When Jane Goodall first encountered chimpanzees using twigs as tools in 1960 - something that scientists had assumed only humans could do - she wrote an excited telegram to her colleague Louis Leakey:
"Now we must redefine tool, redefine Man, or accept chimpanzees as humans," it said.
Scientists went with the first option, and half a century later, the list of tool users has expanded to include other primates, elephants, dolphins and sea otters, among others. But the phenomenon is still relatively rare: it's found in less than 1 per cent of all species.
Which is why the discovery of a new tool user, reported today in the journal Nature, is big news. The Hawaiian crow holds twigs in its teeth to dig insects and other tasty morsels out of hard-to-reach spots - and it's only the second member of its genus known to do so.
"The exciting wider context is, if you have only a single species using tools, you're trying to explain a singularity, and scientifically you're not really winning with that," said Christian Rutz, a behavioural ecologist at the University of St Andrews and the lead author of the study. "But a second species provides leverage for cautiously asking evolutionary questions about how they evolved . . . and perhaps even to start speculating about the origin of tool use in humans."
"I think the plot is thickening," he added.
The discovery is the result of "a true eureka moment," according to Rutz - events that are few and far between in a field characterised by decades of slow, incremental study and unglamorous surveys of data.
For more than a decade, Rutz has been studying New Caledonian crows, the first member of the genus Corvus known for natural tool use. Without anyone teaching them how, chicks from the Pacific island species would instinctively pick up twigs with their beaks and use them to scrape up food.
They could even break off branches and fashion them into hooks or barbs that suited their needs. But - as far as anyone knew - the birds were a biological oddity. Science had found no other crows like them.
Still, "I had a suspicion that there may be undiscovered tool users out there," Rutz said. "There are over 40 species of crows and ravens, and so many of them are understudied, I thought, 'okay, maybe one of them.'"
The trouble was figuring out which one. Corvids are understudied for a reason - many members of the genus live in small, threatened communities on hard-to reach islands. Rutz couldn't exactly fly around to each one and hope to happen upon an avian tool user by chance.
That's when he had his eureka moment: Unlike many other corvids, New Caledonian crows have straight beaks. Presumably, this is helpful for tool use, since straight beaks make it easier to manipulate twigs than the more common curved ones. They also have incredibly big eyes, resulting in the largest field of binocular vision (the field of view that can be seen by both eyes at the same time, enabling depth perception) of any bird.
"I figured, if we search for straight bills and large eyes we can find other candidate species," Rutz said.
A quick image search revealed his best target: the large, all-black Hawaiian crow, known on its home island as 'alalā. It has a straight, blunt beak, and though its eyes are relatively small, they are extremely forward-facing, an adaptation that typically allows for depth perception. Rutz called up the programme manager at a captive breeding facility in Hawaii run by the San Diego Zoo.
"I said, 'Look this may sound a bit crazy but I have a hunch your birds may be tool users,'" he recalled. "And the guy replied, 'oh yeah, they do all sorts of funny things with sticks.' "
A few days later, Rutz was in Hawaii.
Though there are plenty of anecdotes about wily birds using sticks, rocks (or even cars) to help them scoop food out of cracks and break open shells, there are few examples of true species-wide avian tool use.
So Rutz and his colleagues designed a series experiments aimed at testing whether the crows really were natural tool users.
First, they observed more than 100 adult crows and found that 93 per cent would spontaneously pick up stick and use it to dig when presented with hard-to-reach food. In another test, hatchlings were kept apart from adults of their species, to determine whether the tool-using behaviour was innate or learned. Even without training or an adult to set an example, the young crows first picked up and played with sticks, then started to figure out how to use them.
"That strongly suggests that the species has genetic predispositions that lead to development of functional tool use," Rutz said.
This doesn't mean that the crows have a "tool-use gene," or that they're programmed from birth to know how to dig with a stick.
"It's not as simple as that," Rutz said. "More likely, they have a developmental programme that is under genetic influence that makes them engage with objects and explore their environment."
In other words, the birds are probably predisposed to be curious about objects - not unlike human toddlers. But actually putting them to use is up to the individual, not genetics.
The Hawaiian crows are separated by their New Caledonian brethren by more than 6000km of ocean, making it likely that tool use is an example of convergent evolution: two species separately evolving the same trait.
Both species share some commonalities with another well-known natural tool user, the Galapagos woodpecker finch. All three are tropical species, living on small islands with very few predators and not much competition for their favourite food: insects embedded in holes in logs and trees.
"Now we can cautiously start constructing evolutionary arguments about the origins of tool use," Rutz said. "And of course, ultimately this enables much broader comparisons, with non-tool using species, with primates. We can ask whether similar ecological conditions seemed to drive the evolution of this behavior in different parts of the animal kingdom. That's where it gets exciting."
The world came dangerously close to never making this discovery at all.
Hawaiian crows have been extinct in the wild since the early 2000s - the only representatives of the species remaining are those bred in captivity by conservationists from the San Diego Zoo. There are just 109 known Hawaiian crows still alive, just enough for Rutz to study them.
"Let this discovery serve to emphasise the importance to conserving these and other animal species," said Goodall, the mother of animal tool use research, in a statement provided by St Andrews. "These discoveries shows how much there is still to learn about animal behaviour."