We used a different experiment box in the choice test from that in Experiment 1. The depth of the pool area used in the choice test was deeper than that in the door-opening sessions in Experiment 1. Deeper water might be more uncomfortable for rats. It might induce the result in the choice test; the door opening for helping was more valuable than that for food. However, the number of sessions needed to shape the door-opening behavior in the rats that learned it by helping was not statistically different from that in the rats that learned it by food reward. This suggests that the power of reinforcement for the door-opening behavior was not different between the two ways of shaping and that it is unlikely that deeper water caused the results in the choice test. However, the effect of the water depth is still a concern that we should address in future studies.