Intermale social aggression: reinstatement in castrated rats by implants of testosterone propionate in the medial hypothalamus

Male hooded rats were castrated, subcutaneously implanted with testosterone-filled silastic tubes, and individually housed with an intact adult female rat. An unfamiliar male intruder was introduced into each colony on a weekly basis and the aggressive behavior of the resident male was recorded. When the intermale social aggressive behavior of the resident male toward the intruder reached a high level in terms of a composite aggression score, the subcutaneous testosterone tubes were removed. Weekly tests of aggression toward unfamiliar intruders continued until the aggression of the resident male dropped to a low level for two successive weeks in terms of our composite aggression score. Bilateral implants of pellets of testosterone propionate were then made into the medial hypothalamus or adjacent tissue. A control group was implanted with cholesterol pellets into the medial hypothalamus. During four weekly tests following the implant, rats with testosterone propionate implants in the medial hypothalamus showed increases in lateral attacks, lateral attack duration, bites, and piloerection. The increase in aggression was not consistently displayed by animals with testosterone propionate implants dorsal or anterior to the medial hypothalamus or by animals with cholesterol implants in the medial hypothalamus. These results suggest that the medial hypothalamus or closely adjacent tissue contains testosterone-sensitive neural circuitry modulating intermale social aggression.     

A comparison of the aggressive behaviour of isolated intact and castrated male golden hamsters towards intruders introduced into the home cage

Intact and castrated adult male golden hamsters were isolated into individual cages. Six weeks later their aggressive response was observed towards an intact male of the same body weight placed into their home cages for a 10 min test period. Both groups of isolated resident males attacked, and showed more aggression than, the intruders. However, intact residents exhibited significantly more aggression than castrated residents, and showed a significantly lower latency to attack. Because of this, and the relatively high levels of aggression generated by the test situation, it is considered a particularly useful one in which to examine the possible effects of hormonal manipulation.     

Intermale social aggression in rats: suppression by medial hypothalamic lesions independently of enhanced defensiveness or decreased testicular testosterone

Medial hypothalamic lesions or sham lesions were made in castrated adult male rats with subcutaneous implants of testosterone-filled silastic capsules. Seven days following surgery all animals were given a test of defensiveness (reactivity) toward an experimenter. The following day, groups composed of one lesioned male rat, one sham-lesioned male rat, and one intact female rat were placed in large cages. Beginning two weeks later, unfamiliar intruders were introduced into each colony on a weekly basis and the aggressive behavior of the residents recorded. All 12 of the sham-lesioned animals but only 2 of 12 lesioned animals displayed substantial intermale social aggression toward intruders. Analysis of individual elements of intermale social aggression indicated that the lesioned animals were deficient in attack, bite, and piloerection but not in on-top behavior. The deficit in intermale social aggression was not correlated with defensiveness toward the experimenter or body weight of the lesioned animals. It is argued that the medial hypothalamus plays a role in the modulation of intermale social aggression which is independent of its role in modulating defensiveness or testosterone production. These results also demonstrate that intermale social aggression develops even when testosterone levels are held relatively constant by replacing testicular testosterone with an artificial testosterone source.     

Attack directed by groups of castrated male mice towards lactating or non-lactating intruders: A urine-dependent phenomenon?

The aims of the present studies were to test (1) whether grouped males as well as females displayed more aggressive responses towards lactating than towards non-lactating unfamiliar individuals introduced into their cages; (2) if the responses manifested by the males were maintained following castration; and (3) if olfactory stimuli were involved in any behavioral responses observed. The results showed remarkably that castration of males significantly augmented their aggressive responses towards lactating intruders. No effect of this operation was evident on the attacks they directed towards non-lactating intruders. It seems likely that one or more urinary (?) factors control the castrated male's aggression towards intruders. Grouped animals vigorously attack ovariectomized intruders previously rubbed with either saline solution or urine from lactating mice but the aggressive responses towards spayed females can be reduced by rubbing them with urine from non-lactating mice. Some experimental questions raised by these data are discussed.     

