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.

Defensive burying, nest relocation, and pup transport in lactating female rats.

A wire-wrapped wooden dowel was inserted through the wall of the nest compartment of a two-compartment box. There were four conditions. Some lactating female rats were shocked by the dowel when they first touched it, and some were not, after which the dowel was either immediately withdrawn from the chamber or left in place for the duration of the ensuing 30-min test period. Three defensive behaviours were observed during the tests: the mothers buried the shock source with bedding from the floor of the chamber, they transported their pups to the adjoining "safe" chamber, and they built a new nest in the safe chamber from material salvaged from their original nest. The shocked mothers that were confronted with the shock source throughout the test period displayed significantly more of each of these three defensive behaviours than did the mothers in the other three conditions. The methods used in this study provide a simple, reliable, safe paradigm for studying maternal defensive behaviour; the results indicate that the defensive capacities of the rat extend far beyond stereotypical flight, freeze, and fight responses, which have been the focus of most research on rodent defense.

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.

Female rats in a competitive situation: medial hypothalamic lesions increase and ovariectomy decreases success and aggression.

Competition for food was observed between pairs of ovariectomized female rats with medial hypothalamic lesions or sham lesions. A second series of tests observed food-competition between ovariectomized and sham-ovariectomized females. Competing pairs were continuously housed together and maintained on a 23-hr food-deprivation schedule. Competition occurred as each rat attempted to maintain access to a spout containing liquid food that only one animal could lick at a time. Female rats made hyperdefensive by medial hypothalamic lesions maintained access to a food spout significantly longer than their sham-lesioned cagemates. The lesioned animals were also significantly more aggressive than their sham-lesioned cagemates. Sham-ovariectomized rats maintained access to the food spout significantly longer than their ovariectomized cagemates. The intact cagemates were also more aggressive. These results suggest that defensive aggression heightened by medial hypothalamic lesions is displayed in a competitive situation by females as has been demonstrated previously with males. Further, ovariectomy in females appears to decrease aggression and success in a competitive situation as does gonadectomy in males. These results suggest that homologous biological mechanisms modulate aggressive behavior in male and female rats.

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.

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.

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.

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.

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.