Habituation of aggressive behavior in mice: A parametric study

The decline of aggression as a result of repeated episodes of fighting may represent a habituation phenomenon. The frequency of biting and sideways threat by male mice toward conspecific intruders declined over ten 5-min confrontations. Aggression returned to about 50% of original levels when a new intruder was introduced. Variations in length or frequency of confrontation indicated that the rate of decline of attack bites and sideways threats depended on these confrontation parameters. The condition that provided the fewest opportunities to fight provided the least decline in the frequency of attack and threat. The frequency of attack bites and sideways threats elicited by the presentation of a new intruder after repeated confrontations was greater than the frequency of attack bite and sideways threat in the last confrontation with the original intruder, when the resident and intruder were relatively inexperienced. Spontaneous recovery of attack and threat occurred but rate of recovery did not depend on confrontation parameters. The results indicate that the decrement of aggression in mice in repeated confrontations may be the result of habituation but fatigue is an important cofactor.     

Offensive and defensive bite-target topographies in attacks by lactating rats

The attacks by resident lactating Wistar rats on sexually naive conspecifics of both sexes were examined. Male and female intruders were equally attacked in terms of frequency and number of bites, but the topographies of biting seen in these encounters were different. Similarly to male-male agonistic interactions, females were attacked in a fashion which avoided bites to the head and snout (“offensive” attack), whereas males were frequently bitten on such vulnerable regions (“defensive” attack). This dichotomy in bite pattern suggests that different motivations and functions underlay maternal aggression in these situations. The defensive attack on males may be a deterrent to infanticide since only male intruders counterattack lactating females and kill their pups. The attack on females may be concerned with resource competition.     

Different patterns of biting attack employed by lactating female mice (Mus domesticus) in encounters with male and female conspecific intruders

Attacks by resident lactating female mice were examined in a variety of situations. Relatively few attack bites to vulnerable body regions were seen when pairs of unfamiliar lactating females fought, establishing social status prior to communal nesting. Sexually naive male and female intruders were equally prone to attack by lactating females, but patterns of bite attack generated by them were very different; males received the more damaging attacks. More signs of "fear" were seen in the lactating females' responses to male rather than female intruders. Varied motivations may underlie attacks by lactating females directed to conspecific intruders. Defensive patterns of biting by lactating females are more consistently directed towards males, intruders that are more likely to harm or destroy the litter. Although attacks by females rarely thwarted infanticide by male intruders, the behavior may acutely protect parental investment.     

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.     

Does fear modulate defensive and offensive types of maternal attack in mice?

Swiss CD‐1 lactating mice show a different pattern of attack toward intruders of differing sex, displaying defensive attack against the male (bites on the head and ventrum associated with fear) and offensive attack against the female (bites on the back and flanks with no elicitation of fear). This dichotomy may reflect diverse functions of maternal aggression: the attack toward males (the more infanticidal gender in laboratory strains) has been interpreted as a counterstrategy to infanticide, whereas the attack toward females may serve to establish a social hierarchy or to space rivals of the same sex. In terms of proximal mechanisms, fear may be a key factor involved in the modulation of the different patterns of attack. In Experiment 1 we compared the pattern of attack of lactating females in Swiss CD‐1 and Wild mice toward male and female intruders in relation to fear components of behavior of the attacking dams. Results showed that in Swiss mice, male intruders were attacked with a defensive type of attack accompanied by high levels of fear, whereas female intruders did not elicit fear in the attacking animal but were attacked with an offensive pattern. In Wild mice, both types of intruders were attacked with a defensive pattern; notwithstanding, fear was evident only toward male intruders. This suggests that fear is not totally responsible for the expression of the defensive type of attack. To test the hypothesis that defensive attack toward male and female intruders may be related to the infanticidal potential of the intruder, Experiment 2 examined levels of infanticide in both male and female Swiss CD‐1 and Wild mice. Swiss female mice showed virtually no infanticidal behavior, whereas Swiss males and both sexes of Wild mice showed similarly high levels of infanticide (55%–75%). From a game theory perspective, the defensive pattern of maternal attack toward female intruders in Wild mice is discussed as “extreme” defense of a high value resource and thus, functionally, a competitive form of aggression. Aggr. Behav. 26:193–203, 2000. © 2000 Wiley‐Liss, Inc.     

Conspecific aggression in the laboratory rat.

