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.     

Rats (Rattus norvegicus) find occupancy of a restraint tube rewarding

Two experiments evaluated whether rats' occupancy of a restraint tube is reinforcing. In Experiment 1, each rat in the 0-min group moved freely in a chamber where a wall blocked access to a restraint tube. After 10 min the wall was removed, permitting 15 min of chamber access and tube entry. The other 2 groups were locked in the tube for 10 and 20 min respectively before release into the chamber for 15 min. Across sessions, rats locked up for 10 and 20 min entered the tube more frequently than rats in the 0-min group, and during the first 2 sessions rats in the 20-min group stayed in the tube longer than the other groups. Over sessions this difference disappeared. However, for all groups and sessions the mean percentage of session time in the tube exceeded chance expectations. This result suggests tube occupation was reinforcing. In Experiment 2’s Phase 1, rats could enter an open tube. On exiting, the tube door closed. A lever press opened the door for the rest of the 1-hr session. In Phase 2, these rats were locked in the tube for 10 min before the door opened. Upon exiting, the door closed. As in Phase 1, a lever press opened the door for the rest of the session. The latency between pressing and tube entry decreased over sessions, indicating that tube entry reinforced lever pressing. These results are difficult to reconcile with accounts of rat empathy based on the thesis that tube restraint distresses occupants.     

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.     

Prior exposure to stress reduces the diversity of exploratory behavior of novel objects in the rat (Rattus norvegicus)

Although a large literature has documented the varied effects of stress on an organism, relatively little attention has been devoted to investigating stress effects in ecologically relevant situations. Our experiment was conducted to assess the effects of stress on rats' (Rattus norvegicus) exploration of novel objects in a seminaturalistic and familiar environment that is relatively free of the constraints that have been placed on rats in prior investigations of stress. The results show that prior exposure to stress decreased the rats' diversity of exploration but did not affect general activity in comparison with animals not exposed to stress. We propose that the effect of stress on the qualitative aspects of exploratory behavior may be due to effects on organisms' sensitivity to predation.     

Reexamination of spectral mechanisms in the rat (Rattus norvegicus)

The spectral mechanisms of the rat were studied in electrophysiological and behavioral experiments. The photopic electroretinogram (ERG) was recorded (a) to rapidly flickering lights, (b) during the cone phase of dark adaptation, and (c) with a flicker photometric procedure. Each procedure indicated the presence of a single cone mechanism having a lambda max of about 510 nm. Increment-threshold spectral sensitivity functions measured in a behavioral test situation yielded the same conclusion. A behavioral experiment failed to produce any evidence for the presence of color vision in this rodent. The consistent conclusion from both behavioral and electrophysiological experiments is that the rat retina contains only a single photopic spectral mechanism.     

Acoustic startle threshold of the albino rat (Rattus norvegicus)

The startle threshold of the albino Sprague-Dawley rat runs parallel to the curve of the hearing threshold. The difference between the startle and hearing threshold is 87 dB (SPL) at a background noise level of 75 dB (SPL). At 110 dB (SPL), the threshold has a range from 2 kHz to 50 kHz with a minimum at 10 kHz and a second minimum at 40 kHz. Amplitude and latency of the startle response are not only dependent on the sensation level of the acoustic stimulus but also on the frequency. At threshold, only the head movement component of the startle response is elicited.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Unconditioned sexual incentive motivation in the male Norway rat (Rattus norvegicus)

Sexual incentive motivation was evaluated in a procedure consisting of a large open field where incentive animals were confined behind wire mesh openings. When sexually inexperienced male rats (Rattus norvegicus) were exposed to the receptive female-male incentives, they spent more time close to the female. If the incentives were receptive female-nonreceptive female, the receptive female was preferred. However, when the alternatives were nonreceptive female-male, no preference was obtained. Castration abolished preference for the receptive female, and treatment with testosterone propionate restored it. Estradiol plus oil is as efficient as estradiol plus progesterone for giving the ovariectomized female incentive properties. The living female can be replaced with female odor. Sexual experience did not have any long-term effects on the female's incentive value, but immediately preceding limited sexual activity enhanced it     

Development of the auditory orientation response in the albino rat (Rattus norvegicus)

The development of head orientation to auditory stimulation was examined in rat pups at Postnatal Days 8, 11, 14, 17, and 20. The animals were tested in a quiet environment with single bursts of 65 dB (SPL) broad-band noise. A reflexive head turn toward the sound was first seen on Postnatal Day 14 and subsequently on Days 17 and 20. This result demonstrates that the onset of directional auditory responses occurred between Day 11 and Day 14. The role of binaural cues in early sound orientation was examined in 17-day-old pups with monaural ligation of the external meatus. These animals were unable to localize a sound source and consistently turned toward the side of the unligated ear regardless of the position of the stimulus. Thus binaural cues were shown to be important for head orientation to sound in early development. In a separate study, head orientation to high and low frequency tone pips was examined. Directional responses were first seen on Day 12 for a 16-kHz tone and Day 14 for a 2-kHz tone. These results indicate an earlier onset for orientation to high frequency sounds in the rat.     

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.     

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.     

Two forms of minority-group test bias as psychometric artifacts with an animal model (Rattus norvegicus)

Controversy abounds over attributing group differences on tests to nature, nurture, or test bias. Limitations of correlational sampling from natural populations necessitate experimental methods to resolve underlying issues. In classical psychometrics test items are selected from a larger item pool through analysis of item responses in a sample of subjects. Rats of six inbred strains (n = 366) were tested in multiple mazes to provide a large item pool. Six populations were created, each with differing proportions of each strain. Items selected through independent item analyses within each population yielded six tests. An independent cross-validation sample (n = 146) provided scores on all six tests. This sample was also tested in another set of maze problems defined as the criterion to be predicted. Strain means and intrastrain predictive validities for the six tests varied with strain representation in the population used for item selection (p less than .001). Conventional item-selection procedures clearly produced two forms of minority test bias.     

Prenatal expression of species-typical action patterns in the rat fetus (Rattus norvegicus)

We investigated sensory and behavioral responsiveness of the rat fetus. On Days 19, 20, or 21 of gestation, rat fetuses received intraoral infusions of a biologically important stimulus, milk, or a novel chemical stimulus, lemon. Using a technique to directly observe behavior in utero, we found that rat fetuses discriminate between intraoral infusions of milk and lemon, exhibiting different levels and patterns of overall activity after infusion. Milk was found to evoke a low magnitude, delayed increase in overall fetal activity from Day 19 through Day 21, whereas lemon evoked a high-magnitude, spiked pattern of activity that diminished from Day 19 to Day 21. Late in gestation these two stimuli elicited species-typical action patterns. Milk infusions elicited a stretch response much like the one shown by pups at the nipple; lemon infusions elicited face wiping typical of older pups and adults exposed to aversive gustatory stimuli.