Evidence that the use of tools to forage for oysters was an established cultural trait among chimpanzees in 17th century Sierra Leone.

We discuss a historical report of chimpanzees Pan troglodytes using stone tools to forage for large oysters Crassostrea tulipa at the Sierra Leone River estuary in 1678, a behaviour that was also noted there in 1682, apparently independently and with a stick used as the tool, but which has not been reported in any modern study. The animals in question can be identified as chimpanzees by a combination of descriptive, faunistic and linguistic evidence, including their local name. These two accounts together indicate that this behaviour was being used by several chimpanzees in that area over a period exceeding four years, and was therefore probably an established cultural trait of the local chimpanzee population at that time. Chimpanzees are no longer found at the coast in the Sierra Leone estuary, so this trait is probably now extinct there, and opportunities for chimpanzees to encounter oysters are now limited throughout their African range. This behaviour would therefore be unknown if not for these two 17th-century reports, which emphasises the continuing value of such accounts as sources of natural history information.