Chimpanzee ‘folk physics’: bringing failures into focus

Differences between individuals are the raw material from which theories of the evolution and ontogeny of cognition are built. For example, when 4-year-old children pass a test requiring them to communicate the content of another''s falsely held belief, while 3-year-olds fail, we know that something must change over the course of the third year of life. In the search for what develops or evolves, the typical route is to probe the extents and limits of successful individuals'' ability. Another is to focus on those that failed, and find out what difference or lack prevented them from passing the task. Recent research in developmental psychology has harnessed individual differences to illuminate the cognitive mechanisms that emerge to enable success. We apply this approach to explaining some of the failures made by chimpanzees when using tools to solve problems. Twelve of 16 chimpanzees failed to discriminate between a complete and a broken tool when, after being set down, the ends of the broken one were aligned in front of them. There was a correlation between performance on this aligned task and another in which after being set down, the centre of both tools was covered, suggesting that the limiting factor was not the representation of connection, but memory or attention. Some chimpanzees that passed the aligned task passed a task in which the location of the broken tool was never visible but had to be inferred.     

A potent effect of observational learning on chimpanzee tool construction

Although tool use occurs in diverse species, its complexity may mark an important distinction between humans and other animals. Chimpanzee tool use has many similarities to that seen in humans, yet evidence of the cumulatively complex and constructive technologies common in human populations remains absent in free-ranging chimpanzees. Here we provide the first evidence that chimpanzees have a latent capacity to socially learn to construct a composite tool. Fifty chimpanzees were assigned to one of five demonstration conditions that varied in the amount and type of information available in video footage of a conspecific. Chimpanzees exposed to complete footage of a chimpanzee combining the two components to retrieve a reward learned to combine the tools significantly more than those exposed to more restricted information. In a follow-up test, chimpanzees that constructed tools after watching the complete demonstration tended to do so even when the reward was within reach of the unmodified components, whereas those that spontaneously solved the task (without seeing the modification process) combined only when necessary. Social learning, therefore, had a powerful effect in instilling a marked persistence in the use of a complex technique at the cost of efficiency, inhibiting insightful tool use.     

Do Chimpanzees Use Weight to Select Hammer Tools?

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.     

Social tradition and the use of tool-composites by wild chimpanzees

Many different kinds of tool use by wild chimpanzees (Pan troglodytes) in their natural habitat have been documented over the last 30 years.¹ Most instances involve the use of a single type of tool for a single task. Even when a chimpanzee uses more than one tool for a single target, the tools usually are used to perform the same function; for example, when the first object employed to perform a task breaks, it is replaced by a similar object. Use of more than one kind of tool for a single task, a tool-composite, by wild chimpanzees (Pan troglydytes) demonstrates high intelligence and motor control which requires foresight, understanding of relations among tools and task, and behavioral coordination. Application of tool-composites has been reported infrequently and may be due to their use in complicated environmental and situational contexts which chimpanzees encounter less frequently throughout their daily activities. © 1997 Wiley-Liss, Inc.     

Transport of Functionally Appropriate Tools by Capuchin Monkeys (Cebus apella)

Capuchin monkeys (Cebus sp.) are notable among New World monkeys for their widespread use of tools. Like chimpanzees, they use both hammer tools and insertion tools in the wild to acquire food that would be unobtainable otherwise. Recent evidence indicates that capuchins transport stones to anvil sites and use the most functionally efficient stones to crack nuts. We further investigated capuchins’ assessment of functionality by testing their ability to select a tool that was appropriate for two different tool‐use tasks: A stone for a hammer task and a stick for an insertion task. To select the appropriate tools, the monkeys investigated a baited tool‐use apparatus (insertion or hammer), traveled to a location in their enclosure where they could no longer see the apparatus, made a selection between two tools (stick or stone), and then could transport the tool back to the apparatus to obtain a walnut. We incorporated tool transport and the lack of a visual cue into the design to assess willingness to transport the tools and the monkeys’ memory for the proper tool. Six brown capuchins (Cebus apella) were first trained to select and use the appropriate tool for each apparatus. Four animals completed training and were then tested by allowing them to view a baited apparatus and then travel to a location 8 m distant where they could select a tool while out of view of the apparatus. All four monkeys chose the correct tool significantly more than expected and transported the tools back to the apparatus. Results confirm capuchins’ propensity for transporting tools, demonstrate their capacity to select the functionally appropriate tool for two different tool‐use tasks, and indicate that they can retain the memory of the correct choice during a travel time of several seconds.     

