We examined if apes spontaneously remember one-time, distinctive events across long delays when probed by discriminant cues. Apes witnessed an experimenter hide a cache of food, which they could then retrieve. They retrieved one of two food types; one more distinctive than the other. Two, 10 or 50 weeks later, the apes returned to the same enclosure and found a piece of the previously hidden food on the ground. An experimenter who had not hidden the food was also present. Apes immediately searched the location where the food was previously hidden (no food was here), showing recall of the event. One week later, apes returned to the same enclosure, with the same food on the ground, but now the experimenter that had hidden the food was present. Again, apes immediately searched the hiding location. Apes that had not witnessed the hiding event did not search. There was no significant effect of food type, and retention declined from exposure to the two-week delay, then levelled, consistent with the forgetting curve in humans (Ebbinghaus, H. 1964 Memory: a contribution to experimental psychology (transl. H.A. Ruger & C.E. Bussenvis). New York, NY: Dover. (Original work published 1885.)). This is the first study to show apes can recall a one-time, non-goal-directed event longer than two weeks ago and that apes' recall declines in accordance with a standard retention function.

It has been claimed that the ability to recall personal past events is uniquely human. We review recent evidence that great apes can remember specific events for long periods of time spanning months and even years, that such memories can be enhanced by distinctiveness (irrespective of reinforcement), and follow a forgetting curve similar to humans. Moreover, recall is enhanced when apes are presented with features that are diagnostic of the event, consistent with notions of encoding specificity and cueoverload in human memory. These findings are consistent with the involuntary retrieval of past events in humans, a mode of remembering that is thought to be less cognitively demanding than voluntary retrieval. Taken together, these findings reveal further similarities between the way humans and animals remember past events and open new avenues of research on long-term memory in non-human animals.

Non-human primates are capable of recalling events that occurred as long as three years ago, and are able to distinguish between similar events; akin to human memory. In humans, distinctiveness enhances memory for events, however, it is unknown whether the same occurs in non-human primates. As such, we tested three great ape species on their ability to remember an event that varied in distinctiveness. Across three experiments, apes witnessed a baiting event in which one of three identical containers was baited with food. After a delay of two weeks, we tested their memory for the location of the baited container. Apes failed to recall the baited container when the event was un-distinctive (Experiment 1), but were successful when it was distinctive (Experiment 2), although performance was equally good in a less-distinctive condition. A third experiment (Experiment 3) confirmed that distinctiveness, independent of reinforcement, was a consistent predictor of performance.These findings suggest that distinctiveness may enhance memory for events in non-human primates in the same way as in humans, and provides further evidence of basic similarities between the ways apes and humans remember past events.

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