This illustrates the causes and consequences of tool shape variation in a non-human

Hooked stick tools crafted by New Caledonian crows vary markedly in size and shape

Hominins have been making tools for over three million years [], yet the earliest known hooked tools appeared as recently as 90,000 years ago []. Hook innovation is likely to have boosted our ancestors’ hunting and fishing efficiency [], marking a major transition in human technological evolution. The New Caledonian crow is the only non-human animal known to craft hooks in the wild []. Crows manufacture hooked stick tools in a multi-stage process, involving the detachment of a branch from suitable vegetation; “sculpting” of a terminal hook from the nodal joint; and often additional adjustments, such as length trimming, shaft bending, and bark stripping []. Although tools made by a given population share key design features [], they vary appreciably in overall shape and hook dimensions. Using wild-caught, temporarily captive crows, we experimentally investigated causes and consequences of variation in hook-tool morphology. We found that bird age, manufacture method, and raw-material properties influenced tool morphology, and that hook geometry in turn affected crows’ foraging efficiency. Specifically, hook depth varied with both detachment technique and plant rigidity, and deeper hooks enabled faster prey extraction in the provided tasks. Older crows manufactured tools of distinctive shape, with pronounced shaft curvature and hooks of intermediate depth. Future work should explore the interactive effects of extrinsic and intrinsic factors on tool production and deployment. Our study provides a quantitative assessment of the drivers and functional significance of tool shape variation in a non-human animal, affording valuable comparative insights into early hominin tool crafting [].

Artefacts of apes, humans, and others: towards comparative assessment and analysis.

Strong between-site variation in New Caledonian crows’ use of hook-tool-making materials.

The crafting of hook tools by wild New Caledonian crows.

Manufacture and use of hook-tools by New Caledonian crows.

Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials.

The crafting of hook tools by wild New Caledonian crows.

Manufacture and use of hook-tools by New Caledonian crows.

Animal Tool Behavior: The Use and Manufacture of Tools by Animals.

Manufacture and use of hook-tools by New Caledonian crows.

Pelagic fishing at 42,000 years before the present and the maritime skills of modern humans.

Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia.

Results and Discussion

5 Shumaker R.W.

Walkup K.R.

Beck B.B. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. 5 Shumaker R.W.

Walkup K.R.

Beck B.B. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. 10 Sanz C.

Morgan D.

Gulick S. New insights into chimpanzees, tools, and termites from the Congo Basin. 11 Pruetz J.D.

Bertolani P. Savanna chimpanzees, Pan troglodytes verus, hunt with tools. 12 Gruber T.

Muller M.N.

Strimling P.

Wrangham R.

Zuberbühler K. Wild chimpanzees rely on cultural knowledge to solve an experimental honey acquisition task. 5 Shumaker R.W.

Walkup K.R.

Beck B.B. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. 13 McGrew W.C. Is primate tool use special? Chimpanzee and New Caledonian crow compared. 9 Gowlett J.A.J. Artefacts of apes, humans, and others: towards comparative assessment and analysis. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 6 Hunt G.R.

Gray R.D. The crafting of hook tools by wild New Caledonian crows. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. Tool use is extremely rare across the animal kingdom, and the ability to make tools from raw materials is rarer still []. Tool manufacture usually involves the detachment of material and some basic modifications []. Chimpanzees, for example, break off and fray plant stems to make termite-fishing probes [], trim the tips of branches to produce pointed hunting tools [], or fold and chew leaves into “sponge”-like bundles to soak up drinking water [] (for reviews of chimpanzee tool-manufacture behavior, see []). These actions require notable skill, but they contrast with the precise crafting of pre-determined three-dimensional shapes that is characteristic of early hominin tool making []. Interestingly, the New Caledonian (NC) crow provides a non-human example of such behavior: in some populations, these birds fashion hooked foraging tools from branched vegetation in an elaborate multi-stage process [].

4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 6 Hunt G.R.

Gray R.D. The crafting of hook tools by wild New Caledonian crows. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 14 St Clair J.J.H.

Rutz C. New Caledonian crows attend to multiple functional properties of complex tools. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 15 Rutz C.

Sugasawa S.

van der Wal J.E.M.

Klump B.C.

