Testosterone implanted in the preoptic area of male Japanese quail must be aromatized to activate copulation

Intracranial implantation of minute pellets of gonadal steroids was combined with aromatase inhibitor treatment to determine if aromatization within the preoptic area (POA) is necessary for androgens to activate sexual behavior in the Japanese quail (Coturnix japonica). In this species, implantation of pellets of testosterone propionate (TP) or estradiol benzoate (EB) in the POA of castrated males restores male-typical copulatory behavior. In Experiment 1, adult male castrated quail were implanted intracranially with 200-micrograms pellets of equimolar mixtures of crystalline TP + cholesterol (CHOL), TP + 1,4,6-androstatriene-3,17-dione (ATD, an aromatase inhibitor), EB + ATD, or CHOL and behavior-tested with intact males and females. Copulation was stimulated by POA implants containing TP or EB (three of six CHOL + TP males and two of seven ATD + EB males copulated vs zero of four CHOL males), but copulation was not inhibited by combining ATD with TP (three of four ATD + TP males copulated). In Experiment 2, adult male castrated quail were injected systemically with ATD or oil for 6 days prior to and 14 days after intracranial implantation of 200-micrograms pellets containing the same amounts of TP or EB as in Experiment 1. The ATD injections completely blocked copulatory behavior in males with TP implants in the POA such that ATD/TP and Oil/TP mount frequencies differed significantly, but failed to block copulation in males with EB implants in the POA (proportions of males copulating were ATD/EB, 6/8; ATD/TP, 0/6; Oil/TP, 4/7). The cloacal foam gland, an androgen-sensitive secondary sex character, was unaffected by the dose of ATD used. We conclude that activation of copulatory behavior by TP implants in the POA is not due to nonspecific effects of high local testosterone concentrations but rather to aromatization. These results support the hypothesis that cells within the POA aromatize testosterone to estrogens, which directly stimulate the cellular processes leading to activation of male-typical copulatory behavior.     

Sex differences,laterality, and hormonal regulation of androgen receptor immunoreactivity in rat hippocampus

Sex differences, laterality, and hormonal regulation of androgen receptor (AR) immunoreactivity in rat hippocampal CA1 pyramidal cells were examined using the PG21 antibody. Adult male rats were either castrated or sham-operated at least 2 weeks prior to sacrifice. Gonadally intact females were sacrificed on the day of proestrus. Animals received an injection of either testosterone propionate (TP) or vehicle 2 h prior to sacrifice. Within CA1, both the intensity of staining and the number of AR+ cells were assessed. AR immunostaining was detected in all the groups with marked variation among them. The overall ranking of staining intensity was: gonadally intact males > females given TP > castrated males given TP > females > castrated males given vehicle. The number of AR+cells within subregions of CA1 showed the same basic pattern: among control-treated animals, gonadally intact males have more than females, but castrated males have the least, and acute TP treatment increases the number in both sexes. The increased level of AR immunoreactivity in CA1 of castrated males following acute TP treatment suggests that testicular androgens in adulthood normally increase AR immunoreactivity there, producing a sex difference favoring males in gonadally intact animals. We also found a higher number of AR+ CA1 cells on the left than on the right, but only in gonadally intact males and in females given TP. These results suggest that a laterality of AR distribution in the rat hippocampus may lead to lateralities in hippocampal structure and function.     

Influence of androgen in the neonatal period on ejaculatory and postejaculatory behavior in the rat

The objective of the present report was to investigate the influence of androgen in the neonatal period on the development of ejaculatory and postejaculatory behavior. At birth, male rats were either castrated (neonatally castrated males), implanted with a Silastic tube of the aromatase inhibitor androsta-1,4,6-triene-3,17-dione for the first 10 days (ATD males), or left untreated (normal males). Female rats were either injected with 0.5 mg testosterone propionate (TP) on Days 1 (day of birth) and 2 (androgenized females) or left untreated (normal females). All gonadally intact animals were castrated at 60 days of age. Following TP administration, all animals were tested for ejaculatory and postejaculatory behavior under both shock and nonshock conditions. All animals were capable of showing the intromission pattern; however, the ejaculatory pattern was exhibited regularly only by those animals exposed to androgen at birth (normal males, androgenized females, and ATD males). The normal males required fewer intromissions to achieve ejaculation than the other two groups exhibiting this reflex. This result is discussed in terms of peripheral genital stimulation deficits and the differentiation of neural tissue responsible for masculine copulatory behavior. Androgenized females and ATD males displayed a refractory period, characterized by 22-kHz vocalizations, equal to or longer than that found in normal males. These results indicate that defeminization is not necessary for the display of normal ejaculatory and postejaculatory behavior.     

