Humans have become completely dependent on the use of tools. Every day, we use a multitude of objects for interacting with our environment. While we consider many tools disposable, we greatly value others and look after them when they are not needed: a treasured pen is carefully placed in a pen holder, a trusted hammer is secured to a tool belt, and an expensive electronic device is stored in a padded pouch. Such ‘safekeeping’ of tools has at least two main functions. First, it ensures that we can easily find these tools when we need them, avoiding search costs, and second, it minimises the likelihood of tool damage or loss, avoiding replacement costs. To our knowledge, there are no studies that investigated if non-human animals also handle ‘valuable’ tools more carefully.

Using tools for foraging has the benefit of providing access to some nutritious food sources that would otherwise be difficult or impossible to obtain, such as embedded arthropods, honey in tree cavities, or the content of hard-shelled items like nuts and eggs (Bentley-Condit and Smith, 2010; Shumaker et al., 2011; Sanz et al., 2013). But tool use also requires investment in terms of time and energy. A suitable tool needs to be found, or raw materials sourced for tool manufacture, which can be time-consuming if the preferred items are scarce in the environment (Koops et al., 2015; Almeida-Warren et al., 2017). Tool manufacture and/or modification can also be demanding, requiring extensive processing of materials (Hunt and Gray, 2004; Klump et al., 2015a; Lapuente et al., 2017), and if mistakes are made, procurement of fresh raw materials (Tebbich et al., 2012). And finally, during tool deployment, the animal may be at risk of losing its tool, either to thieving conspecifics or by accidentally dropping it (Nishida and Hiraiwa, 1982; Tebbich et al., 2012; Klump et al., 2015b). Animals can offset some of these costs by re-using tools and by keeping them safe between periods of use, either holding onto or storing them. Such tool ‘safekeeping’ has indeed been observed anecdotally in otters (Hall and Schaller, 1964), chimpanzees (Nishida and Hiraiwa, 1982), and Galapagos woodpecker finches (Tebbich et al., 2012), and first controlled studies have examined the behaviour’s context-dependent expression in New Caledonian crows Corvus moneduloides (hereafter ‘NC’ crows) (Klump et al., 2015b) and Goffin’s cockatoos (Auersperg et al., 2017). Specifically, we have previously shown experimentally that NC crows respond to an increase in tool recovery costs (foraging at height) with elevated levels of safekeeping behaviour (Klump et al., 2015b), a result that was subsequently replicated in Goffin’s cockatoos (Auersperg et al., 2017) – a species that does not seem to routinely use tools in the wild (O’Hara et al., 2021). Interestingly, when NC crows’ preferred safekeeping method of holding tools underfoot was made more challenging, because subjects had to handle demanding prey before re-using their tools, they resorted to storing tools more frequently in holes, thereby preventing their accidental loss (Klump et al., 2015b). It remains unknown, however, whether NC crows’ safekeeping behaviour is also sensitive to a tool’s manufacture costs and/or its potential utility in terms of foraging efficiency. In other words, do NC crows keep ‘valuable’ tools safer than more basic ones?

