The joystick was necessary because monkeys, unlike most humans, don't respond well to being told to move their arms in a certain way.

Initially, the other monkeys responded in kind, pulling their own levers 50 percent of the time.

When a raptor alarm call is given, the monkeys respond by moving towards the forest floor and away from aerial attack.

The monkey responds differently to attitude and tone of voice.

The other monkeys might have been responding to a visual signal or they may have seen the leopard themselves.

The monkey won't respond.

The monkey responded ecstatically, swinging its head from side to side and leaping up and down so enthusiastically that the branch swung dangerously close to the earth.

The behavior suggests that the monkeys have a sense of fair treatment and respond negatively when their expectations are violated, the researchers say.

"We find that monkeys respond in the same way as rodents and that the same biological changes may be in play here."

Sometimes the monkeys respond with intimidating gestures and sounds; sometimes they bite.