Playful defensive responses in adult male rats depend on the status of the unfamiliar opponent

Adult male rats reared as pairmates from weaning were tested in a neutral arena with both members of another pair (one at a time). The unfamiliar pairs were found to engage in play fighting, although they were more likely to escalate the encounter into serious fighting than were pairs of familiar rats. Based on their within‐home pair behavior, each pairmate was designated as a dominant or a subordinate. When the test encounters between unfamiliar males were analyzed with regard to whether the pairings consisted of two dominants, two subordinates, or a mixed pair, the pattern of play fighting was found to be attenuated. Both dominants and subordinates were more likely to initiate playful encounters, to respond defensively during these encounters, and to do so using adult‐typical tactics of defense when paired with an unfamiliar rat that was dominant in its home cage. The mechanisms by which the home status of unfamiliar male rats can be identified by another male are discussed, particularly with regard to the role that play fighting may serve for this function. It is concluded that the data support the hypothesis that play fighting can be used by adult rats for social testing, which in this case seems to involve ascertaining the opponent's fighting capability. Aggr. Behav. 25:141–152, 1999. © 1999 Wiley‐Liss, Inc.     

Estimation of the probability of fighting in fallow deer (Dama dama) during the rut

Fighting between males is a frequent component of the rutting behavior of Cervidae. Frequent conflicts are exhausting; fighting may be risky and can lead to serious injuries or even death. We focused on the process of assessment of the opponent's fighting ability and escalation of the combat, estimating the probability of fighting based on the encounter components such as groaning and parallel walk. In this study, we observed the agonistic behavior of fallow deer bucks (Dama dama) during the rut over four seasons. During this time, we recorded 205 encounters between bucks. Non-contact display, which allows contestants to assess their opponents fighting ability, occurred in 83% of the encounters. The highest predicted probability of a fight was found when both of the males vocalized and turned into the parallel walk. The chance of a clear outcome decreased when the males were fighting in comparison to when they did not fight. The initiator of the competitive encounter won 41% of the cases, while the attacked buck won 23% of the encounters. If the contestants avoided fighting, however, the initiator won 78% of encounters. Therefore, the initiator was more successful when no fight occurred compared to when the encounters escalated into fighting. In most cases where ritualized behavior occurred, one of the opponents left after vocalization or parallel walk occurred. Thus, vocalization and parallel walk increased the probability for a clear outcome. The probability of a fight was lowest in situations where the males displayed asymmetric behavior. Increased symmetry of the contestants' behavior was strongly correlated with a higher probability of a fight. Thus, these results indicate that fallow deer bucks use efficient tactic during the rut, which, in turn, minimizes the chance of injury while fighting during the breeding season.     

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.     

Male–male combats in a polymorphic lizard: residency and size,but not color,affect fighting rules and contest outcome

Theoretical models predict that the outcome of dyadic agonistic encounters between males is influenced by resource‐holding potential, resource value, and intrinsic aggressiveness of contestants. Moreover, in territorial disputes residents enjoy a further obvious competitive advantage from the residency itself, owing to the intimate familiarity with their territory. Costs of physical combats are, however, dramatically high in many instances. Thus, signals reliably reflecting fighting ability of the opponents could easily evolve in order to reduce these costs. For example, variation in color morph in polymorphic species has been associated with dominance in several case studies. In this study, we staged asymmetric resident‐intruder encounters in males of the common wall lizard Podarcis muralis, a species showing three discrete morphs (white, yellow, and red) to investigate the effects of asymmetries in color morph, residency, and size between contestants on the outcome of territorial contests. We collected aggression data by presenting each resident male with three intruders of different color morph, in three consecutive tests conducted in different days, and videotaping their interactions. The results showed that simple rules such as residency and body size differences could determine the outcome of agonistic interactions: residents were more aggressive than intruders, and larger males were competitively superior to smaller males. However, we did not find any effect of color on male aggression or fighting success, suggesting that color polymorphism in this species is not a signal of status or fighting ability in intermale conflicts. Aggr. Behav. 35:274–283, 2009. © 2009 Wiley‐Liss, Inc.     