The formation of the brush-sticks: modification of chimpanzees or the by-product of folding?

Based on field research and experimental treatments of trees, we investigated the formation of the brush-like shape of digging sticks used by chimpanzees (Pan troglodytes troglodytes). Evidence obtained in the field consisted of digging sticks found in Mboete, Equatorial Guinea, which is a newly reported locality for this type of tool, and Campo, Cameroon. Digging sticks used by chimpanzees in these areas had a brush-like shape at one end, which was quite different from the other end that was probably used for digging. In our tree-breaking experiment, 8 out of 17 species acquired a typical brush-like shape without human modification when broken off, and the shapes of the stumps were similar to those found in the field. Other species did not acquire the brush-like shape naturally or even after human modifications, and the stumps had different shapes from those found in the field. Our findings suggest that the brush-like shapes of digging sticks are often naturally formed when broken off from trees, depending on the nature of the fibre structure, and that the brush-like end is not used as the digging tool. We conclude that the vegetation surrounding termite mounds might influence how chimpanzees combine different types of tools, i.e., digging stick, brush-stick and fishing tool, for obtaining termites.     

Context-dependent tool use in New Caledonian crows

Humans and chimpanzees both exhibit context-dependent tool use. That is, both species choose to use tools when food is within reach, but the context is potentially hazardous. Here, we show that New Caledonian crows used tools more frequently when food was positioned next to a novel model snake than when food was positioned next to a novel teddy bear or a familiar food bowl. However, the crows showed no significant difference in their neophobic reactions towards the teddy bear and the model snake. Therefore, the crows used tools more in response to a risky object resembling a natural predator than to a less-threatening object that provoked a comparable level of neophobia. These results show that New Caledonian crows, like humans and chimpanzees, are capable of context-dependent tool use.     

Factors determining tool-using ability in two captive chimpanzee (Pan troglodytes) colonies

The influence of several determinants of performance in a reaching tool task were studied: rearing history, object experience, and gender. Forty-five chimpanzees between the ages of 7 and 36 years served as subjects. They were chosen from two facilities, each group having different levels of tool experience. Each group was made up of individuals from three rearing conditions: wild born, captive mother reared, and captive nursery reared. Results indicated that wild-born subjects were better at the task than were both groups of captive-born subjects, who performed similarly. Previous experience with reaching tools was not a significant factor in the determination of tool-using ability. Gender differences were not apparent for either group, and the range of ages of chimpanzees tested was not related to tool-using ability.     

Comparative investigations of manual action representations: evidence that chimpanzees represent the costs of potential future actions involving tools

The ability to adjust one's ongoing actions in the anticipation of forthcoming task demands is considered as strong evidence for the existence of internal action representations. Studies of action selection in tool use reveal that the behaviours that we choose in the present moment differ depending on what we intend to do next. Further, they point to a specialized role for mechanisms within the human cerebellum and dominant left cerebral hemisphere in representing the likely sensory costs of intended future actions. Recently, the question of whether similar mechanisms exist in other primates has received growing, but still limited, attention. Here, we present data that bear on this issue from a species that is a natural user of tools, our nearest living relative, the chimpanzee. In experiment 1, a subset of chimpanzees showed a non-significant tendency for their grip preferences to be affected by anticipation of the demands associated with bringing a tool's baited end to their mouths. In experiment 2, chimpanzees' initial grip preferences were consistently affected by anticipation of the forthcoming movements in a task that involves using a tool to extract a food reward. The partial discrepancy between the results of these two studies is attributed to the ability to accurately represent differences between the motor costs associated with executing the two response alternatives available within each task. These findings suggest that chimpanzees are capable of accurately representing the costs of intended future actions, and using those predictions to select movements in the present even in the context of externally directed tool use.     