Attacks by dominant colony males of a domesticated rat strain on conspecific strangers introduced into the colony include both a full range of threat displays and actual biting attack. These dominant males attack and bite both anesthetized and unanesthetized strangers, but threat displays and bites to anesthetized rats are limited. Lesions resulting from these bites are nonrandomly distributed, with most damage to the head and upper back and very few bites to ventral surfaces. When introduced strangers were left in the colonies for several days, 66% were killed, indicating that domesticated rats are capable of lethal attacks on conspecifics without external provocation.     

Behaviour of Young Growing Pigs in a Resident‐Intruder Test Designed to Measure Aggressiveness

A total of 125 growing pigs (47 days old) were tested for aggressive responses on two occasions using a resident‐intruder (R‐I) design. Our aims were twofold: (1) to attempt to replicate earlier work on pigs showing that resident aggression is a consistent individual characteristic, unaffected by weight or sex of the resident or intruder and (2) to develop behavioural measures to characterise the wide range of aggressive responses in the test. Resident pigs, housed since birth with littermates, were placed individually in a divided‐off portion of their home pen, and a smaller, unfamiliar intruder (approximately 66% of the resident's weight) was introduced. The test ended 5 min after the first investigation of the intruder by the resident or when one of the pigs began to attack the other (by delivering a sudden, rapid series of bites). On days 1 and 2, 33.6% and 43.2% of tests, respectively, ended in an attack by a resident. Intruder attacks were rare. Pigs were consistent in whether they attacked or not over the two tests, although attack latencies for pigs attacking in both tests were not correlated. Females were more likely to attack and attacked more quickly than males on the first test day but not in the second test or overall. Intruder sex had some effect on the test outcome (males were attacked more rapidly in the second test only). Resident and intruder weight had no effect. Aggressive pigs (meaning pigs that attacked vs. pigs that did not and fast‐attacking pigs vs. slow‐attacking pigs) showed a number of differences in behaviour during the R‐I test: (1) they took longer to make initial contact with the intruder in their first test; (2) they showed a higher frequency of aggressive acts (single head knocks, bites, and shoves); (3) they spent a greater proportion of the test engaging in social contact with the intruder rather than non‐social behaviours; (4) their social behaviour involved more postures directed toward the head as opposed to flank‐ or rear‐directed postures or re‐establishing social contact; and (5) they showed closer physical contact with intruders during social encounters, as characterised by their lower head positions. Some of these behavioural measures could be used to improve the measuring power of the test in the future. Improved behavioural measures would enable aggressiveness scoring among pigs that did not attack instead of classifying them all together as “non‐attacking.” Aggr. Behav. 28:401–415, 2002. © 2002 Wiley‐Liss, Inc.     

Fear and aggression in the rat

The effect of fear on two types of aggression in rats was investigated by adding a cat stimulus to a colony in which the dominant male was attacking an intruder (offensive aggression), or, to a tube test situation in which defensive biting was measured before and during tail shock. The cat completely abolished offense in the colony; when the cat was presented and removed before a strange rat was placed in the colony, attack on the intruder was also reduced. In contrast, defensive biting was unchanged or even slightly potentiated by the presence of the cat, demonstrating a separation of the effects of fear on offensive and defensive aggression.     

Attack and defense in conspecific fighting in tree shrews (Tupaia belangeri)

Principles of conspecific defense have been analyzed for rodents, in which specific target sites for biting by attackers on defenders serve as an important determinant of the actions involved in both attacker and defender behavior. In an effort to determine the generality of these principles, attack and defensive behaviors and target sites for biting attack were evaluated in a nonrodent species, the tree shrew (Tupaia belangeri). Brief daily and repeated conspecific dyadic encounters between adult, socially experienced males (dominants, attackers), and adult, socially naive males (subordinates, defenders) that had been transferred into the territory of the dominants, produced a polarization of attack and defense. The dominant males showed chase, chase attack, jump attack, and biting behaviors, while the subordinates displayed flight and freezing. The vast majority of bites, as well as wounds and bruises, were on the subordinates’ backs. These patterns are very similar to those previously found in rats and mice and suggest that the organization of fighting, with targets of biting (or other painful) attack serving as an important determinant of both attacker (dominant) and defender (subordinate) behavior, may show considerable generality across nonrodent as well as rodent species. Although relatively few wounds were found after 28 days of repeated and daily encounters, the subordinate tree shrews show a variety of behavioral, neuroendocrine, and central nervous changes, indicating that they are stressed by these encounters per se. Aggr. Behav. 27:139–148, 2001. © 2001 Wiley‐Liss, Inc.     

