Socially induced renal pathology of captive wild rats (Rattus villosissimus)

Long-haired rats, Rattus villosissimus, were studied in large cages. Groups of adult rats (each of 3 males, or 1 male and 2 females) were observed during intermittent encounters with a male intruder for up to 9 days. Two further groups, each of 8 males and 8 females, were maintained for 70 days without introduction of intruders. Controls were kept in small cages in which intolerant behavior was rare. Behavior during attack resembled that of male R. norvegicus, but social relationships were less stable. Only observations on males are described in detail. Some rats collapsed under attack, though unwounded, and died when not removed. Collapse occurred sometimes after a few hours, but sometimes after many days of exposure. Exposure to attack was accompanied by a decline in body weight and by some adrenal hypertrophy. Two kinds of renal pathology are described: focal glomerular hypercellularity (FGH), probably due to glomerulonephritis, and dilated distal convoluted tubules. Neither condition occurred in the controls. FGH occurred in 3 of 12 rats (25%) that remained apparently healthy during the 9 days of continuous exposure, in 21 of 23 intruders (91%) that were exposed to intermittent attack over 9 days or less without becoming debilitated, and in 8 of 11 such rats (73%) that collapsed. All rats examined from 70-day colonies had FGH, whether collapsed or not. Dilated tubules occurred in 6 of 32 intruders (19%) exposed to intermittent attack, and in 2 of 6 animals that collapsed during 70 days of exposure. Renal pathology, especially glomerulonephritis, was therefore a correlate of social intolerance; but there was no evidence that it was a significant cause of death.     

Social dominance and individual aggressiveness

Male rats exhibiting high, moderate, or low levels of offensive aggressive behavior in interactions with intruders in their home cage were grouped in mixed-sex colonies with 1 male of each aggression-level group in each colony. Agonistic interactions measured 1, 2, 3, 7, 14, 21, and 22 days after colony formation indicated that highly aggressive males on pretests continued to be more aggressive, becoming the dominant colony male in five of seven colonies and attacking intruders more often than less aggressive males. In the two remaining colonies the moderately aggressive male became dominant. This relationship, which was consistent over a number of indices, including offensive and defensive behaviors, and wound counts and wound sites, was seen even when a substantial weight differential favored the less aggressive animal. Dominance relationships were rapidly established and within-group fighting declined significantly over the 21-day test period. Pretest offensive levels also influenced the behavior of subordinates, with high or moderately aggressive subordinates showing more defense in interactions with dominants and receiving more wounds than did low-aggression subordinates. Dominant males also showed more defense in interacting with those subordinates which had been more aggressive during pretests. This pattern of results suggests that aggression level of the subordinate as well as the dominant may be an important factor determining the intensity of agonistic interactions in male rats.     

Responses of dominant and subordinate male rats to the odors of male and female conspecifics

The resident-intruder paradigm was used to examine the effects of social dominance and individual recognition on odor preferences and urine-marking in male rats. Resident males were significantly more aggressive than intruders and spent more time investigating the odors of familiar intruders. Resident males urine-marked most over the odors of females and familiar intruders while intruders marked least over the odor of the familiar resident. Intruders did not avoid investigating nor marking over the odors of familiar resident males or other conspecifics. These results suggest that individual odors of male rats may be more salient than a general odor of dominance, and that the dominant males increase their investigation and marking over the odors of familiar subordinates but not unfamiliar subordinates. The importance of olfactory learning during aggressive interactions is discussed.     

Analysis of ultrasonic vocalizations emitted by residents during aggressive encounters among rats (Rattus norvegicus)

This investigation was concerned with the extent to which aggressive resident rats emit 40-70-kHz vocalizations and the effect of these signals on intruders. In Experiment 1, deafened and intact intruder males were given two encounters with resident animals. Deafened intruders engaged in a higher duration of immobile or freezing postures than intact animals. Experiment 2 indicated that the augmentation of freezing found among deafened intruders was not due to an inability to detect ultrasounds made by residents since intruders encountering devocalized resident males showed no reliable differences in specific motor patterns from intruders paired with intact residents. The results further demonstrated that 40-70-kHz vocalizations are produced almost entirely by intruding animals since there were no significant changes in occurrence of these calls when resident males were devocalized. Under the constraints of the testing procedures employed, the role of ultrasonic communication during the initial formation of agonistic relations could not be determined experimentally.     

