Memory for distant past events in chimpanzees and orangutans

Determining the memory systems that support nonhuman animals' capacity to remember distant past events is currently the focus an intense research effort and a lively debate [1-3]. Comparative psychology has largely adopted Tulving's framework by focusing on whether animals remember what-where-when something happened (i.e., episodic-like memory) [4-6]. However, apes have also been reported to recall other episodic components [7] after single-trial exposures [8, 9]. Using a new experimental paradigm we show that chimpanzees and orangutans recalled a tool-finding event that happened four times 3 years earlier (experiment 1) and a tool-finding unique event that happened once 2 weeks earlier (experiment 2). Subjects were able to distinguish these events from other tool-finding events, which indicates binding of relevant temporal-spatial components. Like in human involuntary autobiographical memory, a cued, associative retrieval process triggered apes' memories: when presented with a particular setup, subjects instantaneously remembered not only where to search for the tools (experiment 1), but also the location of the tool seen only once (experiment 2). The complex nature of the events retrieved, the unexpected and fast retrieval, the long retention intervals involved, and the detection of binding strongly suggest that chimpanzees and orangutans' memories for past events mirror some of the features of human autobiographical memory.