All rats were housed in pairs for 14 days to acclimate and handled for 5 min every other day by a male experimenter
before starting the experiment. We randomly divided the ten pairs of rats into two groups (five pairs each) and shaped the door-opening behavior using different procedures between the two groups. For one group, the door opening resulted in helping the cagemate, and for the other group, it resulted in food rewards in a cup, six pieces of chocolate cereal (Kellogg’s Japan Chocowa TM ). In this shaping phase, two of the three areas of the experimental box were used. For the group in which the door-opening behavior was shaped by helping, the ground and pool areas were used. For the group in which the door opening was shaped by food reward, the ground and food areas were used. The learning criterion was to open the door within 60 s in three out of four consecutive sessions. After fulfillment of the learning criterion, we carried out the choice test using all three of the areas (Fig. 1b). In the choice test, the helper rats in the central ground area could open the doors at either side of the ground area. One of the doors opened into the area with the soaked cagemate, and another opened into the food rewards. We observed which door the rat opened first and measured the time it took to open the door. The test was ended 300 s after the rats opened both the doors or 600 s after the test started. One test trial was carried out per day, and in total, ten trials were carried out for each rat. The positions of the pool and ground areas
were quasi-randomly changed (allowing the repetition4 times) by rotating the entire apparatus. The numbers of each arrangement were the same (five trials each).