St Clair J.J.H. Tool bending in New Caledonian crows. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 16 Troscianko J.

Rutz C. Activity profiles and hook-tool use of New Caledonian crows recorded by bird-borne video cameras. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 17 Hunt G.R.

Gray R.D. Diversification and cumulative evolution in New Caledonian crow tool manufacture. 18 Tebbich S.

Teschke I.

Cartmill E.

Stankewitz S. Use of a barbed tool by an adult and a juvenile woodpecker finch (Cactospiza pallida). 19 Fox E.A.

bin’Muhammad I. New tool use by wild Sumatran orangutans (Pongo pygmaeus abelii). 20 van Schaik C.P.

Ancrenaz M.

Djojoasmoro R.

Knott C.D.

Morrogh-Bernard H.C.

Nuzuar, Odom K.

Atmoko S.S.U.

van Noordwijk M.A. Orangutan cultures revisited. 21 Humle T.

Yamakoshi G.

Matsuzawa T. Algae scooping remains a puzzle. 22 Boesch C.

Kalan A.K.

Agbor A.

Arandjelovic M.

Dieguez P.

Lapeyre V.

Kühl H.S. Chimpanzees routinely fish for algae with tools during the dry season in Bakoun, Guinea. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 5 Shumaker R.W.

Walkup K.R.

Beck B.B. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. 3 O’Connor S.

Ono R.

Clarkson C. Pelagic fishing at 42,000 years before the present and the maritime skills of modern humans. 2 Yellen J.E.

Brooks A.S.

Cornelissen E.

Mehlman M.J.

Stewart K. A middle stone age worked bone industry from Katanda, Upper Semliki Valley, Zaire. To make a hooked tool, NC crows carefully remove a branch from a suitable plant (often by making cuts just above and below the joint), snip off unwanted leaves and trim the shaft, and finally “sculpt” a neat terminal hook from the wooden material of the nodal joint [] (according to our earlier analyses, active hook “processing” occurs in ∼80% of tool-manufacture episodes; []). Frequently, the crows add further design features [], by vigorously bending the tool shaft (which induces curvature that may improve tool “ergonomics” []) or by stripping off bark at the functional end (which may alter its mechanical properties []). During foraging, birds use the hooked tip for snagging arthropods hiding in deadwood and vegetation []. Some animal species use plant materials that have pre-existing or coincidentally formed hooks as tools. NC crows excise foraging tools from the barbed edges of screwpine leaves [], woodpecker finches have been observed to use thorny blackberry twigs to extract embedded prey [], and orangutans occasionally reach for vegetation with naturally hooked branches []. Interestingly, for algae harvesting, chimpanzees not only select stems with natural barbs or hooks (bends can become more pronounced as a result of heavy use) but also produce hooks by stripping away side branches or leaves (leaving behind stem bases) []. But at present, the NC crow remains the only non-human species known to sculpt hooks in the wild [], providing a valuable comparison to our ancestors’ production of fishhooks [] and barbed spears [].

4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 6 Hunt G.R.

Gray R.D. The crafting of hook tools by wild New Caledonian crows. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 8 St Clair J.J.H.

Klump B.C.

van der Wal J.E.M.

Sugasawa S.

Rutz C. Strong between-site variation in New Caledonian crows’ use of hook-tool-making materials. 14 St Clair J.J.H.

Rutz C. New Caledonian crows attend to multiple functional properties of complex tools. Figure 1 A Wild New Caledonian Crow Holding a Hooked Stick Tool Show full caption Apart from the distinctive terminal hook, which is used for snagging arthropod prey, the tool exhibits two additional design features that are typical for this particular population: pronounced curvature of the tool shaft, and stripped bark near the functional end. Photo: Pedro Barros da Costa. Despite considerable interest, surprisingly little was known until recently about the curious hook-tool-making of NC crows. The discovery of the behavior in the 1990s was based on the observation of just four tool-manufacture episodes [], and it was only years later that two birds could be lured to a baited feeding table, affording close-up views of the production of another ten tools []. Over the past few years, our team has identified three adjacent crow populations—in dry forest, farmland, and a beachside settlement, respectively—where birds routinely forage with hooked tools ( Figure 1 ), creating exciting opportunities for systematic studies []. Our work with free-ranging and temporarily captive crows has revealed that, although hooked tools generally share certain design features, they vary appreciably in overall shape (such as the degree of shaft curvature) and specific dimensional properties (such as hook depth). For example, while some tools have only a very small extension at the functional tip, others exhibit well-defined, deep hooks. This raises questions about the functional significance of tool morphology: Are some hooked tools more efficient than others, and if so, what does it take to make such a tool?

