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.     

Enhancement of food aversion by exposure to a poisoned conspecific in Norway rats (Rattus norvegicus)

Two experiments were conducted to determine whether exposure to a poisoned conspecific enhances prior food aversion in rats. In Experiment 1, subjects were serially exposed to two foods, cocoa‐flavored and cinnamon‐flavored ones, and were then poisoned 1 h later. On the next day, they were exposed to a poisoned conspecific that had eaten a cocoa‐flavored food. On the subsequent choice test, subjects had an enhanced aversion to cocoa‐flavored food. The result was replicated in Experiment 2, in which a cinnamon‐flavored food was assigned as a target. The results are discussed in relation to previous findings.     

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.     

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.     

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.     

Food choice in rats (Rattus norvegicus): the effect of exposure to a poisoned conspecific

Three experiments were conducted to examine the effects of exposure to a poisoned conspecific on subsequent food aversion in rats. In Experiment 1A, rats that had been aversively conditioned to a cocoa-flavored food were exposed to a poisoned conspecific that had eaten the same food. On the subsequent choice test, the animals increased their aversion to that food. These results were reconfirmed in Experiment 1B, in which a cinnamon-flavored food was used as the stimulus. In Experiment 2, subjects were first exposed to a poisoned conspecific and then conditioned to the food which the conspecific had eaten. On the test, they showed no sign of increased aversion to that food.     

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.     

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.     

Developmental expression of defensive responses during exposure to conspecific adults in preweanling rats (Rattus norvegicus).

I examined preweanling rats' (Rattus norvegicus) expression of ultrasounds and secretion of ACTH when exposed to unfamiliar adult male rats or to their mothers. Pups at 7 days of age produced similar levels of ultrasonic vocalization near both unfamiliar males and mothers. However, these pups could discriminate familiar from unfamiliar adults because ACTH was significantly higher in pups near adult males than in those near mothers. At 14 days of age, pups avoided adult males but not their mothers; therefore, adult males represented a significant threat. Importantly, 14-day-old rats significantly reduced ultrasound production only when near adult males. Pups at 21 days of age no longer emitted ultrasounds when socially isolated or when near conspecific adults. In addition, 14- and 21-day-old rats produced similar elevated ACTH levels across stimulus conditions. Results show significant changes in preweanling rats' responses to conspecific adults.     

Grooming and yawning trace adjustment to unfamiliar environments in laboratory Sprague-Dawley rats (Rattus norvegicus)

The authors studied grooming and yawning caused by mild stress in laboratory Sprague-Dawley rats (Rattus norvegicus). Two groups received 3 and 6 sequences of 5 foot shocks at random intervals (RI) and fixed intervals (FI), respectively. A 3rd group was not shocked (NS). The groups were exposed for 60 min twice. Grooming did not differ among groups, but yawning diminished with RI. Yawning increased and grooming decreased with the 2nd exposure, except in RI in which grooming increased. In NS and FI, grooming prevailed during the first 20 and 30 min, respectively, whereas yawning dominated the remainder of the time. In RI, grooming occurred more than yawning. An upward shift on this scale causes grooming to substitute yawning, whereas a downward shift causes the reverse effect.     

Rats (Rattus norvegicus) do not prefer salted solid food

Many studies have demonstrated that when sodium-replete rats are given a choice between water and isotonic saline, they consume more of the isotonic saline. Their ingestive responses to a choice between salted and unsalted food, however, have not been studied in the same detail. Because humans consume salt in or on food rather than in pure water, this lack of information calls into question the use of the rat as the animal model for human excess salt consumption. In Experiment 1, 16 Sprague-Dawley rats were given 1-hr access to salted and unsalted foods (potato chips, peanuts, soup) commonly consumed by humans in the salted form. In each choice situation, rats consumed more of the unsalted variety of solid food. In Experiment 2, the concentration of salt in a wider variety of foods was varied. Fifteen rats were allowed a choice of a given salt concentration or the unsalted food. In no case was the salted solid food eaten in excess of the unsalted solid food, and in general, more of the unsalted solid food was eaten. In a third experiment, two groups of 8 rats were given exposure from weaning to either salted or unsalted potato chips. Three-months of exposure to salted chips or unsalted chips did not alter the rats' relative intake of salted chips. When given a choice, more unsalted chips were consumed by both groups. These experiments indicate that sodium-replete Sprague-Dawley rats generally prefer unsalted solid foods to salted ones.     

