An empirically derived scoring system for intermale aggression in mice

Scoring systems used to assess intermale aggression have been characterized by arbitrary scales and wide variability in the behaviors selected for measurement. The use of such different systems severely limits the ability of investigators to make meaningful comparisons among studies and indicates that there is a need for a common, statistically derived evaluative system for intermale aggression. We measured the frequency and duration of five major components of agonistic behavior exhibited by intact males toward olfactory bulbectomized stimulus males and then analyzed the data using a number of univariate and multivariate procedures. The results were used to generate two statistically based scoring systems, one a short-form index and the other a composite index for more detailed studies of aggression. It is hoped that these statistically derived systems will be adopted by other investigators to increase methodological congruence in the field.     

Genetic analysis of “Spontaneous” intermale aggression in mice

“Spontaneous” intermale aggression was investigated in seven inbred strains of mice. A positive interstrain correlation and cross-correlation was found for two indices of fighting intensity, ie, accumulated attacking time and number of attacks. The strain aggressiveness level (percent of mice fighting in each strain) does not correlate with the intensity of aggressive behavior. It has been shown by using a genetical analysis performed on C57BL/6 and BALB/c strains, their reciprocal F₁ hybrids, and back-crosses that these indices of intermale aggression are under different genetic control. Aggressive behavior intensity depends on the additive effects of genes. The control of strain aggressiveness level revealed that a high level of aggressiveness was dominant. No reciprocal effects were found. The level of aggressiveness and the intensity of fighting seem to represent different aspects of aggression and may be controlled by different genetic mechanisms.     

Relationships between testosterone metabolism in the brain,other endocrine variables and intermale aggression in mice

Relationships between some endocrine variables (plasma testosterone and estradiol; in vitro gonadal biosynthesis from pregnenolone precursor and adrenocortical biosynthesis from progesterone precursor; in vitro activity of 17β-hydroxysteroid de-hydrogenase (17β-HSDH) and of aromatizing enzymes in the subcellular fractions of the brain) and isolation-induced inter-male aggressiveness were studied. A significant positive correlation was found between fighting behavior, measured by the number of wins in a series of paired encounters, and the activity of brain 17β-HSDH. A significant inverse correlation was found between fighting behavior and the activity of aromatizing enzymes in the nuclear fraction of the brain. No significant correlations were found between fighting and the other endocrine variables investigated. These results may suggest that the conversions testosterone → androstenedione and testosterone → estradiol in the brain play a role in the control of inter-male fighting in the mouse. These results may also account for the effect of repeated wins and defeats on brain steroidogenesis.     

Mousekilling,intermale fighting,and conditioned emotional response in rats

Several possible relationships between two forms of aggression in rats were studied. First, mouse killing and spontaneous intermale fighting were found to be correlated. Rats which attacked other rats were those most likely to kill mice. To determine whether aggressive and nonaggressive rats were also differentially responsive to other situations involving emotional arousal, but not aggression, mouse-killers and nonkillers were compared in a conditioned emotional response (CER) situation. Mouse-killers showed greater suppression to the conditioned stimulus (CS) and to the situational cues of the apparatus. Therefore, a common arousal mechanism may underlie a number of diverse agonistic responses. Nevertheless, extensive mouse-killing experience did not increase the tendency of rats to fight with either adult males or juvenile males.     

Studies on wild house mice VI: Differential effects of the Y chromosome on intermale aggression

The aim of this study is to determine the effects of different parts of the Y chromosome of wild house mice on aggression. To reach this goal, intercrosses were made between two selection lines for attack latency (SAL and LAL) and their congenic strains (SAL. LY and LAL. SY). This procedure resulted in F₁ hybrids that carried the same autosomes, but differed in their X chromosome and the two different parts of their Y chromosomes, the different parts of the Y chromosome being a recombining part called the pseudoautosomal region (PAR) and a non-recombining part (non-PAR). We conclude that both parts of the Y chromosome contribute slightly but significantly to variation in aggression. The major effect is accomplished by the PAR of the aggressive parent; a mirror effect is achieved by the non-PAR of the aggressive parent in interaction with the PAR. © 1994 Wiley-Liss, Inc.     

