734 Lynn and Brown Table 1. The Mean Weight (g) of Male and Female Rats in Each Age Group (Means SEMs) Adolescent (pnd 34–39) Late adolescent (pnd 51 –55) Young adult (pnd 65– 69) Older adult (pnd 104 –109) 109.0 200.7 266.9 351.0 15.1 15.1 15.1 16.0 110.1 157.4 172.1 203.7 14.3 16.0 14.3 15.1 for data analysis, and an alpha level of <.05 was used throughout. Male Female RESULTS Weight Data The analyses revealed signiﬁcant main effects of age (F3,64 ¼ 43.91, p < .001) and sex (F1,64 ¼ 44.32, p < .001), with weight increasing with age and males weighing more than females (Tab. 1). Age and sex also signiﬁcantly interacted (F3,64 ¼ 9.00, p < .001), as male rats gained more weight than did females. Emergence Test (i) Latency to exit the box: There was a signiﬁcant main effect of sex (F1,22 ¼ 5.66, p ¼ .026) on latency to exit, with females being quicker to leave the box than males (males: 13.6 2.3 s; females: 10.0 2.2 s). There was no main effect of age (F3,22 ¼ 1.05, p ¼ .392). (ii) Total number of re-entries: There was a signiﬁcant effect of age on the total re-entries into the box (w23 ¼ 7.85, p ¼ .049), but no effects of sex (w21 ¼ .10, p ¼ .919). Trend tests revealed a signiﬁcant linear decrease in cage re-entries with age (p ¼ .007; adolescents ¼ 4.3 .4, late adolescents ¼ 4.7 .4, young adults ¼ 4.6 .4, older adults ¼ 3.2 .4). (iii) Total peeks from the box: There was neither a main effect of age (F3,22 ¼ 2.40, p ¼ .096) nor of sex on total number of peeks (F1,22 ¼ 1.22, p ¼ .282). Open Field (i) Total locomotion: There was a signiﬁcant main effect of sex (F1,23 ¼ 8.10, p ¼ .009), with females locomoting more than males, and a main effect of age (F3,23 ¼ 3.51, p ¼ .031). Trend analyses revealed a signiﬁcant linear increase in total locomotion with age (p ¼ .011; Fig. 1). After covarying body weight, the main effect of age on total locomotion persisted (F3,66 ¼ 4.52, p ¼ .006). Developmental Psychobiology FIGURE 1 Total locomotion in the open ﬁeld (means SEMs). Asterisk ( ) indicates a signiﬁcant main effect of age (p .05). White bars represent males; hatched bars represent females. (ii) Total distance moved: This measure did not differ between the sexes (F1,23 ¼ 2.79, p ¼ .108), but there was a main effect of age (F3,23 ¼ 3.39, p ¼ .035; Fig. 2). Trend analyses revealed that the TDM increased linearly with age (p ¼ .011). This main effect of age on TDM persisted after considering body weight as a covariate (F3,66 ¼ 3.04, p ¼ .035). (iii) Percentage of time spent mobile: There was no main effect of age (F3,23 ¼ .23, p ¼ .874) or sex (F1,23 ¼ 2.94, p ¼ .100) on this measure. FIGURE 2 Total distance moved (cm) in the open ﬁeld collapsed across subjects’ sex (means SEMs). Asterisk ( ) indicates a signiﬁcant main effect of age (p < .05).