Short-day increases in aggression are inversely related to circulating testosterone concentrations in male Siberian hamsters (Phodopus sungorus)

Affective responses to changes in day length in Siberian hamsters (Phodopus sungorus)

The goal of these experiments was to test the hypothesis that day length influences anxious- and depressive-like behaviors in reproductively photoperiodic rodents. Male and female Siberian hamsters (Phodopus sungorus) were exposed to long (16 h light/day; LD) or short (8 h light/day; SD) photoperiods beginning at the time of weaning (day 18). Two weeks later hamsters were subjected to a series of behavioral tests to quantify anxiety-and depressive-like behaviors. In an elevated plus maze, SD males exhibited longer latencies to enter an open arm, entered fewer open arms, and spent less time exploring open arms relative to LD hamsters. SD males were likewise slower to enter either of the distal arms of a completely enclosed T-maze, and in a hunger-motivated exploratory paradigm SD males were slower to enter an open arena for food as compared to LD males. In a forced-swimming model of behavioral despair, SD males exhibited immobility sooner, more often, and for a greater total amount of time relative to LD males. Total activity levels, aversiveness to light, olfactory function, and limb strength were unaffected by SD, suggesting that the behavioral changes consequent to SD are not attributable to sensory or motor deficits, but rather may arise from changes in general affective state. The anxiogenic and depressive effects of SD were largely absent in female hamsters. Together the results indicate that adaptation to short photoperiods is associated with increased expression of anxiety- and depressive-like behaviors relative to those observed under LD photoperiod conditions.

Short day lengths enhance skin immune responses in gonadectomised Siberian hamsters

In Siberian hamsters and other photoperiodic rodents, exposure to short photoperiods simultaneously inhibits gonadal hormone secretion and enhances some measures of immune function. The present study tested whether gonadal hormones mediate the effects of short days on skin immune function (delayed-type hypersensitivity reactions) in male Siberian hamsters. The magnitude of delayed-type hypersensitivity reactions was greater in hamsters exposed to short days relative to those in long days. Comparable effects of photoperiod were obtained in castrated hamsters bearing empty or testosterone-filled implants. The data suggest that contemporary gonadal hormone secretion is neither necessary, nor sufficient to mediate the effects of short photoperiods on skin immune function.

Adrenal hormones mediate melatonin-induced increases in aggression in male Siberian hamsters (Phodopus sungorus)

Among the suite of seasonal adaptations displayed by nontropical rodents, some species demonstrate increased territorial aggression in short compared with long day lengths despite basal levels of testosterone. The precise physiological mechanisms mediating seasonal changes in aggression, however, remain largely unknown. The goal of the present study was to examine the role of melatonin, as well as adrenal hormones, in the regulation of seasonal aggression in male Siberian hamsters (Phodopus sungorus). In Experiment 1, male Siberian hamsters received either daily (s.c.) injections of melatonin (15 microg/day) or saline 2 h before lights out for 10 consecutive days. In Experiment 2, hamsters received adrenal demedullations (ADMEDx), whereas in Experiment 3 animals received adrenalectomies (ADx); control animals in both experiments received sham surgeries. Animals in both experiments subsequently received daily injections of melatonin or vehicle as in Experiment 1. Animals in all experiments were tested using a resident-intruder model of aggression. In Experiment 1, exogenous melatonin treatment increased aggression compared with control hamsters. In Experiment 2, ADMEDx had no effect on melatonin-induced aggression. In Experiment 3, the melatonin-induced increase in aggression was significantly attenuated by ADx. Collectively, the results of the present study demonstrate that short day-like patterns of melatonin increase aggression in male Siberian hamsters and suggest that increased aggression is due, in part, to changes in adrenocortical steroids.