Effects of neonatal gonadal steroids on adult CA3 pyramidal neuron dendritic morphology and spatial memory in rats

The hippocampus is implicated in spatial cognition, which is sexually dimorphic and developmentally sensitive to gonadal steroids. Previously we have shown a sex difference in CA3 pyramidal cell layer volume and neuronal soma size that was reversible with neonatal castration in males or prenatal treatment of females with either testosterone propionate (TP) or a nonaromatizable androgen, dihydrotestosterone propionate, but not estradiol benzoate, all of which correlated with adult water maze navigation. The present study further investigates developmental androgen sensitivity of CA3 pyramidal neurons by measuring dendritic morphology and its relation to adult spatial ability. Female rats were injected with TP on postnatal day (P) 3 and P5 or ovariectomized (OVX) on P2, and male rats were castrated on P2, with or without testosterone replacement (Cas+T). Sham surgery controls were also included. Animals were tested on a water maze in adulthood, sacrificed, and CA3 pyramidal neurons were Golgi-stained and reconstructed in three dimensions using a computer-interfaced morphometry system. High-androgen groups (control males, Cas+T, TP females) performed better in spatial navigation and exhibited CA3 neurons with longer dendrites, a larger number of dendritic branches, and volumes of influence compared to low-androgen groups (control females, castrated males, OVX). Collectively, these findings indicate that the critical time period for organizational effects of androgens on the CA3 pyramidal neurons includes both prenatal and postnatal life, during which time androgens regulate developmental events such as somal growth and neuronal differentiation, all of which significantly contribute to establishing the sex difference in adult spatial navigation.     

Comparison of the effect of castration on the development of postural and non-postural muscles of mice.

The effect of castration on the development of muscle mass of postural and non-postural muscles was studied in 18 male mice (9 castrated, 9 uncastrated). Results obtained indicated that the castrated males grew faster and were bigger in body size and weight at maturity than the intact males. The bigger body size of castrated males was not due to larger muscle mass but was probably due to increased subcutaneous fat deposition. Atrophy of muscles usually observed following castration was significantly greater in the non-postural (biceps brachii) muscle of the forelimb as compared to the postural (triceps brachii) muscle of the forelimb. Conversely, the amount of reduction in muscle mass was similar in both postural (soleus) and non-postural (tibialis cranialis) muscles of the hindlimb.     

The reproductive pattern of male dusky salamanders (genus Desmognathus) is neither associated nor dissociated

Many seasonally breeding vertebrate species have an associated reproductive pattern: mating behavior, gonadal activity, and peak circulating androgen levels occur simultaneously. In these species, androgens influence the expression of male mating behavior. Other species have a dissociated reproductive pattern: mating behavior occurs at a different time than peak gonadal activity. In such species, it is hypothesized that mating behavior is not dependent on androgen levels [Crews, D., 1984. Gamete production, sex hormone secretion, and mating behavior uncoupled. Horm. Behav. 18, 22-28]. The salamander Desmognathus ochrophaeus mates in the spring and fall while spermatogenesis occurs during the summer, suggesting that it has a dissociated reproductive pattern and that androgens do not mediate mating behavior. To assess whether mating behavior is regulated by gonadal androgens, we castrated males to reduce endogenous androgens and implanted testosterone propionate (TP) to restore androgen levels. Castrated males mated significantly less than did control males. Castrated males given TP mated as much as control males. Compared to controls, circulating androgen levels (both testosterone (T) and dihydrotestosterone (DHT)) were reduced in castrated males and elevated in castrated males given TP implants. We also found that plasma corticosterone (CORT) levels were strongly and positively correlated with T levels. Together, these data indicate that, although spermatogenesis is dissociated in time from mating behavior, androgens are associated with the expression of mating. Thus, the associated-dissociated dichotomy does not adequately describe the reproductive pattern of D. ochrophaeus. We discuss the limitations of the associated-dissociated framework in clarifying hormone-behavior relationships in reptiles and amphibians.     

