Influence of combined estradiol and testosterone implants on the aggressiveness of nonaggressive female rats.

Female rats that had been cohabiting with a sterile male or with another female for 6 weeks were tested for aggression toward an unfamiliar female once each week for 3 weeks. Females that were not aggressive as a result of cohabitation with a sterile male were ovariectomized. Half were implanted with a Silastic tube containing estradiol (1 mm long hormone-filled space) and a tube containing testosterone (5 mm long hormone-filled space). The other half were implanted with empty tubes. All females that had been cohabiting with another female were ovariectomized and implanted with an estradiol- and a testosterone-filled tube. Three additional weekly tests of aggression were given beginning 1 week postoperatively. Females given hormone replacement displayed only a slight increase in aggression postoperatively. Females not given hormone replacement declined in aggressiveness. These results indicate that hormone replacement levels sufficient to maintain aggression in highly aggressive females following ovariectomy are not sufficient to produce a high level of aggression in females that have not become aggressive following cohabitation with a sterile male or that have been cohabiting with another female.

Aggression by a female rat cohabiting with a sterile male declines within 27 h following ovariectomy.

Females that had become aggressive as a result of cohabiting with a sterile male were ovariectomized and implanted with empty Silastic tubes. Control groups were either sham ovariectomized or ovariectomized and implanted with Silastic tubes providing replacement levels of estradiol and testosterone. Twenty-seven hours following surgery, all animals were tested for aggressiveness toward an unfamiliar female rat. The aggression of ovariectomized females without hormone replacement declined to a minimal level postoperatively and was significantly lower than that of sham-ovariectomized females or ovariectomized females with estradiol and testosterone replacement. Both sham-ovariectomized females and ovariectomized females given hormone replacement displayed a level of aggression close to that observed preoperatively. The aggression of a female rat cohabiting with a sterile male appears to be highly sensitive to the removal of gonadal hormones. This corresponds with observations made on pregnant females and contrasts with those made on lactating females.

Aggression in humans: what is its biological foundation?

Although human aggression is frequently inferred to parallel aggression based on testosterone in nonprimate mammals, there is little concrete support for this position. High- and low-aggression individuals do not consistently differ in serum testosterone. Aggression does not change at puberty when testosterone levels increase. Aggression does not increase in hypogonadal males (or females) when exogenous testosterone is administered to support sexual activity. Similarly, there are no reports that aggression increases in hirsute females even though testosterone levels may rise to 200% above normal. Conversely, castration or antiandrogen administration to human males is not associated with a consistent decrease in aggression. Finally, changes in human aggression associated with neuropathology are not consistent with current knowledge of the neural basis of testosterone-dependent aggression. In contrast, human aggression does have a substantial number of features in common with defensive aggression seen in nonprimate mammals. It is present at all age levels, is displayed by both males and females, is directed at both males and females, and is not dependent on seasonal changes in hormone levels or experiential events such as sexual activity. As would be expected from current knowledge of the neural system controlling defensive aggression, aggression in humans increases with tumors in the medial hypothalamus and septal region, and with seizure activity in the amygdala. It decreases with lesions in the amygdala. The inference that human aggression has its roots in the defensive aggression of nonprimate mammals is in general agreement with evidence on the consistency of human aggressiveness over age, with similarities in male and female aggressiveness in laboratory studies, and with observations that some neurological disturbances contribute to criminal violence. This evidence suggests that human aggression has its biological roots in the defensive aggression of nonprimate mammals and not in hormone-dependent aggression based on testosterone.

Ovariectomy does not attenuate aggression by primiparous lactating female rats.

Nulliparous female hooded rats were allowed to cohabit with a sexually active male in a large living cage. Aggression toward an unfamiliar female was assessed during the second and third week of pregnancy. Within 12 to 24 h following parturition females were ovariectomized (n = 7) or sham-ovariectomized (n = 6) in a manner that balanced previous aggression scores. Aggression was assessed at 48 h following ovariectomy and at three weekly intervals thereafter. Ovariectomized and sham-ovariectomized females did not differ in the number of attacks, number of bites, duration of on-top, or frequency of piloerection on any test day following parturition. These results indicate that circulating levels of ovarian steroids do not influence the level of aggression by a primiparous lactating female toward an unfamiliar female conspecific.

Cohabitation with a sterile male facilitates the development of retrieval behavior in nulliparous female rats exposed to pups.

