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.     

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.     

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.     

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.     

Effect of competitor density on the aggressiveness of juvenile white seabream (Diplodus sargus cadenati de la Paz,Bauchot and Daget, 1974)

The number of aggressive interactions displayed by juveniles of Diplodus sargus cadenati increases progressively in proportion to the number of intruders, with an upper threshold of more than 16 to 18 intruder fish. Beyond this density, resident aggressiveness decreases. The number of aggressive interactions that the resident fish devotes to expelling each intruder shows a clear and significant tendency to decrease as competitor density increases. The resident fish is incapable of rapidly evaluating the outweighing of its capacity for defense. Moreover, the strategy of aggressive defense of the resource against the density of competitors does not seem to be only profitability criteria dependent, but rather there is also a significant influence of the ability to fight shown by the resident. Aggr. Behav. 29:279–284, 2003. © 2003 Wiley‐Liss, Inc.     

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.     

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.     

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.     

Intruding male red swamp crayfish,Procambarus clarkii,immediately dominate members of established communities of smaller,mixed-sex conspecifics

The effects of competing asymmetries (intruder size advantage vs. prior residence) on dominance relationships were investigated in a laboratory setting. Sexually mature (Form I) male red swamp crayfish, 25%-27% larger than the average member of several mixedsex communities of 20-25 sexually mature conspecifics, individually intruded upon these communities on successive days. Each community was invaded once a day, with each of these large intruders invading every community once during the experiment. Five days after the last large intruder invasion, novel intruder group males, approximately the same size as the average community member, individually invaded the same communities, all communities being invaded once during a single day of testing. These novel intruders were used to differentiate the effects of intruder size from those effects of being put into a novel environment. During each intrusion, the frequencies of dominance, submission, aggressive standoffs, and nonaggressive interactions between the intruder and members of the community were recorded. The large intruders on each day immediately and virtually completely dominated all encountered community members, and the large intruders were significantly more dominant than the novel, smaller-sized intruders. The size advantage of the large intruders overwhelmed prior residence in influencing dominance outcomes, even in these well established communities. © 1995 Wiley-Liss, Inc.     

Intruders of differing reproductive status alter aggression differentially in early and late pregnant mice

Independent groups of early and late pregnant mice, housed individually, were observed with an intruder in their home cage. The intruders were either males, virgin, early, or late pregnant females. Male intruders were sniffed less, but more frequently and more severely attacked, than any type of female intruder, both by early and by late pregnant residents. While early pregnant females behaved similarly with the three types of female intruders, late pregnant animals treated them differently: they showed practically no aggression to late pregnant intruders, more to early pregnant ones and were most aggressive towards virgin female intruders. The possible relation of these findings to reproductive behaviour is discussed.     

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.     

The development of territorial-induced intermale agonistic behavior in albino laboratory mice

Male mice of the CF-1 strain (Mus musculus) were allowed to take up lone residence in a small territory consisting of a 60-cm enclosure attached by a tubular runway to a standard mouse cage with food, water, and bedding. A group of ten mice, each of which resided in its own separate enclosure for 24 hours, were more aggressive toward intruders than other groups of ten mice following six-hour residence periods, or no such residence. Aggression toward intruders increased in repeated weekly tests of the six-hour residents, but after four weeks of testing did not reach the maximum stable level displayed by the 24-hour residents over four weeks of testing. In another experiment, the 24-hour residence period of groups of 20 CF-1 male mice was disturbed by briefly removing the mouse from the enclosure, before introducing the intruder, at various intervals prior to testing. Removal of the resident five minutes before testing resulted in a marked decrease in aggression toward intruders. Although lesser decreases in aggression followed intervals of 30, 45, and 60 minutes, a 180 minute interval resulted in no appreciable effects compared to undisturbed controls. It is concluded that exposure to the stimuli provided by the enclosure results in an aggressive readiness in the resident mouse which reaches a high level within a 24-hour period.     

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.     

Ethological analysis of the male rat's socioagonistic behavior in a resident-intruder paradigm

The object of the present study was to assess the rat's socioagonistic behavior by means of ethological procedures. Two groups of male Wistar rats were used: 33 residents and 66 intruders. To allow reliable characterization of social and agonistic patterns, social interactions were increased by the isolation of residents and, on the other hand, both offense in residents and defense in intruders were increased by differences in weight, agonistic experience, and habituation to the enclosure. Encounters were videotaped, and the animals' behavior was analyzed by a 27-pattern ethogram and a software package made up to this end. Several patterns parameters were quantified, and two ethological mathematical models were employed: sequential analysis of preferential directions and cluster analysis based on similarities between patterns. Following mean latencies of the most frequent elements, the first four pattern sequences appeared to be the same in both groups of animals. From the fifth pattern on, residents displayed mainly dominance and threat, whereas intruders showed defensive and submissive patterns. Attack was also displayed by residents, but less frequently than dominance and threat. The alpha status was established by two sorts of domination, either through dominance and threat or by attack. The mean frequency of the intraindividual transition of patterns was higher in residents than in intruders. Pathway graphs, derived from sequential analysis, showed some common dyads, triads, or quadrads of patterns, but they basically defined different behavior structures in residents and intruders. Dendrograms, obtained by cluster analysis, allowed classification of patterns into behavior categories. The categories in residents were exploration, dominance, threat, and attack, and they encompassed 18 elements; in intruders the categories were exploration, dominance, and defense, and they consisted of nine patterns. A submissive category composed of three patterns could be also deduced considering similarity results. To summarize, six behavior categories were obtained: one “socioindividual” (exploration) and five “agonistic” (dominance, threat, attack, submission, and defense). In conclusion, the present study shows how ethological procedures may help to elucidate the rat's socioagonistic behavior and to classify the observed patterns into behavior categories.     

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.     

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.     

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.     

Contradictory asymmetries in body and weapon size,and assessment in fighting male prawns,Macrobrachium rosenbergii

Asymmetries in weapon size and body size, both contributing to an animal's fighting ability, may be contradictory in two fighting Macrobrachium rosenbergii males: One male may be larger in body size but smaller in cheliped size than its rival. The aim of the study was to determine the relevance of these two asymmetries in the assessment of relative fighting ability, and to reveal the effect of the asymmetry level on the structure of contests. Thirty contests were staged between male prawns from the ‘blue-clawed’ morphotype and the ‘orange-clawed’ morphotype. The blue-clawed males were 0%, 10%, or 20% larger in cheliped length but 93%, 45%, or 25% smaller in weight, respectively (n = 10), as compared to their orange-clawed opponents. The blue-clawed males with 10% and 20% advantage in cheliped length almost invariably won the contests. When similar in cheliped length, the 93% smaller blue-clawed males still won half of the contests. This indicated that the prawns ignored large asymmetries in body size even when no asymmetry in cheliped size existed. The results suggest that only cheliped size is used by M. rosenbergii males in the assessment of fighting ability. The effect of asymmetry level on contest structure was apparent only with the increase from 10% to 20% difference in cheliped size: Both the frequency and the intensity of fighting were lower in contests between prawns with 20% difference, as compared to 0% and 10% difference in cheliped length. Aggr. Behav. 23:81–91, 1997.© 1997 Wiley-Liss, Inc.