NC crows in one of our study populations (‘farmland’ site in Rutz et al., 2012) use two different types of stick tools. Non-hooked stick tools (Figure 1a, top panel) are twigs or leaf petioles sourced from the forest floor, or branches removed from live vegetation (Hunt and Gray, 2002); they are ubiquitous, abundant, and usually require no modification before they can be used (Rutz and St Clair, 2012). Hooked stick tools (Figure 1a, bottom panel), on the other hand, are almost always made from one particular plant species, the shrub Desmanthus virgatus, and birds need to locate this patchily distributed material, select a suitable stem, and manufacture the tool in an elaborate process, including the careful ‘sculpting’ of a hooked tip (Hunt and Gray, 2004; Klump et al., 2015a; St Clair et al., 2016; Klump et al., 2019). Although tool procurement costs have not yet been quantified in NC crows, it is evident that hooked stick tools will on average require a greater investment of time in both search and manufacture behaviours than non-hooked stick tools (for further discussion, see ‘Concluding remarks’). In an earlier study, we compared the foraging performance of crows using different tool types on standardised food extraction tasks and found that self-manufactured hooked stick tools made from D. virgatus were much more efficient than non-hooked stick tools sourced from leaf litter (St Clair et al., 2018). Given the extra costs involved in sourcing suitable material and manufacturing a hooked stick tool (plant material requires elaborate processing [Klump et al., 2015a]; producing more efficient, deep-hooked tools often involves additional actions [Sugasawa et al., 2017]; choosing the wrong plant species may be costly [Klump et al., 2019]) as well as the enhanced foraging performance of this tool type (St Clair et al., 2018), we predicted that NC crows would prefer these tools over non-hooked ones when given a choice, and ‘value’ them more highly, taking better care of them when not in active use. In the present study, we tested these predictions in two companion experiments with wild-caught, temporarily captive crows.

Figure 1

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In Experiment 1, we offered subjects choices of hooked and non-hooked stick tools, and recorded which tool type they selected for a single prey extraction. Experiment 2 examined whether crows treated tool types differently between successive prey extractions, and also (in separate treatments) manipulated tool material and manufacture costs. The first two treatments in Experiment 2 (2.A and 2.B, see Figure 1b) were designed to mimic natural foraging conditions, providing a comparison between the safekeeping of non-hooked stick tools crow-sourced from assorted twigs and leaf petioles, and hooked stick tools crow-manufactured from D. virgatus. While this achieved good ecological validity, it inevitably confounded tool type (non-hooked vs. hooked) with material (twigs and leaf petioles vs. D. virgatus) and manufacture effort (no manufacture vs. elaborate processing of raw material). We therefore ran two additional treatments with researcher-supplied tools (2.C and 2.D, see Figure 1b) to test whether any of these aspects by themselves affect crows’ tool-placement behaviour.

To our knowledge, our study is the first to use animals’ tool-handling behaviour to make inferences about the value they ascribe to different tool types. This novel experimental paradigm allows an assessment of how animals value tools without the need for elaborate training, unlocking considerable research potential for observational and experimental studies across a wide range of tool-using species (including those for which safekeeping has already been reported anecdotally; see above).

In Experiment 1, subjects showed a striking preference for hooked stick tools. In Treatment 1.A, almost all crows picked up, transported, and deployed the hooked stick tool and extracted the prey item with it (p<0.001, n = 15–17; Figure 2). In Treatment 1.B, crows significantly preferred the hooked stick tool for transport, deployment, and extraction (p<0.001, n = 9–12), but not at the pick-up stage (p≈1, n = 12; Figure 2). Interestingly, in Treatment 1.B (in which most non-hooked options were made from D. virgatus), fewer crows chose the hooked stick tool than in Treatment 1.A (in which most non-hooked options were leaf petioles) (Figure 2).

Figure 2 with 1 supplement see all

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In Experiment 2, crows kept their tools safe in the vast majority of cases (Figure 3), both in the two naturalistic treatments (92% of 257 cases, pooled across Treatments 2.A [non-hooked stick tools crow-sourced from leaf litter] and 2.B [hooked stick tools crow-manufactured from D. virgatus]) and the two additional treatments with researcher-supplied tools (94% of 185 cases, pooled across Treatments 2.C [non-hooked stick tools researcher-made from D. virgatus] and 2.D [hooked stick tools researcher-made from D. virgatus]). As predicted, subjects were significantly more likely to express safekeeping behaviour (storing tools underfoot or in holes) when foraging with hooked stick tools they had manufactured from D. virgatus (95% of 172 cases in Treatment 2.B) than when foraging with non-hooked stick tools they had sourced from leaf litter (87% of 85 cases in Treatment 2.A; model #1: GLMM [generalized linear mixed model]: χ² = 5.47, p=0.02, n = 257; Figure 3), and they also stored hooked stick tools in holes more often than non-hooked stick tools (Treatments 2.B vs. 2.A; model #2: GLMM: χ² = 19.78, p<0.001, n = 257; Figure 3).

