Animal Cognition

, Volume 14, Issue 4, pp 555–563

Physical cognition and tool-use: performance of Darwin’s finches in the two-trap tube task

Authors

    • Department of Behavioural Ecology and Evolutionary GeneticsMax Planck Institute for Ornithology
  • S. Tebbich
    • Department of Cognitive BiologyUniversity of Vienna
Original Paper

DOI: 10.1007/s10071-011-0390-9

Cite this article as:
Teschke, I. & Tebbich, S. Anim Cogn (2011) 14: 555. doi:10.1007/s10071-011-0390-9

Abstract

The trap tube is a classic test of causal reasoning abilities in animals in the physical domain. Recently, a modified version of this task improved its diagnostic capacity and allowed testing of non-tool-using animals. We used this modified two-trap tube task to compare the cognition of two Darwin’s finch species: the woodpecker finch, Cactospiza pallida, a tool-using species, and the small tree finch, Camarhynchus parvulus, a closely related non-tool-using species. Not all woodpecker finches use tools in nature, and we therefore also tested non-tool-using individuals to assess the effect of experience on trap tube performance. No small tree finches and only two non-tool-using woodpecker finches solved the initial task which was operated using a pre-inserted piston. One tool-using woodpecker finch solved the task when allowed to use its own tool instead of the pre-inserted piston. The fact that none of these subjects transferred their knowledge when the features of the task changed, suggests that in this species, neither experience using tools nor the genetic composition of a tool-user are associated with the general physical cognitive skills required to solve the trap tube task.

Keywords

Tool-useCactospiza pallidaTrap tubeFolk physicsPhysical cognitionCausalityDarwin’s finches

Introduction

The trap tube task of Visalberghi and Limongelli (1994) has been one of the most prevalent assays of causal reasoning in solving a physical problem in animals. This task requires a subject to use a tool to extract a reward contained in a clear plastic tube without pushing it into a vertical trap located along the length of the tube. The central question has traditionally been the cognitive level of the strategy employed in solving the task, i.e., did the successful animal solve this task by abstracting a generalized causal rule rooted in the physical principles of the task (gravity and/or surface continuity) or via a simple associative rule based on the observable features of the task, i.e., ‘push from the side that is furthest from the food’? To differentiate between these possibilities, the authors devised a control test for successful animals in which the tube is inverted 180 degrees, rendering the trap non-functional. If the subjects have learned about the function of the trap, they should immediately cease to respond to the position of the non-functional trap.

To date, primates, including capuchin monkeys, Cebus paella, chimpanzees, Pan troglodytes, orang-utans, Pongo pygmaeus, bonobos, Pan paniscus, and one gorilla, Gorilla gorilla (Visalberghi and Limongelli 1994; Limongelli et al. 1995; Povinelli 2000; Mulcahy and Call 2006; Martin-Ordas et al. 2008) and several bird species such as woodpecker finches, the New Caledonian crow, Corvus moneduloides, and three non-tool-using parrot species (Tebbich and Bshary 2004; Bluff et al. 2007; Liedtke et al. 2010) have been tested in this paradigm. The results of these tests have not provided any conclusive evidence that these species are capable of using anything more cognitively complex than simple procedural rules to solve the task (Bluff et al. 2007; Penn et al. 2008; Penn and Povinelli 2007; Visalberghi and Tomasello 1998).

Limitations of the inverted control task and the two-trap tube

The finding that humans also continue to avoid the inverted trap highlighted major conceptual flaws of this task, for instance that there is no incentive to stop avoiding the trap in the control condition and that inverting the tube changes the task features so drastically that subjects might perceive it as a new task and therefore disregard the trap (Silva et al. 2005).

Seed et al. (2006) and Tebbich et al. (2007) responded by devising a modified tube task that featured two traps: a functional trap sealed with a black disc at the bottom and a non-functional trap, to preclude the use of some simple cues such as ‘pull away from the trap’. They also designed a new set of transfer tasks in which the arbitrary visual task features were systematically varied while the underlying causal task properties were conserved. Presumably, a solution to the sequence of all transfer tasks can only be attained through the extraction of more sophisticated and generalized rules such as the fact that unsupported objects always fall (the observable effect of gravity) and that objects cannot pass through barriers.

