Apes can remember friends they haven't seen for over 25 years, a new study shows. That's longer than a dolphin's memory.

• Scientists set up a screen at sanctuaries for great apes and showed them pictures of other apes.
• They found they looked at photos of apes they used to know longer than random apes.

After a long day, these great apes like to sit down, have some juice, and admire pictures of their old friends.

Perhaps not the most surprising of preferences for an intelligent animal, except that these bonobos and chimps appear to remember old friends from as far back as 26 years, according to research published this week in the journal Psychological and Cognitive Sciences.

The only other animals besides humans who we know have that kind of long-lasting memory are dolphins, who have been shown to recognize acquaintances from 20 years ago.

We've known these animals are highly social, but it hasn't been established how memory plays into that sociality. "They're incredible. They're very like us in many exciting ways," co-author Christopher Krupenye, an assistant professor of psychology at Johns Hopkins University, told Business Insider.

The screen, the juice, and the eye-tracking

The scientists had suspected for a while that apes whom they hadn't seen for years remembered them when they returned, Krupenye said. This led them to wonder just how far back the animals' social memory might go.

To figure out what the animals remembered, the researchers tested 26 apes from Edinburgh Zoo in Scotland, the Kumamoto Sanctuary in Japan, and Planckendael Zoo in Belgium.

In each location, the scientists set up a screen just outside the apes' enclosures and put a bottle with a nozzle that they could drink diluted fruit juice from. This way, they would be relatively still while viewing the images so that the researchers could track their eye movement.

"It seems to be quite enriching for them. A lot of them seem to actually really enjoy participating," lead author Laura Simone Lewis, a postdoctoral fellow in the Department of Psychology at UC Berkeley, told BI.

Using eye-tracking, the scientists could glean what pictures the apes were staring at for longer — photographs of long-lost friends or strangers. Photos of familiar apes included those who had died, had previously lived with the subject, or had been transferred to a new group.

They found that the animals spent on average 0.25 seconds longer staring at photos of apes they used to know than strangers. The scientists interpreted the longer time spent on a photo as a remembrance.

"If they didn't recognize those individuals in any way you would just expect that they're equally looking to both pictures on the screen, right?" Krupenye said.

The scientists also found that the apes looked longer at other apes that they had a positive bond with, as opposed to those they might've been in conflict with.

So, Lewis explained, they seemed to have a stronger preference for "what we might call their friends."

The future for apes

This study isn't just touching, Krupenye said.

"It's also the case that apes are extremely endangered and if we aren't proactively working to conserve them, then we really risk losing the species in our lifetimes," Krupenye said.

It's something that the scientists considered when setting up the study, Lewis said.

They chose to make participation voluntary, not forcing the chimps and bonobos to come up to the screen, but instead just placing it outside their enclosure for them to interact with as they pleased.

It's important, Lewis added, to be deliberate when working with endangered animals.

As long as the apes are around and well cared for, Lewis said she's looking forward to all the information we can learn from them.

She said in the future, we might begin to understand more about how apes remember, and whether or not they miss these other animals they no longer see.

"I think it's exciting to think about the studies that could come next. To understand what does their full memory look like."