Burst lick rate was measured in 16 adult albino rats deprived of water for 23 h. Females licked faster than did males, and the mean lick rates of both sexes increased from Day 1 to Day 6 of the experiment. Both sexes licked faster at night than during the day , but this difference was statistically significant only for females . Results are viewed as challenging the hypothesized invariance of mammalian lick rates.
Laboratory-raised Virginia opossum have been found to readily acquire a lever-press response for water reinforcement. Fixed ratio behavior is comparable to that observed in other species. Fixed interval behavior, however, tends to move fairly rapidly toward an economical response style in which very few responses are emitted per reinforcement. Neither introduction of limited hold procedures nor lengthening of the deprivation schedule had any effect upon the FI responding.
Cage activity of four Virginia opossums was measured twice daily (morning and night) under six levels of ambient illumimr tion (dark, less than 1, 2.5, 5, 25, 50 ft-c). Analysis of variance indicated that the Ss were significantly more active under the three lower levels of illumination than under the three higher ones and were significantly more active at night than in the daytime. In addition the Illumination by Time of Day variable was significant. This interaction between environmental stimuli and phases of the circadian rhythm cycle is discussed in terms of Bindra's hypothesized inverted V-shaped curve relating physiological arousal and performance.
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