The new task could also be used to test non-tool-using animals: a rod was pre-inserted into the tube, and the reward was located between two clear discs that were attached to the rod so that the reward moved in the direction that the rod is pulled or pushed. The construct of the rod and two centrally attached clear discs will be referred to, hereafter, as a ‘piston’. Seed et al. (2006) tested eight rooks, Corvus frugeligus, a non-tool-using corvid species, with this paradigm. Seven of the eight rooks solved the initial problem and also succeeded in the first transfer task. Only one was successful in the subsequent pair of transfer tasks, suggesting that it had formed a generalized rule to solve the task. However, in another trap tube study with rooks which included some of the novel transfer tasks used by Seed et al. (2006), only three of seven birds solved the initial problem and none solved the transfer task (Tebbich et al. 2007).

New Caledonian crows (Taylor et al. 2009a, b), parrots (Liedtke et al. 2010) and chimpanzees (Seed et al. 2009) have also been tested in modified versions of the two-trap tube task. Three of six New Caledonian crows which solved the initial task and subsequent transfer tasks using their own tools were successful on two transfer tasks but failed the third. This indicated that they did not have a full appreciation of the underlying physical problem though the fact that all crows solved the analogous trap table task indicates that they had possibly learned something more general about surface continuity. None of the parrots were able to solve the initial version of a single trap tube task with a pre-inserted piston and though two of the parrots solved the two-trap tube task when able to move the reward directly with their beaks, the study does not allow determination of the rule they extracted to solve this task (Liedtke et al. 2010). Six chimpanzees were tested in a two-trap tube task that could be operated without using a tool, by inserting a finger into small holes in the tube wall. One individual solved the initial task and all three transfer tasks which ‘supports the notion that they had used some form of mental representation’ to solve the problem (Seed et al. 2009). Moreover, the authors found a significant effect of tool inclusion and experience on success: those animals that did not have to use a tool and that had prior experience in a similar task were most successful.

The link between tool-use and general physical intelligence

In humans, tool-use usually involves a generalized appreciation of the physical world in the causal terms of gravity, the functional relationship between an object and the tool used to manipulate it as well as understanding the functional properties of tools. On the surface, the tool-use of some animals also appears to be a remarkable feat, requiring sophisticated high-level cognitive strategies. The findings that tool-use correlates with increased brain size in primates (Reader and Laland 2002) and birds (Lefebvre et al. 2002) suggests that this behaviour is indeed cognitively demanding, possibly because its deployment requires complex information-processing abilities. Reflecting this, much research on animal tool-use during the past two decades has focused on elucidating the cognitive level of information-processing associated with this unusual behaviour in various tool-using species, mostly primates. In this study, we were specifically interested in testing the hypothesis that a tool-using species might have access to a greater appreciation of general physical interactions than a non-tool-using species, either by virtue of their genetic composition or through experience.

To this end, we compared the performance of tool-using woodpecker finches and the closely related non-tool-using small tree finches in the two-trap tube paradigm with a pre-inserted piston. These are closely related species belonging to the Darwin’s finch clade which is endemic to the Galápagos archipelago. Woodpecker finches use cactus spines or twigs to extract arthropod larvae and adults from crevices and holes (Eibl-Eibesfeldt 1961). However, Tebbich et al. (2002) demonstrated that the propensity and ability to use tools is linked to the environment: in coastal areas where food is scarce and hard to access, all individuals are capable of using tools and do so frequently, whereas in humid areas at higher altitudes where food is abundant and easy to access, woodpecker finches hardly use tools and even the ability to use tools seems to vary (Tebbich et al. 2001).

