The influence of preexperimental experience on social discrimination in rats (Rattus norvegicus)

The authors used laboratory rats (Rattus norvegicus) of known relatedness and contrasting familiarity to assess the potential effect of preexperimental social experience on subsequent social recognition. The authors used the habituation-discrimination technique, which assumes that multiple exposures to a social stimulus (e.g., soiled bedding) ensure a subject discriminates between the habituation stimulus and a novel stimulus when both are introduced simultaneously. The authors observed a strong discrimination if the subjects had different amounts of preexperimental experience with the donors of the 2 stimuli but a weak discrimination if the subjects had either equal amounts of preexperimental experience or no experience with the stimuli. Preexperimental social experience does, therefore, appear to influence decision making in subsequent social discriminations. Implications for recognition and memory research are discussed.     

Recognition of food by individual, food-naive, weaning rats (Rattus norvegicus)

To control and observe first feeding experiences, pups were reared without access to food. At weaning age, individual pups received simultaneous access to a palatable, noncaloric diet and a similar diet containing starch. Each diet contained a flavor cue. Within 20 min of sampling both diets pups preferentially ingested the caloric alternative. Further tests indicated that pups learned a preference for flavor cues paired specifically with ingestion of starch. In caloric discrimination tests in which the diets were not distinguished by artificial flavors, pups did not show the rapid preference for the caloric diet seen in earlier experiments. We suggest that recognition and preference for the caloric diet is based on its rapid postingestive effects rather than readily preferred sensory features of the starch. Our findings demonstrate the abilities of individual, food-naive pups to rapidly recognize food and indicate a role for associative learning in the onset of independent feeding.     

Transitive inference in rats (Rattus norvegicus).

Although Piagetian theory proposes that the ability to make transitive inferences is confined to humans above age 7, recent evidence has suggested that this logical ability may be more broad based. In nonverbal tests, transitive inference has been demonstrated in preschool children and 2 species of nonhuman primates. In these experiments, I demonstrate evidence of transitive inference in rats (Rattus norvegicus). I used an ordered series of 5 olfactory stimuli (A < B < C < D < E) from which correct inferences were made about the novel B versus D pair. Control procedures indicated that performance did not depend on the recency with which the correct answer was rewarded during training and may be disrupted by the addition of logically inconsistent premises (F > E and A > F). The possibility that logical transitivity may reflect a form of spatial paralogic rather than formal deductions from a syllogistic-verbal system is discussed.     

Food stealing by young Norway rats (Rattus norvegicus)

Six experiments were undertaken to explore factors affecting young rats' (Rattus norvegicus) frequencies of stealing food from conspecifics when identical food is available in surplus. It was found that (a) rats would walk across a bed of pellets to steal the particular pellet a peer was eating, (b) frequency of stealing within a pair did not decrease over days, (c) rats stole unfamiliar foods more frequently than familiar foods, (d) younger rats stole from older rats more frequently than older rats stole from younger ones, (e) hungry rats stole more frequently than replete rats, and (f) rats that had stolen a pellet of unfamiliar food from an anesthetized conspecific subsequently exhibited an enhanced preference for that food. Results suggest that food stealing is a mode of active seeking of information about what foods to eat.     

Auditory categorization of complex sounds by rats (Rattus norvegicus)

Little research has explored the auditory categorization abilities of mammals. To better understand these processes, the authors tested the abilities of rats (Rattus norvegicus) to categorize multidimensional acoustic stimuli by using a classic category-learning task developed by R. N. Shepard, C. I. Hovland, and H. M. Jenkins (1961). Rats proved to be able to categorize 8 complex sounds on the basis of either the direction or rate of frequency modulation but not on the basis of the range of frequency modulation. Rats' categorization abilities were limited but improved slowly and incrementally, suggesting that learning was not facilitated by selective attention to acoustic dimensions.     

Preference for novel flavors in adult Norway rats (Rattus norvegicus)

The authors fed rats 1 of 2 distinctively flavored, roughly equipalatable diets for 3 days then offered them an ad libitum choice between the 2 diets. For 3 days, subjects exhibited a reduced relative intake of whichever diet they had previously eaten (Experiment 1). Such reduction in relative intake was as effective as a toxicosis-induced conditioned aversion in determining subjects' food choices (Experiment 2). The strength of exposure-induced reduction in relative intake did not depend on similarity of the 2 diets offered for choice either to each other or to subjects' maintenance diet (Experiment 3) but did require continuous exposure to a diet (Experiment 4). These experiments provide the first evidence of a robust, exposure-induced decrease infood preference in rats lasting for days rather than minutes.     