Quantitation of Satellite Cell Proliferation in Vivo Using Image Analysis

A nonisotopic, double fluorescence technique was developed to study myogenic satellite cell proliferation in posthatch turkey skeletal muscle. Labeled satellite cell nuclei were identified on enzymatically isolated myofiber segments using a mouse monoclonal antibody (anti-BrdU) followed by fluorescein-5-isothiocyanate (FITC) conjugated goat anti-mouse IgG secondary antibody. Myofiber nuclei (myonuclei + satellite cell nuclei) were counterstained with propidium iodide (PI). The myofiber segment length, myofiber segment diameter, and the number of PI and FITC labeled nuclei contained in each segment was determined using a Nikon fluorescence microscope, a SIT video camera and Image-1 software. Data collected by three different operators of the image analysis system revealed 5.0 ± 1.4 satellite cell nuclei per 1000 myofiber nuclei and 5284 ± 462 μm³ of cytoplasm surrounding each myofiber nucleus in the pectoralis thoracicus of 9-week-old tom turkeys. BrdU immunohistochemistry coupled with the new approach of PI staining of whole myofiber mounts is an effective combination to allow the use of an efficient semi-automated image analysis protocol.     

Influences of pre- and postnatal testosterone treatment on defeminization of sexual receptivity in pigs

Sexual receptivity was evaluated in female and male pigs that had experienced varying periods of exposure to testosterone pre- and postnatally. For prenatal exposure, pregnant sows were treated with testosterone propionate (TP) from Day 29-35 or Day 39-45 of gestation at a dosage that caused virilization of the external genitalia of their female offspring. Eighty-three percent of the females that received TP prenatally had regular estrous cycles, but reached puberty later than control females. Only 26% of the females that received TP both pre- and postnatally (4-6 mo of age) were observed in estrus by 10 mo of age. After ovariectomy and acute treatment with estradiol benzoate (EB), the proportion of females that showed the immobilization response (receptivity) was similar for all groups of females independent of pre- or postnatal TP treatment. Females treated prenatally from Day 39-45 showed the immobilization response for fewer days after treatment with a high dosage of EB than did controls. On the basis of these observations, we conclude that receptivity in female pigs is not affected greatly by testosterone treatment at the stages of development that were investigated. Males castrated at birth and treated with a single injection of EB after 9.5 mo showed the immobilization response. In contrast, few males castrated at 8 mo or castrated at birth and treated with TP from 3 to 6 mo showed the immobilization response after EB treatment. These observations provide direct evidence for a postnatal component of testosterone-dependent defeminization of receptivity in male pigs.     

Activation of sexual behavior by implantation of testosterone propionate and estradiol benzoate into the preoptic area of the male Japanese quail (Coturnix japonica)

Intracranial implantation of minute pellets of gonadal steroids was performed to determine neuroanatomical loci at which steroids activate sexual behavior in the Japanese quail (Coturnix japonica). In this species, systemic treatment of castrated males with either testosterone propionate (TP) or estradiol benzoate (EB) restores male-typical copulatory behavior (head grabbing, mounting, and cloacal contact movements). In addition, EB activates female-typical receptive behavior (crouching). Adult male castrated quail were implanted intracranially with 300-micrograms pellets containing TP, EB, or cholesterol (CHOL) and behavior was tested with intact males and females. Either TP or EB pellets in the preoptic area (POA) activated male-typical copulatory behavior. Mounting was specifically activated without concomitant activation of other steroid-sensitive sexual and courtship behaviors. TP and EB implants in adjacent nuclei containing receptors for these steroids and CHOL implants in POA had no effect on male-typical copulatory behavior. Eighteen percent of all males tested for female-typical receptivity crouched, but no specific effect of EB was seen at any site. The similarity of the POA sites for activation of mounting by TP and EB is consistent with the hypothesis that cells within the POA aromatize testosterone to estrogens, which directly stimulate the cellular processes leading to behavioral activation.     