In contrast to chimpanzees, the ability to use tools in woodpecker finches is not dependent on social learning, rather they have a specific genetic predisposition to acquire tool-use in a sensitive learning period early in ontogeny through a process which involves trial-and-error learning (Tebbich et al. 2001). The evidence comes from the fact that juvenile, hand-raised woodpecker finches from humid areas learned to use tools regardless of whether they had a tool-using tutor or a tool-using parent, whereas adult individuals from the same area seemed to be unable to use tools in captivity and also failed to learn the technique from a conspecific tutor.

Woodpecker finches are selective in the tools they use and can modify them to suit a task at hand (Tebbich and Bshary 2004). Moreover, one woodpecker finch successfully solved the inverted control task of the single trap tube task, whereas the other five tested failed, but careful analysis revealed that the successful bird seems to have observed the effect of tool manipulation on the movement of the reward. Thus, another goal of this study was to re-evaluate the performance of woodpecker finches with the new two-trap tube paradigm.

We predicted that if tool-use was associated with enhanced general physical cognition in woodpecker finches, then they should outperform small tree finches in the trap tube task. However, if generalized physical intelligence evolved in a different context, small tree finches could match or even exceed their performance.

The fact that not all woodpecker finch individuals use tools (Tebbich et al. 2001, 2002) provided an opportunity to assess the effect of tool-using experience on general physical cognition: experience with tools improves performance in at least some physical tasks involving the use of tools (Hauser et al. 2002). Thus, we also compared the performance of tool-using and non-tool-using woodpecker finches in the two-trap tube with a pre-inserted piston. Our modified prediction was that if only experience with tools (and not the genetics of a tool-user) leads to a sophisticated appreciation of physical interactions, only the tool-using woodpecker finches should outperform small tree finches but not non-tool-using woodpecker finches.

It has been shown that task presentation has a strong effect on performance (e.g., Mulcahy and Call 2006; Seed et al. 2009). These studies highlight the fact that the physical problem itself is only one source of difficulty. The other major challenge is dealing with the intermediate manipulation of an object in order to move the food reward.

To compare non-tool-using animal’s performance in the two-trap tube with that of tool-users, in several studies non-tool-users were trained to pull a pre-inserted piston that moved the food in order to allow them to operate the trap tube apparatus. It is possible that pulling a piston alters the cognitive load necessary to solve the task compared with tasks in which an animal can manipulate a tool freely. It could well be that manipulation of a freely manipulable tool is a greater challenge than the pulling of the pre-inserted piston. However, it might also be that when allowed to use their own tools, tool-using woodpecker finches are able to grasp the physical interactions better because the problem is posed in a more natural context.

In our third experiment, we therefore allowed four additional tool-using woodpecker finches to use a freely manipulable straight stick tool—similar to their natural tools—to retrieve the reward rather than the pre-inserted piston. Comparison of these tool-using woodpecker finches with tool-using woodpecker finches that used the pre-inserted piston allowed a comparison of the cognitive load imposed by the two different methods of operating the task.

General methods

The study was carried out at the Charles Darwin Research Station on Santa Cruz Island in the Galápagos Archipelago, Ecuador from October 2007–March 2008 and September 2008–January 2009. Some test subjects participated in other experiments testing physical cognition, before or after the trap tube experiments that are described here (Teschke et al. unpublished). A summary clarifying the order of experiments and the participation of each bird in each experiment is given as electronic supplementary material (ESM Table 1). ESM Table 2 of the electronic supplementary material details the history of the birds and their experience in other experiments which required them to use a tool. A total of 18 woodpecker finches and 9 small tree finches were mist-netted for this study.

Following capture, finches were first kept in a small habituation cage (0.5 × 0.5 × 1 m) for ≤5 days. Thereafter, the birds were maintained in outdoor aviaries (3.9 × 2 × 3 m or 2 × 1 × 2 m) where they were kept singly and visually isolated from each other on a diet of mashed hard-boiled egg, grated carrot, mixed with commercial bird food mix (Orlux®). Additionally, the birds received fresh fruit and fresh moths daily following testing. Subjects were kept at 100% of their free-feeding weight. Aviaries were furnished with natural branches and an experiment table on which the apparatus were presented.

