Getting back to the first two aims of present study, we are hereby able to show that orangutans can spontaneously innovate a hook-tool out of a novel pliant material to solve a novel problem. Two out of the five subjects (Padana and Pini) were continuously successful from their very first naïve U-trial onwards (straight wire and string were placed next to the apparatus). Furthermore, both subjects improved their tools’ design when a stronger angle was required to lift the basket out of the tube due to a stronger fixation to the tube floor. In the unbending task, four out of the five orangutans spontaneously unbent the wire from their very first naïve U-trial onwards (v-shaped wire and string were placed next to the apparatus). The remaining subject, the adult male Bimbo, was continuously successful after receiving PP-trials (provided wire was already straightened). Similar to Goffin’s cockatoos13 and New Caledonian crows23, two orangutans were able to make different tool types out of the same material, as they were consistently successful in both, the bending and the unbending task.

In the bending task the two adult females, Padana and Pini, spontaneously bent hook tools out of straight wire and retrieved the basket within the first two (Pini) and six minutes (Padana) of their very first trial (note that the New Caledonian crow Betty was the first time successful in her second trial, three of the four rooks on their first trial, one on the fourth trial (but note that both corvids had received pre-experience with ready-made hook tools in the same context and same apparatus beforehand) and the two non-habitually tool using Goffin’s cockatoos in their ninth and tenth test trial (one after having received pre-experience)9,12,13. In the very first trial both orangutans chose and inserted the wire within the first 30 seconds. Pini even bent the tip of the wire by using her mouth/teeth in a steep angle three seconds after picking up the wire for the first time (see movie S3) and Padana bent a hook directly after probing with the unmodified wire (see movie S2). Both subjects modified the tip of the tool several times. Padana also straightened the rest of the tool twice and both always inserted the tool in correct orientation and spent the rest of the time probing with the tool. Therefore, it seems unlikely that they learned to create a hook tool by trial and error, since the actions performed by both orangutans seemed quite spontaneous and goal directed. The corvid’s pre-experience with the task, the ecological pre-dispositions and foraging adaptions10 may have influenced their innovation speed. In case of the orangutans, their morphology (the habitual coordinated use of the hands and mouth for tool-manufacture is likely to allow them to execute more precise bending and probing manipulations than a beak and claws) and the fact that, like all great apes species they are habitually constructing sleeping nests38 could have facilitated the fast innovative process. In order to create the basic layer of the nest, large branches are pulled, bent at the base and broken towards the centre in order to lock them by weaving and twisting motions, usually followed by adding smaller branches in the same style or by detaching them completely and placing them on top39. Nevertheless, the hook-tool manufacture still required ‘bending actions’ in a novel context, using a novel material and using different techniques than described during nest-building (using the teeth to bend over the end of the wire while holding it in one hand, bending while holding the by using both hands using, bending the wire over the rim of the tube afterwards inserting it correctly), which thereby constitutes a case of innovative tool manufacture.

Both orangutans were continuously successful throughout all following test trials. In contrast to Pini who showed no improvement in terms of overall efficiency in the course of the trials (note that it took her only two minutes to lift the basket in the very first trial), Padana’s time until success and her probing time with the modified tool decreased. This is likely due her improvement of the tools’ designs, since the angles of the tools were bent at an approximately 20° steeper angle when comparing the first ten to the last ten trials. Pini never tried to insert the distractor material (string) in any of the test trials, Padana inserted the string only once, but both orangutans inserted the modified wire within the first few seconds directly after the start of the trial. Furthermore, subjects spent more time probing with the functional modified tool, than with the modified but non-functional or with the non-modified wire. Both orangutans used different bending techniques across trials (e.g. either using the teeth to bend one end while holding it with one hand, bending by using both hands, bending the wire over the rim of the tube afterwards inserting it correctly), which indicates that their success was not a result of a learned chain of manipulative actions23. Furthermore, since both orangutans were immediately continuously successful within the short time frame of the first trial, it is unlikely that random interactions followed by associative learning would have been sufficient to succeed in this task. To be successful several unrewarded steps must be executed in a specific order1: bending only one end of the wire while keeping the rest of it straight (2) producing a hook whose angle is functional3 inserting it in correct orientation4 pulling only when the basket’s handle is hooked and5 pulling until the basket is within reach.