Parental responsiveness is feminized after neonatal castration in virgin male prairie voles, but is not masculinized by perinatal testosterone in virgin females

We previously found a large sex difference in the parental responsiveness of adult virgin prairie voles (Microtus ochrogaster) such that most males are spontaneously parental, whereas most females are not. Because this sex difference is independent of the gonadal hormones normally circulating in adult virgin voles, the present study examined whether perinatal hormones influence the development of this sex difference. Males were treated prenatally (via their pregnant dam) with both the androgen receptor blocker flutamide (5 mg/day/dam) and the aromatase inhibitor ATD (1 mg/day/dam), or oil, for the last 2 weeks of gestation. Half of the subjects from each group were castrated on the day of birth and the other half received a sham surgery. As adults, intact males were castrated and all males received a silastic capsule filled with testosterone. Prenatal treatment with flutamide and ATD had no effect on males' behavior toward pups, but neonatal castration significantly reduced the percentage of males acting parentally. In a second experiment, females were exposed to testosterone propionate (TP; 50 microg/day/dam) or oil via their dam during the last 2 weeks of gestation. For the first neonatal week, half of the females from each group were injected with TP (1 mg/day) and the other half oil. As adults, females were ovariectomized and half from each group received a testosterone-filled capsule and the other half received an empty capsule. None of the perinatal TP treatments increased females' parental responsiveness, although females from all groups that received testosterone capsules as adults were highly parental. Therefore, although postnatal testicular hormones are necessary for high parental responsiveness in males, the behavior of females is not influenced by perinatal exposure to testosterone.     

Threshold for behavioral response to testosterone in old castrated male rhesus macaques

The sexual behaviors of old, intact (N = 5) and old, castrated (N = 6) rhesus macaque males were compared in six series of pair tests with receptive females. The castrated monkeys were tested when untreated and when given five doses of testosterone propionate (TP; 0.004, 0.016, 0.064, 0.256, and 1.024 mg/kg of body weight) in consecutive months. The serum testosterone (T) level was determined for each male before and after each series of tests. When untreated, none of the castrated males ejaculated, and yawning was significantly less in these monkeys than in intact males-no other behavioral measures differed significantly. Within 2 weeks of daily injections of 0.004 mg of TP/kg, two males ejaculated, and all differences in measures of ejaculation were eliminated. A third male ejaculated after 1 week of treatment with 0.016 mg of TP/kg. Yawning values did not differ during and after treatment with 0.064 mg of TP/kg. Although final mean serum T levels were six times higher in castrated (24.3 ng/ml) than in intact males (4.2 ng/ml), sexual performance levels did not exceed those of intact males.     

The influence of androgenic and estrogenic hormones on sexual behavior in castrated adult male pigs

The objectives of these studies were to evaluate the influence of testosterone propionate (TP), estradiol cypionate (EC), dihydrotestosterone propionate (DHTP), EC + TP, EC + DHTP, and TP + DHTP on traits of masculine sexual behavior in castrated adult male pigs of different breeds. Masculine sexual behavior was restored and maintained by TP, whereas EC initially activated sexual behavior, including copulation and ejaculation, but was unable to sustain copulatory behavior for the 8- to 18-week periods that were evaluated. Treatment with DHTP was ineffective for stimulation of sexual behavior; thus, it is suggested that testosterone promotes some aspects of masculine sexual behavior in male pigs via aromatization to estrogen, but both androgen and estrogen are required for maintenance of the full complement of masculine sexual behavior traits.     

Sexual preferences of male hamsters (Mesocricetus auratus) for conspecifics in different endocrine conditions

The behavioral responses of sexually experienced male hamsters toward a pair of anesthetized conspecifics were investigated. Males spent significantly more time licking, sniffing, and mounting neonatally and adult castrated males than intact males. Adult castrated males receiving oil injections were preferred over castrates receiving exogenous testosterone propionate (TP). Ovariectomized females were preferred over intact males, adult castrated males, or spayed females receiving exogenous TP. It was concluded that the absence of an androgen-dependent factor(s) renders an animal more sexually attractive.     

Exercise-induced satellite cell activation in senescent soleus muscle.

The purposes of this study were 1) to determine satellite cell mitotic activity and myofiber nuclear density in the soleus muscle of aged rats and 2) to examine the effect of exercise training on these same parameters. Twenty-four-month-old specific pathogen-free female Fischer 344 rats were assigned to either a training or a control group. The trained group performed 10 wk of progressive treadmill running that resulted in a significant increase (P less than or equal to 0.05) in vastus lateralis muscle malate dehydrogenase activity compared with control rats. Training produced a doubling of soleus muscle satellite cell mitotic activity (trained 1.28 +/- 0.33, control 0.52 +/- 0.13 thymidine-labeled satellite cells per 1,000 nuclei; P less than or equal to 0.05). Training also resulted in a doubling in the number of damaged fibers in the soleus muscle (P less than or equal to 0.05). Mean myofiber nuclear density was unaltered by exercise training but varied as a function of soleus muscle fiber size. Nuclear density of a subpopulation of small fibers (cross-sectional area less than one standard deviation below the mean cross-sectional area of all fibers examined) was significantly higher (P less than or equal to 0.05) than in other fibers in the soleus muscle. A high nuclear density and small size suggest that these fibers were immature. In addition, the soleus muscle from trained rats had significantly more (P less than or equal to 0.05) small fibers with high nuclear density than muscle from control animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of testosterone in the VMN on copulation,partner preference,and vocalizations in male rats