Tool-using ability of woodpecker finches was always assessed prior to participation in experiments. For this purpose, we drilled holes into natural logs (logs about 50 mm long × 120 mm wide; holes 40 mm deep × 10 mm wide) and baited the holes with the preferred rewards of the subjects. Birds were observed in sessions of 20–30 min in which a baited log was placed in a subject’s aviary along with abundant tools and tool material, and we observed to see whether the subject used tools to retrieve the food reward [details provided in Teschke et al. (unpublished)]. An individual was categorized as a non-tool-user if it did not show successful tool-use within 530 min (ca. 9 h) of accumulated observation. All tool-users were observed to use tools within the first 90 min of observation but on average within the first 28 min.

The categorization of tool-users versus non-tool-users based on this method was substantiated by the fact that those six woodpecker finches that were held in long-term captivity and had been classified as non-tool-users by the method detailed earlier, were never seen to use tools in >1-year duration in captivity. Furthermore, our findings that only two of eight woodpecker finches captured in the humid zone were observed to use tools, while all arid zone birds were found to use tools reflects the frequency of natural occurrence of tool-use in woodpecker finches. In the humid zone, Tebbich et al. (2002) observed only six instances of tool-use during 430 min of continuous focal observations in contrast to 134 instances of tool-use observed during 845 min in the arid zone near the coast. In this habitat, 20 of the 21 individually identified subjects used tools.

Experiments were conducted in the home aviaries of the birds, and food was removed from their aviaries 2 h preceding testing. Rewards were the preferred food of each individual (generally mealworms or moths). Apparatus were always baited out of sight of the subject and for each trial, placed onto the experimental table within the home aviary. The experimenter then left the room and observed the trial via a camcorder (JVC GZ-MG130EK Hard disk camcorder). All experiments were recorded with the camcorder.

Ethical note

Permission to catch the birds and to conduct the study was given by the Galápagos National Park. Eight of the woodpecker finches were held in long-term captivity (≥1 year) for breeding purposes related to conservation (ESM Table 1). All other birds were held for the minimum amount of time required to complete the experiments and then released at their site of capture following conclusion of our experiments.

Training

Prior to beginning the experiments, all subjects were trained to pull a rod protruding from a transparent Perspex tube (175 mm long × 20 mm wide) that was open only at one end and was fixed to a wooden block (300 mm × 120 mm). In this training phase, the side to which the open tube end pointed was randomized and counterbalanced. A rod with two discs was attached near the end, and food between them was inserted into the tube so that the subject could rake the food reward out of the tube by pulling the rod. Subjects had to retrieve the food in 6 consecutive trials before advancing to test trials.

Experiment 1: two-trap tube

Methods

Subjects were six tool-using woodpecker finches, eight non-tool-using woodpecker finches and five small tree finches. One small tree finch only completed 50 trials due to illness and is therefore not included in the results. All birds were tested in this task during the first field season.

The two-trap tube apparatus consisted of a horizontal, transparent tube (165 mm long × 20 mm wide) with two traps: a functional one with a black disc sealing the bottom and a non-functional trap with either a black disc sealing the top and flush with the inner hollow of the tube (Fig. 1a: Tube A) or open at the bottom with no disc at all (Fig. 1a: Tube B). These were the same initial testing conditions used by Seed et al. (2006). The tubes were held up at each end by a vertical, transparent Perspex panel (140 mm high × 120 mm wide). These were inserted 120 mm apart into a wooden base (300 mm long × 120 mm wide × 25 mm high). The tubes were mounted approximately 65 mm above the wooden base.
https://static-content.springer.com/image/art%3A10.1007%2Fs10071-011-0390-9/MediaObjects/10071_2011_390_Fig1_HTML.gif
Fig. 1

a Trap tubes used in experiment 1 and b those used in experiments 2 and 3. For both experiments, Tube A is on the left while TubeB is on the right. In experiment 3, the stick was not pre-inserted into the tube, rather two sticks were placed in front of the apparatus. Arrows indicate the path of the reward for a correct response

Half of the birds of each test group received Tube A while the other half always received Tube B. Each trap was mounted 25 mm laterally from the centre. Two discs were attached to the pre-inserted rod as in the training trials, and the reward was always placed centrally within the tube between the discs and could thus be moved by pulling the rod from either side. The ends of the rod protruded 20 mm from each tube end.

