Effect of environment (I, improved; S, standard) on active social nonagonistic behavior over time. (A) Frequency of social active nonagonistic behaviors between improved- and standard-housed (S) male BALB/c mice over time (I < S; F1,7 = 41.317, P = 0.001, months 1 through 5). (B) Frequency of social active nonagonistic behaviors between improved- and standard-housed male C57BL/6 mice over time (I > S; F1,7 = 8.042, P = 0.030, months 1 through 5).(C) Frequency of social active nonagonistic behaviors between improved- and standard-housed female BALB/c mice over time (I < S; F1,7 = 10.651, P = 0.017, months 1 through 5). (D) Frequency of social active nonagonistic behaviors between improved- and standard-housed female C57BL/6 mice over time (I < S; F1,7 = 7.308, P = 0.035, months 1 through 5).