Photoperiod affects neuronal nitric oxide synthase and aggressive behaviour in male Siberian hamsters (Phodopus sungorus)

Many nontropical animals display physiological and behavioural changes in response to seasonal environmental cues including photoperiod (day length). Male Siberian hamsters (Phodopus sungorus) housed in short photoperiod undergo testicular regression accompanied by reduced circulating testosterone and decreased reproductive behaviour. By contrast to the majority of small mammals studied, aggressive behaviour is elevated in short-day Siberian hamsters when blood testosterone concentrations are not detectable. Because gonadal steroid hormones influence neuronal nitric oxide synthase (nNOS), and this enzyme has been implicated in aggressive behaviour, we hypothesized that nNOS expression would be decreased in short-day male Siberian hamsters and negatively correlated with the display of territorial aggression. Adult male Siberian hamsters were individually housed in either long (LD 16:8 h) or short (LD 8:16 h) photoperiods for 10 weeks. Hamsters were assigned to one of two categories by assessing testicular volume and plasma testosterone values: (i) photoperiodic responsive (i.e. regressed testes and low testosterone concentrations) or (ii) photoperiodic nonresponsive (i.e. testes size and circulating testosterone concentrations equivalent to hamsters maintained in long days). At week 10, aggression was assessed using a resident-intruder test. Latency to initial attack, frequency of attacks and duration of total attacks were recorded during a 10-min aggression trial. Brains were collected immediately after behavioural testing and stained for nNOS expression using immunohistochemistry. All short day-housed hamsters were significantly more aggressive than long-day animals, regardless of gonadal size or testosterone concentrations. Short-day animals, both reproductively responsive and nonresponsive morphs, also had significantly less nNOS-immunoreactive cells in the anterior and basolateral amygdaloid areas and paraventricular nuclei compared to long-day hamsters. Together, these results suggest that seasonal aggression in male Siberian hamsters is regulated by photoperiod, through mechanisms that are likely independent from gonadal steroid hormones.

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.

Photoperiodic regulation of circulating leukocytes in juvenile Siberian hamsters: mediation by melatonin and testosterone

The reproductive system of Siberian hamsters (Phodopus sungorus) undergoes rapid phenotypic responses to changes in day length that occur around the time of weaning. The present experiments tested whether the immune system of Siberian hamsters is similarly photoperiodic early in life and whether photoperiodic changes in melatonin or gonadal hormone secretions mediate any such responses to day length. Circulating blood leukocyte concentrations (WBC) were measured in juvenile male Siberian hamsters that were gestated in long-days (LD), transferred to short-days (SD) on the day of birth, and subsequently either remained in SD or were transferred from SD to LD at 18 days of age (day 18). WBC values were comparable between LD and SD hamsters on day 18. Between day 18 and day 32, SD hamsters exhibited a 3-fold increase in WBC, whereas LD hamsters failed to undergo a significant increase in WBC during this interval. WBC of LD hamsters was significantly lower than that of SD hamsters on day 25 and on day 32. In LD housed males, peripheral injections of melatonin delivered so as to extend the nocturnal duration of elevated endogenous melatonin secretion (i.e., provided in late afternoon) on days 18-31 increased WBC as measured on day 32. Peripubertal (day 17) gonadectomy abolished the immunosuppressive effect of LD exposure on WBC, and treatment with silastic implants containing testosterone suppressed WBC independent of photoperiod treatment. These data indicate that juvenile Siberian hamsters are immunologically responsive to photoperiod and that the leukocyte responses to day length are the result of melatonin-mediated effects of photoperiod on testicular hormone secretion.

N-methyl-D-aspartate receptors in the amygdala are necessary for the acquisition and expression of conditioned defeat

Here, we describe a biologically relevant model called conditioned defeat that is used to examine behavioral responses to social defeat in Syrian hamsters. In this model experimental animals that are normally aggressive experience social defeat and consequently display high levels of submissive/defensive behavior even in response to non-threatening conspecifics. N-methyl-D-aspartate (NMDA) receptors within the amygdala play an important role in conditioned fear; therefore, the purpose of this study was to examine whether NMDA receptors within the amygdala are necessary for the acquisition and expression of conditioned defeat. Specifically, the present study examined whether bilateral infusions of the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5; 0.625, 1.25, 2.5, 5.0, 10.0 microg) into the amygdala would block the acquisition of conditioned defeat. Subsequently, we examined whether bilateral infusions of AP5 (0.625, 1.25, 2.5, 5.0 microg) into the amygdala prior to testing would block the expression of conditioned defeat. Infusions of AP5 into the amygdala immediately before the initial social defeat significantly reduced submissive/defensive behavior when hamsters were tested the following day with a non-aggressive intruder. Similarly, infusions of AP5 into the amygdala immediately before exposure to a non-aggressive intruder significantly attenuated the display of submissive/defensive behavior. These data demonstrate that NMDA receptors are necessary for both the acquisition and expression of conditioned defeat. We believe that conditioned defeat is a unique and valuable animal model with which to investigate the neurobiology of fear-related changes in social behavior.