The effects of negative reinforcement for irritable aggression on resident-intruder behavior

This experiment demonstrated that rats trained to display elevated levels of shock-induced aggression in a negative reinforcement paradigm displayed more boxing behavior than yoked control groups in a later test in which intruder rats were placed in the home cage of resident rats. Resident or intruder status did not affect the influence of the negative reinforcement procedure on the observed resident-intruder behavior of trained animals; however, naive intruders paired with trained residents displayed increased defensive behavior, suggesting that negative reinforcement for shock-induced aggression affected the behavior of these residents.     

Maternal aggression in mice and rats towards male and female conspecifics

Although postpartum aggression is primarily studied in laboratory mice and rats, it is unclear how the two species compare in terms of the factors associated with peak levels of aggressive behavior. Using the same experimental protocol, we assessed the relative effect of intruder sex and time since parturition on the frequency of maternal aggression in Long-Evans rats and CFW mice. Females were studied for 2 consecutive cycles of pregnancy and lactation. During the first lactation, aggression was tested 2 times per week for 3 weeks in order to select animals that attacked at least once. During the second lactation, both pup care and aggressive behavior were assessed in detail. Testing occurred twice in each lactation week, with postpartum days 1–7, 8–14, and 15–21 considered weeks 1, 2, and 3, respectively. Maternal behavior towards 3 pups was observed for 5 minutes, followed by a confrontation with an intruder. Lactating females encountered female intruders once per week, and male intruders in the alternate weekly test. The same behaviors were measured in the 2 species, except for the tail rattle exhibited by mice and the aggressive posture shown by rats. Lactating rats and mice show similar decreases in pup care behavior as lactation progresses in time; yet the factors associated with peak levels of aggression differ between species. In Long-Evans rats, female intruders receive more attacks, threats, and aggressive postures than males. Frequency of attack bite and sideways threat declines in each passing week of lactation. Lactating mice are more aggressive toward male intruders throughout the lactation period. Mice still attack and threaten during the third week of lactation, but less often in comparison to the first week. Therefore, peak levels of aggression vary in mice and rats both as a function of intruder sex and lactation week.     

Response acquisition and fixed-ratio escalation based on interresponse times in rats

The effectiveness of a fixed‐ratio (FR) escalation procedure, developed by Pinkston and Branch (2004) and based on interresponse times (IRTs), was assessed during lever‐press acquisition. Forty‐nine experimentally naïve adult male Long Evans rats were deprived of food for 24 hr prior to an extended acquisition session. Before the start of the session, three food pellets were placed in the magazine. Otherwise, no magazine training, shaping, nor autoshaping procedure was employed. The first 20 presses each resulted in the delivery of a 45‐mg food pellet. Then, the FR increased (2, 4, 8, 11, 16, 20, 25, 30) when each IRT in the ratio was less than 2 s during three consecutive ratios. Sessions lasted 13 hr or until 500 pellets were earned. On average, rats reached a terminal ratio of 11 (mean) or 16 (median) during the first session. Seven rats reached the maximum value of FR 30 and only one rat did not acquire the response. In most rats, a break‐and‐run pattern of responding characteristic of FR schedules began to develop in this acquisition session. Subsequently, the ratio‐escalation procedure continued during daily 2‐hr sessions. In these sessions, the starting ratio requirement was set at the terminal ratio reached in the previous session. Using this procedure, over half (26) of the rats reached the FR 30 requirement by the fourth session. These data demonstrate that a ratio‐escalation procedure based on IRTs provides a time‐efficient way of establishing ratio responding.     

An animal model of excessive eating: schedule-induced hyperphagia in food-satiated rats.