Olfactory discrimination of individual littermates in rats (Rattus norvegicus)

Adult male rats (Rattus norvegicus) were trained in a Y-maze to discriminate the presence of a littermate from its absence. Transfer of training in subsequent tests indicated that (a) the animals were capable of distinguishing among individuals when relatedness and familiarity were held constant, (b) this ability was not due to training, and (c) their performance was based solely on odor cues. The results are discussed in relation to social behavior in rats.     

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.     

Sex differences in investigatory and grooming behaviors of laboratory rats (Rattus norvegicus) following exposure to novelty

Prior research suggested that during exposure to novel stimuli, rodent investigation and self-grooming behaviors may be sexually dimorphic and interact with ambient illumination. To test this notion we compared the behavior of adult male and female groups of Long-Evans hooded rats in normal room lighting (860 lx) and in very dim, red light (0.2 lx) following exposure to a novel juvenile conspecific. Illuminance level had little or no effect, but investigatory and subsequent self-grooming behaviors of males were substantially greater than those of females, and females engaged in greater ambulatory activity than did males. In a second experiment adult males and females were exposed to a novel inanimate object. No reliable sex differences were observed. We conclude that social novelty, as provided by exposure to a juvenile conspecific, stimulates greater investigation and postinvestigatory self-grooming than exposure to a novel inanimate object and that exposure to novel conspecifics presents a useful method for the investigation of sex differences, gonadal hormone effects, and interactions of hormones with neurotransmitter systems governing motor control systems.     

Defensive behavior of laboratory and wild Rattus norvegicus

Analysis of the defensive behaviors of wild rats to an inescapable approaching threat stimulus (the experimenter) indicated a pattern of freezing to distant stimuli, giving way to vocalization, jumps, and jump-attacks at shorter defensive distances. Comparisons of the defensive reactions of wild-trapped and laboratory-bred wild rats to a variety of threatening stimuli, in escapable as well as inescapable situations, indicated that the two wild strains were similar and consistently more defensive than laboratory rats to both human and conspecific threat stimuli. These results thus suggest that the defensive behaviors of rats have been substantially reduced during the process of domestication, with relatively little of this reduction being attributable to housing in standard laboratory conditions.     

Responses of observer rats (Rattus norvegicus) to complex, diet-related signals emitted by demonstrator rats

We explored the effects of complex, food-identifying signals emitted by demonstrator Long-Evans rats (Rattus norvegicus) on food preferences of their observers. In Experiments 1 and 2, observers identified each of 2 or 3 foods their demonstrators had eaten before interacting with observers. In Experiment 3, individual observers interacted with groups of demonstrators. Some of these demonstrators had eaten one food, some another. Observers then chose between the two foods. The greater the proportion of demonstrators in a group that had eaten a diet, the greater the proportion of that diet the observers ate. In Experiment 4, each observer interacted over several weeks with a series of demonstrators and preferred each of the foods its demonstrators had eaten. In sum, the food preferences of observers were affected by several different types of complex, food-identifying signals like those one might expect rats to encounter outside the laboratory.     

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.     

Maternal aggression: disruption by perioral anesthesia in lactating Long-Evans rats (Rattus norvegicus)

Role of perioral somatosensation in the display of maternal aggression in Long-Evans Norway rats (Rattus norvegicus) was investigated during early lactation by local anesthesia of the mystacial pads with lidocaine, which blocks conduction in the infraorbital nerve. Acute perioral anesthesia markedly reduced the likelihood that maternal aggression was expressed; this effect of perioral anesthesia was not ameliorated by a repeated test with the same treatment or by pretreatment fighting experience. The dams that fought with the intruder during perioral anesthesia did so after greatly increased sniffing contact and with decreased vigor compared with controls. Males that were attacked tended to freeze, whereas males that were not attacked were active. Thus, trigeminal afferents are critical for the initiation and normal execution of maternal aggression responses in Norway rats.     

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.     

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.