a pair of movable stones as anvil and hammer.
A sequential analysis of nut-cracking behaviour was conducted using this notation system. As all of the stones in the Bossou field-experiment site had been identified and numbered, the number of stones was put in the two numeral positions, before and after the alphabetical code for action (n1 n2). An unbroken nut was given the number 0 and the kernel and shells of a broken nut were numbered 1 and 2, respectively. The action codes are listed in table 1. Using this notation system [40], the sequence of nut-cracking behaviour can be encoded sequentially. The sequential codes can then be used to analyse the transition patterns of manipulatory actions. Figure 4 shows the sequential codes and transition patterns identified in an adult female chimpanzee and a juvenile offspring who had just begun to succeed in nut cracking. The sequential codes shown in the figure were taken from a 10 min video (some parts of the video are provided as electronic supplementary material, Movie S1). The adult chimpanzee showed an efficient pattern of sequential behaviour. She first grasped the hammer and adjusted the angle of the anvil stone. Once she had it settled, she showed an efficient chain of behaviour: pick up nut, put nut on anvil, hit nut on anvil with hammer, contact hammer with anvil (until the next percussions), pick up kernel of opened nut and eat the kernel. She used her left hand for hammering and her right to adjust the anvil and manipulate the nut or kernel. Thus, she showed consistent laterality and highly sophisticated coordination of both hands. Conversely, the juvenile was less efficient: the nut sometimes fell from the anvil during percussion, and his percussion was not sufficient, leading him to hit the nut on the anvil with the hammer many times without successfully opening it. He sometimes switched the hammering hand after failures.
Chimpanzees seem to adopt different behavioural strategies to overcome difficulties in cracking open nuts (figure 5). Younger chimpanzees tend to show ineffective strategies, such as putting multiple nuts on an anvil simultaneously or switching the hammering hand. Younger chimpanzees also tend to try many behavioural strategies even if the strategy is not directly related with the error to be solved. For example, when a nut falls from the anvil, a young chimpanzee may respond to this error by changing hammer stone. Adult chimpanzees develop some behavioural-adjusting strategies that are effective to solve the specific error that they are facing, such as changing to a heavier hammer after consecutive hit errors with a lighter hammer or changing the angle of an anvil by rotating or pushing it after a nut falls from the anvil. However, we should be careful when considering the underlying mechanisms of these behavioural strategies. Adult chimpanzees may acquire such strategies through cumulative experiences of nut cracking. In other words, adults may alternatively use error-correction strategies that previously proved to be successful, such as adjusting the anvil by rotation, changing the hammer or hitting with more power, without appropriate physical understanding as indicated in the second row in figure 5.
Schematic flow of behavioural strategies in the context of nut cracking. Rectangles show observable events and behavioural strategies taken by chimpanzees. Oval-dotted rectangles are not observable cause of the events. The cause and observable events/strategies are interconnected based on the theoretical assumption and probability. The top row shows two major errors occurs in nut-cracking sequence. The second row shows the possible causes of the errors (note that these causes are not obvious and recognizable for chimpanzees). The third row shows the actual behavioural strategies observed after the errors. The fourth row shows the result of behavioural strategies taken by the old female chimpanzee during her metatool formation (discarded hammer worked as a wedge stone). (Online verison in colour.)
Sequential analyses of actions involved in nut-cracking behaviour revealed the behavioural strategies adopted by wild chimpanzees when they face to errors in its execution. Efficient coping strategies to deal with errors appeared later in their lives, which suggests that these may be gradually acquired after long-term interaction with the objects and the achievement of an efficient level of skill [16,44]. Future studies are needed in order to accumulate more observational and developmental data, as not so many cases of wedge-stone use have been reported in chimpanzees, and the actual process of making wedges has not been reported elsewhere. The sequential and grammatical analyses focusing on error-correction strategy will be useful in helping to answer the question of the existence of causal understanding and insightful intentionality (such as active insertion of a wedge stone as occurs in humans) during the complex tool use observed in chimpanzees. 2ff7e9595c
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