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.     

Effects of the nonagouti pelage-color allele on the behavior of captive wild Norway rats (Rattus norvegicus)

The objective of the present study was to determine the extent to which the nonagouti pelage-color allele influences selected behaviors (including docility) of the wild Norway rat. Agouti and nonagouti (black) littermates were compared in tests for handling, open-field behavior, platform jumping, and response to a novel food item, all of which clearly differentiate wild and domestic rats. Nonagouti rats were significantly easier to approach, capture, and handle than their agouti sibs. However, differences between agouti and nonagouti rats for the other variables studied were not significant. Although the presence of the nonagouti allele cannot fully account for the behavioral differences between wild and domestic Norway rats, it may have facilitated the domestication of this species by improving ease of handling.     

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.     

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.     

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.     

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.     

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.     

Protein deficiency magnifies social influence on the food choices of Norway rats (Rattus norvegicus)

The food choices of protein-deprived juvenile rats were more profoundly affected by interaction with conspecifics than were the food choices of protein-replete juvenile rats. When choosing among four different-flavored, protein-deficient diets, protein-deprived rats ate significantly more of the diet eaten by a conspecific demonstrator than did protein-replete rats. These data suggest that the food choices of the relatively less successful members of a population are most affected by social interaction. Consequently, the mean effect of social interaction on diet selection in a population of Norway rats is likely to be positive.     

Effects of poisoning on predatory and ingestive behavior toward artificial prey in rats (Rattus norvegicus)

Ingestion of novel prey by vertebrate predators that is followed by illness typically decreases the predators' eating the prey more readily than their pursuing or killing it. We used artificial prey-food items (rolling ball bearings that predicted food pellets) to model and extend this finding in laboratory rats. During daily experimental sessions presentations of a novel bearing-food combination were intermixed with presentations of a familiar bearing-food combination. The poisoned animals received lithium chloride injections after each session. Experiment 1a showed that: (a) Poisoning markedly decreased ingestion of the novel food without decreasing the frequency of contact (nose, seize, carry, or chew) with the novel bearing predicting that food. (b) However, poisoning did significantly decrease the average duration of contact with the novel bearing. (c) Poisoning did not decrease behavior toward the familiar food and bearing. Experiment 1b showed that after moderate exposure to pairings of the novel food and bearing, poisoning decreased behavior toward both novel and familiar food and bearings. The results indicate that important aspects of predation can be simulated and examined in the laboratory by using artificial prey-food items. These results also support the conclusions that predatory behavior and ingestion reflect separable modes of behavior and that the presence of novel prey interferes with the generalization of poisoning effects to familiar prey.     

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.     

Thermoregulatory behavior in infant Norway rats (Rattus norvegicus) and Syrian golden hamsters (Mesocricetus auratus): arousal,orientation, and locomotion

The responses of 2- and 8-day-old rats (Rattus norvegicus) and hamsters (Mesocricetus auratus) to thermal stimulation were assessed in 4 experiments. In Experiment 1, the surface underlying the pup was cooled, and the latency to escape to a region of warmth was measured. Experiment 2 required pups to locomote farther to gain access to warmth. Experiment 3 was similar to Experiment 1 except the underlying surface was heated. Finally, in Experiment 4, locomotor behavior was assessed during isothermal cooling in which there was no possibility for escape. In general, hamsters exhibited more rapid and robust responses to thermal stimulation than rats. A framework for interpreting these results is presented emphasizing how differences in locomotor and thermogenic capabilities influence thermoregulatory behavior under different task conditions.     

Contributions of endothermy to huddling behavior in infant Norway rats (Rattus norvegicus) and Syrian golden hamsters (Mesocricetus auratus)

Infant Syrian golden hamsters (Mesocricetus auratus) do not exhibit endogenous heat production before 3 weeks of age and do not huddle effectively during cold exposure, gaining little thermoregulatory benefit from the presence of multiple littermates. In contrast, infant Norway rats (Rattus norvegicus) produce heat endogenously and are effective at maintaining elevated body temperatures by huddling. Therefore, the ineffective huddling of infant hamsters may be due to the absence of endogenous heat production. The huddling behavior of infants in mixed huddles of 8-day-old hamsters and weight-matched 4-5-day-old rats was observed to explore this possibility. The results indicate that hamsters, even when cold, effectively gain access to heat-producing rats, supporting the idea that endothermy contributes to the behavior of huddling by providing heat to each individual and thermal stimuli to other infants to support aggregation.     

Loudness bisection and masking in the rat (Rattus norvegicus).

The bisection method of animal psychophysical scaling was examined as a measurement procedure. The critical assumptions of bisection scaling, as described by Pfanzagl (1968), were tested to determine if a valid equal-interval scale could be derived. A valid scale was derived in which loudness for the rat (Rattus norvegicus; n = 13) was a power function of sound pressure for 4-kHz tones. Masking noise reduced the discriminability of tonal stimuli but did not affect the bisection point. This result is consistent with an interval scale representation of loudness and demonstrates scale meaningfulness. Loudness bisection data that have been reported in the literature for 3 species (humans, rats, and pigeons) are in substantial agreement with our results.     

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.     

Development of Exploration and Investigation in the Norway Rat (Rattus norvegicus)

A cross-sectional study of the development of spontaneous exploratory behavior in the laboratory rat was conducted. Long-Evans hooded rat pups aged 11 to 28 days (n = 158, from 25 litters) were placed into a dimly illuminated arena for 10 min on 2 consecutive days. Analysis of videotaped records from the second session for each rat pup showed a relatively sudden emergence of locomotor exploration between 12 and 16 days of age. The behavioral topography of interactions with objects developed less uniformly, with some features (e.g., bout length) relatively unchanging and other features (e.g., number of bouts, number of different behaviors displayed) increasing with age. The data for locomotor exploration are consistent with an explanation that depends on maturation-driven emergence of species-typical behavior, but that explanation cannot account for the pattern of development for object investigation. These results provide further evidence that some important aspects of the character of exploratory behavior depend, at least partially, on experience, and that exploration is not in fact a single behavior system. In addition, they provide important baseline information for further studies of the structure and function of exploratory behavior.     

Milk availability modulates weaning in the Norway rat (Rattus norvegicus)

Two experiments were conducted to investigate the role of milk availability in the development of independent feeding and drinking in the Norway rat pup. Beginning on Day 14 postnatally, pups were exposed to different levels of milk supply by changing litter size from 8 to 4 versus 12 pups (Experiment 1) or limiting temporally the pups' access to a lactating dam to 8, 14, or 24 hr daily (Experiment 2). Both manipulations accelerated weaning in milk-poor pups in comparison with same-age pups with relatively greater milk supplies. By adding solid food and water to their diet, early weaning pups compensated for the negative energy consequences of milk reduction and achieved premanipulation growth rates. Milk availability thus appears to affect weaning, and it is suggested that the developmental changes in the nutritive energy balance between mother and offspring contribute to the emergence of independent ingestion.