7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 14 St Clair J.J.H.

Rutz C. New Caledonian crows attend to multiple functional properties of complex tools. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 14 St Clair J.J.H.

Rutz C. New Caledonian crows attend to multiple functional properties of complex tools. 23 St Clair J.J.H.

Klump B.C.

Sugasawa S.

Higgott C.G.

Colegrave N.

Rutz C. Hook innovation boosts foraging efficiency in tool-using crows. 24 Hunt G.R. Social and spatial reintegration success of New Caledonian crows (Corvus moneduloides) released after aviary confinement. 8 St Clair J.J.H.

Klump B.C.

van der Wal J.E.M.

Sugasawa S.

Rutz C. Strong between-site variation in New Caledonian crows’ use of hook-tool-making materials. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. Figure 2 Causes and Consequences of Morphological Variation of Hooked Stick Tools Crafted by Wild-Caught New Caledonian Crows Show full caption (A) Age-dependent variation in tool shape. Points are subject-level averages (calculated for at least three different tools), with color coding indicating bird age (black, adult; gray, immature; pink, juvenile). Schematic illustrations of tools were produced from canonical variate analysis, representing tool shapes at the extremes of canonical variates 1 and 2. Sample size (main experiment): 117 tools made by 14 crows. (B) Hook length (blue) and hook depth (red) of tools (mean ± SEM [mm]) manufactured using the “cut” or “pull” method, or a mixed technique (see text and Figure S1 for details). Sample size (main experiment): 41 tools made by 13 crows. (C) Tool length (green), length to maximum curvature point (orange), hook length (blue), and hook depth (red) of tools (mean ± SEM [cm or mm]; see D for details) crafted by crows from plant materials of increasing rigidity (scores 1–8). Sample size (companion experiment): 28 tools made by 7 crows. (D) Morphological landmarks and measurements superimposed on a representative crow-made hooked stick tool. Summary statistics of measurements (mean ± SD) for crow-made tools were as follows: tool length, 14.31 ± 4.75 cm; length from the non-hooked end to the maximum curvature point, 9.29 ± 3.86 cm; hook length, 4.43 ± 2.13 mm; hook depth, 1.26 ± 1.11 mm. Sample size (main experiment): 122 tools made by 17 crows. (E) Extraction time (natural log-transformed; mean ± SEM [s] per tool) for dead spiders (left) and vermiform pieces of meat (right; data for wide and narrow holes pooled) as a function of hook depth (mm). Sample size (main experiment): 21 tools made and deployed by 11 crows; 13 human-made tools deployed by 8 crows. While the fitted lines are for linear mixed models that exclude unsuccessful bait-extraction attempts (16 holes, with 8 tools made by 5 crows) (see main text for statistical results), a corresponding mixed-effect Cox proportional hazards model including these cases confirmed the significant effect of hook depth (z = 2.34, p = 0.02; see STAR Methods for details). In the present study, we investigated experimentally what extrinsic and intrinsic factors (crow age, manufacture method, and plant properties) determine the morphology of hooked tools, and how tool morphology in turn affects crows’ foraging efficiency. Using recently established protocols [], we trapped crows in our farmland study site and held them temporarily in field aviaries—a method that has been shown to be both scientifically productive [], and well tolerated by this species []. During experimental trials, we allowed subjects to manufacture hooked stick tools from a choice of their preferred plant material, forked stems of the shrub Desmanthus virgatus [], and observed how they subsequently used these tools to extract bait from naturalistic foraging tasks ( Figure 2 ; see STAR Methods and [] for details). Tools were recovered after trials, identified from video footage, and measured using digital reference photographs.

2 2 = 2.33, p = 0.31)—that appeared less variable in overall shape than those made by younger individuals (SD of canonical variates 1 and 2; adults: 0.45 and 0.43; immatures: 1.12 and 1.07; juveniles: 1.02 and 1.12) ( 6 Hunt G.R.

