An acute social defeat stressor in early puberty increases susceptibility to social defeat in adulthood

Syrian hamsters readily display territorial aggression. If they lose even a single agonistic encounter, however, hamsters show striking reductions in aggressive behavior and increases in submissive behavior, a distinct behavioral change that we have previously termed conditioned defeat. This acute social defeat stressor is primarily psychological and is effective in both males and females. Therefore, we maintain that this procedure presents an ideal model for studying behavioral and physiological responses to social stress. Here, we demonstrate that social avoidance following social defeat is a particularly useful dependent measure because of its sensitivity and stability between sexes and across the estrous cycle. In addition, we demonstrate that peripubertal hamsters exposed to a single, 15 min social defeat exhibit significantly more social avoidance 24 h later when compared with no-defeat controls. Later, defeated and non-defeated hamsters display similar agonistic behavior in adulthood indicating that the peripubertal defeat does not alter adult territorial aggression. After experiencing an additional social defeat in adulthood, however, the hamsters that experienced the pubertal defeat respond to the adult defeat with increased social avoidance when compared with hamsters that were defeated only in adulthood and with no-defeat controls. These data are the first to show that a single social defeat in puberty increases susceptibility to later social defeat in both males and females.     

Conditioned defeat in male and female Syrian hamsters

A brief exposure to social defeat in male Syrian hamsters (Mesocricetus auratus) leads to profound changes in the subsequent agonistic behavior exhibited by the defeated animals. Following defeat in the home cage of an aggressive conspecific, male hamsters will subsequently fail to defend their home territory even if the intruder is a smaller, nonaggressive male. This phenomenon has been called conditioned defeat. In Experiment 1, we examined the duration of conditioned defeat by repeatedly testing (every 3-5 days) defeated hamsters with a nonaggressive intruder. We found that conditioned defeat occurs in all defeated male hamsters and persists for a prolonged period of time (at least 33 days) in the majority of male hamsters tested despite the fact that these animals are never attacked by the nonaggressive intruders. In Experiment 2, we examined whether conditioned defeat could be induced in female Syrian hamsters. While conditioned defeat occurred in some females, they displayed only low levels of submissive/defensive behavior and, in contrast to males, the conditioned defeat response did not persist beyond the first test. These results suggest that in male hamsters conditioned defeat is a profound, persistent behavioral change characterized by a total absence of territorial aggression and by the frequent display of submissive and defensive behaviors. Conversely, social defeat in female hamsters does not appear to induce long-term behavioral changes. Finally, in Experiment 3, we determined that plasma adrenocorticotropin-like immunoreactivity increases in females following social defeat in a manner similar to that seen in males, suggesting that the disparate behavioral reactions of males and females are not due to sex differences in the release of, or response to, plasma adrenocorticotropin.     

Winning agonistic encounters increases testosterone and androgen receptor expression in Syrian hamsters

Winning aggressive disputes is one of several experiences that can alter responses to future stressful events. We have previously tested dominant and subordinate male Syrian hamsters in a conditioned defeat model and found that dominant individuals show less change in behavior following social defeat stress compared to subordinates and controls, indicating a reduced conditioned defeat response. Resistance to the effects of social defeat in dominants is experience-dependent and requires the maintenance of dominance relationships for 14 days. For this study we investigated whether winning aggressive interactions increases plasma testosterone and whether repeatedly winning increases androgen receptor expression. First, male hamsters were paired in daily 10-min aggressive encounters and blood samples were collected immediately before and 15 min and 30 min after the formation of dominance relationships. Dominants showed an increase in plasma testosterone at 15 min post-interaction compared to their pre-interaction baseline, whereas subordinates and controls showed no change in plasma testosterone. Secondly, we investigated whether 14 days of dominant social status increased androgen or estrogen alpha-receptor immunoreactivity in brain regions that regulate the conditioned defeat response. Dominants showed more androgen, but not estrogen alpha, receptor immuno-positive cells in the dorsal medial amygdala (dMeA) and ventral lateral septum (vLS) compared to subordinates and controls. Finally, we showed that one day of dominant social status was insufficient to increase androgen receptor immunoreactivity compared to subordinates. These results suggest that elevated testosterone signaling at androgen receptors in the dMeA and vLS might contribute to the reduced conditioned defeat response exhibited by dominant hamsters.     

