Episodic memories combine what happened, when and where. They are an essential part of visualising a different time, and were thought to be uniquely human until Nicola Clayton at the University of Cambridge proved that western scrub jays have simple episodic memories - allowing them to track how long it takes for food they have stashed to rot.
Chimps can remember where they hid food, but it's not clear whether they can track the amount of time that has passed since a memory was formed. To investigate, Marusha Dekleva of Utrecht University in the Netherlands tested nine captive chimps on a task similar to the one Clayton used with scrub jays. She showed each chimp four containers: two were empty; one contained either apple sauce or yoghurt, which the chimps like; and one held red peppers, which they like less.
Dekleva let the chimps pick a food container either 15 minutes, 1 hour or 5 hours later. But there was a twist: the apple sauce disappeared by the 1-hour mark, leaving the container empty, and by the 5-hour mark the yoghurt had gone, but the peppers were still available. Chimps are good learners and were expected to adjust their choice of containers over time.
They didn't. Instead, they remembered which containers held more food early in the study, and picked those no matter how much time had passed. This got them some food, but far less than they could have obtained by adapting their behaviour (PLoS One, DOI: 10.1371/journal.pone.0016593).
If chimps genuinely lack episodic memories, our ancestors must have evolved them after we diverged from chimps. "Episodic memory is linked to the ability to plan for future events and it could well be that this prospective ability gave humans advantages," says Dekleva.
In fairness to chimps, they may claim mitigating circumstances. As Daniel Povinelli at the University of Louisiana in Lafayette points out, chimps do not habitually store food as scrub jays do. He says they might have done better if asked to learn when different plants yield fruit - a point which Dekleva concedes. For now at least, the jury is still out on whether chimps can travel through time in their heads.
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