Gray R.D. The crafting of hook tools by wild New Caledonian crows. 25 Gruber T.

Zuberbühler K.

Clément F.

van Schaik C. Apes have culture but may not know that they do. 26 Logan C.J.

Breen A.J.

Taylor A.H.

Gray R.D.

Hoppitt W.J.E. How New Caledonian crows solve novel foraging problems and what it means for cumulative culture. 27 Rutz C.

Klump B.C.

Komarczyk L.

Leighton R.

Kramer J.

Wischnewski S.

Sugasawa S.

Morrissey M.B.

James R.

St Clair J.J.H.

et al. Discovery of species-wide tool use in the Hawaiian crow. Adult crows manufactured distinctive tools (adults versus immatures: Mahalanobis distance [MD] = 3.67, p < 0.001; adults versus juveniles: MD = 2.68, p = 0.07; immatures versus juveniles: MD = 2.04, p = 0.09)—with pronounced shaft curvature and medium-sized hooks (hook depth, mean ± SD; adults: 1.21 ± 0.64 mm; immatures: 1.04 ± 0.92 mm; juveniles: 1.83 ± 1.59 mm; hook depth by itself did not differ significantly between age classes: χ= 2.33, p = 0.31)—that appeared less variable in overall shape than those made by younger individuals (SD of canonical variates 1 and 2; adults: 0.45 and 0.43; immatures: 1.12 and 1.07; juveniles: 1.02 and 1.12) ( Figure 2 A). This suggests that tool-making skills, or preferences for certain tool shapes, change as birds mature []. For example, older individuals may converge on similar designs through individual trial-and-error learning (see []) and/or the social transmission of tool-related information (either via observation of other birds or interaction with their tools []). Future work should investigate the ontogeny of hook-tool-making, ideally under controlled experimental conditions [].

7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. NC crows in our study population use two main methods for releasing (basic) hooked stick tools from plants—“cutting” and “pulling” ( Figure S1 ; for video clips illustrating these actions, see Additional File 1 in []). Our main experiment demonstrated that, with relatively standardized plant materials, cutting (13 cases, only one of which was without subsequent hook processing) enables the production of significantly deeper hooks (comparison between cut and pull: t = –2.21, p = 0.03; trend for longer hooks, t = –1.73, p = 0.09; Figures 2 B and 2D); while this increases tool efficiency (see below), it necessitates two separate actions—one cut above and one below a branching joint. In contrast, pulling (20 cases, two of which were without hook processing) leads to shallower hooks on average, presumably as less material remains at the tool tip for sculpting after detachment, but it has the advantage that a single action yields a basic tool. Sometimes, a single cut is combined with a pull (cut-pull: 6 cases; pull-cut: 2 cases; Figure S1 ), producing hooks of intermediate depth ( Figure 2 B). A trade-off between tool efficiency and manufacture costs provides a potential explanation for the coexistence of different release techniques in our study population (for further discussion, see below).

7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 23 St Clair J.J.H.

Klump B.C.

Sugasawa S.

Higgott C.G.

Colegrave N.

Rutz C. Hook innovation boosts foraging efficiency in tool-using crows. 28 Hunt G.R.

Uomini N. A complex adaptive system may be essential for cumulative modifications in tool design. 29 Hunt G.R.

Gray R.D. Species-wide manufacture of stick-type tools by New Caledonian crows. 30 Holzhaider J.C.

Hunt G.R.

Campbell V.M.

Gray R.D. Do wild New Caledonian crows (Corvus moneduloides) attend to the functional properties of their tools?. 6 Hunt G.R.

Gray R.D. The crafting of hook tools by wild New Caledonian crows. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 14 St Clair J.J.H.

Rutz C. New Caledonian crows attend to multiple functional properties of complex tools. 15 Rutz C.

Sugasawa S.

van der Wal J.E.M.

Klump B.C.

St Clair J.J.H. Tool bending in New Caledonian crows. 17 Hunt G.R.

Gray R.D. Diversification and cumulative evolution in New Caledonian crow tool manufacture. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 6 Hunt G.R.

