A team of researchers at the University of St Andrews has discovered how New Caledonian crows (Corvus moneduloides) make one of their most sophisticated tool designs — sticks with a neatly-shaped hooked tip. The study is published in the journal Current Biology.

The hook is widely regarded as one of humankind’s most important innovations, with skilful reshaping, a useless piece of raw material is transformed into a powerful tool.

While our ancestors started making stone tools over 3 million years ago, hooks are a surprisingly recent advance — the earliest known hooked tools appeared as recently as 90,000 years ago.

New Caledonian crows are the only non-human animal known to craft hooks in the wild.

The birds manufacture hooked stick tools in a multi-stage process, involving the detachment of a branch from vegetation; ‘sculpting’ of a hook from the nodal joint; and often additional adjustments, such as length trimming, shaft bending, and bark stripping.

Although tools made by a given population share key design features, they vary appreciably in overall shape and dimensions.

Using wild-caught New Caledonian crows, University of St Andrews’ Professor Christian Rutz and colleagues investigated causes and consequences of variation in hook-tool morphology.

“We suspected that tools with pronounced hooks are more efficient, and were able to confirm this in controlled experiments with wild-caught crows,” Professor Rutz said.

“The deeper the hook, the faster birds winkled bait from holes in wooden logs.”

This finding raised the intriguing question of what it takes to make such well-formed hooks.

Professor Rutz and co-authors started planning their study by imagining how humans would approach a comparable task.

“When a craftsperson carves a tool from a piece of wood, two things ensure a quality product: good raw materials and skill,” Professor Rutz said.

The team found that the same, apparently, applies to New Caledonian crows.

The researchers discovered that the depth of the hook was influenced by both the properties of the plant material, and the technique crows used for detaching branches.

When birds made controlled cuts with their sharp bills, the resulting hooks were significantly deeper than when they used a ‘sloppier’ alternative method of simply pulling off branches.

Careful cutting may leave more wooden material at the tip of the stick from which the hook can subsequently be ‘sculpted.’

Surprisingly, adult crows, which are expected to have considerable tool-making experience, did not produce the deepest hooks and regularly employed the ‘quick-and-dirty’ manufacture technique.

“Making very deep hooks may not be the best strategy in the wild,” Professor Rutz noted.

“It probably takes more time and effort to make such tools, and experienced birds may try to avoid these costs. It is also possible that deep hooks break more easily when inserted into narrow holes and crevices,” he said.

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Shoko Sugasawa et al. Causes and Consequences of Tool Shape Variation in New Caledonian Crows. Current Biology, published online December 7, 2017; doi: 10.1016/j.cub.2017.11.028