Experimental procedure

At the start of each trial, a reward was placed centrally within the tube and between the discs of the pre-inserted piston. The baited apparatus was then placed on the experiment table in the subject’s aviary, and the subject was given 5 min to either pull the stick so that the food fell into the trap and was lost or until it successfully extracted the food. Subjects were allowed to switch pulling sides during a trial. We scored whether they were successful or not in attaining the food.

Birds were tested in blocks of 10 trials, receiving one to two blocks per day on 5 to 7 consecutive days per week. If the subject did not approach the apparatus or make a decision within 5 min, a habituation trial was conducted in which the bird was required to take a reward which was placed centrally on top of the apparatus before the trial was repeated. If the bird needed more than 3 habituation trials in one block, the block was ended prematurely.

Success criterion

Birds were given between 140 and 180 trials to meet the success criterion: a bird had to make 15 or more correct choices within two consecutive blocks of 10 trials to meet the success criterion. Specifically, the number of correct responses in one of the two blocks had to be at least 7 consecutively correct and in the other at least 8 or in one block all 10 correct. This criterion was derived using a Monte Carlo simulation which is more reliable than simple binomial statistics because it reduces the likelihood of type I errors (details in Tebbich et al. 2007).

The orientation of the apparatus changed trial by trial so that the correct side was always on the right side on half of the trials and on the left side on half the trials according to a randomized, balanced schedule.

A bird that met the success criterion in its initial task was then given no more than 20 trials to solve the transfer task, which was simply the version of the tube that it had not seen before.

Side bias correction

Animals often develop strong side biases in two-choice tests, since only choosing one side results in a 50% success rate. It is therefore standard practice in experimental psychology to apply a correction procedure to prevent the animals from using such a strategy. Thus, in our experiments, when a bird made 6 consecutive choices on one side, we only presented the reward on the non-preferred side in subsequent trials until the bird chose this side once. At this point, we reverted to the regular, randomized and balanced schedule.

Results and discussion

None of the birds were able to solve the initial version of the task, and therefore, no birds were tested in the transfer task. ESM Table 1 contains the exact number of trials given to each bird. We speculated that the failure to solve the initial task might be attributed to two features of the apparatus. First, the tube might have been too long for the birds to learn about the contiguity between their pulling the piston and moving the reward, and possibly, they were not able to transfer this knowledge from the training sessions in which they learned how to operate the piston and the apparatus due to the perceptual differences between the apparatus used in training and testing. Second, it is possible that the transparency of the apparatus made it difficult for the birds to discern the relevant features of the apparatus, particularly the trap. Therefore, for the next experiment, we modified the tube to deal with these points as described in the next section.

Experiment 2: modified two-trap tube

Methods

This experiment was conducted over both field seasons. We tested six small tree finches, six non-tool-using and six tool-using woodpecker finches in this experiment. All woodpecker finches had been tested in experiment 1, having received between 140 and 180 trials, while two of the small tree finches had already received 140 trials each in the previous experiment (ESM Table 1). Subjects were given between 139 and 200 trials to meet the success criterion on the initial task and up to 30 trials to solve the transfer task. As in experiment 1, the transfer task was simply the tube that a successful bird had not seen before. The success criterion and experimental procedure were the same as for the previous task, and the two tube conditions (Fig. 1b) were the same as in the last experiment.