Olfactory stimuli and intermale aggression in androgen-treated castrated sheep

The ability of olfactory stimuli to elicit aggression in male sheep was assessed in testosterone-treated castrates. Ablation of the olfactory bulbs (Exp. 1) did not reduce the capacity of sheep to participate in aggressive behaviour after testosterone treatment. Topical application of urine from castrates treated with propionated testosterone, 5α-dihydrotestosterone, 17β-oestradiol, or an oil vehicle to the fleeces of untreated castrates (Exp. 2) did not result in different levels of aggressive responsiveness in testosterone-treated castrates.     

Opposite strain-dependent differences for intermale aggressive behavior elicited by individual housing and housing with a female in the mouse

Male mice of the C57BL/6 strain show a significant increase in aggression toward a nonaggressive male conspecific following 72 hr of individual housing. However, this increase was no longer evident following 2 weeks of individual housing. When housed with a female, C57BL/6 mice show significantly more aggression than singly housed mice of the same strain after 72 hr as well as 2, 4, 8 weeks of differential housing. Male C57BL/6 mice housed with a female also show significantly higher levels of aggression than DBA/2 mice living in the same housing condition after 4 or 8 weeks of differential housing. Finally, male DBA/2 mice individually housed for 8 weeks are significantly more aggressive than mice of the same strain housed with a female for the same time. These results indicate that the increase in aggressive behavior observed following isolation and cohabitation with a female in the mouse is not the same phenomenon. © 1994 Wiley-Liss, Inc.     

Intermale fighting and predatory behavior in house mice: An analysis of behavioral content

Two forms of aggression, intermale and predatory, wwere compared on the basis of behavioral content. Male wild mice were presented with opponents of two types, nonaggressive male mice and crickets, and the amounts of time spent in each of 18 behaviors were recorded. Results from analyses of variance and principal components analyses indicated that the structure of behavioral content differs greatly for intermale fighting and predatory behavior. It was, therefore, concluded that the integrated patterns of beahvior were controlled by separate mechanisms for predatory versus intermale aggression.     

Effects of early exposure to intermale aggression on the aggressiveness of adult male mice varying in their genetic disposition for aggressive behavior

This study examined whether adult male aggression is influenced by either visual or olfactory exposure in early postnatal life to brief episodes of aggression. Another focus of interest was the interplay between a genetic disposition for aggressive behavior and early exposure experiences. The subjects used in the study were male mice of the 49th generation of selection for high (Turku Aggressive, TA) and low (Turku Non-Aggressive, TNA) levels of aggressiveness. Moderately aggressive males of the parental strain (Normal, N) were also used. Subjects of each strain were exposed from 21 to 32 days of age to fighting males either behind a wire mesh or glass screen. Control subjects were isolated during the entire experimental period. At 90–100 days of age, each subjects was tested three times for its aggressiveness. Exposure to fighting males behind a wire mesh screen enhanced later aggressiveness of juvenile male mice. Juveniles exposed solely to visual cues were comparable to isolates, both groups showing less adult aggression. Early experience and the genetic disposition for aggression were correlated; TA males showing the greatest increase in aggressive behavior. The role of early olfactory learning is discussed. © 1993 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.     

Variation in aggressive behavior and anatomo-physiological correlates generated by crowding without physical contact in the house mouse

Behavioral, physiological (i.e., endocrine), and anatomical consequences of crowding in mice were studied in a situation where animals were in auditory, visual, olfactory, and tactile contact but restrained from full physical interactions, to prevent overt aggression. Males that cohabited with females undisturbed by neighboring conspecifics showed greater propensity to attack same-sex intruders and had higher plasma testosterone levels than did their “crowded” counterparts, that is, males cohabiting with females and housed adjacent to other male/female pairs. In this respect, the latter animals resembled submissive males. However, a significant increase in weight of androgen-dependent target organs (i.e., seminal vesicles and preputial glands) was found in crowded males. These data indicate that despite the observed inhibition of social aggression these males are not physiologically comparable (homologous) to male mice that experienced defeat and the stress of submission during fighting. The intriguing possibility that different conversion pathway of testosterone are accelerated, as a result of social communication, in males living in these two environments and the behavioral implications of these possibilities are discussed. Finally, the parental behavior of crowded animals, although not freely interacting with each other, was disrupted, causing a marked decrease in reproductive success. In this situation a high incidence of infanticide of their offspring by both parents was observed, whereas this behavior was virtually absent in non-crowded male/female pairs.     

