Thomas Struhsaker, a zoologist at Duke University, has been studying the effects of selective logging on rain forest wildlife in eastern Africa. A Tanzanian biologist told him about the monkeys' charcoal habit in 1981. Over the years, as the human population grew, Struhsaker noticed that the monkeys ate more and more charcoal. The monkeys live in an area with almond, mango, and other exotic fruit trees. The leaves of these trees are rich in protein but also contain toxic compounds like tannic acids.
Most animals avoid the leaves.
When a monkey eats charcoal after chomping on leaves, its meal goes down a little easier. The charcoal selectively holds on to large tannic acid molecules, allowing them to pass through the body while smaller nutritious proteins are absorbed by the gut. The monkeys snatch charcoal from kilns and also nibble on charred wood and tree stumps. Struhsaker isn't sure how they acquired the habit. Baby monkeys, at least, learn from imitating their mothers, and the mothers themselves may have learned from eating soil containing charcoal particles.