The experiment assessed the development of working memory and the use of spatial stimuli. Four groups of weaning rats, 20, 21, 23, 25 days old at the beginning of the experiment (n=12 for each group), were trained in an 8-arm radial maze, 4 trials a day, for 5 days. Both intramaze cues and extramaze spatial stimuli were available. Rewards were placed on the subject-specific cued-arms. On the sixth day, the spatial test in which intramaze cues were eliminated and the cue test in which the use of extramaze stimuli was limited were given to assess the strategies employed to achieve the task. The results indicate that (1) developmental differences were recognized between Groups 21 and 23, and these differences were not compensated after receiving the same number of training, (2) at the age of the experiment, working memory and the use of spatial stimuli were not yet sufficiently developed, and (3) the subjects mainly used the cue strategy when both place and cue strategies could be available.

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Males and females of the domesticated house musk shrew (Suncus murinus), housed under the isolated or grouped condition from 30 to 60 days of age, were tested in 5 times for interactive behaviors with a partner of same sex. Then, they were paired with a partner of opposite sex in the 6th session and observed for sex-related aggressive interactions. Isolation effect of enhancement of aggression was clearly shown in the same-sexed pairs, while repeated encounters did not influence on aggression. On the contrary, isolation effect was not found in the interaction between opposite-sexed mates. This result suggested the mechanism of the aggression in the same-sexed pair was different from that in the opposite-sexed pair in this species.

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The experiment with 24 rats assessed the persistence of memory on formally defined rule structure of food quantity sequence using serial pattern learning followed by transfer test. In original training, monotonic (M) group was presented with the monotonic decreasing series consisting of two, three, or four different items of 45mg food pellets, whereas Random (R) group was provided with the random series in which items were arranged with no consistent relations among them. After the original training was finished, each of these basic groups was divided into two groups. Transfer training was begun on the day after the end of original training for groups MI and RI, whereas on the 21st day after the end of original training for groups MD and RD. In transfer training, all groups received a novel monotonic decreasing series containing 16-9-3-1-0 pellets. Throughout transfer training, groups MI and MD anticipated the 0 pellet item in this series significantly better than groups RI and RD, and any reliable difference was not observed between groups MI and MD, or groups RI and RD. These results indicate that rats' memory for the sequentially arranged food quantities with simple rule structure has strong persistence against to time lapse. Implications of these findings are discussed in relation to the partial reinforcement delayed extinction effect which shows persistent extinction performance after regularly alternating schedules.

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