The remarkable tool making of crows offers scientists a chance to search in other species for the equivalent of human handedness, say New Zealand researchers.
New Caledonian crows make probes from tree leaves to poke at hard-to-reach prey, explains Gavin Hunt of the University of Auckland. After analyzing the leaves left over from the production of 3,700 crow tools, Hunt and his colleagues conclude that the birds prefer working on the left edge of a leaf instead of the right. The researchers report their finding in the Dec. 13 Nature.
The crow, Corvus moneduloides, lives only on New Caledonia’s Grande Terre and adjacent Marc Island in the Pacific. In 1996, Hunt described these crows fashioning a wide range of tools from sticks or the edges of Pandanus leaves.
The Pandanus tree rises to a whorl of stiff, straplike leaves spiraling either clockwise or counterclockwise. Hunt visited trees at 19 sites along most of the length of Grand Terre. He sampled from both kinds of spirals to collect leaves that still bore the outline cut by crows.
To create a tapered probe, the bird begins tearing a strip near the base of the leaf and occasionally stops to snip the leaf crosswise to widen the tool.
The clockwise trees offer a bird easier access to a leaf’s left side. Hunt found that in his clockwise sample, crows had worked about 1,500 left edges and 400 right edges. On the counterclockwise trees, right edges are more accessible. Despite the inconvenience, the crows had worked left edges on some 950 leaves and right edges on only 900.
Other tests of the side preferences of animals, such as parrots, have focused on small groups. Hunt says that the crow work “is the first study that’s been able to say that across the species, yes, there’s a preference for one side.”
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To work the left side of a leaf, the bird views the action through its right eye and snips with the right side of its beak. Hunt and his colleagues propose that the left side of the brain controls the sequence of actions, as the human left hemisphere controls language. They also suggest that the crows’ preference could be equivalent to human right-handedness.
That proposal sounds “plausible, because it comes in the context of an already extraordinary group of animals,” comments animal behaviorist Alex Kacelnik of Oxford University in England. New Caledonian crows live in his lab and have impressed him by choosing correctly among various materials to find the best tool for a task.
Kacelnik says he suspects that learning plays a role in the crows’ repertoire and might complicate analysis. If crows were imitating left-leaf-edge pioneers, they might give the impression of an intrinsic left bias. But brain biology might have created the abundance of left-edge teachers. Only rearing experiments can sort out the question, he says.
The leaf analysis interests primate biologist William C. McGrew of Miami University in Oxford, Ohio, because “it’s the best data set on natural laterality of motor function, outside the apes,” he says.
McGrew cautions, however, that he’s “not so sure we should get too carried away with analogy to language and right-handedness, based on one task.” He says he’s looking forward to research exploring whether crows favor one side when they use the tools.