Neural processing and not ecological factors may influence 'handedness' in these birds.
New Caledonian crows (Fig. 1) fashion tapered tools from either the left or the right edge of the long narrow leaves of pandanus trees or screw pines1,2, which they use to extract invertebrates in rainforest vegetation2. Although right-handedness is thought to be uniquely human3, we show here that crows from different localities display a widespread laterality in making their tools, indicating that this behaviour is unlikely to be attributable to local social traditions or ecological factors. To our knowledge, this is the first demonstration of species-level laterality in manipulatory skills outside humans.
We can detect laterality in the manufacture of stepped tools (Fig. 2a) by New Caledonian crows (Corvus moneduloides) because this differs according to whether the left or the right leaf-edge is used1. Birds carry out a well-defined sequence of precise cutting and ripping actions on both edges, with bill action being away from the tree trunk: they must therefore work from either right to left (left edges) or left to right (right edges). The right side of the head is generally towards the leaf edge in left-edge usage, and the left side in right-edge usage. A tool's finished shape is recorded in its unmistakable 'counterpart', the outline remaining on the leaf edge (Fig. 2a).
We collected 3,727 stepped-tool counterparts from 733 pandanus trees at 19 sites scattered throughout mainland Grande Terre, New Caledonia, the only island where crows are known to make these tools. The sites were mostly at least 10 km apart and distributed along 300 km of the narrow, 400-km-long island at altitudes of 160–1,600 m above sea level. This sampling covered the geographical range of pandanus-toolmaking (G. R. H., personal observation).
The crow's use of left or right leaf-edges depends in part on the direction in which the leaves spiral. Clockwise-spiralling leaves provide easier access to left edges, and anticlockwise-spiralling leaves provide easier access to right edges. This access effect was overridden, however, by an island-wide preference for manufacturing tools from left edges (Fig. 2b).
Population-level footedness in manipulating food while feeding is known in parrots4. In great apes, population-level handedness has been reported in several manipulative activities in wild gorillas5 (Gorilla g. beringei) and in captive chimpanzees (Pan troglodytes)6,7,8, but has not been shown so far in chimpanzees in the wild9,10. Although there have been no systematic comparisons, it has been suggested that the level of complexity of a motor task and the associated neural-processing demands may correlate with the degree of lateralization8,11.
The rather simple nature of chimpanzees' tool-making contrasts with the greater complexity of crows' stepped-tool manufacture1. The shape of stepped tools is highly regular and mostly determined by the crows themselves, rather than by the raw material, implying that a neural programme is involved in their manufacture1. Laterality in crows may therefore reflect the complexity of stepped-toolmaking and a specialization of the right-eye/left-hemisphere system for non-spatial, sequential actions, as previously demonstrated in birds and mammals8.
It has been proposed that right-handedness in humans may be a consequence of the evolution of language, which is also predominantly left-hemispheric3. Our results favour the more general possibility that species-level lateralization is an adaptation for the efficient neural programming of complex sequential processing8,12, of which language and right-handedness in humans and stepped-tool manufacture in crows are examples.
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Hunt, G., Corballis, M. & Gray, R. Laterality in tool manufacture by crows. Nature 414, 707 (2001). https://doi.org/10.1038/414707a
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