Abstract
BackgroundNew Caledonian crows use a range of foraging tools, and are the only non-human species known to craft hooks. Based on a small number of observations, their manufacture of hooked stick tools has previously been described as a complex, multi-stage process. Tool behaviour is shaped by genetic predispositions, individual and social learning, and/or ecological influences, but disentangling the relative contributions of these factors remains a major research challenge. The properties of raw materials are an obvious, but largely overlooked, source of variation in tool-manufacture behaviour. We conducted experiments with wild-caught New Caledonian crows, to assess variation in their hooked stick tool making, and to investigate how raw-material properties affect the manufacture process.ResultsIn Experiment 1, we showed that New Caledonian crows’ manufacture of hooked stick tools can be much more variable than previously thought (85 tools by 18 subjects), and can involve two newly-discovered behaviours: ‘pulling’ for detaching stems and bending of the tool shaft. Crows’ tool manufactures varied significantly: in the number of different action types employed; in the time spent processing the hook and bending the tool shaft; and in the structure of processing sequences. In Experiment 2, we examined the interaction of crows with raw materials of different properties, using a novel paradigm that enabled us to determine subjects’ rank-ordered preferences (42 tools by 7 subjects). Plant properties influenced: the order in which crows selected stems; whether a hooked tool was manufactured; the time required to release a basic tool; and, possibly, the release technique, the number of behavioural actions, and aspects of processing behaviour. Results from Experiment 2 suggested that at least part of the natural behavioural variation observed in Experiment 1 is due to the effect of raw-material properties.ConclusionsOur discovery of novel manufacture behaviours indicates a plausible scenario for the evolutionary origins, and gradual refinement, of New Caledonian crows’ hooked stick tool making. Furthermore, our experimental demonstration of a link between raw-material properties and aspects of tool manufacture provides an alternative hypothesis for explaining regional differences in tool behaviours observed in New Caledonian crows, and some primate species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-015-0204-7) contains supplementary material, which is available to authorized users.
Highlights
New Caledonian crows use a range of foraging tools, and are the only non-human species known to craft hooks
Experiment 1 – Variation in tool-manufacture behaviour Each crow was provided with an extraction task and several forked stems of Desmanthus virgatus (Fig. 1a) that were judged to be suitable for hooked stick tool manufacture
Following an initial observation that tools recovered in our study site often lacked bark at the hooked end and exhibited pronounced curvature [43], we confirmed with our behavioural experiments that these design features are actively induced by crows (Additional file 3: Movie 2), rather than merely being a by-product of tool detachment or deployment
Summary
New Caledonian crows use a range of foraging tools, and are the only non-human species known to craft hooks. New Caledonian (NC) crows (Corvus moneduloides) manufacture a diversity of tools, which they use for extracting embedded prey [1, 2] Two of their tool types have ‘hooks’: pandanus tools (made from the leaf edges of screw pines, Pandanus spp.) and hooked stick tools (made from forked plant stems) [1, 2]. While the former have multiple barbs that occur naturally on the plant material (as in the blackberry tools used by woodpecker finches [3]), the latter have a single terminal hook that is actively crafted by the bird [1, 4], representing the only known example of hook production by a non-human species [5]. Only 14 manufacture episodes have been documented: four for unmarked, free-ranging crows [1], and another 10 for two wild subjects that visited a baited feeding table [4]
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