The tubes from the last experiment were modified to make the task easier to solve: the tubes were shortened (105 mm long) and the lower half of the tubes was painted dark grey to accentuate the features of the task. Furthermore, the Perspex supporting panels were trimmed (85 mm high × 55 mm wide) and moved closer to one another (80 mm apart), thereby lowering the tube slightly (45 mm above wooden base). A wider wooden base into which the supports were inserted was constructed (200 mm wide × 300 mm long). The piston rod was shortened and still protruded 20 mm from each end of the tube. Subjects who had been trained and tested in experiment 1 were not given further training prior to this experiment unless more than 2 weeks had elapsed between the experiments. In this case, we checked to make sure the subject recalled how to operate the apparatus by testing them briefly with the training apparatus.

Results and discussion

Only two non-tool-using woodpecker finches (purplegreen and lightgreen) solved the initial task (Fig. 2a, middle row), while none of the tool-using woodpecker finches or small tree finches were successful. There was no significant difference in the proportional number of successful individuals when tool-using and non-tool-using woodpecker finches were compared (Fisher’s exact test: P = 0.455) nor was there a difference in the proportional success between pooled tool-using and non-tool-using woodpecker finches and small tree finches (Fisher’s exact test: P = 0.529). However, both of the birds that solved the initial task immediately failed the following transfer task (Fig. 2b), suggesting that they had solved the initial task using simple rules based on the arbitrary visual features of the task such as ‘pull towards the upper black disc’.
https://static-content.springer.com/image/art%3A10.1007%2Fs10071-011-0390-9/MediaObjects/10071_2011_390_Fig2_HTML.gif
Fig. 2

a Performance of all birds in experiment 2 as per cent of trials correct per block. Birds initially tested with Tube A are plotted on the left column while those originally tested with Tube B are plotted on the right. The horizontal dotted line represents chance performance (50% of trials correct). Top row tool-using woodpecker finches (TU); middle non-tool-using woodpecker finches (NTU); bottom small tree finches (STF). b Transfer performance of two non-tool-using woodpecker finches and the retesting of purplegreen with its original trap tube (Tube A)

The interpretation is more complicated in the case of purplegreen. This bird responded correctly in 8/10 trials in its first block of the transfer test with the new tube (the first 6 consecutive transfer trials were correct), but made only 5/10 correct responses in each of the following two blocks. This was surprising, given that the bird started out so well. To see whether this failure might have been due to a lack of motivation, we retested it with its original tube. In the next 100 trials with its original task, it never reached criterion (Fig. 2b). This inability to form a stable procedural rule over time could be due to a lack of motivation, though based on the quick approach of all birds to the apparatus in most trials, this does not seem likely. Another possibility is that reverting between tubes confused the subject and affected its memory about what it had previously learned. Finally, it is also conceivable that the bird simply reached the criterion by chance. Liedtke et al. (2010) also observed that some birds reached criterion early on and then performed unreliably in subsequent trials.

The success of the two non-tool-using woodpecker finches in the initial task of experiment 2 might be attributed to their longer period in captivity preceding experiment 2 than small tree finches and tool-using members of the same species. On average, non-tool-using woodpecker finches began this experiment 242 days after being caught while tool-using woodpecker finches and small tree finches began it 60 and 53 days, respectively, after being caught. The two successful non-tool-using woodpecker finches began testing 115–74 days, respectively, after being caught.

An alternative explanation for the fact that woodpecker finches were able to learn something specific about the task and not small tree finches is that all woodpecker finches tested in experiment 2 had previously been exposed to at least 140 trials of experiment 1, whereas 4/6 small tree finches had never been tested with a trap tube at the beginning of this experiment. Thus, some small tree finches might also have solved the initial task if they had previously had exposure to experiment 1. An effect of experience has been reported for chimpanzees: chimpanzees with prior experience in a two-trap problem were more successful in a new variation of this problem than inexperienced individuals (Seed et al. 2009).

Experiment 3: modified two-trap tube without pre-inserted tool

Methods

Four tool-using woodpecker finches were tested in this experiment, all during the second season. We used the same apparatus and procedure as in experiment 2 (Fig. 1b), with the exception that the pre-inserted piston was not available. Instead, we always provided two sticks (80 mm long) in front of and central to the apparatus which the birds had to insert and manoeuvre themselves in order to extract the food reward in each trial. We provided two sticks because the birds would sometimes lose a stick during the trial, and we scored success as well as number of inserts and switches between sides.

