By Bob Grant
An entourage of subordinate chimps is gathered eagerly around Steward, the big alpha-male chimpanzee, at the Yerkes National Primate Research Center outside of Atlanta. They're watching Steward brandish a Plexiglas stick at an oblong polycarbonate box sitting behind a wide mesh fence - a rudimentary candy vending machine. As the subordinates look on, he liberates M&M after M&M from the contraption - called Pan-pipes after the chimpanzee genus, Pan - designed to make chimps work for their treats.
He's doing so by raising an internal trap door with the Plexiglas stick. In the upper pipe on the Pan-pipes is a piece of candy, trapped by a blockage (see first image below). Once the blockage is removed, the candy can roll into the lower pipe and down a chute into a waiting chimp's hand. In this case, Steward is using a technique called ...