The present study tested whether testosterone propionate (TP) implanted in the ventromedial nucleus (VMN) of the hypothalamus could initiate performance, motivational, or sociosexual components of sexual behavior in castrated male rats. Twenty-seven intact male Long Evans rats were pretested for copulation, partner preference, and 50-kHz vocalization and were subsequently castrated. Approximately 3 weeks after castration, males were retested to confirm that these behaviors had declined, and groups were assigned. Groups 1 and 2 were implanted with bilateral stainless steel cannulae directed at the VMN that were either filled with TP (TVMN group) or remained empty (Blank group). A third group (TSC) was implanted subcutaneously with two 10-mm Silastic capsules filled with testosterone. Restoration of behavior was measured for 2 weeks after implants. We found that copulation and 50-kHz vocalization were not restored by TP in the VMN alone. However, partner preference returned to preoperative levels in both the TVMN and TSC groups, indicating that TP in the VMN was sufficient to restore sexual motivation. Following behavioral testing, prostate glands and seminal vesicles were weighed and confirmed that TP did not leak into the periphery in the TVMN group. Immunostaining for androgen receptors also verified that TP spread was confined to the immediate area surrounding the cannula tip. These results suggest that androgen activation at the VMN is sufficient to induce the motivational components of male sexual behavior, whereas activation of other brain sites is required for copulation and ultrasonic vocalization.     

Critical Periods of Sensitivity of Sexually Dimorphic Spinal Nuclei to Prenatal Testosterone Exposure in Female Rats

Male rats normally have more neurons than do females in two nuclei of the lumbar spinal cord, the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN). Female rats exposed to testosterone propionate (TP) on the 2 days of gestation (Days 18 and 19) when males normally experience a surge in plasma testosterone showed a maximal increase in both SNB and DLN neuronal number. TP exposure just prior to, or following, Days 18 and 19 led to smaller increments. Administration of a small (5 μg) dose of TP after birth, while having no effect by itself, synergized with prenatal TP to enhance the number of SNB neurons. DLN neurons were less responsive to postnatal TP. The somal and nuclear size of SNB, but not DLN, neurons was increased by perinatal TP. Paradoxically, the number of DLN neurons with large somas (1358 μm²or larger) was reduced by perinatal TP, a finding congruent with a previous report that females and feminized males have more of these large DLN neurons than control males. Our data suggest an exquisite sensitivity of the developing spinal nuclei to the timing of hormonal surges normally found in fetal males. Exposure to androgens during a brief prenatal period is needed to assure responsiveness to the low amounts of androgen circulating during neonatal ontogeny, when the process of sexual differentiation is completed.     

Effects of a gonadotrophin-releasing hormone antagonist on gonadotrophin secretion and gonadal development in neonatal pigs

Male (N = 8) and female (N = 8) pigs were assigned to receive saline or a potent GnRH antagonist ([Ac-D2Nal1,D4-Cl-Phe2,D-Trp3,D-Arg6, D-Ala10]- GnRH*HOAc; 1 mg/kg body weight) at 14 days of age. The GnRH antagonist caused LH to decline (P less than 0.01) from 1.7 ng/ml at 0 h to less than 0.5 ng/ml during 4-32 h in males and females. Concentrations of FSH in gilts declined slowly from 75 +/- 8 to 56 +/- 5 ng/ml (P less than 0.05) at 32 h. In males FSH was low (5.7 +/- 0.5 ng/ml) at 0 h and did not change significantly. To observe the effect of long-term treatment with GnRH antagonist, 10 male and 10 female pigs, 3 days of age, were treated with saline or 1 mg GnRH antagonist per kg body weight every 36 h for 21 days. Concentrations of LH were reduced (P less than 0.01) to 0.2-0.4 ng/ml throughout the experimental period in male and female piglets treated with GnRH antagonist. Plasma FSH increased in control females, but remained suppressed (P less than 0.001) in females treated with GnRH antagonist. Treatment with the GnRH antagonist suppressed FSH levels in males on Days 8 and 16 (P less than 0.05), but not on Day 24. Treatment of females with the GnRH antagonist did not influence (P greater than 0.10) oestradiol-17 beta concentrations. Administration of GnRH antagonist to males suppressed testosterone and oestradiol-17 beta values (P less than 0.01) and reduced testicular weight (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Stressor-associated alterations in porcine plasma prolactin