Sex and estrous cycle differences in the display of conditioned defeat in Syrian hamsters

We have reported that there is a sex difference in the behavioral response to social defeat in hamsters. While previously defeated male hamsters fail to display normal territorial aggression and instead produce submissive/defensive behavior, a phenomenon that we have termed conditioned defeat (CD), only a small portion of previously defeated females exhibit CD. In Experiment 1, we tested the hypothesis that CD varies over the estrous cycle and found that previously defeated female hamsters tested on diestrus 2 and proestrus were more likely to exhibit CD than were females tested on diestrus 1 and estrus. In Experiment 2, we found that regardless of hormonal status, non-defeated females displayed normal territorial aggression, indicating that the behavioral changes observed in Experiment 1 were not due to a cyclic variation in submissive behavior independent of a previous defeat encounter. In Experiment 3, we found that females tested 4 days after defeat responded similarly to those tested 1 day after defeat suggesting that the hormonal status of females on the day of testing is a more important determinant of the behavioral response to defeat than is the hormonal status on the day of defeat training. Finally, in Experiment 4, we monitored anxiety-like behaviors in diestrous 1 and proestrous females in an open field arena and found that there was no effect of cycle on any of the observed behavioral measures, suggesting that the observed differences in CD are not the result of differences in generalized anxiety-like behaviors across the estrous cycle.     

Seasonal changes in and effects of familiarity on agonistic behaviors of rat-like hamsters (Cricetulus triton)

The seasonal changes in agonistic behaviors and effects of familiarity on agonistic behaviors in wild-caught adult rat-like hamsters (Cricetulus triton) were observed in dyadic encounters in a neutral arena. The aggression of opposite- and same-sex encounters became higher or remained the same during the non-breeding season. This indicates that the hamsters were solitary during both seasons. Familiarity increased the aggression in male–male encounters and decreased the aggression in female–female encounters during both seasons. Familiarity also increased the aggression in female–male encounters during the non-breeding season and had no effect on the aggression in female–male encounters during the breeding season. These results may be related to the hamsters social structure. The more agonistic acts both male and female hamsters had, the more frequently they marked using flank glands during both seasons. This implies that flank gland marking can be used to advertise status and can be assessed by opponents to reduce the agonistic costs.     

Effects of repeated aggressive encounters on approach to a female and plasma testosterone in male mice

Effects of repeated experience of aggression accompanied by social victories or social defeat in 10 daily agonistic confrontations on testosterone levels in and the behavioral response of CBA/Lac male mice exposed to a receptive female from behind a perforated transparent partition have been examined. Testosterone levels were not changed significantly in the mice that had consistently been victorious over 10 days (winners) or in the mice that had consistently been defeated over 10 days (losers). Losers and controls (mice that had been caged individually for 5 days) responded with increased levels of behavioral activity near the partition and elevated testosterone. Winners showed a significantly poorer behavioral and hormonal response. It is concluded that the repeated display of aggression by male mice led to a reduction in both their behavioral and neuroendocrine responses to an estrous female.     

Behavioral and neuroendocrine consequences of social subjugation across adolescence and adulthood

BACKGROUND: Social subjugation is a very significant and natural stressor in the animal kingdom. Adult animals defeated and subjugated during establishment of dominance hierarchies or territorial encounters can be highly submissive in future agonistic interactions. While much is know about the biological and behavioral consequences of winning and losing fights in adulthood, little is known about adolescence; a developmental period noted for impulsivity and heightened agonistic behavior. The present studies were undertaken to determine if the behavioral and neuroendocrine consequences of social subjugation are comparable in adolescent versus adult Syrian golden hamsters (Mesocricetus auratus). Male siblings were studied from adolescence into adulthood following exposure to counterbalanced episodes of either a benign stressor, i.e., isolation in a novel cage, or the more severe stressor of social subjugation. RESULTS: As adults, hamsters with a history of social subjugation in adolescence show high levels of aggression toward intruders as compared to siblings subjugated in adulthood. Sibling controls subjugated in adulthood are highly submissive with little or no aggressive behavior. However, when subjugated in adulthood, hamsters with the earlier history of subjugation are no different than their sibling controls, i.e., adult subjugation promotes submissive behavior. Sexual motivation is high in adult hamsters with adolescent subjugation and testosterone levels remained stable over adulthood. In contrast, sibling controls subjugated in adulthood show lower levels of sexual motivation and reduced levels of testosterone. Release of cortisol during agonistic encounters is blunted in animals subjugated in adolescence but not adulthood. Measures of anxiety are reduced in hamsters with adolescent subjugation as compared to their sibling controls. CONCLUSION: These data demonstrate a pronounced difference in behavior and neuroendocrinology between adolescent and adult hamsters in their response to social subjugation and suggest adolescence is a resilient period in development.     