Perioral somatosensory determinants of nursing behavior in Norway rats (Rattus norvegicus)

Prior to assuming the upright crouching posture over their pups during nursing bouts, lactating rats typically engage in several oral behaviors, including nuzzling, licking and rearranging pups. By acutely depriving dams of various aspects of perioral stimulation from pups (with anesthesia of the mystacial pads or of the tongue, with mouth suturing, or with muzzling), we found the following: (1) Distal stimulation from pups maintains proximity-seeking behavior, but is insufficient to stimulate nursing behavior. (2) Lack of tongue feedback decreases pup licking and hastens the onset of crouching. (3) Snout, but not tongue, contact with pups is required for hovering over them. (4) The position of the dam while hovering over her litter enables the pups to gain access to her ventrum, thereby provoking her upright, crouching posture. (5) Older pups are capable of bypassing the dam's perioral attentions and stimulating crouching directly by burrowing under the dam's ventrum.     

Social identification of toxic diets by Norway rats (Rattus norvegicus)

In the present experiment, a naive rat (an observer) interacted with two conspecifics (demonstrators) that had recently eaten a diet unfamiliar to the observer, ate two unfamiliar foods in succession, one of which was the food its demonstrators had eaten, suffered toxicosis, and finally, was offered a simultaneous choice between the two diets it had eaten prior to toxicosis induction. During the choice test, observers exhibited an aversion to that diet their respective demonstrators had not eaten. This result indicates that exposure of a rat to conspecifics that have eaten a diet can act, as does actual ingestion of a diet, to reduce that diet's subsequent associability with toxicosis. I discuss this finding as suggesting that interaction with conspecifics may provide an alternative to individual trial and error learning in identification of toxic foods by rats that ingest a number of novel foods in succession before becoming ill.     

Cooperative behavior of laboratory rats (Rattus norvegicus) in an instrumental task

Cooperation is a cognitively demanding, complex social behavior, found primarily in primates. Here we investigated mutualism in rats (Rattus Norvegicus), a simple form of cooperation in which two subjects work on operant task, receiving immediate and simultaneous sucrose reward for a joint action. To receive the sucrose reward, familiar pairs of rats were required to nose poke simultaneously. Following 44 training days, we examined the relation of social contact and ultrasonic vocalizations to the rat's cooperative behavior by testing the effects of inserting opaque, wire-mesh, or no partition--between subjects. Cooperative behavior (simultaneous nose-poking): (a) increased gradually during initial training; (b) decreased with the opaque partition (restricting visual, acoustic, and physical communication); (c) increased with a wire mesh partition restricting only physical contact); and (d) increased with the number of 50 kHz USV "happy" calls and the intensity of social interaction. The possibility of studying the development of cooperative behavior in laboratory rats using a simple procedure based on commercially available equipment may prove useful in modeling determinants of social behavior.     

Penile responses of rats (Rattus norvegicus) in extended ex copula tests

Prolonged tests of rats' penile responses outside the context of copulation (ex copula) were undertaken to facilitate analysis of these penile components of copulatory behavior. Penile erections and flips were evoked for 1 hr following penile sheath retraction (SR), and rats were retested for 30 min after a rest interval of 5 min to 4 hr. The number of penile responses declined sharply over the course of the hour. Following a 5-min rest, there was little likelihood of additional responses. Within 2 hr some measures of penile response potential approached their original high asymptotes; other measures were still depressed after 4-hr rest. In control tests, the penile sheath was unretracted (SU) during the first 60 min, and hence few or no penile responses occurred. This condition caused no reduction in the number of erections and flips in the subsequent 30-min SR test; hence the reduction in response potential during and after SR tests was due to the responses displayed, not to the conditions of restraint. Ejaculation occurred frequently in the SU condition but rarely in SR tests, results suggesting that sheath retraction may normally inhibit ejaculation in ex copula tests, and perhaps during copulation as well.     

