Evidence for large long-term memory capacities in baboons and pigeons and its implications for learning and the evolution of cognition.

Previous research has shown that birds and primates have a rich repertoire of behavioral and cognitive skills, but the mechanisms underlying these abilities are not well understood. A common hypothesis is that these adaptations are mediated by an efficient long-term memory, allowing animals to remember specific external events and associate appropriate behaviors to these events. Because earlier studies have not sufficiently challenged memory capacity in animals, our comparative research examined with equivalent procedures the size and mechanisms of long-term memory in baboons and pigeons. Findings revealed very large, but different, capacities in both species to learn and remember picture-response associations. Pigeons could maximally memorize between 800 and 1,200 picture-response associations before reaching the limit of their performance. In contrast, baboons minimally memorized 3,500-5,000 items and had not reached their limit after more than 3 years of testing. No differences were detected in how these associations were retained or otherwise processed by these species. These results demonstrate that pigeons and monkeys have sufficient memory resources to develop memory-based exemplar or feature learning strategies in many test situations. They further suggest that the evolution of cognition and behavior importantly may have involved the gradual enlargement of the long-term memory capacities of the brain.