Individual variability in visual recognition memory of black-handed spider monkeys (Ateles geoffroyi)
Recognition memory is an ability that allows animals to respond differentially to stimuli, individuals, or situations experienced in the past and plays an important role in foraging and social behavior. This ability has never been tested in black-handed spider monkeys (Ateles geoffroyi), but their social structure and diet force them to remember other individuals and food items. Therefore, they are a species in which to test whether their visual recognition memory depends on the retention interval (RI) and the list-length effect. Seven adult spider monkeys (Ateles geoffroyi) solved a delayed match-to-sample task where they had to touch a list of picture samples with different lengths (three, four, and five pictures). After that, they waited for different RIs (0, 15, and 30 s) to touch one of two pictures, signaling which was presented in the list. The group results indicate a high level of variability within individuals that overlap in each RI and position of the pictures for the three list-lengths. Individual plots and a nested ANOVA indicate effects of the RI, the list-length effect, and the position of the pictures on the list. The individual plots also show different strategies of the individuals to solve the task, such as only primacy, only recency, or both primacy and recency; but these strategies change as the RI increases. Based on these results, we recommend changes to the task and the statistical analysis for a better understanding of the underlying mechanism of recognition memory.