Figure 3

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The following factors by themselves did not significantly affect safekeeping behaviour: tool type (Treatments 2.C vs. 2.D; non-hooked stick tools researcher-made from D. virgatus vs. hooked stick tools researcher-made from D. virgatus; model #3: GLMM: χ² = 1.04, p=0.31, n = 185); tool material (Treatments 2.A vs. 2.C; non-hooked stick tools crow-sourced from leaf litter vs. non-hooked stick tools researcher-made from D. virgatus; model #7: GLMM: χ² = 0.94, p=0.33, n = 142); and manufacture effort (Treatments 2.B vs. 2.D; hooked stick tools crow-manufactured from D. virgatus vs. hooked stick tools researcher-made from D. virgatus; model #9: GLMM: χ² = 0.06, p=0.81, n = 300). For reference, the comparison between Treatments 2.B and 2.C was non-significant (model #5: GLMM: χ² = 0.87, p=0.35, n = 229).

Interestingly, closer inspection of the data revealed that the presence of the hook alone had a measurable effect on the mode of safekeeping (i.e., how, rather than whether, tools were kept safe). Crows stored (supplied) hooked stick tools researcher-made from D. virgatus significantly more often in holes than (supplied) non-hooked stick tools researcher-made from D. virgatus (Treatments 2.D vs. 2.C; model #4: GLMM: χ² = 7.24, p=0.007, n = 185). This increased tendency to store hooked tools in holes was also observed when comparing crows handling hooked stick tools self-manufactured from D. virgatus and (supplied) non-hooked stick tools researcher-made from D. virgatus (Treatments 2.B vs. 2.C; model #6: GLMM: χ² = 4.11, p=0.04, n = 229). The mode of safekeeping (i.e., how tools were kept safe – specifically, whether or not they were stored in holes) was neither affected by tool material (Treatments 2.A vs. 2.C; non-hooked stick tools crow-sourced from leaf litter vs. non-hooked stick tools researcher-made from D. virgatus; model #8: GLMM: χ² = 0.07, 0.79, n = 142) nor manufacture effort (Treatments 2.B vs. 2.D; hooked stick tools crow-manufactured from D. virgatus vs. hooked stick tools researcher-made from D. virgatus; model #10: GLMM: χ² = 0.03, p=0.86, n = 300).

The physical properties of tools are key determinants of foraging efficiency (e.g., Sanz et al., 2009; Fragaszy et al., 2010; Lapuente et al., 2017; Lamon et al., 2018; St Clair et al., 2018), and many habitual tool users, including the NC crow, have been shown to exhibit some degree of tool selectivity (e.g., Aumann, 1990; Chappell and Kacelnik, 2002; Carvalho et al., 2008; Visalberghi et al., 2009; Gumert and Malaivijitnond, 2013; Sirianni et al., 2015; Visalberghi et al., 2015). In Experiment 1, we established that NC crows strongly prefer hooked stick tools when they have access to both hooked and non-hooked stick tools, and that they are able to differentiate between these tool types even when they are made from the same material (see also St Clair and Rutz, 2013). We had previously shown that, when presented with experimentally altered ‘puzzle’ tools (St Clair and Rutz, 2013), crows pay attention to the presence of a hook, prioritising it over two other, potentially efficiency-enhancing design features – a curved tool shaft and a functional end with bark removed (for discussion, see St Clair and Rutz, 2013; Sugasawa et al., 2017). While both of our choice tests clearly confirmed a strong preference for hooks, more birds chose the hooked stick tool when the non-hooked options were of a noticeably different material (Treatment 1.A), rather than when both tool types were made from D. virgatus and thus looked superficially similar (Treatment 1.B). This could be an indication that NC crows sometimes employ a ‘rule of thumb’ when selecting tools (Hunt et al., 2006; Hunt, 2021), initially paying more attention to the tool material than to the tool type (see Klump et al., 2019 and discussion below).

