Chimpanzees enjoy completing puzzles in much the same way as humans do, researchers have found.
Scientists at the Zoological Society of London Whipsnade Zoo found that the primates get the same feeling of satisfaction from completing a brainteaser as person does after finishing a crossword or Sudoku puzzle.
They studied six chimpanzees by providing them with a complex task. Researchers used plumbing pipes to create a maze which the chimps had to push dice through.
For the study, published in the American Journal of Primatology, the animals had to push the dice through the pipes by prodding sticks through holes in them until the dice fell into an exit chamber.
Researchers then repeated the task with Brazil nuts, which the chimps could eat upon completing the task.
The chimps were not trained on how to use the device and it was placed into their normal social grouping, so the animals were able to solve the cognitive problem by choice; completing the task was completely voluntary.
Fay Clark, ZSL researcher, said: "We noticed that the chimps were keen to complete the puzzle regardless of whether or not they received a food reward.
"This strongly suggests they get similar feelings of satisfaction to humans who often complete brain games for a feel-good reward.
"For chimps in the wild, this task is a little bit like foraging for insects or honey inside a tree stump or a termite mound; except more challenging because the dice do not stick to the tool."
The group included two females and four males. The boys are all half-brothers, named Phil Grant and Elvis.
The cognitive test was part of the Zoo's enrichment programme, which also involves hiding tasty treats in boxes and providing the animals with pillows and blankets every night, so the group can make their own beds - in the wild chimps build their own nests every night.
It is thought this encourages the animals to behave more naturally.
Researchers made the task harder by connecting more pipes and making them opaque so the chips could only see the dice or nuts through small holes.
They believe chimps are encouraged to solve puzzles when there is no food reward for the same reason humans do - satisfaction.