Chimpanzees prey on army ants with specialized tool set

Several populations of chimpanzees have been reported to prey upon Dorylus army ants. The most common tool‐using technique to gather these ants is with “dipping” probes, which vary in length with regard to aggressiveness and lifestyle of the prey species. We report the use of a tool set in army ant predation by chimpanzees in the Goualougo Triangle, Republic of Congo. We recovered 1,060 tools used in this context and collected 25 video recordings of chimpanzee tool‐using behavior at ant nests. Two different types of tools were distinguished based on their form and function. The chimpanzees use a woody sapling to perforate the ant nest, and then a herb stem as a dipping tool to harvest the ants. All of the species of ants preyed upon in Goualougo are present and consumed by chimpanzees at other sites, but there are no other reports of such a regular or widespread use of more than one type of tool to prey upon Dorylus ants. Furthermore, this tool set differs from other types of tool combinations used by chimpanzees at this site for preying upon termites or gathering honey. Therefore, we conclude that these chimpanzees have developed a specialized method for preying upon army ants, which involves the use of an additional tool for opening nests. Further research is needed to determine which specific ecological and social factors may have shaped the emergence and maintenance of this technology. Am. J. Primatol. 72:17–24, 2010. © 2009 Wiley‐Liss, Inc.     

Tool-use to obtain honey by chimpanzees at Bulindi: new record from Uganda

Honey-gathering from bee nests has been recorded at chimpanzee (Pan troglodytes) study sites across tropical Africa. Different populations employ different strategies, ranging from simple ‘smash-and grab’ raids to use of sophisticated tool-sets, i.e., two or more types of tool used sequentially in a single task. In this paper I present evidence of tool-use, and the probable use of a tool-set, for honey-gathering by unhabituated chimpanzees at Bulindi, a forest–farm mosaic south of the Budongo Forest in Uganda. Between June and December 2007, 44 stick tools were found in association with 16 holes dug in the ground, corresponding to the period when stingless bees (Meliponula sp.) appeared in chimpanzee dung. In 11 cases the confirmed target was a Meliponula ground nest. Two potential tool types were distinguished: digging sticks encrusted with soil, and more slender and/or flexible sticks largely devoid of soil that may have functioned to probe the bees’ narrow entry tubes. Reports of chimpanzees using tools to dig for honey have been largely confined to Central Africa. Honey-digging has not previously been reported for Ugandan chimpanzees. Similarly, use of a tool-set to obtain honey has thus far been described for wild chimpanzee populations only in Central Africa. Evidence strongly suggests that Bulindi chimpanzees also use sticks in predation on carpenter bee (Xylocopa sp.) nests, perhaps as probes to locate honey or to disable adult bees. These preliminary findings from Bulindi add to our understanding of chimpanzee technological and cultural variation. However, unprotected forests at Bulindi and elsewhere in the region are currently severely threatened by commercial logging and clearance for farming. Populations with potentially unique behavioral and technological repertoires are being lost.     

Wild‐caught great tits Parus major fail to use tools in a laboratory experiment,despite facilitation

Studies investigating tool use in animals that are not known tool users in the wild are important in helping to understand how and under what circumstances this ability might arise. Tool use appears to be uncommon in great tits (Parus major), with only a single documented observation in which a wild great tit used conifer needles to extract larvae from crevices in trees. In a laboratory‐based experiment, we examined whether wild‐caught great tits could learn to use tools in a similar manner. We presented the birds with two different tool use tasks in which they would need to use either a stick or a hook to extract an otherwise inaccessible meal worm from a transparent plastic tube. First, the birds passed a simpler training criterion (pulling a tool with an attached food reward) that aimed to reduce the difficulty of the task. Nevertheless, none of the individuals learnt to use tools in either of the two tasks. This result stands in stark contrast to the abilities of some corvids and parrots, which can learn to use tools in captivity, even though some of them are not tool users in the wild. We believe that tool use might be difficult for some birds to learn since the skills required for this ability seem not to be part of their natural foraging behaviour.     