Ovariectomy attenuates aggression by female rats cohabiting with sexually active sterile males

Female rats were individually housed with a single castrated male with a testosterone implant that maintained sexual and aggressive behavior. At weekly intervals, the resident male was removed and an unfamiliar female intruder was introduced into the colony. Attacks, bites, on-top, and piloerection of the resident female toward the intruder were scored. Females whose level of aggression toward the intruder was substantial and stable were either ovariectomized or sham-ovariectomized. Aggression tests resumed 1 week postoperatively and continued for an additional 3 weeks. The results confirm that female cohabiting with a sterile male become aggressive. They also demonstrate that ovariectomy greatly attenuates but does not entirely abolish aggression toward an unfamiliar female intruder. The results appear to contribute to a growing body of evidence suggesting that the biological substrate and behavioral form of aggression by females housed with males (including that following parturition) is a hormone-dependent aggression which parallels testosterone-dependent social aggression of males housed with females.     

Olfactory control of aggression in lactating female housemice.

Wild female lactating mice were tested for aggression toward strange wild male mice doused with either the test female's own urine (Owns), the urine of other wild females (Novels), or water (Controls). Treatment effects were assessed against an aggression index comprised of 5 different behaviors shown to be related by a factor analysis. Females aggressed more toward Novel-scented opponents than toward either Owns or Controls, the latter 2 conditions failing to differ statistically. It was concluded that an olfactorily-mediated mechanism controls the recognition of intruders by aggressive lactating housemice.     

Intensity of aggressive interactions modulates testosterone in male marmosets

Androgen is associated with the expression of male-typical behavior, including aggressive behavior, but high levels of androgen may be incompatible with other behavioral systems, such as paternal care. In a variety of species of birds that display paternal care, testosterone (T) levels in males are maintained at low levels, and these levels rise only in response to direct agonistic challenges. This idea has not been thoroughly studied in mammals with biparental care, and we exposed male marmosets (Callithrix kuhlii), a monogamous and biparental primate to aggressive interactions with unfamiliar intruders. Urinary levels of T and cortisol (CORT) were monitored prior to and following these interactions. Baseline T was not correlated with variation in aggression in either residents or intruders, and CORT was not affected by the encounters. However, males responded to an encounter with male intruders with changes in T that correlated with the level of aggression displayed by the resident male during the trial. Encounters with male intruders that elicited high frequencies of aggressive displays by the male resident were associated with increased T 2-6 h and 24 h following the encounter, and encounters that had few aggressive displays resulted in no change or a decrease in T concentrations. Intruders did not demonstrate a significant relationship between T and aggression. Thus, the magnitude of the hormonal response is dependent on the intensity of aggression during a male-male encounter, suggesting that elevated androgens are likely to be a consequence, rather than a cause, of aggressive interactions in marmosets.     

Activation of aggression in female rats by normal males and by castrated males with testosterone implants

Female hooded rats were continuously housed with an intact male, a castrated male with subcutaneous testosterone implants, or two other females. At weekly intervals over a 10-week period, the cagemate(s) and pups were removed and aggression by the female toward an unfamiliar female intruder was observed over a 15-min period. On the 11th week each female was subjected to this intruder test in an unfamiliar cage. On the 12th week, a final test was conducted in each female's living cage with a male rather than a female as the intruder. The aggressive behaviors recorded were attacks, bites, on-top, and piloerection. Females housed with normal males displayed a significant increase in aggression prior to parturition. Their aggressiveness persisted through the 10th test with peaks at parturition and the start of lactation. Females housed with castrated males also displayed significant increases in aggression but without the peaks associated with parturition and lactation. Their aggressiveness also persisted throughout the test period. Females housed with other females showed a small increase in aggression over weeks. All groups showed virtually no aggression in the unfamiliar cage. All females displayed some aggression toward a male intruder but the level of aggression was highest in maternal females. The results demonstrate that aggression qualitatively similar to that displayed following parturition and during lactation can be elicited in nulliparous females.     

