The approaching squall announced itself as a gentle stirring of the heavy air; then a distant hissing. Only minutes earlier, it had leapt over the darkened summits of the Mahale Mountains and started down towards Lake Tanganyika, advancing through the jungle like a wall of noise - the sound of rain beating on the luxuriant foliage.

Two dozen chimps in the treetop paid it no heed. For the past half hour they had been stalking a pack of red colobus monkeys whose alarmed screeching left no doubt about the chimps' intentions. Although largely vegetarian, chimpanzees relish meat, and at Mahale it forms a great than normal share of their diet during the rainy season - October through April - when fruit is less abundant. Now as the storm intensified and we huddled beneath our umbrellas, craning to catch a view, the chimps closed in for the kill.