Reversal Learning:

  • All 12 animals successfully reached the learning criterion during the initial testing and following the first reversal, with some able to master up to five reversals.
  • Learning patterns varied across individuals, with some demonstrating gradual learning, others appearing to respond randomly until a sudden insight occurred, and some exhibiting a more erratic learning process.
  • The animals needed more sessions to reach the learning criterion at the first reversal compared to the initial learning. Afterward, a trend toward a decrease in the number of sessions required for subsequent reversals has been observed.

 Mean (+/- SE) number of sessions to reach the learning criterion for each reversal. Number of individuals decreases across the reversals. The star represents a significant difference between two means (Wilcoxon signed rank test).

Learning-Set Formation:

  • The spider monkeys’ performance improved across the 20 stimulus pairs, requiring fewer sessions to reach the learning criterion as they advanced through the pairs.
  • The monkeys’ performance in the first session also improved across the 20 stimulus pairs, with a marked increase in the number of correct responses.

Mean number of sessions (+/- SE) required by the spider monkeys (n=10) for each stimulus pair of shapes during the visual learning-set formation task. Stars represent significant differences between means. The red line represents the curve fitting.

Long-term Memory Test:

  • All 10 animals chose the shape that had been rewarding for them in the first pair of shapes during the one-trial long-term memory test after a 12-week interval.
  • The spider monkeys’ responses were significantly not due to chance, suggesting that they demonstrated long-term memory retention in this task.