Play fighting and the development of agonistic behavior in male and female rats

Two longitudinal studies were conducted to quantify the social behaviors exhibited by both male and female Long-Evans rats from the immediate postweaning period until young adulthood. In Experiment 1, male sibling pairs engaged in a high level of play fighting during the early juvenile period but such activity declined to a level significantly lower than that of female and mixed-sex pairs after 54 days of age. In Experiment 2, social exchanges during maturation were examined during the presence and absence of the piloerection response in an effort to distinguish play fighting from agonistic interactions. In male pairs, piloerection was rarely seen before 55–75 days of age but thereafter occurred with increasing frequency especially among dominant males. Furthermore, subordinate males retreated from their dominant partners and remained in an escape chamber for a significant amount of time only during encounters involving the exhibition of piloerection. This finding suggests that piloerection can be useful in identifying play and aggressive interactions. In female and heterosexual pairs, piloerection was observed infrequently during social encounters occurring throughout maturation. In addition, when given the opportunity to escape, females were less likely to retreat from play activity if their partner was another female than a male.     

Ontogeny of aggressive-sexual behavior between males in lines of Japanese quail (Coturnix coturnix japonica) selected for male mating frequency

Japanese quail from lines bidirectionally selected for high and low male mating frequency and from the random-bred base population were observed in an ontogenetic study of aggressive-sexual behavior. Quail were reared in sex-intermingled flocks until 28 days of age at which time half of the males from each line were housed as all-male flocks and half were transferred to individual cages. At 41 days of age, individually caged males exhibited mounting behavior to win encounters with other males. By 45 days, males from the high and control lines exhibited significantly more aggressive-sexual behavior than those from the low lines. Line x Rearing Experience interactions for mounting behavior at 45 and 56 days of age were due to the individually caged high- and control-line males' winning encounters by mounts; those maintained in flocks rarely mounted. Although some low-line males won encounters by mounts, occurrence of this behavior was infrequent. Males maintained in flocks were placed in individual cages at 57 days of age. When these males were compared with those housed in individual cages from 28 days of age, at 84 days of age and older, the Line x Rearing Experience interaction observed previously disappeared, and only differences among genetic lines were evident. Genetic and rearing experience effects are discussed as influencing agonistic and sexual behaviors.     

Aversive odors of male mice: Experiential and castration effects,and the predictability of the outcomes of agonistic encounters

The relationship between the aversive urine odors of adult male mice and their agonistic interactions was examined. The urine of 28 isolated intact donors was tested for its aversiveness before the donors had won or lost an agonistic encounter, by spotting a portion of an open field with their urine. Group-housed males (n = 28), each tested with the urine of an ultimate winner and loser, as well as a castrate, avoided both intact types. The urine of castrates was not aversive. Another group of subjects (n = 28), tested with urine collected following the encounter between donors, provided identical results. A second experiment examined the aversiveness of urine from 24 isolate mice, and their responsiveness to urine of other isolates, prior to their encounter. The urine was not highly aversive to the isolates However, winners and losers differed in their responsiveness, as ultimate winners exhibited greater aversion. The results are discussed with regard to territorial maintenance via odors, and how differential responsiveness to odors may influence the outcomes of agonistic encounters.     

Dyadic social behaviour in the hamster: Evidence for contextual mediation

Dyadic social interactions among 34 adult male golden hamsters reared in either standard wire cages (Experiment 1) or cages with sandy substrates (Experiment 2) were observed for 15 min on consecutive days in two neutral arenas with either a bare Plexiglas or a sandy substrate. The tests showed that in the arena with the sandy substrate the hamsters spent less time engaged in agonistic encounters, initiated fewer agonistic encounters of all kinds and engaged in fewer agonistic encounters of a potentially physically damaging nature (i.e. violent encounters) than they did in the arena with the Plexiglas substrate. These effects were the same in animals reared under either condition. This finding argues against the behavioural effects being due to the novelty of a sand substrate to laboratory-reared hamsters. Consequently, the social behaviour of hamsters in laboratory settings is highly labile across various contexts. Therefore, it is necessary to examine the behavioural effects of contextual variables if general behavioural principles are to be derived from laboratory indices of social behaviour in hamsters, and possibly a number of other species.     

Fighting experiences and natural killer cell activity in male laboratory mice

There is currently much interest in the potential impact of psychological factors on immune responses. An attempt was made to assess the effects of dominant/subordinate polarity in male mice on the cytotoxic activities of their natural-killer (NK) cells. On the basis of repeated agonistic encounters, categories of subordinate and dominat animals were selected. These animals were compared with manipulated controls (introduced to a novel test cage without an opponent on each occasion) and undisturbed controls (who remained in isolation without manipulation). In terms of NK activity, immunosuppression was observed in both dominant and subordinate categories when compared to undisturbed controls. There were, however, no differences between the fighting exposed subjects and the manipulated controls, suggesting that stress accounts for any changes. © 1994 Wiley-Liss, Inc.     

