INTRODUCTION

Until recently, there was limited evidence of tool use (sensu Beck, 1980) by wild orangutans (Pongo pygmaeus), and no evidence of tool manufacture, despite several long-term field studies in Borneo (Ulu Segama: MacKinnon, 1974; Tanjung Puting: Galdikas, 1982; Kutai: Rodman 1973; Mitani 1985; Suzuki, 1989; Gunung Palung: Mitani et al., 1991) and Sumatra (Ketambe, in Gunung Leuser: Rijksen, 1978; Sugardjito 1986; Utami & Mitrasetia, 1995). Orangutans had been observed using only “found objects” as tools (Byrne, 1995): simple, unmodified raw materials such as leaves to wipe off feces (MacKinnon, 1974), a pad of leaves for holding spiny durian fruit (S. Utami, personal communication), a leafy branch for a bee swatter (E. Fox, personal observation), a bunch of leafy branches held together as an “umbrella” while traveling in the rain (Rijksen, 1978; E. Fox, personal observation), a single stick as a backscratcher (Galdikas, 1982), and a branch or tree trunk as a missile (all studies). Because observations of flexible tool behavior in wild primates had been limited to chimpanzees (Pan troglodytes), it had often been suggested that the manufacture and flexible use of tools arose only in the chimpanzee–hominid clade (McGrew, 1992, 1993).

Such limited tool use by wild orangutans was inconsistent with observations of captive orangutans. In contrast to their wild counterparts, captive and rehabilitant orangutans demonstrated a rich array of flexible tool behavior, at least as complex as that demonstrated by chimpanzees under similar conditions (Lethmate, 1982; Russon & Galdikas, 1993, 1995). Why did captive and rehabilitant orangutans demonstrate such complex tool behavior, while wild orangutans remained relatively unsophisticated?