Tool use by captive chimpanzees at an artificial termite mound

This study examines tool use by a colony of captive chimpanzees at an artificial termite mound. The mound, constructed of concrete, simulates the termite mounds which are used as food sources by wild chimpanzees who extract the termites using grass or twig-type tools. In the present study, tool availability was manipulated, specifically the type of tool, and the distance of tool material from the mound. The type of food available in the mound was also varied. Tool-making and tool-using behavior was examined in relation to individual, age, and sex differences. The artificial mound proved to be a viable simulation of the naturally occurring mounds, with most of the chimpanzees exploiting the food in the mound by using tools over the period of study. Interesting individual differences emerged in the way that the chimpanzees selected and used tools, some preferring to move some distance from the mound to collect “off-the-peg” tools, others preferring to sit and fashion a tool from material available nearer the mound. Also, some chimpanzees used both ends of a tool, while others used only one end. There were significant age differences in activity at the mound, the younger chimpanzees spending more time at the mound, using tools previously used by others, and manipulating the mound holes manually. Sex differences, although not significant, were apparent. The artificial mound provides the chimpanzees with a stimulating and rewarding activity, interest and enjoyment for the public, and an opportunity for researchers to study tool use under more controlled conditions than are possible in the field.     

Complex tool sets for honey extraction among chimpanzees in Loango National Park, Gabon

Homo faber was once proposed as a label for humans specifically to highlight their unique propensity for tool use. However, new observations on complex tool use by the chimpanzees of Loango National Park, Gabon, expand our knowledge about tool-using abilities in Pan troglodytes. Chimpanzees in Loango, when using tools to extract honey from three types of bee nests, were observed to regularly use three- to five-element tool sets. In other words, different types of tools were used sequentially to access a single food source. Such tool sets included multi-function tools that present typical wear for two distinct uses. In addition, chimpanzees exploited underground bee nests and used ground-perforating tools to locate nest chambers that were not visible from the ground surface. These new observations concur with others from Central African chimpanzees to highlight the importance of honey extraction in arguments favoring the emergence of complex tool use in hominoids, including different tool types, expanded tool sets, multifunction tools, and the exploitation of underground resources. This last technique requires sophisticated cognitive abilities concerning unseen objects. A sequential analysis reveals a higher level of complexity in honey extraction than previously proposed for nut cracking or hunting tools, and compares with some technologies attributed to early hominins from the Early and Middle Stone Age. A better understanding of similarities in human and chimpanzee tool use will allow for a greater understanding of tool-using skills that are uniquely human.     

Use of tool sets by chimpanzees for multiple purposes in Moukalaba-Doudou National Park,Gabon

We report our recent findings on the use of tool sets by chimpanzees in Moukalaba-Doudou National Park, Gabon. Direct observations and evidences left by chimpanzees showed that chimpanzees used sticks as pounders, enlargers, and collectors to extract honey from beehives of stingless bees (Meliponula sp.), which may correspond to those previously found in the same site for fishing termites and to those found in Loango National Park, Gabon. However, we observed chimpanzees using a similar set of tools for hunting a medium-sized mammal (possibly mongoose) that hid inside a log. This is the first report of hunting with tools by a chimpanzee population in Central Africa. Chimpanzees may recognize the multiple functions and applicability of tools (extracting honey and driving prey), although it is still a preliminary speculation. Our findings may provide us a new insight on the chimpanzee’s flexibility of tool use and cognitive abilities of complex food gathering.     

Chimpanzees' choices of prey between two sympatric species ofMacrotermes in the Campo Animal Reserve,Cameroon

