No-one likes being stared at, and finding out that you are being eyeballed by a stranger can be unnerving.

But now we know that it isn't just humans who are spooked by the feeling of being spied on.

Monkeys also react badly - and get stressed and become more aggressive when they are being watched by people in zoos, a study has found.

The primates scratched themselves and fought more when they could see human visitors compared to when they were behind a one-way screen.

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Previous research has found that visitors can cause stress to apes and monkeys but not explained why. It now appears that eye contact is to blame.

Researchers filmed a group of ten black-capped capuchins at Melbourne zoo to test their reactions to being observed. Half were put in an enclosure with a one-way screen to prevent them from seeing visitors, while the other five were placed in the unmodified area.

Scientists found that monkeys who were screened off were 68 per cent less likely to show signs of aggression, such as shaking their faces or making physical threats like chasing and fighting.

The primates also spent 38 per cent less time displaying abnormal behaviours such as scratching themselves and pacing, the researchers found.

The authors, led by Sally Sherwen of the University of Melbourne, found that once a bout of aggression started, "typically... most or all individuals would quickly rush towards the commotion and become involved, making it difficult to accurately determine the initiator or the recipient of aggression."

Concentrations of chemicals linked to stress were also a third lower in the monkeys whose view of the people was obscured.

The authors said reducing visual contact with visitors improved the animal's welfare. However, making eye-to-eye contact with the animals was a draw for zoo-goers and the researchers found that visitor numbers dropped when the monkeys could not look back directly at them.

Writing in the journal Applied Animal Behaviour Science, they said: "These findings also raise a possible dilemma for the zoo industry between enhancing animal welfare... and providing for visitor experience."