The human population of Zanzibar, a Tanzanian island off the East African coast, doubles every 15 years or so. The island's red colobus monkeys, however, are dwindling as their habitats are destroyed for firewood and timber. But some monkeys have found a way to coexist with humans: they snack on charcoal.
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. "Each animal," he says, "eats about five grams a day."
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. But charcoal has a well-known ability to adsorb toxins--it is used as a poison control agent, and in Europe people use it in liquid form as a digestive aid. 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. "There must be a quick effect so they can learn by association," he says. Baby monkeys, at least, learn from imitating their mothers, and the mothers themselves may have learned from eating soil containing charcoal particles.
"These are pretty clever animals," says Struhsaker. "They've picked up a habit that allows them to exploit a resource to an extent that was not possible before." Despite this adaptation, red colobus populations are still shrinking in Zanzibar, even in nature reserves, where speeding cars take a large toll. "If they put the potholes back in the road, or built speed bumps, I think the reserve animals would be fine."