Long-term persistence of male copulatory behavior in castrated and photo-inhibited Siberian hamsters

Gonadal steroids are essential for the long-term maintenance of the full repertoire of sexual behavior in male rodents. Typically, all individuals of several species cease to display the ejaculatory reflex within a few weeks of castration. The present study documents the persistence of the ejaculatory reflex 19 weeks after orchidectomy in 40% of male Siberian hamsters maintained in long or short day lengths; testosterone was undetectable in the circulation of these animals. Intact hamsters transferred from a long to a short photoperiod underwent gonadal regression: 50% of these animals continued to display mating behavior culminating in ejaculation throughout 25 weeks of testing. The remaining animals failed to ejaculate after approximately 11 weeks of short day treatment but resumed mating coincident with spontaneous gonadal recrudescence. Activation of sex behavior in the latter cohort appears to depend on gonadal steroids and is in contrast to the copulatory behavior of the substantial proportion of the study population that sustains the full sexual repertoire in the long-term absence of gonadal steroids. Sex behavior of the latter animals may be dependent on nongonadal steroids or mediation by steroid-independent mechanisms.

Peripubertal Immune Challenges Attenuate Reproductive Development in Male Siberian Hamsters (Phodopus sungorus)1

Differential allocation of energy to reproduction versus host defense is assumed to drive the seasonal antiphase relation between peak reproductive function and immunocompetence; however, evidence supporting this assumption is only correlational. These experiments tested whether photoperiod affects immune responses to antigens in peripubertal Siberian hamsters, whether such activation of the immune system exacts energetic and reproductive costs, and whether such costs vary seasonally. Male Siberian hamsters were raised from birth in long (LD) or short days (SD), which respectively initiate or inhibit the onset of puberty. To elicit a specific immune response, hamsters were injected with a novel antigen (keyhole limpet hemocyanin [KLH]) as juveniles. Reproductive development was attenuated and body temperature was elevated in LD hamsters relative to saline-injected control animals. In contrast, KLH treatments affected neither thermoregulation nor reproductive development in photoinhibited SD hamsters. In experiment 2, juvenile male hamsters were challenged with bacterial lipopolysaccharide (LPS) in order to elicit an innate immune response. Febrile and anorexic responses to LPS were greater in reproductively stimulated LD hamsters relative to reproductively inhibited SD hamsters. LPS treatments attenuated somatic and testicular development in LD hamsters, but did not significantly affect circulating testosterone concentrations. In contrast, LPS treatments were without effect on somatic and reproductive development in SD hamsters. These experiments indicate that photoperiod affects antigen-specific antibody production, febrile responses to LPS, and sickness behaviors in juvenile Siberian hamsters, and that peripubertal activation of the immune system exacts energetic and metabolic costs that can diminish the magnitude of somatic and reproductive maturation in LD. The data also underscore the importance of seasonally dependent life history factors in assessing physiological tradeoffs.

Opposing hormonal mechanisms of aggression revealed through short-lived testosterone manipulations and multiple winning experiences

Territorial aggression is influenced by many social and environmental factors. Since aggression is a costly behavior, individuals should account for multiple factors such as population density or reproductive status before engaging in aggression. Previous work has shown that male California mice (Peromyscus californicus) respond to winning aggressive encounters by initiating aggression more quickly in future encounters, and we investigated the physiological basis for this effect. We found that injections that produced a transient increase in testosterone (T) following an aggressive encounter caused males to behave more aggressively in an encounter the following day. Experience alone was not enough to change aggression, as males treated with saline injections showed no change in aggression. The effect of T injections on aggression was androgen-based, as the inhibition of aromatase did not block the T injections from increasing aggression. Aromatase inhibition did, however, increase aggression in the initial aggression tests (before application of T or saline injections), and aromatase activity in the bed nucleus of the stria terminalis (BNST) was negatively correlated with aggression. A previous study suggested that aromatase activity in the BNST decreases after males become fathers. Thus, distinct neuroendocrine mechanisms allow male California mice to adjust aggressive behavior in response to changes in social and reproductive status.

