Chimpanzees and orangutans remember distant past events.

Chimpanzees and orangutans were able to remember past events when presented with sensory reminders, a new study shows.

Both species found where a useful tool was hidden three years after performing a task only four times.
They were also able to recall a unique event two weeks later. 

The team say their work, published in Current Biology, shows memory for past events is not unique to humans. 

Chimps and orangutans were presented with two boxes in different rooms, one of which had useful tools, the other useless ones. In order to get a reward they had to successfully retrieve the useful tools.

Three years later, without witnessing them being hidden, they retrieved the useful tools correctly.
Instant recollection 
It has been well established in humans that sensory cues like songs and smells can help transport our minds back to the past.

The team, led by Gema Martin-Ordas of Aarhus University, Denmark, used the same principle. They found that cues - keeping the experimental set up the same - triggered the apes' memories.
 

They observed that 90% of the apes who experienced the event three years earlier found the tool in the correct location almost instantly.

"Our data, and other emerging evidence, keep challenging the idea of non-human animals being stuck in time," said Dr Martin-Ordas.

"We show not only that chimpanzees and orangutans remember events that happened two weeks or three years ago, but also that they can remember them even when they are not expecting to have to recall those events at a later time.

"What this shows is that the episodic memory system in humans is not as unique as we thought it was, as we share features with non-human primates." 

This could mean the capacity to remember past event could have evolved before humans were present, she told BBC News.
"I think it's important to know who we are and what makes us unique. Learning about what other species can do gives us a more comprehensive picture about humans."

Conscious replay Michael Corballis from the University of Auckland, New Zealand, who was not involved with the work, said the study moves us significantly closer to showing that chimpanzees and orangutans have human-like episodic memory "in defiance of what some of us have maintained".

But while humans can recall what, where and when something happened, the apes were missing the "when" component, he added.

"There is no indication that the animals remembered when the earlier event occurred. This is not to say the animals had no inkling of this, and in any case we humans are often hazy about the locations of events in time.
"My guess is that great apes, and perhaps even rats, have episodic memories similar to our own, probably less rich and detailed, but similar in essence."

The authors of the work note there is no way of telling whether or not the animals had a conscious recollection of these past events. 

But Dr Corballis said this could be overly pessimistic, as previous studies of activity in the hippocampus of rats "do seem to provide evidence of conscious replay of event".