Gray R.D. The crafting of hook tools by wild New Caledonian crows. 31 Takemoto H.

Hirata S.

Sugiyama Y. The formation of the brush-sticks: modification of chimpanzees or the by-product of folding?. 5 Shumaker R.W.

Walkup K.R.

Beck B.B. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. 32 Hansell M. Animal Architecture. 33 Hansell M.

Ruxton G.D. Setting tool use within the context of animal construction behaviour. It has been suggested that hooked stick tools have evolved from basic non-hooked stick tools through a process of “cumulative” modification []. In fact, the cost-efficient single-step pulling method observed in our study population resembles the production of basic non-hooked stick tools—where crows swiftly snap off twigs from nodal joints []—and may therefore represent an evolutionary precursor of more involved multi-step manufacture techniques (as described in []). Over time, crows may have gradually improved tool efficiency further, by processing the hook, stripping the bark off the functional end, and/or bending the tool shaft ( Figure 2 D; []; cumulative refinement of tool designs has previously been suggested for the tools that NC crows make from screwpine leaves—see []). Interestingly, with some plant species, pulling apparently does not produce hooks [], and crows exclusively employ the cutting technique []; similarly, only some plants appear suitable for the production of brush-sticks by chimpanzees []. We suspect that comparable effects of raw-material properties on manufacture methods, and ultimately on artifact morphology, are widespread in animal tool use and construction behavior [].

2 1 = 10.04, p = 0.002; 2 1 = 3.63, p = 0.06) likewise increased with increasing material score (no effect of manufacture method), as did overall tool dimensions (tool length: p = 0.02; length from the non-hooked end to the maximum curvature point: p = 0.05; 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 15 Rutz C.

Sugasawa S.

van der Wal J.E.M.

Klump B.C.

St Clair J.J.H. Tool bending in New Caledonian crows. 14 St Clair J.J.H.

Rutz C. New Caledonian crows attend to multiple functional properties of complex tools. 34 Troscianko J.

von Bayern A.M.P.

Chappell J.

Rutz C.

Martin G.R. Extreme binocular vision and a straight bill facilitate tool use in New Caledonian crows. 10 Sanz C.

Morgan D.

Gulick S. New insights into chimpanzees, tools, and termites from the Congo Basin. 35 Visalberghi E.

Sirianni G.

Fragaszy D.

Boesch C. Percussive tool use by Taï Western chimpanzees and Fazenda Boa Vista bearded capuchin monkeys: a comparison. 31 Takemoto H.

Hirata S.

Sugiyama Y. The formation of the brush-sticks: modification of chimpanzees or the by-product of folding?. 36 Gaino E.

Cianficconi F.

Rebora M.

Todini B. Case-building of some Trichoptera larvae in experimental conditions: Selectivity for calcareous and siliceous grains. 7 Klump B.C.

Sugasawa S.

St Clair J.J.H.

Rutz C. Hook tool manufacture in New Caledonian crows: behavioural variation and the influence of raw materials. 9 Gowlett J.A.J. Artefacts of apes, humans, and others: towards comparative assessment and analysis. 37 Sharon G. The impact of raw material on Acheulian large flake production. 38 Eren M.I.

Roos C.I.

Story B.A.

von Cramon-Taubadel N.

Lycett S.J. The role of raw material differences in stone tool shape variation: An experimental assessment. In a companion experiment using seven NC crows from our main sample, we found that the properties of plant raw materials influenced several aspects of hooked tool morphology. After controlling for the effect of manufacture method, hook depth increased significantly with stem rigidity (χ= 10.04, p = 0.002; Figure 2 C). Hook length (χ= 3.63, p = 0.06) likewise increased with increasing material score (no effect of manufacture method), as did overall tool dimensions (tool length: p = 0.02; length from the non-hooked end to the maximum curvature point: p = 0.05; Figure 2 C). The increase in tool length with increasing material score may simply be due to the fact that tool shafts of higher rigidity were difficult to sever close to the joint. Interestingly, the relative position of the maximum curvature point changed little across material scores (p = 0.62), resulting in a relatively consistent overall tool shape as tool dimensions increased (see STAR Methods ). Although this may reflect allometric properties of the plant material, crows often actively adjust tool curvature through shaft bending [], presumably in an effort to keep the hook centered in the field of binocular vision during deployment []. Although many studies have investigated tool material selectivity in primates (e.g., []), the effects of material properties on the morphology of manufactured tools remain poorly documented (e.g., []). Our work on NC crows has shown that plant properties affect aspects of manufacture behavior [] as well as the morphology of the resulting tools (our present study). Such research on extant tool-using animals provides a valuable window into early human tool making, where the relationships between raw materials, crafting techniques, tool morphology, and tool functionality remain a topic of great interest [].