Aggressive behavior in female golden hamsters: development and the effect of repeated social stress

In male golden hamsters, agonistic behavior matures during puberty, changing from play fighting to adult-like aggression. In addition, this transition is accelerated by repeated social subjugation early in puberty. However, little is known about the development of agonistic behavior in females. In the present study, we compared the development of agonistic behavior in male and female golden hamsters. Furthermore, we also tested the effects of repeated social subjugation on the development of agonistic behavior during puberty. Hamsters were tested for agonistic behavior in the presence of a smaller intruder at different intervals during puberty. Several observations were made. First, the frequency of attacks remained stable in females, while varying in males. Second, the transition from play fighting to adult-like aggression occurred at earlier time periods in females than in males. Finally, a clear transitional period marked by attacks focused on the flanks was observable in males around mid-puberty. However, this transitional period was not apparent in females. In addition, juvenile females were exposed to aggressive adult males or females. In both cases, repeated exposure to stress had no statistically significant effect on the development of agonistic behavior. After 2 weeks of subjugation, exposure to aggressive adults had no effect on serum cortisol levels, indicating that juvenile females habituate to repeated social stress. These data show significant sex differences in the development of agonistic behavior and adaptation to repeated stress in juvenile golden hamsters.     

Aggression Levels Affect Social Interaction in the Non‐Breeding Territorial Aggression of the Weakly Electric Fish,Gymnotus omarorum

Animals typically decide whether to fight or retreat from conspecifics based on their individual estimates of the costs and benefits of fighting. Theoretical models predict how contenders solve a conflict, but the evaluation processes involved in these decisions depend on multiple factors that are difficult to explore experimentally. We addressed these questions using the non‐breeding territorial aggression of Gymnotus omarorum, in which subordinates make three distinctive decisions to signal their submission during a fight: (1) interruption of their electric discharges to hide from the dominant, (2) stop attacking and retreat, and (3) emission of ‘chirps’, transient submissive electric signals. We confirmed that subordinates take into account the aggressive performance of dominants to shape their own agonistic decisions and performance. The intensity of aggression is highly correlated with an agonistic dyad, and the decision of subordinates to retreat is influenced by the attack rates of dominants. When we lowered the aggression of expected dominants with a 5‐HT_1A receptor agonist, the correlation between the two contenders' aggression levels was lost and subordinates completely stopped emitting electric chirp signals. The aforementioned results contribute to the understanding of the decision‐making strategies driven by social challenge inherent to agonistic encounters.     

Cortisol controls the pubertal development of agonistic behavior in male golden hamsters via type II corticosteroid receptors

In male golden hamsters, agonistic behavior undergoes a pubertal transition from play fighting to adult aggression. Previous studies have shown that this aspect of behavioral development is associated with pubertal increases in glucocorticoids and that daily social stress or injections of a synthetic glucocorticoid accelerate the transition. The goals of this study were to confirm the effects of cortisol on the development of agonistic behavior and to investigate the role of type II corticosteroid receptors in this process. First, animals treated with cortisol during early puberty [from postnatal days 31 (P-31) to P-36] showed an accelerated transition from play fighting to adult aggression. In a second experiment, the behavioral effects of cortisol were blocked by a co-treatment with a type II corticosteroid receptor antagonist. These findings are the first to show a facilitating role for type II corticosteroid receptors in the pubertal development of a social behavior. As such, these findings provide new insights into the neuroendocrine mechanisms controlling behavioral development during puberty.     

Effect of photoperiod on vasopressin-induced aggression in Syrian hamsters

Syrian hamsters are photoperiodic and become sexually quiescent when exposed to short "winter-like" photoperiods. In short photoperiods, male hamsters display significantly higher levels of aggression than males housed in long photoperiods. Arginine-vasopressin (AVP) within the anterior hypothalamus (AH) has been reported to modulate aggression in hamsters housed in long photoperiods. Previous studies have shown that AVP can facilitate aggression and its effects appear to be mediated by AVP V(1a) receptors (V(1a)R). In the present study, we investigated whether the increased levels of aggression observed after exposure to short photoperiod were the result of an increased responsiveness to AVP within the AH. Injections of AVP into the AH significantly increased aggression in hamsters housed in a long photoperiod, but had no effect in hamsters housed in a short photoperiod. In addition, injection of a V(1a)R antagonist into the AH significantly inhibited aggression in hamsters housed in long photoperiod, but had no effect in hamsters housed in a short photoperiod. These findings indicate that AVP within the AH increases aggression in hamsters housed in long photoperiods, but not in hamsters housed in short photoperiods.     

