Researchers from Bristol University studied 77 animals in 13 zoos and found that almost half of the elephants displayed abnormal behaviour.
BBC: Elephants 'die earlier in zoos'
The abnormal behaviour declined and the chimps began to play together.
BBC: Lab chimps successfully treated with anti-depressants
But Dr Kranendonk found that the abnormal behaviour actually increased.
"Willy" showed the least abnormal behaviour.
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