Intermale aggression in rats selected for emotional reactivity and their reciprocal F1 and F2 hybrids

The aggressive behavior of two selectively bred rat strains (high “H” and low “L” emotional reactivity) and their reciprocal F1 and F2 hybrids were measured during three consecutive days in home-cage intruder tests. In terms of biting, bite latency, on-top-of and aggressive grooming, L males were more aggressive than H and F1 males. There was no difference between H and F1, nor between HLF1 and LHF1 with the exception that LHF1 showed more on-top-of. About one fourth of F2 males were equal to or more aggressive than L in terms of bite latency and number of bites. These findings suggest that aggressiveness was inherited as a recessive, monogenic trait in these rats.     

The organization of play fighting in the grasshopper mouse (Onychomys leucogaster): Mixing predatory and sociosexual targets and tactics

The body targets contacted, the type of contact made, and the patterns of defense and counterattack elicited by those attacks are examined in the play fighting of captive male and female pairs of grasshopper mice. The nape was the most frequently contacted body target, irrespective of the type of contact made, be it nosing, allogrooming, biting, or striking with a forepaw. The types of defense varied with both body area contacted and type of attack performed. Based on the topography and pattern of contact, it was concluded that grasshopper mice, as is the case for many other muroid rodents, primarily attack and defend targets otherwise contacted during precopulatory encounters. However, grasshopper mice, which are obligate carnivores, also attack and defend predatory targets, although less frequently than sociosexual targets. Surprisingly, predatory attacks were more likely to be counterattacked with predatory attacks, whereas sociosexual attacks were more likely to be counterattacked with sociosexual attacks. Conspecific aggression involves bites directed at the face, lower flanks, and dorsum. Neither the biting of these areas nor the tactics of attack and defense usually associated with such bites were observed during the juvenile interactions. There were no sex differences in either frequency or patterns of attack and defense in play fighting. The data presented for grasshopper mice shed light on the issue of mixing behavior patterns from multiple functional systems during play. Aggr. Behav. 26:319–334, 2000. © 2000 Wiley‐Liss, Inc.     

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.     

Behaviors of Swiss-Webster and C57/BL/6N sin mice in a fear/defense test battery

When confronted by an approaching threat stimulus (experimenter or laboratory rat), Swiss-Webster mice show initial flight, followed by freezing and defensive vocalization and biting, the latter only when escape is blocked. These defense patterns resemble those of the wild rat, suggesting that mice of this strain do not show the reductions in flight and defensive threat/attack that are typical of laboratory rats. C57/BL/6N Sin strain mice showed fewer avoidances to an approaching predator, as well as reduced vocalization and defensive biting, a pattern more similar to that of laboratory rats. As with rats, female mice appeared to be more defensive to a predator. They showed greater reactivity to dorsal contact and more frequent defensive biting and jump attacks than males of the same strains. These patterns of defensive behaviors suggest that, although strain differences in defense are substantial, laboratory mice are suitable for, and may offer several advantages in, the study of the genetic, endocrine, and pharmacological basis of antipredator defense. © 1995 Wiley-Liss, Inc.     

Pain‐induced aggression and changes in social behavior in mice

The effects of neuropathic, formalin, and acetic acid‐induced visceral pain were investigated on the social and aggressive behaviors in the Swiss male mice. Neuropathic pain was induced by tibial nerve transection (TNT). Also, somatic and visceral pain was conducted by intraplantar injection of diluted formalin (1%, 20 μl) and intraperitoneal administration of acetic acid (0.6%, 200 μl), respectively. Fourteen and twenty one days after the TNT surgery, and also, 1 and 7 days following formalin and acetic acid administration, the three‐chambered test was used to determine sociability and preference for social novelty and resident/intruder test was used for the evaluation of the aggressive behaviors. In the sociability phase of the three‐chambered test, all the three models of pain did not change the animal's sociability. However, in the social novelty preference phase, the animals in pain showed deficits in social novelty preference by a significant increase in the time spent with the familiar mice compared to the control groups. Also, animals in pain significantly showed more aggressive behaviors like biting and clinching and have much less attack latency in comparison to the control groups. Pain‐induced changes in the social novelty preference and aggressive behaviors continued in the neuropathic group until the end of the experiment. However, 7 days following the induction of both formalin and visceral pain, animals' social memory, and aggression almost returned to the standard value. These results suggest that long‐lasting pain could lead to social memory impairment and increase aggressive behaviors in mice.     