Female rats were housed with a sterile male or another female. After 3 weeks, half of the females that had been housed with a female were rehoused with an intact male. At the end of 6 weeks female or sterile male cagemates were removed. Intact male cagemates and pups were removed 3 to 12 h following parturition. All females were tested for retrieval of three unfamiliar pups placed in their cage on the day following removal of their cagemate. Three unfamiliar pups were placed with each female and the female's behavior observed for 10 min. Observations were made in this way for 13 days or until the female retrieved all three pups within the 10-min interval. Pups were left with the female on days they were not retrieved. Females housed with a sterile male reached criterion for pup retrieval in 2.9 days, significantly fewer days than were required for females housed with another female (6.6 days) but significantly more than were required for a postpartum female (0.8 days). By demonstrating that cohabitation with a male fosters the development of retrieval, these results support evidence from the study of aggressive behavior that pseudopregnancy facilitates the development of behaviors associated with pregnancy and lactation.

Interaction of estradiol, testosterone, and progesterone in the modulation of hormone-dependent aggression in the female rat.

Female rats that had become aggressive as a result of cohabiting with a sterile male were ovariectomized and implanted with Silastic tubes of estradiol, testosterone, and progesterone, estradiol and testosterone alone, or with empty tubes. The implants were designed to model serum concentrations present during the last week of pregnancy (estradiol, 0.06 ng/ml; testosterone, 2.6 ng/ml; progesterone, 70 ng/ml). Following a test of aggression 1 week postoperatively, estradiol and testosterone implants were replaced with ones designed to maintain the lower hormone levels present following parturition (0.02 ng/ml; 0.6 ng/ml, respectively). Progesterone was not replaced. At the first aggression test, females with estradiol and testosterone alone displayed significantly more aggression than females with these hormones plus progesterone. Both groups were more aggressive than females without hormone replacement. Following the exchange of large implants for small ones, females that previously had progesterone increased in aggression while females that previously had only estradiol and testosterone decreased in aggression. Both groups continued to be more aggressive than the group without hormone replacement. High serum progesterone present near the end of pregnancy appears to moderate the expression of aggression supported by estradiol and testosterone. Conversely, progesterone's decline at parturition appears to produce a rebound facilitation of aggression even though serum estradiol and testosterone simultaneously decline.

Hormone-dependent aggression in male and female rats: experiential, hormonal, and neural foundations.

Hormone-dependent aggression in both male and female rats includes the distinctive behavioral characteristics of piloerection and lateral attack. In males the aggression is dependent on testicular testosterone and is commonly known as intermale aggression. In females, the aggression is most commonly observed as maternal aggression and is dependent on hormones whose identity is only beginning to emerge. The present review examines the experiential events which activate hormone-dependent aggression, the relation of the aggression to gonadal hormones, and the neural structures that participate in its modulation. In males and females, the aggression is activated by cohabitation with a conspecific of the opposite sex, by competitive experience, and by repeated exposure to unfamiliar conspecifics. In the female, the presence of pups also activates aggression. In both males and females, hormones are necessary for the full manifestation of the aggression. The essential hormone appears to be testosterone in males and a combination of testosterone and estradiol in females. The information available suggests the neural control systems for hormone-dependent aggression may be similar in males and females. It is argued that hormone-dependent aggression is behaviorally and biologically homologous in male and female rats.

Aggression by a female rat cohabitating with a sterile male: termination of pseudopregnancy does not abolish aggression.

At the end of that time, each female was assessed for aggressiveness toward an unfamiliar female intruder once each week for 3 weeks. Those females displaying a high level of aggression had their male cagemate changed. For half of the females, the new male cagemate was a castrated male with a testosterone implant. For the other half, the new cagemate was a castrated male without a testosterone implant. Replacement males had been subjected to surgery 9 weeks previously. There were no differences in the aggressiveness of females of the two groups on any of 3 subsequent weekly tests of aggression. In a 3-h evaluation of male sexual behavior, none of the 9 castrated males without testosterone replacement displayed sexual activity with an estrogen/progesterone primed ovariectomized female, but 6 of 9 males with testosterone replacement did. Reanalysis of the aggression data comparing the females whose males had no testosterone replacement and females housed with the 6 males that were sexually active also revealed no differences in aggression over the 21-day test period. Since pseudopregnancy is known to last 13 days, these results indicate that the continuous presence of pseudopregnancy is not required for maintenance of aggression by a female cohabiting with a sterile male.

Hormone-dependent aggression in the female rat: testosterone plus estradiol implants prevent the decline in aggression following ovariectomy.

Female rats were individually housed with a sterile male for the duration of the experiment. Beginning 7 to 10 weeks after the start of cohabitation, each female was tested for aggression toward an unfamiliar female at weekly intervals for 3 weeks. Females that displayed consistent and substantial aggression were given one of the following treatments: ovariectomy followed by both testosterone and estradiol implants, ovariectomy followed by 2 empty implants, or sham ovariectomy followed by 2 empty implants. The implants were subcutaneously placed hormone-filled Silastic capsules. They were expected to produce a serum testosterone concentration of 0.5 ng/ml and an estradiol concentration of 15 pg/ml. Postoperatively, the aggression of each female continued to be assessed on a weekly basis for 3 weeks. Ovariectomized females with hormone implants displayed a level of aggression postoperatively similar to that of sham-ovariectomized females and significantly greater than that of ovariectomized females with empty implants. These results, together with others, suggest that estradiol and testosterone act together to form the hormonal foundation of hormone-dependent aggression by females cohabiting with a sterile male.