Effects of prenatal cocaine and genotype on intermale agonistic behavior in Mus musculus

We exposed the pups of three F₁ genotypes of mice to a daily regime of cocaine by injecting their mothers (all C57BL/10J strain) on days 7–18 of gestation with 20 mg/kg subcutaneously. Pups of the cocaine and control groups did not differ on measures of maternal and pup health or size. Male pups were weaned and isolated at 21 days of age and their behaviors measured in an intermale aggression situation at about 80 days of age. Treated and untreated males of each F₁ genotype were paired in dyads with either a C3H/HeJ (hawk-like) or AKR/J (retaliator-like) standard tester male. Standard tester males were not exposed to cocaine. Cocaine treatment alone reduced the overall level of aggression in dyads, and in interaction with genotype or the standard tester it altered the behavior of dyads in all phases of social interaction: the initiation, content, and outcome. Standard testers used as behavioral probes, differentiated cocaine and control males with respect to their stimulus and behavioral qualities. Aggr. Behav. 23:183–196, 1997.© 1997 Wiley-Liss, Inc.     

Relationship between sexual activity and intraspecific fighting in male mice

Significantly more male mice having cohabited and mated with intact females subsequently displayed intraspecific fighting behavior after castration than males having cohabited with noncycling (ovariectomized) females. Also, intact males that failed to achieve a criterion for aggression during three screening tests subsequently showed a marked increase in fighting after having had copulatory experience relative to males that lived with ovariectomized females. Lastly, spontaneously aggressive males copulated more frequently than nonfighters.     

Elevation and reduction of plasma tryptophan and their effects on aggression and perceptual sensitivity in normal males

Data from experimental animals suggest that 5-hydroxytryptamine (5HT) may have an inhibitory effect on aggression, while clinical studies have found a correlation between pathological aggression and low brain 5HT. To investigate this relationship further we used amino acid mixtures designed to raise or lower the levels of the 5HT precursor, tryptophan. Normal male subjects were given tryptophan-depleted, balanced, or tryptophan-supplemented ammo acid mixtures and tested for aggression 5 hours later. The balanced amino acid mixture served as a control for the tryptophan depletion and supplementation. Testing for aggression was done using the Buss paradigm in which subjects deliver electric shocks to a (nonexistent) partner in response to stimulus tones. Duration and intensity of shock delivered were the measures of aggression, while responsivity to the stimulus tones was the measure of perceptual sensitivity. Neither tryptophan supplementation nor tryptophan depletion had any effect on these measures of aggression or perceptual sensitivity. We conclude that raising or lowering the synthesis of brain 5HT through alterations in tryptophan availability does not influence aggression in normal males as measured by the Buss task.     

Facial attractiveness as a moderator of the association between social and physical aggression and popularity in adolescents

This study examined the relations between facial attractiveness, aggression, and popularity in adolescence to determine whether facial attractiveness would buffer against the negative effects of aggression on popularity. We collected ratings of facial attractiveness from standardized photographs, and teachers provided information on adolescents' social aggression, physical aggression, and popularity for 143 seventh graders (70 girls). Regression analyses indicated that facial attractiveness moderated the relations between both types of aggression and popularity. Aggression was associated with a reduction in popularity for adolescents low on facial attractiveness. However, popularity did not decrease as a function of aggression for adolescents high on facial attractiveness. Aggressors with high facial attractiveness may experience fewer negative consequences to their social standing, thus contributing to higher overall rates of aggression in school settings.     

