Manufacture, morphology and use of hook tools in New Caledonian crows

Abstract

Tool use is widespread across taxa, but complex tool behaviour is limited to only a few species, mainly primates. The New Caledonian crow Corvus moneduloides is endemic to New Caledonia, South Pacific, and uses at least three tool types for extractive foraging. New Caledonian crows can serve as a valuable avian study system for investigating aspects of advanced tool behaviours. In particular, these birds are the only non-human animals known to manufacture hooked tools in the wild, and may therefore provide valuable insights into hominin technological evolution. In this thesis, I investigated the manufacture, morphology and use of hooked stick tools in New Caledonian crows. I conducted a suite of behavioural experiments with wild-caught, temporarily-captive crows, where subjects were presented with tool materials and naturalistic extraction tasks. I identified two methods by which crows release basic hooked tools from branched vegetation, cutting and pulling, and acquired the first detailed observations of two processing behaviours that may improve tool functionality, bending of the tool shaft and stripping of bark. Manufacture behaviour was more variable than previously thought, but there were no systematic age effects. Other experiments highlighted that crows were extremely dextrous in handling their tools, and enabled first detailed analyses of the morphology of crow-made hooked stick tools. Taken together, my work offered valuable insights into the possible evolutionary origins of hook making in New Caledonian crows. I propose that hooked stick tools derive from more basic, non-hooked stick tools. Pulling of side branches may have occasionally produced small hook-like extensions that provided a foraging advantage; later on, cutting release techniques afforded more control over tool shape, and other modifications like shaft bending and bark stripping may have been added. Hook making in New Caledonian crows may provide a rare example of cumulative material evolution outside humans.

Date of Award	30 Nov 2016
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Christian Rutz (Supervisor)