23 St Clair J.J.H.

Klump B.C.

Sugasawa S.

Higgott C.G.

Colegrave N.

Rutz C. Hook innovation boosts foraging efficiency in tool-using crows. 4 Hunt G.R. Manufacture and use of hook-tools by New Caledonian crows. 16 Troscianko J.

Rutz C. Activity profiles and hook-tool use of New Caledonian crows recorded by bird-borne video cameras. 2 2 = 1.56, p = 0.46). Including trials where crows were offered human-made exemplar hooked tools allowed us to extend the range of hook depths over which extraction efficiency could be measured and confirmed the pattern observed with crow-made tools only (z = 2.56, p = 0.01). To our knowledge, this is the first demonstration that variation within a specific design feature of crafted animal tools can affect foraging performance, adding to studies that found similar effects for researcher-deployed replica hominin [ 39 Cheshier J.

Kelly R.L. Projectile point shape and durability: the effect of thickness:length. 40 Key A.J.M.

Lycett S.J. Are bigger flakes always better? An experimental assessment of flake size variation on cutting efficiency and loading. 41 Sanz C.

Call J.

Morgan D. Design complexity in termite-fishing tools of chimpanzees (Pan troglodytes). Having identified three significant drivers of variation in NC crow tool morphology—bird age, manufacture method, and raw-material properties—we next examined whether hook geometry in turn affects foraging efficiency. As a simple performance metric, we measured how long it took our subjects to extract two types of “prey” (dead spiders and worm-like cylinders of meat) from standardized holes drilled into wooden logs []—tasks that resemble foraging scenarios routinely encountered by wild crows []. We found that extraction speed increased significantly with hook depth (z = 2.80, p = 0.005; Figure 2 E), with spiders being extracted more quickly on average than vermiform prey (spider in wide hole versus vermiform prey in wide hole: z = 3.67, p < 0.001; spider in wide hole versus vermiform prey in narrow hole: z = 3.65, p < 0.001; no significant difference in slopes: χ= 1.56, p = 0.46). Including trials where crows were offered human-made exemplar hooked tools allowed us to extend the range of hook depths over which extraction efficiency could be measured and confirmed the pattern observed with crow-made tools only (z = 2.56, p = 0.01). To our knowledge, this is the first demonstration that variation within a specific design feature of crafted animal tools can affect foraging performance, adding to studies that found similar effects for researcher-deployed replica hominin [] and chimpanzee tools [].

42 Bluff L.A.

Troscianko J.

Weir A.A.S.

Kacelnik A.

Rutz C. Tool use by wild New Caledonian crows Corvus moneduloides at natural foraging sites. 43 Holzhaider J.C.

Hunt G.R.

Gray R.D. The development of pandanus tool manufacture in wild New Caledonian crows. Based on the findings from our two experiments, one might expect that experienced adult crows use the more controlled cutting technique, to produce deep hooks that enable faster prey extraction. We found instead that adults frequently used pulling (for details, see Figure S1 ), yielding hooks of intermediate depth (see above). This may be because there are yet-to-be-investigated costs associated with deeper hooks, including increased manufacture effort, high rates of hook damage, and/or reduced performance in very tight crevices such as beneath tree bark. Such hidden costs would imply the existence of an optimal hook depth. Thus, although we still do not know whether foraging performance with hooked stick tools increases with age (and hence experience; see []), it is conceivable that older birds optimize returns from a given tool by trading off extraction speed against one or more other factors. In general, it remains an important challenge for future studies to assess—with larger sample sizes and dedicated experimental designs—how different extrinsic and intrinsic factors interact to drive variation in tool morphology, and ultimately tool and foraging efficiency.