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2015 ; 13
(ä): 97
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Hook tool manufacture in New Caledonian crows: behavioural variation and the
influence of raw materials
#MMPMID26582537
Klump BC
; Sugasawa S
; St Clair JJ
; Rutz C
BMC Biol
2015[Nov]; 13
(ä): 97
PMID26582537
show ga
BACKGROUND: New 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. RESULTS: In 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. CONCLUSIONS: Our 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.