Results and discussion

Only one individual (purpleblack) solved the initial version of this task (Fig. 3), but it immediately failed in the subsequent transfer task which suggests that purpleblack used an arbitrary perceptual feature of the task to solve the initial problem. Furthermore, in contrast to the successful woodpecker finch ‘rosa’ of Tebbich and Bshary (2004), we did not find a difference in the number of tool insertions in the first 20 trials (median insertions = 1, range = 1–3) and last 20 trials (median insertions = 1, range = 1–5) for purpleblack (Wilcoxon signed-ranks test: z = −1.536, P = 0.125), which would have suggested the implementation of the trial-and-error strategy ‘monitor the effect of the tool on the reward’.
https://static-content.springer.com/image/art%3A10.1007%2Fs10071-011-0390-9/MediaObjects/10071_2011_390_Fig3_HTML.gif
Fig. 3

Performance of all birds in experiment 3 as per cent trials correct per block. Transfer performance for purpleblack is shown on the far right. The dotted line signifies chance performance (50%)

In a similar experimental set up, 3 New Caledonian crows solved the initial version of a two-trap tube paradigm and two transfer tasks but failed to solve a tube that was the equivalent of Tube B used in this study. However, these birds subsequently solved the perceptually distinct ‘trap table’. This implies that the crows are in principle able to extract a general rule about surface continuity but that this ability is not evident in trap tube testing (Taylor et al. 2009a, b). Thus, we cannot entirely exclude the possibility that purpleblack learned something more general about the task. Nevertheless, the performance of this subject provides no grounds to reject the null hypothesis, and thus, we adhere to the most parsimonious explanation which is that purpleblack solved the initial task by picking an arbitrary cue to guide its choice, such as ‘pull towards the upper black disc’. In the transfer task (Tube B), the cue was not available or at least altered in a way which might have confused the subject.

The fact that there was not a significant difference in the number of tool-using woodpecker finches to solve the initial task with the pre-inserted piston (experiment 2) and with a freely manipulable tool (Fisher’s exact test: P = 0.455) does not support the hypothesis that the cognitive load imposed in the two different task operation methods differed. However, due to the small sample size, this negative result needs to be interpreted with caution.

General discussion

In this study, we assessed the general physical problem-solving abilities of two closely related Darwin’s finch species: the small tree finch and the woodpecker finch using the two-trap tube task with a pre-inserted piston that could be operated by both species. Furthermore, we assessed the effect of tool-using experience on general physical cognitive abilities in woodpecker finches and controlled for the effect of the unknown cognitive load imposed by the operation of a freely manipulable stick tool and a pre-inserted piston.

The comparison of woodpecker finches and small tree finches in experiment 2 did not provide evidence that woodpecker finches excel at solving the two-trap tube task and by extension that they have a more sophisticated understanding of physical interactions involved in the task. Though only woodpecker finches solved the initial task, all failed in the following transfer task. Furthermore, the proportion of successful woodpecker finches in the initial task was not significantly higher than for small tree finches.

The fact that small tree finches performed equally well or better than woodpecker finches in other tasks testing physical cognition (Teschke et al. unpublished) provides further reason to be cautious in interpreting the poor performance of the small tree finches in the trap tube tasks.

The results parallel the findings of the previous woodpecker finch study (Tebbich and Bshary 2004) in the sense that woodpecker finches learned to avoid a trap but did not seem able to appreciate the function of a trap even after being given numerous opportunities to do so. We also did not observe that tool-experienced woodpecker finches performed better in the trap tube with the pre-inserted piston (experiment 2). Indeed, the only woodpecker finches that were able to solve the first stage of this task were non-tool-users.