Life-span changes in the copulatory behavior of male rats (Rattus norvegicus)

The copulatory behavior of 5-, 10-, 15-, and 20-month-old sexually naive male rats (Long-Evans) was studied during 2-hr tests with receptive females. There was no apparent change in sexual arousal as measured by latency to initiate copulation across age, with subjects from all groups exhibiting comparable latencies to first mount and first mount with intromission. The numbers of ejaculations achieved were also similar across ages. Significant age differences were found for frequency of mounts, with 20-month-olds having the highest mean frequency. The persistent mounting by older males appeared to account for significant group differences in interintromission interval and ejaculation latency. It is suggested that motor deficits may impair the ability of older males to achieve intromission, increasing the number of mount bouts as well as the number of mounts per bout, thus extending the length of each copulatory series.     

Odor cue determinants of urine marking in male rats (Rattus norvegicus)

This study examined the spatial distribution of urine marking by sexually inexperienced male rats under six odor conditions: minimum odor (baseline), conspecific male urine, conspecific female urine, an animal's own urine, male gerbil urine, and amyl acetate. The gerbil urine and amyl acetate conditions were included to test the theory that marking, rather than being elicited by and directed as a display to appropriate conspecifics, occurred to the introduction of any novel or disruptive odor. The subject's own urine was used to test the theory that the animal marked to maintain an optimum level of its own odor in the environment. The results showed that marking was directed predominantly at objects in the environment over which the animal could readily pass. All odor conditions except "own odor" sharply increased marking over baseline levels. Marking decreased with repeated exposure to particular stimuli. The results strongly supported the theory that urine marking, at least in sexually inexperienced male rats, is produced by novel odor stimuli introduced into a familiar environment.     

Sexual free behavior in male rats (Rattus norvegicus).

Male rats (Rattus norvegicus) were given continuous access to estrous female rats for 24 hr each day for 10 days. During the first 12 hr, the rats achieved an average of 10 ejaculations, followed by a 1- to 2-day recovery period with little sexual activity. During the last 7 days, the rats maintained a reasonably stable equilibrium level of 3 ejaculations per day. These occurred predominantly during the dark phase of the diurnal cycle, they frequently occurred in a cluster, and they usually occurred shortly after the introduction of a novel estrous female. Except for quantitative differences, these results are generally consistent with conventional research but systematically extend the generality of the results to the context of the free behavior situation. The availability of sexual activity had no appreciable effect on food and water intake, but it did decrease the amount of running activity.     

Sensory cues that elicit ultrasonic vocalizations in female rats (Rattus norvegicus)

Female rats emit ultrasonic vocalizations during copulation. Female vocalizations are associated with darting and other proceptive behavior. In addition, females frequently call while approaching the male. A series of experiments was undertaken to determine whether female rats emit ultrasonic vocalizations in response to other rats or to their odors. Females were exposed either to bedding soiled by males or females or to devocalized conspecifics separated from the subject by a barrier. Females vocalized more to male cues than to female cues, regardless of whether the odor cues were from soiled bedding or from a devocalized conspecific. In addition, subjects vocalized more when presented with a devocalized female rather than a neutral stimulus. Furthermore, devocalized adult males, separated by a barrier, were more effective than either devocalized castrated or juvenile males in eliciting vocalizations; in turn, castrates and juveniles were more effective than neutral stimuli. Female calling was likely induced by odors from the male. In one experiment, the female subject was positioned in such a way that she could neither see or touch the stimulus male. Presentation of tape-recorded male vocalizations did not affect calling by the female. Vocalizations emitted by females in response to male odors may attract males or may facilitate subsequent copulatory behavior by the male.     

Numerical competence in rats (Rattus norvegicus): Davis and Bradford (1986) extended

The task of rats (Rattus norvegicus) was to enter 1 box of a defined ordinal number among an array of boxes. In Experiments 1 and 2, the rats correctly chose the 4th box from arrays of 6 and 12 boxes, respectively. In Experiments 3 and 4, in which the ordinal number of the correct box was increased in a graduated fashion, they were able to select the correct box even when its position was higher than 10th among 12 and 18 boxes, respectively. In Experiment 5, the possibility that the rats had used cues to the openability of the box doors was ruled out. In Experiments 6 and 7, the rats succeeded in the task even when the total number of boxes was varied from trial to trial. In Experiment 8, both small- and large-sized boxes were used to control for the possibility of using the cumulative length of the boxes as a cue. Overall, the results suggest that performance was based on numerical cues. Intentional acts were occasionally observed but appeared not to be essential for solving the task.     

