Introduction

Imagine the following set of incidents:

1. A chimpanzee at Assirik repeatedly bashes the hard-shelled fruit of a baobab tree against one of its exposed roots. Eventually the fruit cracks open and the ape eats its contents. Earlier the chimpanzee ate the fruits of two other kinds of palm trees, but as no oil palms were available, these could not be eaten.

2. A chimpanzee at Gombe sits for an hour in the crown of an oil palm tree, patiently extracting the fruits. These are prised out one by one but processed by the mouthful: the fibrous outer husk is chewed to a wad and sucked dry, then both it and the undamaged nut inside are spat out or swallowed.

3. A chimpanzee at Kasoje walks through a grove of fruiting oil palms. Overhead in the trees, vervet monkeys consume the fruits while below bush pigs crunch the discarded nuts. The ape ignores the palms, though earlier on the same day several other domesticated plants used by the local humans, such as banana, mango and sugar cane, were eaten.

4. A chimpanzee at Lopé climbs an oil palm tree in a clearing in the rain forest. The oily husks are eagerly eaten, especially as this is the lean time of the dry season, but other equally nutritious kinds of nuts, also common and accessible, remain untouched.