Exogenous vasotocin alters aggression during agonistic exchanges in male Amargosa River pupfish (Cyprinodon nevadensis amargosae)

Pupfishes in the Death Valley region have rapidly differentiated in social behaviors since their isolation in a series of desert streams, springs, and marshes less than 20,000 years ago. These habitats can show dramatic fluctuations in ecological conditions, and pupfish must cope with the changes by plastic physiological and behavioral responses. Recently, we showed differences among some Death Valley populations in brain expression of arginine vasotocin (AVT). As AVT regulates both hydromineral balance and social behaviors in other taxa, these population differences may indicate adaptive changes in osmoregulatory and/or behavioral processes. To test whether AVT is relevant for behavioral shifts in these fish, here we examined how manipulations to the AVT system affect agonistic and reproductive behaviors in Amargosa River pupfish (Cyprinodon nevadensis amargosae). We administered exogenous AVT (0.1, 1, and 10 microg/g body weight) and an AVP V1 receptor antagonist (Manning compound, 2.5 microg/g body weight) intraperitoneally to males in mixed-sex groups in the laboratory. We found that AVT reduced the initiation of aggressive social interactions with other pupfish but had no effect on courtship. The effects of AVT were confirmed in males in the wild where AVT (1 microg/g body weight) reduced the aggressive initiation of social interactions and decreased aggressive responses to the behavior of other males. Combined, these results show that AVT can modulate agonistic behaviors in male pupfish and support the idea that variation in AVT activity may underlie differences in aggression among Death Valley populations.     

Neurobiological correlates premeditated (learned) aggression: seeking new experimental approaches]

Theoretical possibility of experimental modeling of learned (premediated) aggression developing in human after experience of aggression is considered. The sensory contact technique increases aggressiveness in male mice and allows aggressive type of behavior to be formed as a result of repeated experience of victories in daily agonistic confrontations. Some behavioral domains confirm the development of learned aggression in males similar to those in humans. The features are: repeated experience of aggression reinforced by victories; elements of learned behavior after period of confrontations; intent, measured by increase of the aggressive motivation prior agonistic confrontation; decreased emotionality estimated by parameters of open field behavior. Relevant stimuli provoke demonstration of aggression. This review summarized data on the influence of positive fighting experience in daily intermale confrontations on the behavior, neurochemistry and physiology of aggressive mice (winners). This sort of experience changes many characteristics in individual and social behaviors, these having been estimated in different tests and in varied situations. Some physiological parameters are also changed in the winners. Neurochemical data confirm the activation of brain dopaminergic systems and functional inhibition of serotonergic system in winners under influence of repeated experience of aggression. The expression of the neurochemical and behavioral changes observed in winners has been found dependent on the mouse strain and on the duration of their agonistic confrontations. Similarities in mechanisms of learned aggression in humans and mice are considered.     

Gonadal hormones modulate the display of conditioned defeat in male Syrian hamsters

It has been widely reported that gonadal hormones influence the display of aggression in Syrian hamsters; conversely, much less is known about whether gonadal hormones modulate submissive/defensive behaviors in these animals. Following social defeat, male hamsters no longer display normal territorial aggression but instead display submissive/defensive behavior in the presence of a smaller opponent, a phenomenon we have termed conditioned defeat (CD). The purpose of the present study was to examine the effect of gonadal hormones on the display of CD in male hamsters. In Experiment 1, males were castrated or sham-operated. The castrated males were significantly more submissive following social defeat relative to their intact counterparts. The increased submissive behavior in the castrated males during CD testing was particularly surprising, given the fact that they were attacked significantly less during CD training. In Experiment 2a, males were castrated and given hormone replacement. Castrated males treated with testosterone or dihydrotestosterone displayed significantly less submissive behavior following social defeat than did those treated with cholesterol or estradiol. Finally, in Experiment 2b, there was no effect of hormone replacement on aggressive behavior in non-defeated hamsters suggesting that the decrease in submissive behavior in males treated with dihydrotestosterone or testosterone is specific to being previously defeated. Taken together the data indicate that the presence of androgens reduces the display of submission in defeated male hamsters. More importantly, these findings suggest that androgens may have a protective effect against the development of depression-like or anxiety-like behaviors following exposure to an ethologically relevant stressor.     