Responsiveness to estrous chemostimuli in male rats (Rattus norvegicus) of different ages

Previous findings indicate that Norway rats (Rattus norvegicus) respond differentially to estrous chemostimuli in early infancy and that adult male sexual responsiveness to such chemostimuli depends on infantile experience with suckling odors (Fillion & Blass, in press). The development of behavioral responsiveness to estrous chemostimuli in male rats from late infancy to adulthood was examined in this study. Preweanling, adolescent, and adult males were observed for 20 min in the presence of an anesthetized estrous or an anesthetized diestrous female. This was, for adolescent and adult subjects, the first heterosexual encounter since weaning. Rats in each age-group responded differentially according to female state, but in distinct, age-related ways. Preweanling spent more time investigating the female's body if she was estrous. Adults expressed the same affinity but by more focused investigation directed perivaginally. Adolescents showed no female-directed bias but groomed themselves more if the female was estrous. These data identify developmental continuities and discontinuities in male rat behavior toward sexually receptive females.     

Habituation of the hiding response to cat odor in rats (Rattus norvegicus)

Cat odor-induced hiding was examined in rats (Rattus norvegicus) using an apparatus with a "hide box" at one end and a piece of a worn cat collar at the other end. Rats spent most of their time hiding on exposure to the cat collar, but this response gradually habituated over repeated daily exposures. Hiding was reversed by administering the anxiolytic drug midazolam (0.375 mg/kg). Rats showed increased anxiety on the elevated plus-maze after exposure to the collar. This response was absent in habituated rats, suggesting that habituation of hiding reflects decreased odor-induced anxiety. It was established that rats located in the hide box of the apparatus might not have detected the odor of the cat collar placed at the other end of the apparatus. This implies that habituation occurs after relatively modest levels of odor exposure. Overall, the results cast some doubt on claims that predatory odors in rats are akin to phobic stimuli in humans.     

Effects of spatial food distribution on search behavior in rats (Rattus norvegicus)

To analyze how search strategies are adapted according to the geometric distribution of food sources, the authors submitted rats to a search task in which they had to explore 9 food trays in an open field and avoid visiting already-depleted trays. Trays were spatially arranged in 4 independent configurations: a cross, a 3 x 3 matrix, 3 clusters of 3 trays each, and a random configuration. Rats exhibited differential search efficiency as a specific effect of the susceptibility of the configurations to being explored in a principled way: crosses were first, matrices or clusters were in the middle, and random configurations were last. Although no exhaustive searches or highly principled patterns were observed in any of the configurations, performances improved as the sessions went by. Thus, structural affordances of the environment influence the construction not only of search strategies but also of information linked to where the reward is.     

Sex differences in urinary odors produced by young laboratory rats (Rattus norvegicus)

Responses of adult female rats to male and female pup urine were examined in three experiments. Investigatory sniffing of male and female pup urine deposits by maternal rats was compared in a series of simultaneous choice tests given at 3-day intervals between Day 2 and Day 17 postpartum. Male urine was consistently preferred. Introduction of male, but not female, pup urine to the nest was also found to significantly elevate maternal licking of anogenital regions of pups. Thus, urine from pups of various ages contains sex-identifying odors that differentially elicit spontaneous maternal interest. The odor of male urine may provide a sufficient stimulus to account for the greater anogenital licking that males of this species normally receive. Nonmaternal, naive, adult females behaved like maternal rats, preferring male urine in the choice test. This indicates that the maternal condition of a dam is not necessary either for the olfactory discrimination or for the male odor preference.     

Contrasting lever-press avoidance behaviors of spontaneously hypertensive and normotensive rats (Rattus norvegicus)

Lever-press avoidance performances of Spontaneously Hypertensive (SHR) and Wistar-Kyoto (WKY) rats were observed in two experiments. The first compared them at shock intensities of 1 or 2 mA. The SHRs had the highest percentage of avoidances, made the most coping responses (avoidances plus escapes), and received the fewest shocks (ps less than .05). They also had shorter avoidance and initial-escape latencies (ps less than .05). In the second experiment these strains were trained with signal-shock intervals that produce good (60 s) or poor (10 s) avoidance acquisition. Analysis of the Strain x Conditions x Blocks interaction showed that SHRs performed better with the longer interval (p less than .05), whereas WKYs' performance was similar with both (p greater than .05). Thus WKYs differed from all other strains previously exposed to these conditions. The findings are interpreted in terms that emphasize the SHRs' tendency to move, compared with the WKYs' tendency to freeze in response to aversive stimuli, rather than characterizing SHRs as hyperreactive.