Importantly, Experiment 2 demonstrated that crows’ preference for hooked stick tools was reflected in their tool-handling behaviour, with hooked stick tools being kept safe more frequently than non-hooked stick tools (Figure 3). Tool procurement inevitably involves costs, particularly when materials are scarce or manufacture is time consuming (see ‘Introduction’). To date, little is known about the magnitude of these costs, or the extent to which non-human animals – including NC crows – actually mitigate them (but see Klump et al., 2015b; Auersperg et al., 2017). The storage and re-use of tools necessarily reduces the frequency with which they must be replaced, and may allow considerable savings of time and energy. We had predicted that, given their higher procurement costs (see ‘Introduction’) and increased foraging efficiency (St Clair et al., 2018), hooked stick tools would be perceived as more ‘valuable’ by crows compared to non-hooked stick tools. Subjects generally looked after all of their tools very well, securing them either underfoot or inserting them into a hole – safekeeping modes we had previously documented both in captivity and in the wild (Klump et al., 2015b). While the overall level of safekeeping in Experiment 2 was striking (>90%), hooked stick tools were kept safe significantly more often than non-hooked stick tools. Treatments 2.A and 2.B attempted to replicate conditions crows from our study population would experience in the wild, with hooked stick tools self-manufactured from D. virgatus and non-hooked stick tools sourced from leaf litter. Since wild birds incur the additional cost of finding suitable plant material for tool manufacture (D. virgatus has a patchy distribution in our study site; unpublished data), one might expect them to handle hooked stick tools even more cautiously than our captive subjects did; in other words, our experiment might lead us to underestimate the difference in safekeeping behaviour.

We can of course not rule out that our subjects – all confirmed hooked stick tool makers (see ‘Study site and subjects’) – were inexperienced non-hooked stick tool users (see St Clair et al., 2018). If birds do not regularly use non-hooked stick tools in the wild, they might handle this unfamiliar tool type less cautiously, which could lead to reduced levels of safekeeping during experimental trials. Although we cannot at present exclude the possibility that tool-type familiarity played a role, it is worth noting that, in Experiment 2, most birds extracted prey in each of the treatments, subjects only foraged with one tool type at any given time, and perhaps most importantly, that only scores for tool placements following successful food extractions were included in our analyses (see ‘Video scoring and statistics’).

In an attempt to disentangle the relative contributions of tool type, tool material, and manufacture effort, we compared treatments in Experiment 2 where tools only differed in a single aspect. When tool material and manufacture effort were held constant (i.e., all tools were researcher-supplied and made from D. virgatus), crows still appeared to take less care of non-hooked stick tools (91% of tools kept safe in Treatment 2.C) than of hooked stick tools (95% of tools kept safe in Treatment 2.D), but this effect was small and non-significant. This outcome may simply arise from a lack of statistical power (following Colegrave and Ruxton, 2003, we did not conduct post-hoc power analyses), or it may reflect actual crow behaviour, with subjects applying the rule of thumb of treating all tools made from D. virgatus as though they are hooked. While we have observed the manufacture of non-hooked stick tools from D. virgatus when birds are held in short-term captivity (and they appeared to treat these tools similarly to self-manufactured hooked stick tools; Klump et al., 2015b), non-hooked stick tools are apparently rarely made from D. virgatus in the wild (personal observation; St Clair et al., 2016). In any case, such an approach might both save time and carry little cost (Clark and Dukas, 2003), and would be consistent with a growing body of evidence indicating that NC crows employ simple ‘heuristics’ when making tool choices (Hunt, 2021). Crows certainly use co-occurring features as a cue for initially recognising and orienting hooked tools: as noted above, in Experiment 1, more birds chose a non-hooked stick tool when the supplied tools were stems of D. virgatus (Treatment 1.B) rather than different materials (Treatment 1.A), and in an earlier study, crows given puzzle tools with normally co-occurring features at different ends were more likely to pick them up in the wrong orientation (i.e., with the hooked tip pointing backwards; see St Clair and Rutz, 2013).