Details are presented relating to chimpanzees' choices between two sympatric species of termites,Macrotermes lillijeborgi andM. vitrialatus, as food in the Campo Animal Reserve, southwest Cameroon, West Africa. An attempt was made to determine the various factors that affected such choices. The two species of termites seemed to have almost the same value in terms of ecological factors. However, chimpanzees fed almost exclusivelyonM. lillijeborgi, using digging sticks and fisching probes, during the study period which extended from the end of August to the middle of January, with their feeding activity showing peak at the beginning of the rainy season. By contrast,M. vitrialatus was rarely eaten in spite of the ease with which such prey could be obtained, namely, by desctruction of termite mounds by hand, without the need for tools. The reason that the chimpanzees discriminated between the two species of termite cannot be explained in terms of ecological factors such as size of prey, seasonal differences in termite activity, etc. Sticks used as tools were fairly uniform in size and character, mainly because of physical constraints related to the structure of termite mounds, and the brush-like ends of sticks seemed to be incidentaly byproducts of the chimpanzee's choice of plant species. Ecological factors could provide chimpanzees with a basis for the use of some kind of tool and help them modify it, while other factors, for example, something akin to leisure or the chimpanzee's interest in use of a tool, could provide an opportunity for inventing some tool-using behavior or for maintaining such behavior. These different factors, not being exclusive of one another, might affect the invention and maintenance of tool using-behavior at different phases. It is possible that chimpanzee's choice of prey may not always be the most efficient or appropriate in a given ecological situation.     

How similar are nut-cracking and stone-flaking? A functional approach to percussive technology

Various authors have suggested similarities between tool use in early hominins and chimpanzees. This has been particularly evident in studies of nut-cracking which is considered to be the most complex skill exhibited by wild apes, and has also been interpreted as a precursor of more complex stone-flaking abilities. It has been argued that there is no major qualitative difference between what the chimpanzee does when he cracks a nut and what early hominins did when they detached a flake from a core. In this paper, similarities and differences between skills involved in stone-flaking and nut-cracking are explored through an experimental protocol with human subjects performing both tasks. We suggest that a ‘functional’ approach to percussive action, based on the distinction between functional parameters that characterize each task and parameters that characterize the agent''s actions and movements, is a fruitful method for understanding those constraints which need to be mastered to perform each task successfully, and subsequently, the nature of skill involved in both tasks.     

Drinking tools of wild chimpanzees at Bossou

Use of drinking tools by wild chimpanzees (Pan troglodytes) and the context in which the tools were used were studied at Bossou, Republic of Guinea, West Africa. During the middle to late dry season and early wet season liquids are available occasionally in the holes of trees. Chimpanzees drank water or sap using a leaf (or fiber) as a sponge or spoon. When the chimpanzees were on the ground, they tended to use one of a few kinds of soft, hairless leaves, if they were available nearby. Females, particularly juveniles and adolescents, were thought to be the main users of the drinking tool. In a few episodes, a tool set was used to procure liquid. Once a chimpanzee used a stick to push a leaf sponge into a water hole and to pull it out from the hole. In addition, three chimpanzees used a pestle to squeeze sap from an oil-palm tree before using a fiber sponge. © 1995 Wiley-Liss, Inc.     

Chimpanzees (Pan troglodytes) tolerate some degree of inequity while cooperating but refuse to donate effort for nothing

In cooperative hunting, a carcass cannot be divided equally, and hunts may be unsuccessful. We studied how chimpanzees respond to these two variables, working for unequal rewards and no rewards, which have been rarely included in experimental cooperative tasks. We presented chimpanzees with a task requiring three chimpanzees to work together and varied the reward structure in two separate experiments. In Experiment 1, two individuals received more rewards than the third, making the outcome unequal. We wanted to know if cooperation would continue or break down, and what mechanisms might maintain performance. Experiment 2 used equal rewards, but this time one or more locations were left unbaited on a proportion of trials. Thus, there was a chance of individuals working to receive nothing. In Experiment 1, the chimpanzees worked at a high rate, tolerating the unequal outcomes, with rank appearing to determine who got access to the higher-value locations. However, equal outcomes (used as a control) enhanced cooperative performance, most likely through motivational processes rather than the absence of inequity aversion. In Experiment 2, performance dropped off dramatically when the chimpanzees were not rewarded on every trial. Their strategy was irrational as donating effort would have led to more rewards in the long run for each individual. Our results lead to a hierarchy of performances by condition with equity > inequity > donating effort. Chimpanzees therefore tolerate mild inequity, but cannot tolerate receiving nothing when others are rewarded.