One tool-using woodpecker finch solved the initial task when allowed to insert a stick tool (experiment 3) but none were able to do so using the pre-inserted piston (experiments 1 and 2). This might be due to the more natural context of task presentation in experiment 3—even chimpanzees and orangutans perform better in the original trap tube task when allowed to apply a ‘species-specific tool-using action’ (Mulcahy and Call 2006, p. 194). However, the failure of this subject in the transfer task does not allow us to infer that a sophisticated appreciation of the underlying problem was the reason for this bird’s success.

When one considers the relative uniformity in the natural tool-use of woodpecker finches whereby tools are used only in one context and in one way, it makes sense deploying tools in this species does not necessitate generalization of physical interactions—simple situation-specific rules probably suffice to get the job done. However, the low number of individuals that were able to solve the initial task even using simple context-specific rules shows that an easier task would be needed to detect a significant species difference with such a low sample size. One possibility would be to present them with a task which allows direct movement of the food with the beak as in the study by Liedtke et al. (2010).

The fact that large-brained rooks are able to extract generalized rules in a task where small tree finches do not even succeed in successfully applying a simple procedural rule in first stage of the task suggests a qualitative difference in the cognition of these non-tool-using species. Likewise, the comparison of the performance of tool-using woodpecker finches and New Caledonian crows in the two-trap tube task that was operated with a freely manipulable tool also shows that larger brained New Caledonian crows have a higher propensity to form a general rule while woodpecker finches only can form a situation-specific rule. Finally, one chimpanzee that was tested in a further variation of the two-trap tube paradigm was able to solve a series of transfer tasks that had no simple perceptual cue in common. The summary of these results suggests that large brain size might be a better predictor of the ability to form a generalized rule pertaining to the physical properties of the task than tool-use. Only the performance of parrots does not fit in with this interpretation. Parrots are relatively large brained, but they failed to even extract a simple procedural rule to solve the initial single trap tube task with a pre-inserted piston when given between 100 and 200 trials to do so. Even woodpecker finches with their presumably smaller proportional brain sizes solved the initial task using a simple rule (see Liedtke et al. 2010 for a detailed discussion).

Woodpecker finch tool-use is characterized by selectivity, modification, and high frequency in natural populations and furthermore ‘is not a stereotypic behavioural pattern, but is open to modification by learning’ (Tebbich and Bshary 2004, p. 696). Nevertheless, all studies to date which have investigated cognition related to tool-use in woodpecker finches have failed to provide any evidence that this species possesses sophisticated physical cognitive abilities and that they use mental representation and planning in problem solving related to tool-use (Tebbich and Bshary 2004; Tebbich et al. 2010) nor have they yielded evidence that woodpecker finches must learn this seemingly complex technique from other conspecifics (Tebbich et al. 2001). In woodpecker finches, simple cognitive solutions appear to suffice for the ontogenetic development of tool-use and for its deployment. In particular, trial-and-error learning appears vital to the species in acquiring tool-using skills in ontogeny (Tebbich et al. 2001) and also in solving a battery of physical problems.

Acknowledgments

IT was supported by the German research foundation (DFG, Project Nr. TE628/1-1) and ST by the Austrian Science Fund (FWF, Project Nr. V95-B17). The experiments comply with the current laws of the country in which they were carried out. We are thankful to the Charles Darwin Research Station for support and TAME for reduced ticket fares. We are also grateful to Dr. Birgit Fessl for support in all facets of this study. Caroline Raby, Viviana Morales, Mari Cruz Jaramillo, Tania Quisingo Chiza, Paola Buitron Lopez, and Eduardo Sandoval provided valuable field assistance and help with experiments. Patrick Meidl provided vital support in organizing vast amounts of data. Thanks to Andy Burnley for constructing experimental apparatus and to Sue-Anne Zollinger for helping to make the figure depicting the experimental apparatus.

Supplementary material

10071_2011_390_MOESM1_ESM.doc (122 kb)
Supplementary material 1 (DOC 122 kb)
10071_2011_390_MOESM2_ESM.doc (49 kb)
Supplementary material 2 (DOC 49 kb)

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© Springer-Verlag 2011