The average and individual learning curves.

Left top: The average success rate (in percent; y-axis) plotted by session (x-axis) for Level 1a (first session only) and Level 1b (2^nd–18^th sessions) (filled symbols) and Level 2a (open symbols). Sessions were numbered independently for Levels 1 and 2 and aligned at the origin of the abscissa because sessions for later Level 1 and early Level 2—easy and difficult sessions, respectively—were intermingled to keep the animals motivated. Note that in practice, Level 2a training contained trials that resulted in Level 2b difficulty after the first retrieval attempt failed, leaving the food behind the tool. Each data point represents the average of the five animals. The rest of the graphs: Learning curves of individual animals (#1–#5). The success rate (in percent; y-axis) is plotted by session (x-axis) for Level 1a (first session), Level 1b (2^nd–20^th sessions) (filled symbols), and Level 2a (open symbols) for each of the five animals. Level 1a training was given only for one session. For Levels 1b and 2a, the criterion for success was 75% or higher in three consecutive sessions. After the criterion was satisfied, some animals received additional training. Sessions are numbered independently for Levels 1 and 2, and are aligned at the origin of the abscissa.