Testosterone removal in rats results in a decrease in social aggression and a loss of social dominance

Alpha male rats from mixed sex colony groups were tested for aggressiveness toward nonaggressive male intruders. Afterward, they were castrated and implanted with testosterone filled Silastic tubes, castrated and implanted with empty tubes, or sham castrated and implanted with empty tubes. There were significant declines in the aggressiveness (lateral attacks, bites, and piloerection but not on-top) of castrated rats without testosterone replacement but not in castrated rats with testosterone replacement. At a second operation, castrated animals had their testosterone capsules removed or had their empty capsules replaced with testosterone filled capsules. When tested for aggression toward nonaggressive intruders, those alpha males which had testosterone removed declined in aggressiveness while those which had it implanted returned to a level of aggressiveness close to that emitted by sham castrated control animals. Subordinate males became dominant when alpha males were castrated and not given testosterone replacement. In a final series of observations, sham castrated males were found to be more aggressive than castrated males when pitted against one another. It is argued that testosterone plays a primary role in intermale social aggression and that the decline in aggressiveness following castration is typically accompanied by a loss of social dominance.     

A comparison of the contribution of the major histocompatibility complex (MHC) and Y chromosomes to the discriminability of individual urine odors of mice by Long-Evans rats

A go/no-go operant task was used to assess the ability of male Long-Evans rats to discriminate between the urine odors from pairs of intact MHC congenic mice (C57BL/6-H-2K^b/J and C57BL/6-H-2K^bm1/ByJ), intact Y congenic mice (DBA1 and DBA1.C57BL10-Y), and castrated Y congenic mice of these two strains. The MHC congenic strains differ in alleles of the H-2 K locus, while the Y congenic strains differ in the nonrecombining part of the Y chromosome. Analysis of the number of correct responses to a criterion of 85% correct on each block of 20 trials revealed that the ability of the subjects to discriminate between urine odors did not differ whether samples were from pairs of intact MHC congenic mice, intact Y congenic mice, or castrated Y congenic mice. These findings are consistent with the hypothesis that individually unique urine odors may be influenced both by genes in the nonrecombining part of the Y chromosome and by genes in the major histocompatibility complex of chromosome 17. These odors are not androgen dependent. Such urinary chemical signals may be involved in pregnancy block (the Bruce effect), aggression, and other mouse social behaviors.     

Hormone-dependent aggression in male rats is proportional to serum testosterone concentration but sexual behavior is not

Male hooded rats were castrated and implanted with Silastic capsules (1.57 mm i.d.; 3.18 mm o.d.) having a testosterone-filled space 0, 7, 22, 60, or 90 mm long. All animals were returned to their original group cages for a three-week period to allow hormone concentrations and behavioral tendencies to stabilize. Each male was then housed with an intact female in a large cage. Aggression by the male toward an unfamiliar male was tested at weekly intervals for three weeks. Sexual behavior with an estrogen/progesterone-primed ovariectomized female was tested on each of the subsequent two weeks. Serum testosterone was measured during the following week. The frequency of aggression was correlated with serum testosterone concentration up to the normal level and did not increase with higher serum testosterone concentrations. In contrast, sexual behavior was virtually absent in animals with no testosterone replacement and normal in all other groups. These results demonstrate a clear dissociation in the dependence of hormone-dependent aggression and sexual behavior on serum testosterone concentration. In a male cohabiting with a female, sexual experience activates hormone-dependent aggression toward an unfamiliar male but the level of aggression that develops depends on the serum testosterone concentration in the resident male.     