Nineteen rats were maintained throughout the experiment on ad libitum wet mash and water and were trained to press a lever on fixed-interval or fixed-ratio schedules of reinforcement with electrical brain stimulation. Fourteen rats ate at least 150% more mash during intermittent reinforcement sessions than during baseline, massed reinforcement control, and/or extinction sessions. In a 3-hr session, 11 of those 14 consumed more than 22 g of wet mash (13 g dry weight), the equivalent of nearly half an animal's daily food intake. In subsequent control sessions, the electrodes did not support stimulus-bound eating despite attempts to make stimulation parameters optimal. These results indicate that the eating was schedule induced or adjunctive, and suggest that the procedure may provide an animal model of excessive nonregulatory eating that contributes to obesity in humans.     

Ultrasounds produced by rats accompany decreases in intraspecific fighting

Male intruder rats were placed individually into the cage of an established resident on 2 occasions separated by a 7–8 day interval. Residents readily attacked intruders and both animals lost weight during the first encounter. In contrast, no serious fighting occurred on the second encounter, and both intruders and residents maintained their body weight during the 24-hr test. Observation of the intruder's behavior during the first 30 min of each encounter indicated that defensive-submissive postures represent a response to an attack that only temporarily inhibits aggression whereas the emission of 22 kHz calls by the intruder is associated with a relatively permanent decrease in the resident animal's aggressive response.     

Session duration and the VI response function: Within-session prospective and retrospective effects

Two experiments examined the effects of session duration on responding during simple variable-interval schedules. In Experiment 1, rats were exposed to a series of simple variable-interval schedules differing in both session duration (10 min or 30 min) and scheduled reinforcement rate (7.5 s, 15 s, 30 s, and 480 s). The functions relating response rate to reinforcement rate were predominantly monotonic for the short (10-min) sessions but were predominantly bitonic for the long (30-min) sessions, when data from the entire session were considered. Examination of responding within sessions suggested that differences in the whole-session data were produced by a combination of prospective processes (i.e., processes based on events scheduled to occur later in the session) and retrospective processes (i.e., processes based on events that had already occurred in the session). In Experiment 2, rats were exposed to a modified discrimination procedure in which pellet flavor (standard or banana) predicted session duration (10 min or 30 min). All rats came to respond faster during the short (10-min) sessions than during the first 10 min of the long sessions. As in Experiment 1, the results seemed to reflect the simultaneous operation of both prospective and retrospective processes. The results shed light on the recent controversy over the form of the variable-interval response function by identifying one variable (session duration) and two types of processes (prospective and retrospective) that influence responding on these schedules.     

Aggression in domesticated rats reared in a burrow-digging environment

Male and female rats were raised individually in either large boxes filled with soil (burrow environment) or laboratory cages of comparable size (cage environment). In a test of intruder-elicited aggression, intruders exposed to burrow-environment residents lost more body weight, were more likely to have empty stomachs, and received more wounds during a 24 hour aggression test. The results indicate that the physical environment during the period from weaning to maturity is an important determinant of aggression intensity in adult rats.     

Effect of chronic ethanol administration on intermale aggression in rats

An experiment was performed to study the effect of chronic ethanol administration on intermale aggression in rats using a 24-hour resident-intruder test. During the resident-intruder test residents displayed virtually all of the agonistic behaviors, and intruders displayed virtually all of the defensive behaviors. Intruders treated with ethanol displayed more defensive behavior and elicited more agonistic behavior than control intruders. Twenty minutes into the resident-intruder test intruders showed the greatest increase in corticosterone (338% vs. 129%), while residents showed the greatest increase in testosterone (103% vs. 18%). On the 2nd day of the resident-intruder test intruders lost more weight than residents (21.5 g vs. 10.2 g). Plasma corticosterone levels remained elevated for the intruders, and in particular for those intruders displaying defensive behaviors regardless of the resident's behavior. Plasma testosterone levels remained elevated for those residents that were paired with intruders that displayed defensive behaviors regardless of the resident's behavior. The frequency and severity of biting attacks by ethanol residents was significantly greater than that of control residents. In addition, the locus of biting attack shifted from the upper back of intruders paired with control residents to the flanks, tail, lower feet, and ventral surface on intruders paired with ethanol residents.     