Serum estradiol concentration required to maintain body weight, attractivity, proceptivity, and receptivity in the ovariectomized female rat.

Female hooded rats (230 to 260 g) were ovariectomized and given a subcutaneous implant of an estradiol-filled Silastic tube. The length of the tube was varied in order to produce a variety of serum estradiol levels. In the first experiment, animals were weighed over a 6-week period following surgery and then tested for sexual responsiveness to a male. The results demonstrated that ovariectomized females with an implant maintaining a serum estradiol concentration at about 15 pg/ml maintained body weight at the same level as that of intact females. A smaller implant gave rise to a higher weight gain and a larger implant to a lower weight gain. All implants resulted in a continuous state of receptivity. In a second experiment, ovariectomized females were implanted with smaller estradiol-filled implants in order to determine the threshold for maintaining proceptivity and receptivity. The results indicated that with a serum estradiol concentration below 15 pg/ml, the frequency of lordosis and of ear wiggling and darting decreased. Progesterone injections facilitated both proceptive and receptive behavior. In addition, following progesterone injections, the time required for a male to mount a female 10 times was decreased in females with low or no estradiol replacement. These results indicate that a constant concentration of estradiol at about the mean level present throughout the estrous cycle will result in normal body weight regulation and will maintain sexual behaviors that normally occur only during estrus. These results emphasize that Silastic implants of estradiol do not mimic normal endocrine function since, even at low levels, estradiol implants produce continuous receptivity.

Enhanced defensiveness and increased food motivation each contribute to aggression and success in food competition by rats with medial hypothalamic lesions.

Castrated male rats (N = 27) with medial hypothalamic lesions or sham lesions were placed on a 23-h food-deprivation schedule and adapted to a highly palatable liquid food. They were also given two tests of defensiveness toward an experimenter. All animals were then housed in medial hypothalamic lesion/sham lesion pairs and subjected to a series of 6 competition tests (1 per day). Following the competition tests, all animals were given individual food consumption tests and a third test of defensiveness toward an experimenter. Correlational analysis showed that postcompetition defensiveness scores but not precompetition defensiveness scores or individual food consumption were related to aggression during the food competition. Analysis by criterion groups indicated that animals high in precompetition defensiveness and with food consumption in the normal range were not more successful in the competition but were slightly more aggressive than their sham-lesioned competitors. Animals with high postcompetition defensiveness scores and with individual food consumption in the normal range were more successful than their sham-lesioned competitors and the most aggressive of the lesioned animals during the food competition. Animals that were high in food consumption and only moderately defensive were also more successful but only slightly more aggressive in the food competition than their sham-lesioned competitors. These results suggest that a high and stable level of defensiveness, and excessive food intake, each contribute to the success and aggressiveness of rats with medial hypothalamic lesions in a food competition situation.

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.

Aggression by ovariectomized female rats: combined testosterone/estrogen implants support the development of hormone-dependent aggression.

Female hooded rats were ovariectomized and implanted with a single estrogen-filled and a single testosterone-filled Silastic tube. Control animals were ovariectomized and implanted with empty tubes. The implants produced an estrogen concentration of 30 pg/ml and a testosterone concentration of 0.25 ng/ml, levels close to those found in intact females. Two weeks following surgery, all animals were housed in individual cages, placed on a 23-hr food-deprivation schedule, and adapted to a liquid food. They were then housed in hormone-implant/empty-implant pairs and given a series of 3 restricted-access competition tests and 3 free-access competition tests (1/day). The animals were then paired with new partners and given a second series of restricted-access and free-access competition tests. Ovariectomized females with hormone implants were more successful at maintaining access to the liquid food and more aggressive than their competitors without hormone replacement. The aggression was used to maintain access to food during free-access as well as restricted-access competition. Following the competition tests, animals with hormone implants were significantly more aggressive toward an unfamiliar conspecific than were their cagemates with empty implants. The level of success and aggression by females with testosterone + estrogen implants appears greater than that which occurs with either hormone alone and comparable to that observed in intact females.

Hormone-dependent aggression in female rats: testosterone implants attenuate the decline in aggression following ovariectomy.

