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.

Defensive aggression and testosterone-dependent intermale social aggression are each elicited by food competition.

Castrated rats with medial hypothalamic lesions or sham lesions and castrated rats with testosterone implants or sham implants were placed on a 23-hr food deprivation schedule, adapted to a highly palatable liquid food, and then housed in pairs. The pairs were observed in competition for the highly palatable food over a 4-min period on each of six days. On the first three days, the food was dispensed in a way that allowed only one animal at a time to drink while during the second three days both animals could drink simultaneously. The pairs of animals were then separated, individually adapted to a bland liquid food, and paired with a different animal for a second series of competition tests. With highly palatable food as the incentive, rats made hyperdefensive by medical hypothalamic lesions were more successful at maintaining access to the food and more aggressive than their sham-lesioned competitors on tests when food access was restricted to a single animal but not on tests when both animals could drink simultaneously. With bland food as the incentive, lesioned animals were not consistently more successful in maintaining access to the food but were significantly more aggressive than their cagemates. With the highly palatable food, castrated males with testosterone implants were neither more successful in maintaining access to the food nor more aggressive than their cagemates with sham implants. However, when paired with an unfamiliar cagemate in preparation for competition tests with the bland food, most rats with testosterone implants attacked the new cagemate using a lateral attack and displaying piloerection.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Competitive behavior: intact male rats but not hyperdefensive males with medial hypothalamic lesions share water with females.

Male hooded rats with medial hypothalamic lesions or sham lesions were given tests of defensiveness toward an experimenter. The 8 lesioned males with the highest defensiveness scores and 7 sham-lesioned males were each placed in a double cage with a single intact female. For each pair of rats, food was continuously present but water was available for only 1 hr/day through a single water spout. Beginning on the fifth day of water deprivation, each pair of animals was given a 4-min water competition test on 3 consecutive days. Competition for water was created by placing a plastic ring over the hole in the cage where the water spout entered the cage. The ring restricted access to the spout to a single animal and was put in place 5 min before water was given. One hr following the competition test, each pair of animals was given access to a single unencumbered spout for a 1-hr period. Rats with medial hypothalamic lesions drank significantly more and initiated more aggression than their female cagemates during the 4-min competition tests. Sham-lesioned rats neither drank significantly more nor were more aggressive than their female cagemates. These results are consistent with previous observations indicating that the aggressiveness of rats with medial hypothalamic lesions can be elicited by a competition situation.

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.

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.

Competitive behavior in male rats: aggression and success enhanced by medial hypothalamic lesions as well as by testosterone implants.

Castrated male hooded rats were given electrolytic lesions of the medial hypothalamus or sham lesions. Another group of castrated rats was implanted subcutaneously with bilateral testosterone-filled Silastic capsules or empty capsules. Lesioned animals with a high defensiveness (reactivity) score toward the experimenter were each placed in a cage with a sham-lesioned animal of a similar weight. Animals with testosterone implants were likewise housed with an animal of similar weight without a testosterone implant. Following a period of adaptation to a 23-hr water deprivation schedule, each pair of rats was given daily competition tests on each of 6 days. During the tests, a single water spout was placed in the cage for a 4-min period. The spout was surrounded by a plastic ring which prevented more than one animal from drinking at any time. Access to an unencumbered water spout was present following the competition test for 1-hr each day. Rats with medial hypothalamic lesions displayed more aggression than sham-lesioned rats during the competition tests and were able to spend more time drinking. Rats with testosterone implants were more successful in maintaining access to the spout but did not consistently display more aggression than their cagemates without testosterone implants. The aggression of the lesioned rats was defensive while that of animals with testosterone implants corresponded to intermale social aggression. These results demonstrate that a competitive situation can elicit intermale social aggression mediated by testosterone and defensiveness induced by medial hypothalamic lesions.

Maternal aggression and intermale social aggression: a behavioral comparison.

The aggressive behavior of alpha male rats and lactating females were each examined toward an intact adult male rat, a castrated adult male rat, an anesthetized adult male rat, a nonlactating adult female rat, an adult albino guinea pig (male or female), or an albino mouse (male or female). When in their living colony, females displayed high levels of aggressiveness toward all stimulus objects except a mouse. The aggression toward the intruding males occurred whether the female's pups were present or not. Alpha males were aggressive toward the same stimuli except an intruding female rat and a mouse. When tested in an unfamiliar colony, the males but not the females (with or without pups present) were aggressive toward an adult male rat. Half of the females but none of the males displayed defensive burying toward an anesthetized intruder. It is suggested that the attack on an adult female, the absence of attack outside of the resident colony, and the tendency to display defensive burying are features of the aggressiveness of lactating females that are fundamentally different from the aggressiveness of alpha males. The form of the aggression (lateral attack vs. lunge attack) was only quantitatively different in males and females.