In the unbending task Padana, Pini and Dokana were successful within less than one minute in their very first trial, Raja within less than ten minutes. The fact that almost all subjects were able to solve the task immediately and continuously, hardly ever touched or inserted the string and modified the wire directly within the first seconds of the trial, strongly indicates that their actions were executed in a goal-directed way. Additionally, as in the bending task, individuals showed different techniques for unbending the wire including: bi-manual coodination, biting into one end of the wire while pulling with one hand, or pushing the bent wire into the tube and then unbending it inside the tube. Similar to children16, the success rate was higher in the unbending task compared to the hook bending task. This is not surprising since, unlike the hook-bending task, which requires several unrewarded steps, the unbending task requires only unbending and inserting of the tool.

The third aim of this study was to investigate the effect of pre-experience with hooked tools by providing stepwise scaffolding experience to half of the subjects after initial failure. Pre-experience with ready-made hook tools had no effect on subject’s overall success in the hook bending task, although in children using a pre-made tool led to higher task success19. In contrast to a Goffin’s cockatoo that used the provided ready-made hook-tool in correct orientation early on13, both orangutans inserted it in only half of the trials with the hook at the working end. Although Raja and Bimbo (non-experienced in using hook-tools prior to the experiment) were finally continuously able to retrieve the basket with the pre-inserted hook-tool, they were very rarely (Raja in only two trials) or never successful at all in the hook bending task (Bimbo). Furthermore, previous experience with the unbending task seems to have no influence on subject’s performance in the bending task, since none of the subjects that received the unbending task first became consistently successful. This is not surprising since different motor actions and stimuli configurations were necessary to solve the two tasks. The six-year-old Tanah was able to retrieve the basket in six trials, but showed no improvement in latency to success. Further she mostly started to modify the wire after several minutes had passed, indicating that she failed to associate a modified wire with task success. In contrast to the two continuously successful subjects, Tanah’s tool modification seemed to occur in a rather uncoordinated manner by bending the wire over the rim of the tube, by pushing it into the tube or by oral manipulation. Moreover, in the majority of trials she manipulated the distractor material and inserted it.

Other than Goffin cockatoos, the other two species tested so far had seen or used a hook tool in the same context and with the same apparatus before9,12,13. In case of the rooks it was suggested that they had invented a technique to create a ‘known’ tool (a tool that they had already used in the same context; although hereditary predispositions from nest-building can also not be fully excluded), but it cannot be confirmed that they invented a novel tool itself. While it is highly likely that children have encountered hooks before, it is unlikely that they had previously used a pipecleaner in the context of retrieving a basket out of a vertical tube15. Although four of our subjects had experience using a raking tool in a study that was published eight years before (35; see SI, chapter 1), that tool (a wooden rake whose head was 30 × 12 × 1 cm in size) differed substantially in its physical appearance from the one used in the current study. Moreover, subjects had used it in another context and physical orientation (to rake a reward resting on a horizontal trap-table; at the beginning of each trial the rake was pre-positioned with the handle reaching through the wire mesh, which allowed only restricted movement of the rake possibly leading to a limited information gain about the functionality of the tool). Moreover, only two out of the four subjects that had participated in this study manufactured hook tools. It can thus be assumed that this raking-experience was not greatly relevant for task-success. One of the two immediately successful subjects (Pini) had used a rigid unbendable metal hook-tool in order to pull in a plastic bottle filled with juice hanging from the ceiling in front of the test compartment in a study that was conducted ten years before36. It cannot be excluded that this experience could have influenced Pini’s performance in the present study. Although it cannot be confirmed that she invented a novel tool itself, nevertheless she invented a novel technique to create a possibly known tool to solve a novel problem. Further Dokana, that had received the same raking-experience as Pini, was never successful. Since Padana successfully bent hooks and retrieved the basket from her first trial on without having this kind of raking experience, it seems not to be essential for task success. Additionally, subjects Raja and Tanah, who had no raking-experience with hooked raking tools beforehand, successfully manufactured hook tools and retrieved the basket (Raja in two trials, Tanah in six trials).