Postweaning environmental and social factors influencing the onset and expression of agonistic behavior in Norway rats

Two studies were conducted to determine the importance of the postweaning environment and social milieu in regulating the expression of intraspecific aggression in Norway rats. In Experiment 1, male rats were housed either individually or in pairs at 21 days of age. In addition, one-half of the singly housed and paired animals were given experiences with intruders during maturation. At 85 days of age, all animals were given a brief intruder test and then removed from their postweaning environment and provided individually with homecages for a two week period until tested for aggression toward intruders. Results of intruder tests given during maturation indicated agonistic exchanges appeared earlier and more frequently in cages housing a single resident than cages with cohabiting males. However, agonistic exchanges between singly reared residents and intruders had detrimental consequences in adulthood especially under long-term combat situations. That is, although individually reared animals, with early fighting experiences, were capable of initiating intraspecific aggression, such individuals were unable to defend their homecage over a long period of time as evidenced by the high number of wounds and tendency to lose body weight during adult fighting.In Experiment 2, male Norway rats were reared in pairs from 21 days of age and identified as dominant or subordinate on the basis of intracolony social interactions shown during maturation. At 80 days of age, animals were paired with individually reared males in an unfamiliar cage for a 20 day period and examined for agonistic behavior toward intruders at 100 days of age. Group-reared subordinate males exhibited defensive behavior during confrontations with individually reared animals and incurred more wounds and lost more body weight than their cohabiting partner. In addition, subordinate males showed significantly fewer offensive postures toward intruders than individually reared cohabitants. In contrast, group-reared dominant animals did not differ from individually reared males in display of agonistic patterns, in number of wounds, and body weight changes during the period of cohabitation. These findings demonstrate that early rearing factors have pronounced effects on agonistic behavior. Animals experiencing defeat during development are more likely to lose agonistic confrontations in unfamiliar territory than either animals dominant in their early social interactions or animals without the experience of winning or losing agonistic encounters. These results have implications for the understanding of agonistic behavior and predicting outcomes of animal contests, and reveal important differences in agonistic experiences among animals reared in groups.     

Vasopressin differentially modulates aggression and anxiety in adolescent hamsters administered anabolic steroids

Adolescent Syrian hamsters (Mesocricetus auratus) treated with anabolic/androgenic steroids display increased offensive aggression and decreased anxiety correlated with an increase in vasopressin afferent development, synthesis, and neural signaling within the anterior hypothalamus. Upon withdrawal from anabolic/androgenic steroids, this neurobehavioral relationship shifts as hamsters display decreased offensive aggression and increased anxiety correlated with a decrease in anterior hypothalamic vasopressin. This study investigated the hypothesis that alterations in anterior hypothalamic vasopressin neural signaling modulate behavioral shifting between adolescent anabolic/androgenic steroid-induced offensive aggression and anxiety. To test this, adolescent male hamsters were administered anabolic/androgenic steroids and tested for offensive aggression or anxiety following direct pharmacological manipulation of vasopressin V1A receptor signaling within the anterior hypothalamus. Blockade of anterior hypothalamic vasopressin V1A receptor signaling suppressed offensive aggression and enhanced general and social anxiety in hamsters administered anabolic/androgenic steroids during adolescence, effectively reversing the pattern of behavioral response pattern normally observed during the adolescent exposure period. Conversely, activation of anterior hypothalamic vasopressin V1A receptor signaling enhanced offensive aggression in hamsters exposed to anabolic/androgenic steroids during adolescence. Together, these findings suggest that the state of vasopressin neural development and signaling in the anterior hypothalamus plays an important role in behavioral shifting between aggression and anxiety following adolescent exposure to anabolic/androgenic steroids.     