Sex differences in photoperiodic and stress-induced enhancement of immune function in Siberian hamsters

Siberian hamsters breed during the long days of spring and summer when environmental conditions (e.g., ambient temperatures, food availability) are favorable for reproduction. Environmental conditions may also influence the onset and severity of infection and disease, and photoperiodic alterations in immune function may comprise part of a repertoire of seasonal adaptations to help survive winter. In order to test the hypothesis that animals use day length to anticipate seasonal stressors and adjust immune function, we measured antigen-specific delayed-type-hypersensitivity (DTH) responses in the skin of male and female hamsters during long, "summer-like," or short, "winter-like" days, at baseline and following acute restraint stress. Sex steroid hormones were lower, and cortisol was higher, in males and females during short days. Baseline DTH was enhanced in short- compared to long-day males, and acute stress augmented this effect. In contrast, photoperiod alone did not influence the DTH response in females. As predicted, female hamsters exhibited significantly higher DTH responses than males during long days, but not during short days. However, this enhancement was observed in acutely stressed females only. Cortisol concentrations were significantly higher at baseline in females, and increased more in response to stress, compared to males in both photoperiods. These results suggest that photoperiod provides a useful cue by which stressors in the environment may be anticipated in order to adjust immune function. Furthermore, interactions among reproductive status and stress responses appear to mediate the expression of sex differences in immune responses in hamsters.

Photoperiod alters the time course of brain cyclooxygenase-2 expression in Siberian hamsters

Fever is initiated by activation of the arachidonic acid cascade and the biosynthesis of prostaglandins within the brain. Inducible cyclooxygenase (COX-2) is a rate-limiting enzyme in prostaglandin synthesis, and the number of blood vessels expressing COX-2 correlates with elevated body temperature following peripheral lipopolysaccharide (LPS). Despite its importance in host defense, fever is energetically expensive and we hypothesized that fever may be limited by available metabolic resources. During winter, when competing metabolic demands are constrained by low temperatures and food availability, it was predicted that fever duration would be reduced in seasonally breeding Siberian hamsters (Phodopus sungorus). We measured LPS-induced COX-2 expression in blood vessels of hamsters to test whether photoperiodic alterations in fever duration are centrally mediated, or whether they reflect changes in peripheral modulation of body temperature. Hamsters housed in long, 'summer-like' or short, 'winter-like' day lengths for 10 weeks were injected with LPS, and brains were collected 2, 4, or 8 h later. COX-2 expression was comparably increased in long- and short-day hamsters by 2 h and 4 h post-LPS; however, short-day hamsters exhibited significantly fewer COX-2-positive cells and blood vessels by 8 h post-LPS compared to long-day hamsters, corresponding with reduced fever duration in short-day hamsters. Cortisol concentrations increased more than two-fold in short-day compared to long-day hamsters by 4 h; this increase may have contributed to the decrease in COX-2 expression observed by 8 h in short days. We conclude that short photoperiods significantly altered the time course of central COX-2 protein expression in hamsters in a manner consistent with reduced fever duration.

