How much memory capacity do animals have? It's a difficult question to devise a test for, because finding the limit would involve saturating an animal's memory. If the memory capacity is extensive, that could take years. And, in fact, it did take years—over three of them, in fact. But some researchers put in the time, and the results appeared in a recent open access publication.
The memory test involved was very simple: one picture, two buttons. Press the correct button in response to the picture, and you get a reward (the correct button was chosen randomly the first time a given picture was shown). Over the intervening years, more pictures were added, while memory of earlier pictures was retested on a regular basis. The test subjects? Two baboons and two pigeons. The animals were already adults when the testing started, so presumably had already devoted part of their memory capacity to prior experiences.
The pigeons started out strong, with success rates of around 90 percent when there were only a few hundred images involved. But their capacity was strained rather quickly: success rates dropped to 80 percent by the time 1,000 images were in play, and were approaching 60 percent by 3,000 images. The baboons were a whole different story. They started out with an 85 percent success rate, but the rate of decline was minimal: by 6,000 images, they were still in the neighborhood of 80 percent accuracy. The authors weren't sure where their memory would reach its saturation point.
The authors also looked at some of the properties of the memory recall. As the number of trials went up between exposures to a single picture, recall went down slowly (baboons dropped to 70 percent success when an image reappeared after 50,000 intervening ones). But the slopes of the decay were nearly identical between the species, with baboons having a 10 percent lead on the pigeons at all time points. Reaction times—the lag between exposure to the picture and registering a choice—did not increase with increasing use of memory, suggesting that the brain can scan memory stores in parallel. Both pigeons and baboons also consistently took longer before giving an incorrect answer, a lag that suggests than an alternate decision making process kicks in when memory fails. Finally, the authors plotted success against the order in which images were introduced. All four test subjects produced U-shaped curves, suggesting they were best with items that were either among the first introduced, or the most recently seen.
Overall, it's clear that baboons have significantly more memory capacity than pigeons. But aside from capacity, all the properties of recall appeared remarkably similar. The authors suggest that this indicates that the mechanism of storing and recalling memories may be identical in these distantly related species. That, in turn, indicates that the ability to store memories may have developed very early in the evolution of vertebrates, and that much of the change since then has simply involved adding capacity.
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