While heuristics clearly inform NC crows’ initial selection of tools (Hunt, 2021), our experiments (present study; St Clair and Rutz, 2013) also demonstrate a striking ability to attend to hooks (for results for a different tool type, see Knaebe et al., 2015). For example, subjects that picked up a hooked stick tool in the wrong orientation will usually re-orient it very quickly before use (St Clair and Rutz, 2013). It thus seems unlikely that crows would assess only the material when deciding on the appropriate level of safekeeping behaviour for a given tool. In further support of this conclusion, if NC crows exclusively used material as an indicator of tool type during foraging, we would expect them to express more safekeeping behaviour when handling supplied non-hooked stick tools researcher-made from D. virgatus than when handling non-hooked stick tools self-sourced from leaf litter (Treatments 2.C vs. 2.A), and we did not observe a significant effect for this comparison. It is reassuring that our naturalistic (multi-aspect) comparison (Treatments 2.A vs. 2.B) remained robust when subsampled to include only those individuals that also took part in the single-aspect treatments (Treatments 2.C and 2.D), but presumably comparisons involving treatments that differed only in one aspect would have smaller effect sizes. Both ‘hooked’ treatments (2.B and 2.D) elicited higher levels of safekeeping than Treatment 2.C (non-hooked D. virgatus tools), although neither comparison was significant; while we cannot rule out at present that NC crows do not respond to these manipulations, it is conceivable that a larger sample size would have detected significant effects.

It is notable that, when the particular mode of safekeeping was examined (i.e., how tools were kept safe – specifically, whether or not they were stored in holes), rather than the level of safekeeping (i.e., whether or not tools were kept safe), tool type had a significant effect even in the absence of material differences. Specifically, researcher-supplied hooked stick tools were stored in holes significantly more often than researcher-supplied non-hooked stick tools (both researcher-made from D. virgatus; Treatments 2.D vs. 2.C). We have previously shown that this safekeeping mode minimises the risk of dropping a tool, and that birds store tools in holes more often when foraging at height, at ca. 1.3 m above ground, presumably to avoid retrieval costs (Klump et al., 2015b). While the baited log was much closer to the ground in Experiment 2 reported here, a height of ca. 15–20 cm (from the floor to the top of the log) may have been enough to induce a change in tool-placement behaviour since birds had to leave the log to pick up a dropped tool. An interesting alternative explanation is that crows may deliberately place hooked stick tools in holes because they are then unlikely to be picked up with the hook in the non-functional orientation (see above); non-hooked stick tools have less ‘functional polarity’ and there is less benefit to preserving any particular orientation between spells of probing.

We had predicted that crows would show increased levels of safekeeping when foraging with self-manufactured hooked stick tools, compared to researcher-supplied hooked stick tools, as they had paid manufacture costs in the former case, but not the latter. We did not detect such an effect (Treatments 2.B vs. 2.D) and can suggest several reasons for this. The cost of manufacturing hooked stick tools within our experimental setting (Treatment 2.B) may have been too small to induce a behavioural response (it takes an NC crow only a few minutes to make a new tool, and we necessarily eliminated search costs by providing raw material; Hunt, 1996; Hunt and Gray, 2004). Moreover, these manufacture costs may have been partially compensated for by the higher value of researcher-supplied hooked stick tools in terms of both increased foraging efficiency (St Clair et al., 2018) and availability (only 3 tools were provided in Treatment 2.D, while 10 stems of raw material were available in Treatment 2.B). Given this arguably modest cost/benefit ratio, any resulting difference in safekeeping behaviour would also likely be small, and (as discussed above) we may have lacked the statistical power to detect it. Alternatively, there might be no difference in safekeeping behaviour for self-manufactured and researcher-supplied hooked stick tools, as crows may assess manufacture costs not on a tool-by-tool basis, but over the course of many manufactures (remembered/experienced utility; Kahneman et al., 1997), leading them to ascribe an equally high value to all hooked tools, rather than assessing the value of individual tools independently. At any rate, the costs of replacing a lost hooked stick tool are the same regardless of whether the lost tool was self-manufactured or serendipitously discovered; valuing a self-manufactured tool more highly than an identical ‘found’ tool would be logically unsound ‘Concordian’ thinking (where decisions are made based on past investment rather than rationally assessing the net expected future benefit; Curio, 1987), and perhaps we should be unsurprised that our evidence is consistent with crows avoiding this error.

