Abstract
New Caledonian crows (NCCs) are proficient tool manufacturers and flexible problem solvers. A study reporting innovative hook bending by a captive individual in 2002 (Weir et al., 2002), has since been cited as a remarkable case of causal reasoning in animals. Recently, however, tool bending has been found to form part of the species’ natural behavioral repertoire. Yet, further studies examining the flexibility of this behavior in controlled novel problem-solving contexts are lacking. We tested 17 captive NCCs, in one tool bending and three unbending problem-solving experiments. In the first study, two groups of five subjects were trained to select inflexible straight or hooked tools respectively for retrieving food from an apparatus. In the subsequent test, the crows were supplied with the wrong, but now pliable, type of tool. Three subjects made hooks while one subject straightened tools consistently. In the second study, 13 subjects, including five from Study 1, were tested. Ten subjects received an unbending task with a Z-shaped tool without prior training. Both angles needed to be straightened for the tool to become sufficiently long for reward retrieval. Four subjects solved the task at least once, and two did so consistently. Eight subjects, including the previous solvers, were challenged with yet another apparatus, requiring novel unbending behavior. One crow succeeded in both unbending tasks consistently. Our results show that NCCs can flexibly bend or unbend pliant material to create tools for solving novel tasks. The underlying cognitive mechanisms seemingly varied across individuals although trial-and-error learning prevailed.
Highlights
New Caledonian crows (NCCs) are noted for their proficient and flexible tool use and tool manufacture
The first report of innovative tool manufacture in birds, namely the spontaneous bending of a hook by the captive NCC called Betty in 2002, was a significant finding in the field of animal cognition (Weir et al, 2002). This first recorded tool bending occurred in an novel problem-solving context, during a tool choice experiment, in which the hook tool required to lift a basket from a transparent, vertical tube had been van Buuren et al 191 removed by the subject’s interfering mate
Our aim was to assess whether captive NCCs, both wild-caught and reared in captivity, can solve a novel problem by creating a tool with the correct functional properties through modification of pliant materials by bending or unbending
Summary
New Caledonian crows (NCCs) are noted for their proficient and flexible tool use and tool manufacture To examine whether this was mere serendipity, the experimenters reproduced the circumstances with a single straight wire several times and, Betty continued to successfully modify and use the wire in the same manner, i.e., by wedging one end under some tape securing the tube or under her foot and pulling the rest of the wire up to create a hook at the distal end Throughout this series of trials, the specific steps of the behavioral sequence she used were not rewarded, except for the extraction of the rewarded bucket. In a follow-up study, Weir and Kacelnik (2006) tested the same subject in another bending task, but with strips made from tougher aluminum that could not be modified with the previously used technique With this new material, Betty first attempted to use the same technique, i.e., wedging one end of the tool under the base of the apparatus, pulling it up to create a hook. The conclusions were predicated on bending behavior not being part of the natural behavioral repertoire of NCCs, and a novel tool modification for solving a specific task
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