Confirming Aesop – rooks use stones to raise the level of water in a pitcher

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Aesop’s fable “The Crow and the Pitcher” has been confirmed in a wonderful experiment. In the classic tale, a thirsty crow uses stones to raise the level of water in a pitcher until it rises within reach of its beak. This is no mere fiction – rooks, close relatives of crows, have the brains to actually do this.

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The aptly named Chris Bird, along with Nathan Emery, gave four captive rooks (Cook, Fry, Connelly and Monroe) a chance to reach a small worm floating in a cylinder of water, with nothing but a small pile of stones sitting on the side. All of them solved the task, and Cook and Fry succeeded on their first attempt. They were savvy about the stones too, using exactly the right number to bring the water within reach and preferring larger stones over smaller ones.

The accomplishments of Bird’s rooks are even more impressive when you consider that rooks are not natural tool-users. Many of the corvids – crows, ravens and the like – are avid tool-users and the skills of the New Caledonian crow are rapidly becoming the stuff of popular science legend. But rooks are different – even though they too excel in laboratory tests (as Bird and Emery have previously shown), they hardly ever use tools in the wild. Rather than any special tool-using adaptations, their skills must stem from the sort of general intelligence that great apes are thought to possess.

More so than other members of the family, rooks are extremely opportunistic feeders, relying on a varied menu of seeds, insects, dead meat and rubbish. With such catholic tastes, food is never far from their beaks and they may have little need for specialised tricks involving tools. The same might be true for capuchin monkeys, which happily brandish tools in a lab but only ever use them in the wild when food is scarce.  As Aesop’s fable moralised, “Necessity is the mother of invention.”

By varying the starting height of the water, Bird and Emery found that the rooks used the stones in a very precise way, putting in the exact number needed to raise the water to a reachable height. They always examined the tube from different angles before they started, and they waited until they had put a few stones in before having a go at reaching the worm. The number of stones they added before making this first attempt was strongly related to the actual number of stones they eventually needed.

Bird and Emery think that the rooks started off with a plan, translating their initial assessment of the water into an estimate of how many stones they’d need. The true test of that hypothesis would be to cover the cylinder after the rooks first see it so that they can’t get any feedback on their progress as they lob stones in. Nonetheless, the fact that the rooks only made a grab for the worm after they’d dropped a few stones, even from the first trial, does suggest that they had a specific goal in mind.

As they got the hang of the task, the rooks also moved towards using larger stones that would displace more water. The fact that they all started off with smaller stones suggests that they didn’t initially understand the link between size and water displacement, but they quickly learned that the biggest stones were the most effective. Likewise, even though the rooks tried the same technique with a cylinder full of sawdust, they quickly learned that it wouldn’t work for this particular medium.

Bird and Emery’s results provide yet more evidence of the incredible problem-solving abilities of the crow family, abilities that Aesop documented over two thousand years ago. The same talents allow them to manufacture their own tools, combine different tools into one, and even to crack unyielding nuts by placing them in front of cars stopped at a red light.

The four rooks have starred in past experiments, but never quite like this. They have seen plastic vertical cylinders, but not those containing water. They had used stones as tools, but to collapse a rickety platform rather than to raise a worm into reach. They had probably seen stones in water, but never had to actually do anything with them.

However, Bird and Emery suggest that corvids in general are probably intelligent enough to understand the useful properties of water. In past experiments, various species have used a cup to carry water to dry food, used paper as a sponge, or used a plug to create a pool of water.

Reference: Current Biology 10.1016/j.cub.2009.07.033

Image: Video by Bird and Emery; Rook by R.komorowski

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