Chimpanzees solve the trap problem when the confound of tool-use is removed.

The trap-tube problem is difficult for chimpanzees to solve; in several studies only 1 to 2 subjects learn the solution. The authors tested eight chimpanzees on a non-tool-using version of the problem to investigate whether the inclusion of a tool in previous tests of the trap problem may have masked the ability of chimpanzees to solve it. All eight learned to avoid the trap, in 40 to 100 trials. One transferred to two tasks that had no visual cue in common. The authors examined the performance of 15 chimpanzees on a new task in a 2 x 2 design: seven had experience on the two-trap box, eight had not; half of each group was tested with a tool, half without one. An ANOVA revealed a significant effect of tool-inclusion and experience (p < .05). Our results show that including a tool in the trap problem profoundly affects the ability of chimpanzees to solve it. With regard to what the chimpanzees had learned, the results support the notion that rather than using the available stimuli as arbitrary cues, the subjects had encoded information about functional properties.