ABSTRACT
Let us suppose for the moment that we are in a grove of acacia trees on the savannahs of East Africa. In the trees above our heads there is a group of the common African green monkey, sometimes called the vervet. Suddenly, a martial eagle swoops down on the vervets. The first vervet that sees the approaching eagle gives an alarm bark. At the sound of this vocalization all the vervet monkeys suddenly drop from the branches of the trees to the dense undergrowth, where they are safe from the attack of the eagle.