Brain aromatase, 5 alpha-reductase, and 5 beta-reductase change seasonally in wild male song sparrows: relationship to aggressive and sexual behavior

In many species, territoriality is expressed only during the breeding season, when plasma testosterone (T) is elevated. In contrast, in song sparrows (Melospiza melodia morphna), males are highly territorial during the breeding (spring) and nonbreeding (autumn) seasons, but not during molt (late summer). In autumn, plasma sex steroids are basal, and castration has no effect on aggression. However, inhibition of aromatase reduces nonbreeding aggression, suggesting that neural steroid metabolism may regulate aggressive behavior. In wild male song sparrows, we examined the neural distribution of aromatase mRNA and seasonal changes in the activities of aromatase, 5 alpha-, and 5 beta-reductase, enzymes that convert T to 17 beta-estradiol, 5 alpha-dihydrotestosterone (5 alpha-DHT, a potent androgen), or 5 beta-DHT (an inactive metabolite), respectively. Enzyme activities were measured in the diencephalon, ventromedial telencephalon (vmTEL, which includes avian amygdala), caudomedial neostriatum (NCM), and the hippocampus of birds captured during spring, molt, or autumn. Aromatase and 5 beta-reductase changed seasonally in a region-specific manner. Aromatase in the diencephalon was higher in spring than in molt and autumn, similar to seasonal changes in male sexual behavior. Aromatase activity in the vmTEL was high in both spring and autumn but significantly reduced at molt, similar to seasonal changes in aggression. 5 beta-Reductase was not elevated during molt, suggesting that low aggression during molt is not a result of increased inactivation of androgens. These data highlight the relevance of neural steroid metabolism to the expression of natural behaviors by free-living animals.

Testosterone, puberty, and the pattern of male aggression in Syrian hamsters

The interaction between testosterone and pubertal development on aggression in the male Syrian hamster (Mesocricetus auratus) was examined in two experiments. First, gonad-intact prepubertal and adult male hamsters were tested for aggression using the resident-intruder paradigm with an age- and weight-matched intruder. Prepubertal males engaged in a greater number of attacks and had longer attack durations than adults. In the second experiment, prepubertal and adult males were castrated and treated with either a 0-, 2.5-, or 5-mg pellet of testosterone, and their aggression was assessed using the resident-intruder paradigm. Prepubertal males again showed shorter attack latencies and greater attack durations compared to adult males across all levels of testosterone. Testosterone treatment did not significantly affect these behaviors before or after pubertal development. We conclude that (a) pubertal development is associated with a decline in aggression in the male Syrian hamster, and (b) a developmental shift in behavioral sensitivity or responsiveness to testosterone does not underlie the pubertal decrease in aggression.

Short days and exogenous melatonin increase aggression of male Syrian hamsters (Mesocricetus auratus)

Many nontropical rodent species rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behavior. Among these changes, some species of rodents demonstrate increased aggression in short, "winter-like" compared with long "summer-like" day lengths. The precise neuroendocrine mechanisms mediating changes in aggression, however, remain largely unknown. The goal of the present study was to examine the effects of photoperiod and exogenous melatonin on resident-intruder aggression in male Syrian hamsters (Mesocricetus auratus). In Experiment 1, male Syrian hamsters were housed in long (LD 14:10) or short (LD 10:14) days for 10 weeks. In Experiment 2, hamsters were housed in long days and half of the animals were given daily subcutaneous melatonin injections (15 microg/day in 0.1 ml saline) 2 h before lights out for 10 consecutive days to simulate a short-day pattern of melatonin secretion, while the remaining animals received injections of the vehicle alone. Animals in both experiments were then tested using a resident-intruder model of aggression and the number of attacks, duration of attacks, and latency to initial attack were recorded. In Experiment 1, short-day hamsters underwent gonadal regression and displayed increased aggression compared with long-day animals. In Experiment 2, melatonin treatment also increased aggression compared with control hamsters without affecting circulating testosterone. Collectively, the results of the present study demonstrate that exposure to short days or short day-like patterns of melatonin increase aggression in male Syrian hamsters. In addition, these results suggest that photoperiodic changes in aggression provide an important, ecologically relevant model with which to study the neuroendocrine mechanisms underlying aggression in rodents.

