Influence of sustained maternal state upon weaning of rats was studied in successive litters produced by postpartum mating. In preliminary experiment, the mother's behavior during pre- and postpartum period of the 2nd litter were examined using within-subject design. Results showed that frequency of nursing decreased, while that of eating and drinking increased. But frequency of nursing increased after the delivery of the 2nd litter, indicating the restored maternal state. In the experiments, the behavior of mother and pups were compared between the mothers with successive litter and those only with the 1st litter. Suckling of the 1st litter was maintained longer in the mothers of successive litter group. But eating and drinking was not different between the two groups. These results indicate that the termination of suckling is determined by the decrement of milk supply, but the emergence of eating and drinking is not influenced during weaning period.

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The present experiment examined concurrent learning of multiple serial patterns of reward amounts by rats. In Phase 1, rats were trained with 8-8-8-4-0, 4-4-4-2-0, and 2-2-2-1-0 original series, each consisting of varying number of 45 mg food pellets which were presented in the goal box of a straight runway on consecutive runs. Rats showed reliable anticipation of 0 pellet in all the series. In Phase 2, 8-8-8-1-0 novel series was added to the three original series. Anticipation of 0 pellet was more accurate in the novel series than in the original series. In Phase 3, 8-8-8-8-8-4-0, 4-4-4-4-4-2-0, and 2-2-2-2-2-1-0 seven-item series were added to the three original five-item series. Rats showed consistent anticipation of 0 pellet in the seven-item series as well as in the original five-item series. These results indicate that “less than” rule, an abstract relationship defining the decrease in reward amount, served as discriminative cue for 0 pellet, and that neither item-association learning nor serial-position learning played a major role in the learning situation used here.

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The present study examined relationships between rat's hypertension and activity pattern. Behavioral activity of spontaneously hypertensive rat (SHR) and Dahl salt-sensitive rat (DS) was measured under the condition of an open-field and a home-cage. Twelve male hypertensive SHR, and 12 male normotensive Wistar-Kyoto rats (WKY) were used in the experiment 1. In an open-field, SHRs were more active than those of WKYs. In a home-cage, circadian variations of spontaneous activity in SHRs were smaller and less noticeable than WKYs. Six male hypertensive DSs fed with a high salt diet (NaCl 8%) and 6 male normotensive DSs fed with a low salt diet (NaCl 0.4%) were used in the experiment 2. Although no difference was found in open-field activity, circadian activity variations of hypertensive DSs in a home-cage were found to be smaller in the same manner as that of SHRs. This suggested a relationship between cardiovascular system and regulation of spontaneous activity.

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In recent years, environmental enrichment is becoming imperative at zoos. A goal of the environmental enrichment of zoo environments is to make the behavioral repertoire and activity budgets of captive animals as close to those of their wild counterparts as possible. However, since there are few studies concerning zoo animal behavior, it is first necessary to elucidate the differences of behavior between wild and captive animals. This study aimed to describe the behavioral patterns of nine mammalian species (wallaby, lion, wolf, bear, elephant, zebra, deers, Japanese macaque, chimpanzee) reared at three zoos in '95 and '96 using the focal animal sampling method. The differences of animal behavior between the captivity and the wild were examined. The results show that zoo animals have a much simpler repertoire and also exhibited extremely different activity budgets compared with wild counterparts. This was especially the case in feeding behavior. It is concluded that modification of feeding conditions should be one of the high priority targets in zoo environmental enrichment projects.

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Effects of postexposure of sucrose solution on conditioned flavor preference after flavor-sucrose pairing were investigated. Forty-two hungry rats were allowed to drink 5% sucrose solution with a flavor and tap water with another flavor in phase 1. In phase 2, Group US received unflavored sucrose solution postexposure and Group C+received sucrose solution with a third flavor. Group Water was given unflavored tap water. Finally, all subjects received choice tests of two flavors used in conditioning. Groups US and C+showed less preference to the sucrose-paired flavor than Group Water. These results suggest that context-sucrose association did not play a major role in the postexposure effect of sucrose on conditioned flavor preference.

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The experiment assessed the role of the central nucleus of the amygdala (AMY-C) in two kinds of conditioned fear responses (activity and startle response) using delayed conditioning procedure. Eleven AMY-C lesioned and 10 control (intact) rats were used. In the pre-conditioning session, five paired-presentations of CS (light) and burst noise were given, and the amount of startle responses was measured. In the fear conditioning session, 20 pairings of CS and US (a 0.5 sec footshock) were presented, and the amount of activity was measured. The procedure of the post-conditioning session was the same with the pre-conditioning session. The control rats showed increased amount of startle responses in the post-conditioning session compared with the pre-conditioning session, and their amount of activity in the fear conditioning session also increased as training progressed. For the lesioned rats, neither the amount of startle responses in the post-conditioning session nor the amount of activity during the fear conditioning session increased. These results suggest that the central nucleus of the amygdala plays an important role in these conditioned fear responses.

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