Monkeys draw from memory

We’re a little bit closer to understanding what it’s like to be a monkey, and it’s thanks to the same technology that powers your smartphone: the touchscreen.

The latest victory for touchscreens is in the field of memory research. Scientists have been studying this ability in animals for decades – some birds, for example, are remarkably good at keeping track of the little details they use when foraging. Florida scrub jays collect thousands of acorns in the fall, hiding them as reserves to help get through the winter. Proof that scrub jays can keep track of multiple pieces of information about their caches – including the type of food, its perishability, and how long it ago it was stored – came from some clever experiments where jays learned to store worms and peanuts in sand-filled ice cube trays in the lab1. Rufous hummingbirds perform a similar feat. They can keep track of flowers on their daily foraging routes, including when the nectar for each one should be replenished, and time their visits accordingly. How do we know? Biologists taught hummingbirds in the Alberta Rockies to feed at artificial flowers that could be refilled on schedule2.

There is also a long history of research on the mental capacities of our closest animal relatives, primates. Rhesus macaque monkeys, a lab favourite used in countless studies of pharmacology and physiology, can easily keep track of a set of objects and spot the difference if you show them an altered version later on3. Not surprisingly, primates seem to have better memories than birds. Baboons can learn thousands of different photographic images and retain these memories for years – incredibly, when this particular study went to press, the baboons were up to 5000 and still hadn’t maxed out their capacity4.  Pigeons, on the other hand, hit a memory wall at roughly 1000 images4. These abilities might prove useful to primates like the chimpanzees living in the Taï National Park of Côte d’Ivoire, Africa. They make extensive use of their vast forest habitat, visiting hundreds of fruit trees that ripen on different schedules5. The Taï chimps can apparently remember where the especially productive trees are, and will often travel longer distances just to get there5.

But there is something missing from this research. It has to do with a subtle distinction in the way memory works: the difference between recognition and recall. Recognition is the ability to identify something because you’ve experienced it in the past. Recall, which can be more difficult, involves retrieving that memory on demand. Ben Basile and Rob Hampton liken it to the difference between a police lineup and talking to a criminal sketch artist. To recognize something is to see it and sense familiarity; to recollect is to create that experience in its absence.

So far all we have been able to study in animals is recognition. Without language, we can’t get them to describe their memories – until now, that is. Basile and Hampton, two scientists from the Yerkes National Primate Research Center in Atlanta, have figured out how to get monkeys to act like criminal sketch artists6.

Their subjects were rhesus macaques, and the methods they used were based on a common test in human psychology. In the Rey-Osterrieth complex figure test, subjects are shown a complicated line drawing, and then asked to reproduce it from memory using pencil and paper. Basile and Hampton used a touchscreen to put pencils into the hands of monkeys. They began by training macaques to create simple shapes by pressing on different areas of the touchscreen in order to fill them with blocks of colour. At the same time, the monkeys learned that if they correctly reproduced a shape that had previously appeared on the screen, they would earn a food reward.

With this relatively simple paradigm, Basile and Hampton proved that primate memory goes beyond mere recognition – the macaques could reproduce shapes they had seen before6. You can watch a brief video of the macaques in action here. First, you see them tapping a green square (the “go” button), then they are presented with a shape. After taking a quick look, the monkeys tap this shape to make it disappear. The test begins when a lone blue square pops up somewhere else on the screen. I’m not sure whether the two examples in the video are the same monkey or not, but I’m astounded that they both look so deliberate while tapping out their responses (especially since the second monkey is obviously dealing with some distraction, or flagging motivation, or both).

The macaques were able to generalize what they had learned. They’re not drawing faces yet, but the experiment proved that they could successfully reproduce new shapes that they hadn’t been exposed to before6. This result is the best evidence to date that recall is not a uniquely human trait. It means that the ability to recount an experience does not depend on language.

Basile and Hampton also report that, like humans, rhesus macaques find recall somewhat more difficult than recognition. When comparing macaque performance in the recall task to similar recognition-based tasks, they found that recall performance declines more rapidly with a longer delay period between the shape and the test (which would increase the probability of forgetting)6.

You might wonder how much to make of the supposed distinction between recall and recognition. Don’t we do a little bit of both all of the time? And how important is this research when we already knew that monkeys had pretty good memories?

It’s may be hard to separate these two forms of memory in practice, but clinical studies of human memory disorders prove that they are based on different parts of the brain. People can have one ability without the other: in amnesia, it’s possible to have recognition without recall. Amnesic patients do poorly on the Rey-Osterrieth task, which is often used to diagnose this condition. Some people have the opposite problem, like Oliver Sacks’ Man Who Mistook His Wife for a Hat. As Sacks so colourfully describes in his book, in rare cases brain damage can cause a condition of visual agnosia that leaves you unable to recognize a familiar face while somehow maintaining the ability to describe it from memory.

It would be interesting to know if memory is wired similarly in the primate brain. Another open question is the adaptive value of recall for something like a macaque. Is the ability to recreate an experience useful beyond basic recognition for an animal? Basile and Hampton speculate that recollection might allow animals to store information about distant locations or past social interactions, whereas recognition is is probably better for situations that require a rapid response6.

So: monkeys prove that you don’t need language for recall. But I think the more important message is that you don’t need language to test for this ability – and I can’t help but wonder if birds will be next. The key here is the touchscreen: despite years of use in animal research, we may have only begun to scratch the surface of what this device can do. It’s as though behaviourists, long limited to asking yes or no questions, can finally ask animals to report the contents of their minds.


  1. Clayton, N. S. and Dickerson, A. 1998. Nature 395: 272-274.
  2. Henderson, J. et al. 2006. Current Biology 16: 512-515.
  3. Heyselaar, E. et al. 2011. Journal of Vision: 11.
  4. Fagot, J. and Cook, R. G. 2006. PNAS 103: 17564-17567.
  5. Normand, E. et al. 2009. Animal Cognition 12: 707-807.
  6. Basile, B. M. and Hampton, R. R. 2011. Current Biology 21: 774-778.