Examining the relation between PTSD and insomnia on aggression

Posttraumatic stress disorder (PTSD) symptoms and poor sleep have been identified as potential causals factor in aggression, violence, and impulsive behavior. Given the high cost of aggression to society and public health, identifying modifiable factors related to aggression, such as insomnia, may guide treatment strategies to help decrease aggression. Participants were 143 Veterans seeking treatment for PTSD at a VA outpatient PTSD clinic. Linear and logistic regression analyses were used to examine the relation between PTSD and insomnia on aggression. Results from bivariate analyses indicated that while both PTSD and insomnia severity were associated with higher aggression scores independently, when PTSD and insomnia were examined together, PTSD severity was the only significant predictor of aggression. Interaction effects yielded nonsignificant results suggesting that poor sleep did not moderate the PTSD and aggression relation. Results suggest that addressing PTSD symptoms as a first treatment target may be more important for decreasing risk for aggression than targeting insomnia. More research is needed to understand whether treating PTSD and insomnia reduces aggression in Veterans.     

Reciprocal influences between maternal discipline techniques and aggression in children and adolescents

Most studies assessing the link between parental discipline and child aggression have focused primarily on discipline as a cause and aggression as an outcome. In addition to the pathway from discipline to aggression, however, aggressive behavior on the part of the child may lead to future use of discipline by the parent. In this study, structural equation modeling was used to assess reciprocal influences over time between a mothers' use of discipline and aggression in children. Data were drawn from the Springfield Child Development Project, a longitudinal study of middle childhood and adolescence, focusing on antecedents of aggression. The original sample consisted of 440 mother-child dyads living in the city of Springfield, MA. Children in the sample were between 7 and 14 years of age at the first data collection period and between 12 and 19 years of age at the final data collection period. Four hypotheses were tested: (1) a mother's use of aggressive discipline predicts an increase in later child aggression, (2) child aggression predicts an increase in later use of aggressive discipline, (3) the use of reasoning predicts a decrease in later child aggression, and (4) child aggression predicts an increase in later use of reasoning. All hypotheses except number 3 were supported to some degree. Results suggest that children's early aggressive behavior leads to an increase in their mothers' use of both reasoning and aggressive discipline; in turn, increased use of aggressive discipline leads to an increase in aggression during both childhood and adolescence.     

The influence of dibutyryl cyclic cmp on aggression in the mouse

Cytidine 3′,5′-cyclic monophosphate (cCMP) and its associated enzymes have recently been identified in a number of mammalian tissues. The analogy with other cyclic nucleotides leads to speculation that it may prove to be a second messenger for some neurotransmitters and/or hormones. In this study we examined the possibility that cCMP may modulate aggressive behaviour. When dibutyryl cCMP (250 μg and 2.5 mg/k SC) was administered to mice it proved to have a potent antiaggressive influence but had little effect on general activity. It was concluded that cCMP may prove to have an important metabolic function.     

Maternal aggression in mice: The non-specific nature of the exteroceptive maintenance by young

Five hours of pup removal during early lactation (Postpartum Day 6) reduced maternal aggression in mice, while placement of the dam's entire litter (six pups) or a single pup behind a double wire mesh partition in the homecage maintained aggression at a level identical to that of mothers in direct contact with their young. When placed behind the partition, unfamiliar 6, 13, and 20-day-old pups, but not 30-day-old mice, maintained the behavior as effectively as the dam's own young. The relationship of the findings to communal rearing conditions in the mouse is discussed.     

Revealing the nuanced associations between facets of trait impulsivity and reactive and proactive aggression

Although impulsivity has been repeatedly associated with aggression, specific associations between facets of impulsivity and reactive (RA) and proactive (PA) have yet to be fully elucidated. This may be due, in part, to overlapping variance among facets of impulsivity and between RA/PA. The current study systematically examined associations among these variables using both bivariate correlations as well as path analysis. In addition to raw aggression scores, we isolated the variance unique to both RA/PA by regressing RA onto PA (and vice versa), and saving these residual aggression scores. Participants included 384 racially-diverse undergraduates. Results indicated facets of impulsivity uniquely characterize RA/PA, particularly using residual aggression scores. RA was uniquely characterized by higher levels of Negative Urgency followed by low Perseverance, as well as high Premeditation and low Positive Urgency. In contrast, PA was uniquely characterized by higher levels of Positive Urgency, and to a lesser degree, high Premeditation. Results indicate facets of impulsivity represent potentially different underlying pathways to specific subtypes of aggression. As such, impulsivity, particularly in the context of affect, may be especially important to consider in relation to specific subtypes of aggression.