Chimpanzees: The Culture-Zone Concept Becomes Untidy

The discovery that chimpanzees in Cameroon use stone tools reinvigorates a long-standing puzzle: what explains the geographical distribution of chimpanzee traditions? In 1844 a missionary to Liberia called Thomas Savage claimed that wild chimpanzees crack nuts ''with stones precisely in the manner of human beings'' [1]. More than a century later he was proved right. In certain locations chimpanzees smash nuts both skilfully and often. In the two best-studied populations, in Guinea and the Ivory Coast, chimpanzees spend 12–15% of their annual feeding time cracking nuts with stone or wood hammers, and during the three or four high-production months individuals thereby obtain more than 3500 kilocalories of fat-rich seeds per day [2]. Nut-smashing makes at least six species of inaccessible foods available and contributes a major part of the diet, particularly when fruits are scarce. The resulting food supply has even been suggested to foster an accelerated reproductive rate and a relatively gregarious grouping pattern. Chimpanzees might therefore be expected to smash nuts wherever the right combination of foods and raw materials are present. Yet they do not. Throughout Africa there are many populations where rocks, logs and nuts appear sufficiently abundant to allow the nut-smashing adaptation to flourish. In most such places chimpanzees ignore the rich nut offerings [3]. The reason why some chimpanzees fail to exploit these valuable food supplies has appeared uncontroversial until now. The limiting variable seemed to be the rate of invention. This hypothesis derived from evidence that nut-smashing was pervasive throughout far western Africa, but was found nowhere east of the N'Zo-Sassandra River, a supposed geographical boundary for chimpanzees that runs from savanna in the north of the Ivory Coast south to the Atlantic coast. To the east of this river, surveys found no evidence of nut-smashing. To the west it was close to universal, though populations were found to differ in details such as tool choice and food species. The implication was that nut-smashing is a behavioural tradition constrained in its spread by an uncrossable barrier. It was absent elsewhere because it had not been invented a second time [4]. This cultural explanation was supported by surveys showing that there are no relevant ecological or genetic differences between populations that do and do not smash nuts. It also fit evidence that, like other chimpanzee tool-using such as termite-fishing, the behavior is spread by social learning [5]. For example, nut-smashing proficiency improves with age, …