While our two naturalistic treatments (Treatments 2.A and 2.B) clearly show that our subjects take better care of their hooked than their non-hooked stick tools, it remains unclear whether this difference is driven exclusively by the tool type’s performance benefits (St Clair et al., 2018) or whether other factors – such as procurement costs – also play a role. We suggest that performance benefits are important, but cannot explain safekeeping behaviour by themselves – after all, in a ‘perfect world’ in which spare hooked stick tools were always within reach, there would be little motivation for a crow to keep its current tool safe. Future work should address the relative importance of these factors and how they might interact in shaping NC crows’ safekeeping behaviour.

One worthwhile line of investigation may be provided by the ‘larva fishing’ behaviour exhibited by some NC crow populations. Since crows exclusively use non-hooked stick tools when extracting wood-boring beetle larvae from decaying candlenut logs (Hunt, 2000; Bluff et al., 2010b), this would provide a context in which non-hooked stick tools have both lower procurement costs and are functionally superior. While it would be illuminating to record safekeeping behaviour of both tool types in this foraging context in captivity, such a study would demand careful planning: if individuals that are skilled at larva fishing were unfamiliar with hooked stick tools, introducing them to this novel tool type could potentially alter the natural tool behaviour of wild populations (see Bluff et al., 2010a). The same ethical concerns apply to the use of artificial tools and tasks with wild-caught, temporarily captive crows. In any case, it should be possible to experimentally manipulate the procurement costs of different tool types in aviary studies by restricting access to tools or raw materials. For example, crows could be trained to expect that these resources will be withdrawn for part of the day (for work on primates, see Mulcahy and Call, 2006; Dekleva et al., 2012).

Work on the ultimate drivers of NC crows’ tool safekeeping decisions should ideally go hand-in-hand with an investigation of how birds acquire the perceptions of value that arguably underpin this behaviour. Experiments should establish whether individuals’ safekeeping behaviour for different tool types is dynamic, changing with the acquisition of relevant new information, while ontogenetic and social-learning studies may cast light on when, and how, safekeeping behaviour arises in the first place.

Raw count data for Experiment 1 are shown in Figure 2. Raw data for Experiment 2 are shown in Figure 3 and are also supplied as a csv file.

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Author details

1. Barbara C Klump

   1. Centre for Biological Diversity, School of Biology, St Andrews, United Kingdom
   2. Cognitive and Cultural Ecology Group, Max Planck Institute of Animal Behavior, Radolfzell am Bodensee, Germany

   Contribution

   Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Scoring of videos, Validation, Visualization, Writing – original draft, Writing – review and editing

   For correspondence

   bklump@ab.mpg.de

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-3919-452X

2. James JH St Clair

   Centre for Biological Diversity, School of Biology, St Andrews, United Kingdom

   Contribution

   Conceptualization, Investigation, Methodology, Visualization, Writing – original draft, Writing – review and editing, Design of the project that provided data for Experiment 2 (St Clair et al., 2018)

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-2902-4391

3. Christian Rutz

   Centre for Biological Diversity, School of Biology, St Andrews, United Kingdom

   Contribution

   Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Visualization, Writing – original draft, Writing – review and editing, Design of the project that provided data for Experiment 2 (St Clair et al., 2018)

   For correspondence

   christian.rutz@st-andrews.ac.uk

   Competing interests

   Senior editor, eLife

   "This ORCID iD identifies the author of this article:" 0000-0001-5187-7417

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