Male-related chemical cues promote sexual receptivity in the female musk shrew

The role of conspecific chemical cues in the activation of sexual behavior was investigated in the female musk shrew (Suncus murinus). In Experiment 1, virgin female musk shrews were exposed to either clean cages or cages recently vacated by an adult male. Regardless of whether the male used for the sexual behavior test was "familiar" to the female (having spent the 24 h exposure in his vacant cage) or "unfamiliar," females exposed to male-related cues received mounts from males significantly sooner than females exposed to clean cages. In Experiment 2, females housed for 24 h in a cage soiled by an adult male allowed males to mount significantly sooner than females housed in a cage soiled by a castrated male, another female, or a clean cage. These results demonstrate that chemical cues, produced exclusively by adult males, promote sexual receptivity in female musk shrews.     

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.     

Selective interference in rhesus monkey (Macaca mulatta) intragroup agonistic episodes by age-sex class

Adult male interference in agonistic episodes is strongly biased against adolescent and adult male participants, whereas adult female interference is biased in favor of kin and in support of younger animals against older animals. Although natal males also are biased in favor of their kin, their selective targeting of sexually mature males is independent of kinship. Adolescent males target adult males, but only in defense of kin. This selective interference against adolescent and adult males by adult males has the potential to profoundly modify male agonistic participation in intragroup encounters after puberty. Because female support is influenced primarily by kinship, females less consistently interfere against male agonistic participants. Adult males may therefore play an important role in the socialization of male agonistic expression.     

Gender-specific social experiences and the development of aggressive and sexual behavior in male mice

This study examined influences of gender-specific social experiences on the development of aggressive and sexual behavior in male mice. To determine the effects of gender-specific social experience three different types of groups were constituted after the animals had been weaned. The subjects were randomly assigned to different treatments. Female groups were composed of one experimental male and three female cohabitants. Male groups were composed of five experimental males each, and the mixed-sex groups were composed of two experimental males and of two females. The experimental subjects stayed in these groups until the age of approximately three months, when the testing for sexual and aggressive behavior commenced. For the sexuality tests, a receptive female was placed in the home cage of the experimental male for ten minutes. A nonaggressive male was placed in the home cage of the experimental male for seven minutes for the aggression tests. The experimental males were administered both sexuality and aggression tests, the sequence of testing sexual and aggressive behavior was systematically varied in order to control the influence of the two different types of behavioral tests. The results showed that males with only male social experiences showed fewer responses and were less active in both the aggression and sexuality tests than the males from the two other types of groups. Significant positive correlations between activity during aggression and sexual tests were obtained for all three groups. © 1994 Wiley-Liss, Inc.     

Comparison of agonistic behavior in individually-housed male mice with those cohabiting with females

Videotape recordings of male mice group-housed, individually-housed and cohabiting with females, were rated for their agonistic behavior in a “standard opponent” test. Previously mated male mice showed more fighting than isolated or grouped males. Marked differences in other social and non-social behaviors, which could not be accounted for in terms of increased fighting, were not evident. These results suggest that agonistic behavior may be usefully studied by examining male mice that have cohabited with females. One obvious advantage is that such mice cannot be dismissed as being “socially deprived,” as is sometimes claimed for individually-housed mice. Other advantages are that aggressiveness is induced quickly, at high levels, and the mice appear very sensitive to hormone manipulation following castration.     

Anogenital distance and dominance status in male house mice (Mus domesticus)

Factors influencing the tendency to be aggressive were investigated in male house mice using a series of paired encounters. Body size, body lenght, body temperature, age, and anogenital distance were measured on all males. Paired encounters were conducted using a standard mouse cage as an arena. Across 64 males involved in 224 encounters, the tendency to be dominant and win encounters was significantly correlated only with anogenital distance (r = 0.383). These findings suggest that there are significant behavioral effects in male mice that could parallel the intrauterine position and related prental hormone effects that have been elucidated in female house mice and other rodents. © 1995 Wiley-Liss, Inc.     