Experiments were conducted to determine effects of restraint and thermal stressors on plasma prolactin (PRL) in castrated male pigs. A single 20-min restraining period in a restraining cage which prevented both movement and injury increased (P less than 0.05) plasma PRL when applied at either 0800 or 1600 hr. Exposure to 32 degrees C at 0800-1000 hr or at 1600-1800 hr produced more moderate increases (P less than 0.05). A combination of 20 min restraint and 2 hr at 32 degrees C produced a response similar to restraint alone. Twenty minutes after stressor application plasma PRL concentrations in pigs exposed to restraint or restraint +32 degrees C at 1600 h were greater (P less than 0.05) than concentrations measured in all other treatment groups at that time interval. However, there were no statistically significant differences in additional quantitative indices of the plasma PRL responses (maximal level, maximal change, or integrated response above basal levels) among restraint, 32 degrees C, or restraint +32 degrees C, nor between morning and afternoon applications of treatment. Such data do not provide, therefore, any strong evidence for stressor-dependent or circadian differences in plasma PRL response. A second study subjected castrated male pigs to 20 degrees C (controls), 20 +/- 12 degrees C (cyclic temperature, sine wave variation), 5 degrees C constant, and 5 +/- 12 degrees C cyclic for 20 days. After 6 days exposure to 5 degrees C constant or 5 +/- 12 degrees C cyclic there were decreases (P less than 0.05) of 59 and 67% respectively in plasma PRL when compared either with pretreatment levels or with levels in pigs at 20 or 20 +/- 12 degrees C. There were no differences in PRL responses between cyclic vs constant temperatures. These results are the first to indicate that plasma PRL in pigs is affected by acute restraint and thermal stressors.     

Sexual differentiation of behavior in the zebra finch: effect of early gonadectomy or androgen treatment

Treatment of nestling zebra finches with estradiol benzoate (EB) has been shown to masculinize singing in females and demasculinize copulatory behavior in males, suggesting that sexual differentiation of these behaviors is under hormonal control such that testicular hormones induce the capacity for song and ovarian hormones suppress the capacity for mounting. Two experiments were carried out to obtain a more complete picture of sexual differentiation in this species. In Experiment 1, nestlings were injected daily for the first 2 weeks after hatching with testosterone propionate (TP), dihydrotestosterone propionate (DHTP), or a combination of DHTP and EB. As adults, birds were gonadectomized and implanted with TP prior to testing, then tested again after implantation with EB. Singing was not increased in females by any of the treatments. The only effect of either TP or DHTP given alone was defeminization of female proceptive behavior by DHTP. Thus androgens appear to have less influence than estrogens on sexual differentiation of behavior in this species. The combination of DHTP and EB demasculinized mounting in males. In Experiment 2, nestlings were gonadectomized at 7-9 days of age and implanted with TP prior to testing in adulthood. Early gonadectomy had little effect on later behavior; early castrated males sang, danced, and copulated normally and early ovariectomized females neither sang nor mounted.     

Effects of androgen on fighting behavior in male and female Mongolian gerbils (Meriones unguiculatus)

Contrary to the results of most other mammalian species studied thus far, castration in infancy or adulthood has been shown to increase the display of intermale aggression in gerbils tested as adults in dyadic encounters. Males castrated in adulthood were divided into two groups: one that received testosterone propionate (TP) treatment and one that did not. A third group of adult males were sham-operated. Infant subjects were either sham-operated or castrated and tested once without and once with TP. Both infant and adult castrates that received no TP treatment demonstrated significantly more fighting behavior than did sham-operates. Adult subjects treated with TP show significantly less aggression than castrates that did not receive TP. Ovariectomized females were also divided into a TP and no-TP group. Females treated with TP showed significantly less aggression than those that had no treatment showing that TP inhibits aggression in both males and females gonadectomized in adulthood. The results are assessed in terms of the importance of perinatal androgen to “organize” adult behavior patterns.