Though it was dogma for decades, the notion that tool-making distinguishes humans from other animals turned out to be a flattering delusion. But of all the animal artisans, only humans seemed to shape tools according to a set pattern and with a favored hand. Now, an ecologist claims to have found both those traits among crows in the southwest Pacific. He suggests that studying these birds may shed light on the mental transformation of our hominid ancestors as they went from idle rock bashing to purposeful sculpting of tools.
Smaller and more demure than their raucous American cousins, crows living on the island of New Caledonia have a knack for fashioning twigs into hooks. They also snip serrated probes from the barbed, elongated leaves of the screw pine. The crows use both kinds of tools to winkle tasty insects out of holes and crevices.
To learn how crows craft these leaf tools, Gavin Hunt of the University of Auckland in New Zealand studied their raw materials. Notched scars on the remnants of screw pine leaves record each tool's shape. After studying more than 1300 leaf scars from two sites, Hunt discovered that the tools usually resemble the tapered blades of a saw. As they hew this intricate pattern, the birds must alternate angled cuts with horizontal rips. That the birds take such pains--when they could make a serviceable dipstick simply by tearing off a straight strip of leaf--suggests a mental template, Hunt argues in the 22 February Proceedings of the Royal Society.
Although handless, crows manifest "handedness" in their preference for one side of the leaf, Hunt says. At both sites, the crows removed the majority of their tools from the left edge of the leaf, snipping with the right side of the bill and guiding the work with the right eye. "I can't see any obvious explanation for that preference in the structure of the plants or anything else," Hunt says. In humans, handedness goes along with the specialization of one side of the brain for precise manipulation, an asymmetry Hunt hopes to look for in crow brains.
The work is intriguing, says Mary Marzke, a physical anthropologist at Arizona State University, but she wants to know more about how the crows manipulate other objects before she's willing to accept they are natural righties.