Cohabitation with a female activates testosterone-dependent social aggression in male rats independently of changes in serum testosterone concentration

Male hooded rats (350 to 450 g) were sham-castrated, castrated and implanted with testosterone-filled, or castrated and implanted with empty Silastic tubes. Twenty-four hours postoperatively the animals in each group were housed with a female or a male similar in size to the female. Beginning one week following surgery and continuing for three weeks thereafter, the female or male cagemate was removed once each week while a 15-min test of aggression toward an unfamiliar male intruder was conducted. During the aggression tests, lateral attacks, lunge attacks, bites, on-top, and piloerection were recorded. At the first aggression test, males housed with females were significantly more aggressive than their counterparts housed with males. In contrast, different testosterone regimes did not consistently influence the initial activation of intermale social aggression. At the second and third tests, males with testicular testosterone or a replacement were significantly more aggressive than their castrated controls on most measures but males housed with females continued to be more aggressive than the comparable group housed with males. These results suggest that normal fluctuations in serum testosterone concentration associated with sexual interaction are not necessary for the initial activation of intermale social aggression. Both repeated exposure to unfamiliar males as well as cohabitation with a female are effective stimuli for activation of testosterone-dependent social aggression.     

Postweaning social experience and adult aggression in rats

One group of male rats was reared continuously in small, stable colonies of three animals each during a ten-week period beginning when they were 46 days old. A second group of animals was also reared in small groups of three animals each but membership within these colonies was randomly changed at weekly intervals. Thereafter, all subjects were housed in individual cages. A 24 hr test of the subjects' aggression toward an unfamiliar male conspecific at 130-133 days of age indicated that only the heaviest animal in each of the stable colonies was aggressive. In contrast, all animals in the random-composition group were aggressive toward intruders. Scores on a second test of intruder-directed aggression administered when the subjects were nine months old were significantly related to scores on the first aggression test. Subtle variations in social experience during development are critical in establishing long-term patterns of aggression toward conspecifics in laboratory rats.     

Androgen effects on responsiveness to aggression and stress-related odors of male mice

The effect of male gonadal hormones on responsiveness to the aggression and stress-related odors of male mice was examined. The initial experiment indicated that intact male mice would avoid an area of an open field that had been spotted with the urine of aggressive male donors, while castration of the subjects eliminated the response. Hormone replacement was effective in reinstating the aversion, clearly demonstrating the androgen-dependent nature of the response. A second experiment determined that the effects of castration do not generalize to another type of aversive odor, namely the alarm odors of castrate donors. That is, both intact and castrate males exhibited a pronounced aversion to the urine odors of castrates that had been subjected to a prolonged period of stress. These results suggest that gonadal hormone effects on olfactory responsivity are somewhat specific, and more interestingly, that the mechanism behind the effects of gonadal hormones on rodent aggression may lie in their influence on the nature of the response to the relevant olfactory stimuli.     

Competitive experience activates testosterone-dependent social aggression toward unfamiliar males

Male hooded rats (350 to 450 g) were castrated and given subcutaneous implants of testosterone-filled or empty Silastic tubes. Four weeks later, half of the animals with testosterone implants were housed with an animal with an empty implant and left for 6 weeks. The other animals were adapted to a food-deprivation schedule, housed in testosterone-implant/sham-implant pairs and given a series of food-competition tests. Following the competition tests, all animals were observed individually in their living cage for aggression toward an unfamiliar intruder. Within the competitive situation, animals with testosterone implants were more aggressive and more successful at maintaining access to food than their cagemates with sham implants. In the unfamiliar intruder test, animals with testosterone implants that had been subjected to food competition were more aggressive toward an unfamiliar intruder than were animals with testosterone implants that had not been given competitive experience. Animals with testosterone implants given competitive experience were more aggressive than their castrated cagemates, but animals with testosterone implants not given competitive experience were not more aggressive than their cagemates. These results demonstrate that testosterone-dependent social aggression fostered by a competitive situation is elicited by an unfamiliar male intruder. They also confirm other evidence that activation of social aggression does not appear to require increased testicular testosterone secretion.     