Effects of reactivity to dorsal stimulation and social role on aggressive behavior in laboratory rats

Rats were selected on the basis of reactivity to dorsal tactile stimulation and then tested in a resident-intruder paradigm. While reactivity of residents did not influence the occurrence of agonistic behaviors or wounding of residents and intruders, reactivity of intruders did affect offensive and defensive patterns of interactions and the wounds sustained by residents and intruders. Subsequent to resident-intruder testing, rats were tested for shock-induced aggression. The pattern of the results and the results of additional experiments demonstrated that resident-intruder experience could affect subsequent shock-induced aggressive behavior.     

Analysis of ultrasonic vocalizations emitted by intruders during aggressive encounters among rats (Rattus norvegicus)

This investigation was concerned with the identification of the ultrasonic vocalizations produced by intruders during aggressive interactions and the role of these signals in agonistic behavior of rats. In the first experiment, experienced resident males were paired with both devocalized and intact vocalizing naive intruder males. Devocalization of the intruder males resulted in a drastic decrease in 50-kHz vocalizations and the elimination of all 22-kHz vocalizations. This almost total absence of ultrasonic vocalizations was not accompanied by any change in resident aggressive behavior or intruder defensive and submissive behavior. In a second experiment, naive intruders were tested with either deafened or intact resident males. Similarly, preventing residents from hearing intruder ultrasounds had no detectable effect on any aggressive behavior. These experiments are not consistent with the correlative evidence that intruder-produced 22-kHz vocalizations inhibit the aggressive behavior of the resident. The results also show that most of the ultrasonic vocalizations emitted during aggressive encounters are probably produced by the intruder.     

Polydipsia's contribution to temporal discrimination of rats on a fixed interval schedule

This experiment concerned the contribution of polydipsia on the temporal discrimination of rats during a fixed-interval 60-sec. schedule. In this study, the timing accuracy of 12 rats which had access to water during training was compared to that of 12 rats which had no water during training. The rats were trained for 25 sessions on an FI 60-sec. schedule. In early sessions before polydipsia was fully developed, no differences existed between the timing accuracy of the water group and no-water group. As the amount of water drunk by the water group increased as the number of sessions increased, a parallel increase was noted in the timing accuracy of the water group. In the final sessions, a significant difference was found between the timing accuracy of rats in the water group and that of those in the no-water group. It was concluded that polydipsia facilitated the development of the temporal discrimination which is characteristic of a fixed-interval 60-sec. schedule.     

Genotype and environment in the social interactions of wild and domestic “Norway” rats

Wild rats, Rattus norvegicus, (a) trapped as adults or (b) of the second generation in captivity (lab-wild), and domestic rats of two strains, were studied for 28 days in artificial colonies in large cages with attached nest boxes. Controls were kept in mated pairs in small cages. Each colony consisted of six males and six females. The interactions of the males in six colonies of trapped rats were highly “stressful;” 61% died; and most of the survivors lost weight and had greatly enlarged adrenals. In each colony, however, there was a male (an alpha) that gained in weight and spent much time, during the dark hours, in the open on the floor of the cage; and in three colonies there were also other males (“betas”) that gained in weight. The adrenals of alphas and betas weighed about the same as those of the controls. In one of the 12 colonies of domestic rats one male behaved like a wild male; but in the other colonies the males gained in body weight and their adrenal weights resembled those of the controls. In three colonies of lab-wild rats 22% of the males died, but there was no evidence of males of different status. The findings confirm that the “agonistic” behavior of domestic rats is usually much attenuated in comparison with that of the wild type; a number of methodologic implications are discussed.     

