Dr Godelieve Kranendonk, a behavioural biologist leading the study at AAP, a rescue centre for animals in the Netherlands, told BBC News that the results had been astonishing.
"Suddenly, [the chimps] woke up. It was as if they were zombies in their enclosures and now they are happy, playing with each other. They are chimps again - that was really nice to see," she told me.
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Staff at the AAP sanctuary care for the animals until they die. They try to rehabilitate them so that they can live out their remaining years happily.
The chimps are fed a good diet of vegetables, have toys and plenty of space in which to play. But Dr Kranendonk found that the abnormal behaviour actually increased. It was as if the animals did not know how to cope with their new found freedom.
Dr Kranendonk decided to consult Martin Bruene, a professor of human psychiatric disorders at the University of Bochum, Germany. He prescribed a course of anti-depressants for five of the chimps.
All the animals had been used in medical experiments and were infected with Hepatitis C. "Willy" showed the least abnormal behaviour. "Tomas" and "Zorro", on the other hand, would spend a third of their waking hours eating their own vomit.
"Iris" had lost so much weight from vomiting when she first came to the sanctuary that the staff thought she would die.
The most troubled though was "Kenny", a small chimp who was constantly anxious that the others would attack him and spent much of his time screaming in terror.
The chimps were given SSRI (Selective Serotonin Reuptake Inhibitors), which is a class of anti-depressant similar to Prozac and is used to treat human patients for depression, anxiety disorder and obsessive compulsive disorder.
After six to eight weeks, the animals behaviour started improving. The abnormal behaviour declined and the chimps began to play together. After seven months, there was a vast difference.
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The big question though is whether the effect lasts when the chimps are taken off the medication. The early indications are promising. The medication has been steadily reduced and there has been no adverse effect on the chimps' behaviour.
Kenny himself decided that he did not want to take the anti-depressants anymore. His clownish behaviour has continued.
"It seems that while on the medication, the chimps learn to be chimps again," said Dr Kranendonk. "And once they have learned that, they don't need the medication any more."