Territorial prior residence,size asymmetry,and escalation of aggression in convict cichlids (Cichlasoma nigrofasciatum Günther)

The present study was designed to assess the effects of resident versus intruder size differences upon the territorial prior-residence effect and level of territorial aggression in convict cichlids. Prior to a direct territorial dominance encounter, pairs of fish were randomly composed for one of three experimental treatment conditions: 1) the resident having a body-length 20–30% larger than that of the intruder, 2) the intruder having a body-length 20–30% larger than the resident, or 3) the combatants' body-lengths differing by no more than 5%. After a 3-day territorial acclimation period in their individual territories, the subject designated as the intruder was introduced to the resident's territory. For each encounter the pairmember that attacked first and the one that ultimately established dominance were recorded. Also measured during the encounter were the total number of bites, resident bites, intruder bites, and jaw-locking frequency and duration. The results revealed that there was a significant resident advantage in the resident-larger group. The intruder-larger group resulted in a significant intruder dominance advantage. However, no significant dominance advantage occurred in the same-size group. As predicted by game theory, there was significantly less escalation of aggression in contests in which one combatant held both designated asymmetric cues (prior residence, size) than in contests in which one combatant had prior residence, or when these two asymmetric cues were divided between the pairmembers. The size asymmetry is more important in determining dominance than the prior-residence asymmetry, for the particular size-difference range selected in the present study.     

The effects of tail shock on target-biting behavior of confined mice

Mice that are confined in a narrow cylinder readily bite inanimate targets. The following studies were conducted to determine the effects of tail shock on the target biting behavior of two strains of mice. During 20-min daily sessions, ten tail shocks were administered on a fixed-time 2-min schedule. One group of mice received a tone-conditioned stimulus (CS) which terminated with the onset of the shock, and a second group of mice did not. Target bites were collected in eight 15-sec bins over the 2-min trial and cumulated over the session. It was observed that tail shock was followed by an increase in target biting and that there was a comparatively high intershock interval biting rate. In addition, intershock interval biting was suppressed for the duration of the CS. These observations are discussed in terms of the utility of this paradigm for the psychopharmacological assessment of aggressive behavior.     

Supine defense in the intraspecific fighting of male house mice mus domesticus

House mice have been reported rarely to perform the supine behavior pattern as a defensive tactic during intraspecific fighting. However, in this study of intraspecific fighting by male mice, it is shown that mice do indeed rotate to supine. This maneuver is used to evade or extricate themselves from bites to the lower dorsum by the attacking opponent. Once free from the bite the defender does not remain supine, but will immediately turn to prone and flee. Remaining motionless in the supine position may serve a submissive function in other species, but this does not seem to be the case for mice. The present findings illustrate that the supine tactic is a dynamic maneuver for defense of body areas targeted by the opponent. © 1992 Wiley-Liss, Inc.     

Relationship between mousekilling and conspecific aggression in the male rat

In order to investigate the relationships between mousekilling and conspecific aggression, behavioral variables of killer and nonkiller rats were compared in a “resident-intruder” paradigm, in resident as well as in intruder animals. The occurrence of offensive items (offensive sideways, attack) was significantly higher in killer rats when they were residents; their corresponding opponents displayed more defensive behaviors. No significant difference in aggressive behaviors was noted when the comparison was done in the intruders. These results and those of previous studies suggest that there is a correlation between mousekilling and intraspecific offensive behaviors. Some similarities in the situations where both behaviors are elicited–eg, introduction of an unfamiliar intruder into a familiar environment–may contribute to the existence of such a correlation and the possibility of common mechanisms underlying both behaviors is discussed.     

On the dual nature of maternal aggression in rats

Presentation of a natural predator, a cat, was used to differentiate elements of maternal attack by female rats on a male intruder. Following exposure (without direct physical contact) of post-partum females to a cat or to a toy stuffed cat (control group), the females were replaced in their home cages and presented with a male intruder rat. Cat exposure reliably decreased lateral attack to the intruder, as well as locomotion, but had no effect on either jump attack or an upright defensive posture (boxing). Since predator exposure produces a somewhat durable increase in defense, along with inhibition of nondefensive behavior, these results suggest that maternal aggression represents a mixture of offensive, usually related to competition, and defensive (protective) behaviors. The results indicate that maternal aggression, as a parental care behavior, appears to be at least partially resistant to fear. © 1996 Wiley-Liss, Inc.