A bat, flying through the night sky, is thirsty. As it flies, it sends out high-pitched squeaks and listens for the returning echoes. It hears a telltale pattern. It hears no echoes form up ahead and the only ones that reflect back at it are coming from straight below. That only happens when the bat flies over a flat, smooth surface like the top of a lake or pond. The bat dives, opens its mouth to take a sip of refreshing water… and gets a mouthful of metal.
In nature, bodies of water are the only large, smooth surfaces around. Waves of sound that hit the surface of still water would generally bounce away, except for those aimed straight downwards. Stefan Greif and Björn Siemers from the Max Planck Institute for Ornithology have found that bats are instinctively tuned to find water using this unique feature (and yes, the institute does mostly, but not exclusively, bird research).
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.
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.”