New Caledonian crows are famous for their ability to craft sticks into hooked tools, which they use to probe for larvae and insects hidden in trees. But why they do this has been anyone's guess — until now.
As anyone who has ever struggled to open a bag of chips knows, grabbing a pair of scissors allows you faster access to your snack. For crows, a hooked stick dislodges their grub 10 times faster than a regular stick, according to a new study in Nature Evolution and Ecology.
The research establishes a potential evolutionary driver for this type of technological advancement among New Caledonian crows, the only nonhuman species known to manufacture hooked tools.
"If you have a tool that is 10 times more efficient, it will considerably raise your survival prospects — there's more food to raise your offspring," said Christian Rutz, a behavioral ecologist at the University of St. Andrews in Scotland who led the project. "It just pays off."
On New Caledonia, a forest-covered island east of Australia, the jet-black crows pick out sticks to help them hunt for winkle beetle grubs and insects. Some of the crows take an extra step.
Researchers have observed the birds as they select a forked stem from a particular local shrub. Then the crows trim the side stems and carefully carve and peel the end until it forms a "neat little hook," Rutz said.
The birds must think their creations are neat, too — they often carry, store and re-use their tools.
Rutz and his colleagues wanted to know why these birds go the extra mile to perform such a complex behavior.
So they captured 17 wild crows and temporarily housed them in field aviaries. Some of the birds were allowed to fashion their hooked tools from the local shrub that's a favorite material for tool-making. Others were provided nonforked twigs and stalks that researchers collected from the forest.
Both groups of crows were timed as they tried to fish out spiders and pieces of meat placed in holes drilled in a log. Depending on the task, the crows armed with hooks retrieved the bait three to 13 times faster than the crows that stuck with straight sticks.
But this isn't just a study about crafty crows, Rutz said. "It's about why, in very few cases, technology advances."
The oldest known fishing hooks are only about 23,000 years old. About 1,000 generations later, we now have smartphones, supercomputers and an International Space Station. In evolutionary terms, that's a blink of an eye, Rutz said.
Animals that use tools typically use only one type of tool. Chimpanzees employ stones for a variety of tasks, sea otters bang open shells with rocks and Galapagos woodpecker finches use twigs to hunt.
But aside from humans, the New Caledonian crow is the only known species that looks at its primitive tool and thinks, "I can improve this."
"Normally nonhuman tools, they stay what they are," Rutz said. "Now with these crows, we have experimentally shown that their technology advances."
The researchers are eager to find out how the birds gain their tool-making knowledge. Is the ability "hard-wired" in their genes? Do crow chicks learn it by watching their parents or other adults?
Finding the answers won't be easy.
For starters, New Caledonian crows tend to use their tools high up in the trees, making their behavior difficult to observe in the wild. And when Rutz tried to observe them in captivity, the birds didn't sculpt their sticks into hooks.
One way to get to the bottom of question would be to perform a "cross-fostering" experiment. In such a study, chicks from a family of hooked tool-making crows would be exchanged with a nest of more simple tool users. But this isn't an option on New Caledonia, Rutz said.
Depending on the area of the island, different populations of crows will make various types of tools using materials they have available. "It looks like there are local adaptions, there are possibly local cultures," Rutz said. "And that is absolutely fascinating."
Cross-fostering chicks from different areas "has the potential to really irreversibly alter the system we hope to study," he said.