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ABSTRACT.-New Caledonian Crows (Corvus moneduloides) manufacture the most complex foraging tools used by nonhuman animals. Not only do they shape multiple tool designs of different complexity out of raw material using distinct, design-specific manufacture techniques, they are the only species to incorporate hook technology. The three different hook tool designs that they cut out of barbed Pandanus spp. leaves are suggested to have evolved by a process of diversification through cumulative changes rather than independent invention. Here, I describe three examples of an oversized version of the so-called 'narrow' pandanus tool design in an area where narrow tools are also made. My observation of the way a crow used one of these oversized tools in the wild suggests that they may be efficient for foraging in especially deep probe sites. The co-occurrence of two different designs originating from a very similar manufacture technique is consistent with diversification. Furthermore, qualitative data suggest that shape variation in the three previously described pandanus tool designs might be associated with ecological function. These findings strengthen the possibility that pandanus tool designs are an example of rudimentary diversification developed in close association with functional requirements. Received 16 May 2013. Accepted 5 November 2013.
Key words: Corvus moneduloides, cumulative change, design diversification, New Caledonian Crow, pandanus tools, tool manufacture, tool use.
New Caledonian Crows (NC crow; Corvus moneduloides) manufacture complex foraging tools because they shape distinct tool designs out of raw material that incorporate hook technology (Hunt 1996, Hunt and Gray 2002, 2004a, b). One material that they use to do this is Pandanus spp. (screw pine) leaves. The crows manufacture three distinct hook-tool designs from the barbed edges of these leaves: wide tool, narrow tool and stepped tool designs (Hunt and Gray 2003). Variation in the shape of each design mostly exists between sites, with a high degree of shape consistency within individual sites (Hunt and Gray 2003). The shape variation within both the uniformly broad wide tool and narrow tool designs is mostly to do with tool length. More complex shape variation exists among stepped tools because the characteristics of the tapered edge vary as well as tool length (e.g., the number and spacing of steps).
The manufacture of a pandanus tool can be inferred from inspecting missing...