Maternal aggression toward infanticidal males of different social status in wild house mice (Mus musculus domesticus)

Maternal aggression was examined in wild female mice (Mus musculus domesticus) derived from animals trapped in Alberta, Canada. Lactating females were tested for their behavior toward intruder males during the time of postpartum estrus while housed in a two-cage apparatus containing a defensible nest area. Prior to being used as intruders, sexually naive males were screened for their behavior toward a newborn pup (83% exhibited infanticide). Only infanticidal males were then housed in pairs and allowed to establish a dominance hierarchy. Dominance status was further verified by a urine marking test. The dominant and subordinate infanticidal males were then placed into a lactating female's cage and observed for 1 hr. The test was terminated immediately when a male began to attack the pups. Lactating females attacked the males in both groups, but subordinate males received more intense attacks than dominant males. Dominant males elicited significantly more fear/defense behavior than subordinate intruders. All of the dominant males and only one submissive male attacked the pups. Females were thus successful in blocking infanticide only by infanticidal subordinate males. Since females do not persist in attacking males with high fighting ability, one function of maternal aggression could be to assess the fighting, and resource holding, potential of a future mate. © 1994 Wiley-Liss, Inc.     

Effects of female presence on intrasexual aggression in male lizards,Podarcis hispanicus

Resource value and expected gain in reproduction may affect motivation to fight and the likelihood of winning. Previous experiments have showed that males increase their fighting effort when defending a territory that contains females. However, we hypothesized that for an intruding lizard, the value of a new territory may be lower if he already has a female in his own territory, and consequently, aggressivity should be lesser than if he has no access to any female. We staged encounters between males of the lizard Podarcis hispanicus in outdoor terraria to analyze the outcome and detailed behaviors involved in agonistic interactions in the presence or absence of a female in the terraria of resident and intruders. Our results showed that when a female was present, the level of aggressivity of the resident male was higher; the probability of winning the contest also increased, but only if the intruding male had no females in his own terraria. In contrast, when the intruding male was also the owner of another territory containing a female, residents were less aggressive. We suggest that the lack of information on the reproductive state of an unfamiliar female may be enough to decrease fighting motivation of an intruding male, if he has more expectations of success with his own familiar female. We conclude that differences in expected reproductive success with different females may help to decide the outcome of conflicts between males quicker and cheaper. Aggr. Behav. 28:491–498, 2002. © 2002 Wiley‐Liss, Inc.     

The role of endocrines in isolation-induced lntermale fighting in albino laboratory mice. II. Sex steroid influences in aggressive mice

Isolation-induced intermale fighting in laboratory mice can be dramatically reduced under most circumstances by castration. This behavior in castrates may, however, be restored, or even accentuated, by androgen replacement. Experiments on the effects of sex steroids on such fighting in castrated mice, which, for want of a better term, are designated as “aggressive,” have been recently described. These mice are housed with a female until 10 days after siring a litter and are, thereafter, housed individually for a further 14 days before castration and subsequent hormone treatment. Such mice show substantial levels of fighting in “standard-opponent” tests even before isolation. Although castration results in reduced fighting in these mice, this behavior is rarely completely abolished in all individuals. It seems likely that steroid treatment of aggressive mice maintains or intensifies an already present motivation. Treatments in these studies consisted of daily oil-based intramuscular injections for 14 days preceding and throughout behavioral testing. The standard-opponent tests were 7 min encounters with adult, subordinate, grouped males in the cleaned home cages of experimental mice. The steroids investigated included estradiol benzoate (EB), 19-hydroxytestosterone (19-OHT), androstenedione (A), testosterone (T), and Sα-dihydrotestosterone (DHT), either singly or in combination. The results suggest that (a) on a dosage basis, estrogens were at least as effective as androgens in maintaining fighting in castrated aggressive mice; (b) 19-OHT (one of the metabolic intermediates between testosterone and 17 β-estradiol) was also effective but somewhat less so than the same dose of EB; (c) the three naturally occurring androgens investigated all effectively maintained fighting at comparatively low doses (50 μg/day) which compares with a replacement dose of 500 μg/day of T in some studies in traditional castrated mice (e.g., Luttge and Hall, 1973); (d) aromatization is not essential for a behavioral action of androgens as DHT, a nonaromatizable androgen, maintained fighting in these mice; (e) whereas a two-site (central motivational and peripheral penile) action seems probable in the influence of androgens on sexual behavior in castrated rats (e.g., Parrott, 1975), DHT did not augment the action of EB on fighting in castrated aggressive mice, indicating that only a central action of steroids was required in the aggressor.     

The squeal as an indicator of aggression in mice

Fighting behavior of paired male mice was observed while automatic recording equipment counted the squeals. A strong positive relationship was found between the squeals and other measures of fighting. The results are discussed in terms of the validity of squeals as a measure of aggression and the usefulness of the squeal measure and the advantages it provides, i.e., the possibility of continuous monitoring of fighting behavior, elimination of O effects, less handling of Ss, and use of a “home cage” type of environment.     

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.