Melatonin regulates energy balance and attenuates fever in Siberian hamsters

Fever is considered an important host defense response but requires significant metabolic energy. During winter many animals must balance immune function with competing physiological demands (i.e. thermoregulation) to survive. Winterlike patterns of melatonin secretion induce a number of energy-saving adaptations. For instance, Siberian hamsters attenuate the duration of fever during simulated short winter day lengths, presumably to conserve energy. To determine the proximate role of melatonin in mediating this photoperiodic response, hamsters housed in long days were injected with saline or melatonin 4 h before lights off for either 1 or 6 wk and assessed for fever following injections of bacterial lipopolysaccharide. Fever duration was attenuated (32%) only in hamsters that decreased body mass, increased cortisol, and exhibited gonadal regression in response to 6 wk of melatonin. Because melatonin-treated hamsters lost significant body mass, fever was assessed in a second long-day group following ad libitum food intake, food restriction, or 24-h food deprivation. Food restriction sufficient to reduce body mass by approximately 25%, but not to reduce leptin, did not influence fever, and 24-h food deprivation virtually abolished fever. Our data suggest that long-term exposure to long-duration melatonin signals is required to induce the physiological changes necessary for short-day immune responses, perhaps involving interactions with hormones such as cortisol and leptin.

Short day lengths augment stress-induced leukocyte trafficking and stress-induced enhancement of skin immune function

Environmental conditions influence the onset and severity of infection and disease. Stressful conditions during winter may weaken immune function and further compromise survival by means of hypothermia, starvation, or shock. To test the hypothesis that animals may use photoperiod to anticipate the onset of seasonal stressors and adjust immune function, we evaluated glucocorticoids and the distribution of blood leukocytes in Siberian hamsters (Phodopus sungorus) exposed to long day lengths (i.e., summer) or short day (SD) lengths (i.e., winter) at baseline and during acute stress. We also investigated the influence of photoperiod and acute stress on a delayed-type hypersensitivity response in the skin. SDs increased glucocorticoid concentrations and the absolute number of circulating blood leukocytes, lymphocytes, T cells, and natural killer cells at baseline in hamsters. During stressful challenges, it appears beneficial for immune cells to exit the blood and move to primary immune defense areas such as the skin, in preparation for potential injury or infection. Acute (2 h) restraint stress induced trafficking of lymphocytes and monocytes out of the blood. This trafficking occurred more rapidly in SDs compared to long days. Baseline delayed-type hypersensitivity responses were enhanced during SDs; this effect was augmented by acute stress and likely reflected more rapid redistribution of leukocytes out of the blood and into the skin. These results suggest that photoperiod may provide a useful cue by which stressors in the environment may be anticipated to adjust the repertoire of available immune cells and increase survival likelihood.

Dehydroepiandrosterone (DHEA) increases territorial song and the size of an associated brain region in a male songbird

In many species, male territorial aggression is tightly coupled with gonadal secretion of testosterone (T). In contrast, in song sparrows (Melospiza melodia morphna), males are highly aggressive during the breeding (spring) and nonbreeding (autumn and early winter) seasons, but not during molt (late summer). In aggressive nonbreeding song sparrows, plasma T levels are basal (< or = 0.10 ng/ml), and castration has no effect on aggression. However, aromatase inhibitors reduce nonbreeding aggression, indicating a role for estrogen in wintering males. In the nonbreeding season, the substrate for brain aromatase is unclear, because plasma T and androstenedione levels are basal. Aromatizable androgen may be derived from plasma dehydroepiandrosterone (DHEA), an androgen precursor. DHEA circulates at elevated levels in wintering males (approximately 0.8 ng/ml) and might be locally converted to T in the brain. Moreover, plasma DHEA is reduced during molt, as is aggression. Here, we experimentally increased DHEA in wild nonbreeding male song sparrows and examined territorial behaviors (e.g., singing) and discrete neural regions controlling the production of song. A physiological dose of DHEA for 15 days increased singing in response to simulated territorial intrusions. In addition, DHEA treatment increased the volume of a telencephalic brain region (the HVc) controlling song, indicating that DHEA can have large-scale neuroanatomical effects in adult animals. The DHEA treatment also caused a slight increase in plasma T. Exogenous DHEA may have been metabolized to sex steroids within the brain to exert these behavioral and neural effects, and it is also possible that peripheral metabolism contributed to these effects. These are the first results to suggest that exogenous DHEA increases male-male aggression and the size of an entire brain region in adults. The data are consistent with the hypothesis that DHEA regulates territorial behavior, especially in the nonbreeding season, when plasma T is basal.