In the unbending task, all but one subject were continuously successful from their first trial on. The adult male Bimbo was continuously successful from Session 2 on after receiving the first scaffolding step (straight tool was provided next to the apparatus). It took him less than a minute to unbent the wire and get the reward. If success was due to the number of exposures to this task or receiving pre-experience, or a mixture of both cannot be determined.

The fourth aim of this study was to investigate whether successful subjects were able to improve the hook-design feature. The corvid’s hook tools were mostly bent at relatively wide angles9,12,23 and were bent more centred than the Goffin cockatoo’s hook tools13. Tools bent at wider angles were better for the rooks since they could only reach to a certain depth before running out of wire and usually pulled the basket up diagonally12. Betty mostly bent their hook tools by wedging the tip of the wire outside of the tube and then by pulling sideways and the rooks bent the tool by bending it over the rim of the tube and then turning it around9,12. Goffin cockatoos bent tools at steeper angles, probably due to different beak morphology which allowed them to directly bent the tool in their beak by pressing the upper mandible against the rounded corner of the lower mandible13. As the basket was fixed to the bottom of the tube with Velcro fastener, which required the crafting of a steeper hook angle to be able to retrieve the basket, both orangutans bent the wire at a steeper angle compared to the previous standard hook-bending task. Again, both subjects immediately picked up, modified and inserted the wire and never manipulated nor inserted the string. Padana’s tools were bent as well at a slightly steeper angle when comparing the first to the last ten trials, although this did not seem to have an impact on her speed of solving the task. Interestingly Pini’s latency to success as well as the duration of probing with the functional tool decreased rapidly from her eighth trial onwards. Looking at the actual tools, it is very likely that this might be due to improved tool design since fashioned hooks were bent at an even steeper angle from the eighth trial onwards. In contrast to Padana, which lifted the basket by pulling it vertically up in the original hook-bending task, Pini mostly used the previously bent tip of the tool and the sidewall of the tube to lift the unattached basket. Since she became faster from trial eight onward, it is likely that Pini learned by trial and error to bend steep enough angles in the fixed basket condition.

As mentioned previously, hook bending was suggested to represent an “ill-structured problem”15,16, since the final goal has to be kept in mind when exercising all the necessary steps to achieve it (steps include bending the wire at a functional angle while keeping the other part of it straight, inserting it in correct orientation, hooking the handle of the basket, pulling it up until the basket is within reach). Other studies further confirm that children gain the majority of their tool use behaviours by social learning and employ innovation as a last resort40. In contrast to the obvious difficulties younger children face when confronted with the hook-bending task, two orangutans were able to continuously solve the problem from their very first trial on within only two and six minutes. Further Padana’s and Pini’s actions seemed to be executed in a goal directed way. This finding supports the assumption that they used and coordinated relevant pieces of information targeted as a solution to the problem. Nevertheless, only two out of the five orangutans solved the hook-bending problem, although they were given all relevant information in order to solve the task prior to the actual experiment (experience that a wire retains its form after bending manipulations, basket can be pulled up by pulling on the handle). Interestingly, when looking at subject’s performance in other tool use tasks (for details see SI, section f, Table S3), the two most successful orangutans in the hook-bending task also obtained high rankings in terms of tool proficiency compared to the other subjects (Pini was ranked number 1 and Padana number 3). It is assumed that the main obstacle for young children is retrieving the required knowledge from memory17. Taken together, in order to be successful, most likely a mixture of advanced modes of cognitive processing (e.g. high behavioural flexibility, fast individual learning, precise sensorimotor control) that lead to an integration of accessing, retrieving and coordinating knowledge, seem to be the key to innovate a successful solution to the hook-bending problem. Nevertheless, in order to get a more detailed look on the emergence of the ability to solve an ill-structured tool problem, which apparently could be seen as a milestone in some specie’s cognitive development41, further studies that test the standardized hook-bending task in other habitual tool users, such as e.g. the chimpanzee or capuchin monkey, are required. Moreover, to shed light on possible parallels in the ontogeny of the mechanisms underlying tool-making innovations in primates, it would further be necessary to test different age classes in tasks such as the hook-bending paradigm.