Agonistic behavior patterns in mice with streptozotocin-induced diabetes mellitus

To examine the effects of the altered metabolic and hormonal state of diabetes mellitus on rodent social behavior, male Swiss Webster mice made diabetic with streptozotocin were tested in a resident-intruder encounter. Isolated diabetic and control mice were introduced as intruders into the home cages of aggression-trained resident mice. The encounters were videotaped and analyzed for frequencies and durations of agonistic behavior. Diabetic mice responded to the aggression of resident mice with significantly less investigation and aggression and significantly more static defense and escape behaviors than did control mice. Resident mice responded to less aggressive diabetic mice with more aggression and social investigation. Plasma corticosterone levels were significantly higher in diabetic mice compared to controls and positively correlated with submissive behavior in diabetic mice. These findings indicate that social behavior is altered in male diabetic mice and support the hypothesis that elevated pituitary-adrenal cortical activity and/or metabolic changes affect behavior in male diabetic mice.     

A comparison of the effects of androstenedione, dihydrotestosterone and testosterone propionate on aggression in the castrated male golden hamster

Castrated adult male hamsters were isolated for six weeks, and their aggressive responses to intact intruders placed into the home-cage observed whilst receiving oil administration (control), testosterone propionate, dihydrotestosterone and androstenedione. All three androgens elicited significantly higher levels of aggression in the castrated males than oil alone. Of the three, androstenedione was the most effective and dihydrotestosterone the least, although on somatic considerations androstenedione was the least potent androgen.     

Urinary odors and size protect juvenile laboratory mice from adult male attack

For a portion of their ontogeny, the juveniles of many mammalian species appear resistant to aggressive attack by adult male conspecifics. The possibility that urinary odors and small size contribute to this immunity from attack was investigated with C57/Bl, A/J, and DW/J laboratory mice. The results of the 1st of 6 experiments were that juvenile mice of both genders evoked great curiosity from aggressive resident male adults, but juveniles were rarely attacked. In Experiments II and III, exchanging urine between juveniles and adult males suggested that juveniles possess a distinctive odor. In Experiments IV–VI, dwarf adults, normal-sized adults, and juveniles were placed with aggressive adult male residents. The dwarf males were attacked, but not as severely as normal-sized male intruders (Experiment IV). The dwarf males, however, evoked more aggressive behavior than juveniles (Experiment V), and juveniles with the odors from either normal-sized or dwarf adult males were attacked more readily than non-odorized juveniles (Experiment VI). These data suggest that both odor and size contribute to the juveniles' immunity from attack.     

Female-enhanced aggression in male rats: effects of genital anesthesia, castration, or preoptic lesions

Male rats that had cohabited with ovariectomized females for 2 weeks became more aggressive toward male intruders after a novel estrous female had been placed in their home cages for a period of 4 hr on the previous day. No increase in aggression was seen in males exposed to anestrous females. Genital anesthesia did not attenuate the female-enhanced aggression effect. By contrast, no enhancement of aggression was observed in long-term castrated males after 24 hr of exposure to estrous females. Rats with bilateral electrolytic lesions in the medial preoptic area showed a normal female-enhanced aggression effect. The observations that exposure to females facilitates aggression in males subjected to genital anesthesia and in males with preoptic lesions raise the possibility that copulatory cues are not always required for the effect.     

Prenatal testosterone exposure elevates maternal aggression in mice

Pregnant Rockland-Swiss (R-S) female mice were injected with oil, 0.5, 1.0 or 2.0 micrograms of testosterone propionate (TP) on days 12, 14 and 16 of gestation and the maternal aggressive behavior of their resulting female offspring was examined in adulthood. Prenatal exposure to 1 or 2 micrograms of TP, but not 0.5 micrograms of the steroid, significantly increased the number of attacks displayed by parturient mice toward adult male intruders. The behavioral effects on aggression were observed in the absence of effects on external morphology, body weight, or lactational performance. The findings support previous research showing that the development of feminine behavior may be sensitive to prenatal androgens. The possibility that the presence of fetal testosterone augments both male and female aggressive behavior is discussed.