Attack priming and satiation in female golden hamsters: Tests of some alternatives to the aggression arousal interpretation

“Priming” a female hamster by allowing it a single attack on an intruder placed into its home cage transiently decreases the latency and increases the probability of attack on a second trial. Although we have previously argued that this priming effect reflects an increase in aggressive arousal, an alternative interpretation is that the fear elicited by placing a foreign object into the subject's home cage is reduced when it happens again on the second trial. Another interpretation is that priming is an effect of intruder novelty, i.e., the subject perceives a difference between the first and second intruders which causes it to attack the second more quickly. Experiment 1 compared the standard two trial paradigm with different intruders to trials in which (1) the first intruder was withdrawn and used again in the second trial, and (2) the intruder remained in the cage following the first attack. All intruders were pretreated with the analgesic-sedative methotrimeprazine to reduce the variability of their behavior. Neither hypothesis tested in Experiment 1 was supported, strengthening the interpretation of attack priming as a manipulation that affects primarily internal motivational mechanisms specific to aggression. Allowing a hamster to carry out a protracted series of attacks produces a “satiation” effect that is the reverse of priming, i.e., the latency of a subsequent attack is increased and its probability reduced. It is possible that the attack satiation observed in our earlier studies was not the result of processes internal to the subject, but could have been due to habituation to a particular intruder or to certain stimuli emitted by it during the protracted interaction. In Experiment 2 subjects were given three sessions of 10 successive trials using either 1 intruder presented repeatedly, 2 intruders presented alternately, or 10 different intruders presented once each. No difference among conditions was found in this study either suggesting that subject's aggressive behavior is insensitive to whatever changes may occur in intruders' behavior or other stimulus characteristics when they have been treated with methotrimeprazine. The lack of differences among test conditions in both experiments is most likely due to the efficacy of the drug in “standardizing” intruder behavior. Experiment 2 also revealed an interesting difference in two measures of attack latency. The time elapsing between intruder presentation and attack, i.e., the standard measure of latency, decreased from the first to the fourth trail; it then increased steadily over the remaining trials. The cumulative time that the subject remained in contact with the intruder prior to attack, a measure more indicative of attention to the intruder, dropped to an asymptotic value by the second trial. This difference suggests that the satiation effect may be accounted for by subjects' increasing avoidance of the intruders over trails, perhaps as a way of regualting their level of aggressive arousal.     

Attack and defensive behaviors in the albino mouse

Attack by dominant male colony mice on intruders included chasing and lateral attack behaviors, while the corresponding intruder behaviors were flight, boxing, and checking. Both of these are similar to the attack and defensive behaviors of colony rats and intruders. However, mice did not show a significant constraint on bites to ventral areas, and the rat defensive behavior of lying on the back, which is effective because of this constraint, was rare; the corresponding “on-top” behavior of attackers was almost absent in mice. These findings strongly support the view that intraspecific attack and defensive behaviors, and target sites for bites, are interrelated factors facilitating effective but nonlethal agonistic interactions in muroid rodents.     

Lean and mean rats: Some effects of acute changes in the food supply upon territorial aggression

In experiment 1, individually housed rats subjected to short-term food restriction displayed more territorial aggression toward conspecific intruders than controls maintained on a free-feeding diet. In experiment 2, small groups of three adult male rats had access to either a standard laboratory diet or the standard diet plus sucrose. Groups with the sucrose supplement were significantly less aggressive toward intruders than controls. Sucrose availability did not produce appreciable gains in body weight but it did reliably decrease within-colony weight variation. The results suggest the existence of an effective dietary mechanism that enables a social species such as Rattus norvegicus to tolerate each other in dense feeding aggregations when food is abundant. Conversely, when food is limited, social intolerance increases and serves to limit the development of large feeding groups.     

Flavor preferences produced by backward pairing with wheel running

In 3 experiments rats given 8 sessions of preexposure to wheel running acquired a preference for a flavor that was given immediately after each of 4 subsequent sessions of wheel running. Such flavor preference was less likely when rats were given the same conditioning procedure but without preexposure to wheels (Experiment 1) or when access to flavor was delayed by 30 min following a wheel session (Experiment 2). When rats were given a flavor before each wheel session, the resulting conditioned aversion was greater in rats that had no prior exposure to wheel running (Experiment 3). These results show that whether an aversion or preference for a flavor is produced by wheel running depends on an interaction between prior wheel experience and the sequence of events.