非繁殖期大仓鼠对同种气味的反应和个体间的行为关系

在实验室通过对陌生成年大仓鼠（Ｃｒｉｃｅｔｕｌｕｓｔｒｉｔｏｎ）非繁殖期的气味偏好,性别选择和两鼠间的行为关系的实验研究表明,用Ｙ－型迷宫进行选择测试时,与空白组比较,大仓鼠了同种个体鼠垫的气味,在雌雄巢垫气味之间,雌鼠偏好雄性巢垫的气味,雄鼠不表现性别偏好,与空白组比较,被试鼠选择同种个体;在雌雄个体之间,雄性大仓鼠选择同性个体,雌性不表现性别偏好,在观察箱内,无论同性相遇还是异性相遇,两鼠间攻     

Repeated anabolic-androgenic steroid treatment during adolescence increases vasopressin V(1A) receptor binding in Syrian hamsters: correlation with offensive aggression

Repeated anabolic-androgenic steroid treatment during adolescence increases hypothalamic vasopressin and facilitates offensive aggression in male Syrian hamsters (Mesocricetus auratus). The current study investigated whether anabolic-androgenic steroid exposure during this developmental period influenced vasopressin V(1A) receptor binding activity in the hypothalamus and several other brain areas implicated in aggressive behavior in hamsters. To test this, adolescent male hamsters were administered anabolic steroids or sesame oil throughout adolescence, tested for offensive aggression, and examined for differences in vasopressin V(1A) receptor binding using in situ autoradiography. When compared with control animals, aggressive, adolescent anabolic steroid-treated hamsters showed significant increases (20-200%) in the intensity of vasopressin V(1A) receptor labeling in several aggression areas, including the ventrolateral hypothalamus, bed nucleus of the stria terminalis, and lateral septum. However, no significant differences in vasopressin V(1A) receptor labeling were found in other brain regions implicated in aggressive responding, most notably the lateral zone from the medial preoptic area to anterior hypothalamus and the corticomedial amygdala. These data suggest that adolescent anabolic steroid exposure may facilitate offensive aggression by increasing vasopressin V(1A) receptor binding in several key areas of the hamster brain.     

Changes in social behavior of the male golden hamster accompanying photoperiodic changes in reproduction

Testicular regression and decreased serum testosterone levels result when male hamsters are placed in a short-day photoperiod, and these changes are reversed after reexposure to long-days. The present study was undertaken to determine whether these physiological changes lead to modifications in social behavior in the male hamster. Animals in a nonstimulatory short-day (LD 6:18) or a stimulatory long-day (LD 14:10) photoperiod were observed every 3 weeks for 21 weeks. After 9 weeks, half of the LD 6:18 males were returned to LD 14:10 and showed rapid testicular recrudescence. The other half remained in LD 6:18 and showed a slower rate of testicular recrudescence. Assessment of aggression was accomplished by placing an intruder in the home cage of a test animal and recording latency to attack, decisive encounters, number of attacks, and number of aggressive postures. Dominance was evaluated in a neutral arena with opponents of different weights, using the same parameters. Scent-marking was recorded in response to both bedding from an intact male and bedding from an ovariectomized female in a clean cage. Short-day animals undergoing testicular regression showed heightened levels of aggression and an upward shift in their dominance status. Alteration of the photoperiod did not affect the pattern or number of scent marks. As animals underwent testicular recrudescence, there was a return to lower levels of aggression and to a lower dominance status. Males undergoing rapid testicular recrudescence also showed the most rapid decline in aggressive behaviors. These results suggest that levels of agonistic behavior in the male hamster are inversely related to serum testosterone levels. Scent-marking appears to be unreleated to androgen levels in the male hamster and its role in hamster social behavior remains unclear.     

Gonadal hormones modulate the display of submissive behavior in socially defeated female Syrian hamsters

There are striking differences in the behavioral response to social defeat between male and female Syrian hamsters. Whereas males exhibit a prolonged behavioral response to defeat (i.e., conditioned defeat), many females remain aggressive or show only a transient submissive response following defeat. The current study tested the hypothesis that sex steroids underlie this differential behavioral responsivity to social defeat. Female hamsters were ovariectomized and implanted with Silastic capsules containing estradiol (E(2)), testosterone (T), progesterone (P), dihydrotestosterone (DHT), or a blank capsule (no hormone replacement). After a 3-week recovery period, each subject was placed inside the home cage of a larger, more aggressive female for four 5-min defeat trials. The following day, each animal was tested for conditioned defeat by testing it in its own home cage in the presence of a smaller, non-aggressive intruder. Submissive, aggressive, social, and nonsocial behaviors were subsequently scored. Hamsters receiving E(2) or T displayed significantly lower levels of submissive behavior than did animals receiving P, DHT, or no hormone replacement. There were no significant differences in aggressive behavior among groups. These data suggest that gonadal hormones can influence submissive behavior in female hamsters. Collectively, these results suggest that the sex differences observed in conditioned defeat may, in part, be explained by sex differences in circulating gonadal hormones.