Tool use among animals isn't common, but it is spread widely across our evolutionary tree. Critters from sea otters to cephalopods have been observed using tools in the wild. In most of these instances, however, the animal is simply using something that's found in its environment, rather than crafting a tool specifically for a task. Tool crafting has mostly been seen among primates.

Mostly, but not entirely. One major exception is the New Caledonian crow. To extract food from holes and crevices, these birds use twigs or stems that are found in their environment without modification. In other environments, however, they'll remove branches from plants and carefully strip parts of the plant to leave behind a hooked stick. The behavior takes over a minute, and the crows will typically carry the tool with them when they explore new sites, and they will sometimes store it for future use.

Understanding how this complex behavior came about in crows requires us to understand the evolutionary advantages that might be had from a good tool. A group of researchers, mostly from the University of St. Andrews, has now done just that: the researchers have quantified how tool manufacture influences food harvesting. The results show that the use of bird-crafted tools can increase food extraction by up to 12 times the rate the crows could achieve by using unmodified sticks.

The researcers' approach was simple. They constructed a series of logs with drill-holes of various sizes and stuffed food into the bottom of the holes. The crows were either provided with chunks of a bush from which they typically manufacture tools, or the birds were given a bunch of straight sticks that could be used as tools, but not modified. The researchers tracked how long the food in the bottom of a hole stayed there after the birds started trying to extract it. This "survival time" should provide a rough measure of the efficiency of the tool being used.

Overall, there was no contest. Crow-made tools were anywhere from three to 13 times more effective at pulling out food than the twigs the researchers provided. If the researchers hand-made a crow-style hooked tool, then that was between six and nine times more efficient than an unmodified twig. While the range of outcomes was narrower with the human-made tools, the average efficiency was statistically indistinguishable from what the crows achieved when they made their own tools.

The authors say this clearly demonstrates a significant advantage for manufactured tools. And that, they argue, means that toolmaking could be under evolutionary selection.

"Could," however, doesn't mean "is." The authors are excited about toolmaking being a genetic trait that's directly selected for, since that would be relatively easy to search for. But we don't know anything about the origin of toolmaking among these crows, and we do know that other birds learn socially, including learning involving tools (that's been seen in cockatoos). So there's the chance that social behaviors have been under selective pressure and the ability of tool use to spread in the population was just a non-specific side effect.

Nature Ecology & Evolution, 2018. DOI: 10.1038/s41559-017-0429-7 (About DOIs).