Female rats were individually housed with a sterile male for a 4- to 5-week period. Each female was then tested for aggression toward an unfamiliar female intruder at weekly intervals. Those females that displayed a high level of aggression on each of three weekly tests were ovariectomized and given subcutaneous implants of testosterone-filled tubes, ovariectomized and given subcutaneous implants of empty tubes, or sham-ovariectomized and implanted with empty tubes. These implants should produce a serum testosterone concentration of about 0.6 ng/ml, compared to 0.17 ng/ml in intact females. Beginning 1 week postoperatively, the aggression of each female was tested weekly for 4 weeks. Ovariectomized females with testosterone implants displayed a level of aggression significantly higher than that of ovariectomized females with empty implants on 3 of 4 weekly tests. The level of aggression by females with testosterone implants was not significantly different from that of sham-ovariectomized females on the first postoperative test. Additional observations showed that testosterone implants did not produce an increase in aggression in females whose preoperative level of aggression was low. Further, Silastic implants containing estrogen (1 to 2 mm long) sufficient to maintain a serum estrogen level of 20 to 30 pg/ml also attenuated the decline of aggression following ovariectomy. These results suggest that testosterone and estrogen may both contribute to the biological substrate of hormone-dependent aggression in female rats.

Aggression by ovariectomized female rats with testosterone implants: competitive experience activates aggression toward unfamiliar females.

Female hooded rats (250 to 325 g) were ovariectomized and bilaterally implanted with testosterone-filled or empty Silastic tubes. The testosterone-filled space in each tube was 10 mm long and this should produce a serum testosterone concentration 4 to 5 times that of an intact female, but well below that of a male. Three weeks following surgery, half of the animals with testosterone implants were housed with an animal with an empty implant and left for 6 weeks. The remaining animals were placed on a 23-hr food deprivation schedule, housed in testosterone implant/empty implant pairs, and then subjected to a series of food competition tests. Following the competition tests, all animals were individually tested in their living cage for aggression toward an unfamiliar female. In food competition, females with testosterone implants were more successful and more aggressive than their cagemates with empty implants. When tested for aggression toward an unfamiliar intruder, females with testosterone implants given competitive experience were more aggressive toward an intruder than were their cagemates with empty implants or females with testosterone implants not given the competitive experience. Females with testosterone implants but without competitive experience were not more aggressive toward an unfamiliar female than were their cagemates with empty implants. These results suggest that, in ovariectomized females with testosterone implants, hormone-dependent aggression fostered by a competitive situation is displayed toward unfamiliar females.

Medial accumbens lesions attenuate testosterone-dependent aggression in male rats.

Male hooded rats were castrated and implanted with testosterone-filled Silastic tubes appropriate for maintaining a normal average serum testosterone concentration. They were then given lesions of the medial accumbens nucleus or sham lesions. Twenty-four hours postoperatively each male was housed with a female. Beginning 7 days following pairing and continuing once each week for 4 weeks, each lesioned or sham-lesioned male was observed for aggression toward an unfamiliar male intruder. On the day following each test of aggression toward an unfamiliar male, each lesioned and sham-lesioned male was assessed for defensiveness toward an experimenter. Rats with medial accumbens lesions displayed significantly less aggression toward an unfamiliar male intruder during each of the weekly tests than did sham-lesioned animals. The attenuation was most pronounced in animals with lesions damaging the posterior part of the medial accumbens nucleus (also designated as anterior portion of the bed nucleus of the stria terminalis) in the region of the crossover of the anterior commissure. Although medial accumbens lesions are known to make individually housed rats hyperdefensive toward an experimenter, lesion-induced hyperdefensiveness was not observed in the pair-housed animals in the present experiment. It is argued that the medial accumbens/bed nucleus of the stria terminalis area is an important region in the anterior forebrain for the modulation of hormone-dependent aggression.

Testosterone supports hormone-dependent aggression in female rats.

Female hooded rats were ovariectomized and implanted with a single testosterone-filled Silastic tube or an empty tube. The tube size was one which allowed a release of testosterone at the high end of the mean normal serum testosterone concentration for intact females. Following a 7-day recovery period, all rats were placed on a 23-hr food-deprivation schedule and adapted to a highly palatable liquid food over a 5-day period. Each animal with a testosterone implant was then housed with an animal of similar weight but an empty implant. The pairs were subjected to a series of 3 restricted-access competition tests (1/day) followed 4 days later by a series of 3 free-access competition tests. The animals were then separated, adapted to a bland liquid food, and paired with new partners. They were then subjected to the restricted- and free-access food-competition tests but with bland food as the incentive. During the first 6 competition tests there were no significant differences between groups in aggression or in time spent licking at the food spout. During the second series of tests, females with testosterone implants were more aggressive and more successful at maintaining access to the food than were their competitors with empty implants. The difference between groups occurred during the free- as well as the restricted-access tests. The effectiveness of physiological levels of testosterone in supporting aggression is attributed to the use of a test situation that activates as well as elicits hormone-dependent aggression. These results suggest that testosterone may be the hormonal substrate for hormone-dependent aggression in female rats.