BioEd Online

Orang-utans can solve a brain-teaser that would vex many human minds, researchers have found.

Faced with a vertical transparent tube, a quarter filled with water, in which a peanut floats tantalizingly beyond reach, what should you do? Five orang-utans from Leipzig Zoo in Germany all came to the same conclusion. Taking mouthfuls of water from a nearby bottle, they spat into the tube until the peanut floated into reach.

The apes' ingenuity amazed the study's leader, Natacha Mendes from the Max Planck Institute for Evolutionary Anthropology in Leipzig. "Before we started we thought this was really complicated," she says. "If you asked someone in an office to solve this problem many people wouldn't be able to give a quick answer, and some probably wouldn't be able to figure it out at all."

What made the task especially challenging is that the water was concealed inside a drinker, like a hamster's water bottle, and was some distance from the tube.

This suggests that the orang-utans had to think at a more abstract level, says Mendes. "They have to have a mental image of the water in order to solve the problem." The results are reported in Biology Letters¹.

One thing that's still unclear is whether the orang-utans worked out the problem before spitting the first mouthful, or whether it only occurred to them after they had seen the water level rise. Mendes plans an experiment using an opaque tube that prevents the animals from seeing the effects of their actions to tease these alternatives apart.