Sex steroids modulate changes in social and sexual preference during juvenile development in zebra finches

Zebra finches, like many other animals, have close social relationships mainly with the family at young ages but begin to express interest in opposite-sex extra-family animals as they enter the late juvenile period and sexual maturity. This experiment tested a set of hypotheses that sex steroids are involved in this developmental transition. At 25-30 days, subjects were implanted subcutaneously with Silastic tubes that were empty (controls), filled with testosterone propionate, filled with estradiol benzoate, or filled with a combination of ATD (an aromatization inhibitor) and flutamide (an anti-androgen). Once a week between ages 30 and 90 days, they were given three-choice tests where the three stimulus types were the family members, unpaired males, or unpaired females. The preferred category was defined as the one adjacent to the proximity zone in which the subject spent the most time. Control males were more likely to prefer females and less likely to prefer the family as they got older, and control females were increasingly likely to prefer males. Males treated with testosterone or estradiol showed a premature increase in preferences for females. Females treated with ATD plus flutamide failed to show the normal increase in preferences for males shown by controls. These results indicate an involvement of sex steroids in the maturation of sexual preferences in a socially monogamous species that relies on visual and auditory, rather than olfactory, cues for sexual or other social behavior.     

The role of sex steroids in courtship, pairing and pairing behaviors in the socially monogamous zebra finch

The purpose of this study was to test whether sex steroid actions are necessary for courtship and pairing in socially monogamous birds. We examined the effects of an aromatase inhibitor, 1,4,6-androstatriene-3,17-dione (ATD), combined with an anti-androgen, flutamide (F), on the behavior and pairing status of initially unpaired male and female zebra finches (Taeniopygia guttata). In the first experiment, 24 adult males were implanted with either a combination of ATD and flutamide or empty implants. Two weeks after implantation, birds were housed in aviaries containing 3 ATD + F males, 3 control males, and 3 females and allowed 2 weeks to pair, with observations 7 times during the 2-week period. A second experiment tested the effects of these same treatments in females. During the first 4 days of testing, ATD + F males were less likely to attack conspecifics than were control males. ATD + F males were also less likely to "greet," or approach, females than were control males, but other courtship behaviors, including directed singing, were unaffected. ATD + F females did not differ from control females on any courtship behavior measured. Furthermore, these treatments did not affect pairing behaviors (time spent clumping or in a nest box together) or the likelihood of pairing with a partner of the opposite sex. ATD + F treatments in females did, however, increase the likelihood of same-sex pairing. This suggests that, although sex steroids may regulate some courtship behaviors in males, they do not regulate pairing behaviors and have little effect on the likelihood that a male or female will be chosen as a mate by a bird of the opposite sex.     

Experimentally altering pre‐breeding sex steroids reduces extra‐pair paternity in female tree swallows

Extra‐pair paternity commonly occurs in many socially monogamous animal species, yet the reasons why females mate with males outside the social pair bond remain poorly understood. Because sex steroids mediate aggressive and sexual behaviors that peak prior to egg laying in female birds, we tested whether they also influence the rate of extra‐pair paternity in nests of female tree swallows (Tachycineta bicolor). In one treatment, we experimentally elevated testosterone (T) using implants containing exogenous T, and in another treatment, we blocked the estrogenic and androgenic actions of T using implants containing 1,4,6‐androstatrien‐3,17‐dione in combination with flutamide (ATD+F). Females with empty implants served as controls. Nests of females treated with T and those with ATD+F were less likely to contain extra‐pair offspring and had a lower proportion of extra‐pair offspring, compared to control females. Treating females with T also delayed clutch initiation dates and disrupted incubation behavior, as found in previous studies, but clutch sizes of T females did not differ from controls. Females treated with ATD+F tended to lay larger clutches than control and T‐treated females, which may be due to their clutch sizes not declining with later initiation dates as found in the T and control treatments; however, after controlling for brood size, ATD+F females produced nestlings that were lighter at day 16 than control females. Although the effect of T on extra‐pair paternity that we observed in tree swallows is consistent with previous studies in female birds, our results demonstrate that blocking the estrogenic and/or androgenic actions of T during pre‐breeding also lowers extra‐pair paternity. Overall, our study suggests that extra‐pair paternity in tree swallows is mediated by sex steroids of females.     

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.     

Sex differences and effects of neonatal aromatase inhibition on masculine and feminine copulatory potentials in prairie voles

Copulatory behaviors in most rodents are highly sexually dimorphic, even when circulating hormones are equated between the sexes. Prairie voles (Microtus ochrogaster) are monomorphic in their display of some social behaviors, including partner preferences and parenting, but differences between the sexes in their masculine and feminine copulatory behavior potentials have not been studied in detail. Furthermore, the role of neonatal aromatization of testosterone to estradiol on the development of prairie vole sexual behavior potentials or their brain is unknown. To address these issues, prairie vole pups were injected daily for the first week after birth with 0.5 mg of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) or oil. Masculine and feminine copulatory behaviors in response to testosterone or estradiol were later examined in both sexes. Males and females showed high mounting and thrusting in response to testosterone, but only males reliably showed ejaculatory behavior. Conversely, males never showed feminine copulatory behaviors in response to estradiol. Sex differences in these behaviors were not affected by neonatal ATD, but ATD-treated females received fewer mounts and thrusts than controls, possibly indicating reduced attractiveness to males. In other groups of subjects, neonatal ATD demasculinized males' tyrosine hydroxylase expression in the anteroventral periventricular preoptic area, and estrogen receptor alpha expression in the medial preoptic area. Thus, although sexual behavior in both sexes of prairie voles is highly masculinized, aromatase during neonatal life is necessary only for females' femininity. Furthermore, copulatory behavior potentials and at least some aspects of brain development in male prairie voles are dissociable by their requirement for neonatal aromatase.     

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.     

Sexual dimorphism in esterified steroid levels in the gastropod Marisa cornuarietis: the effect of xenoandrogenic compounds

Molluscs can conjugate a variety of steroids to form fatty acid esters. In this work, the freshwater ramshorn snail Marisa cornuarietis was used to investigate sex differences in endogenous levels of esterified steroids. Testosterone and estradiol were mainly found in the esterified form in the digestive gland/gonad complex of M. cornuarietis, and males had higher levels of esterified steroids than females (4-10-fold). Additionally, the ability of several xenobiotics, namely tributyltin (TBT), methyltestosterone (MT) and fenarimol (FEN) to interfere with the esterification of testosterone and estradiol was investigated. All three compounds induced imposex - appearance of male sexual characteristics in females. Exposure to TBT led to a decrease in both esterified testosterone (60-85%) and estradiol (16-53%) in females after 100 days exposure, but had no effect on males. Exposure to FEN and MT did not alter levels of esterified steroids in males or in females, although exposed females developed imposex after 150 days exposure. The decrease in esterified steroids by TBT could not be directly linked with a decrease in microsomal acyl-CoA:testosterone acyltransferase (ATAT) activity, which catalyzes the esterification of steroids. In fact, ATAT activity was marginally induced in organisms exposed to TBT for 50 days (1.3-fold), and significantly induced in males and females exposed to MT for 50 days (1.8- and 1.5-fold, respectively), whereas no effect on ATAT activity was observed after 150 days exposure.     

Differential effects of testosterone metabolites in the neonatal period on open-field behavior and lordosis in the rat

Two experiments were done to compare the effects of neonatal exposure to testosterone and its major metabolites, dihydrotestosterone (DHT) and estradiol (E₂), on the development of sex differences in open-field behavior in the rat. In Experiment 1 female rats administered either testosterone propionate (TP), DHT, or estradiol benzoate (EB) were found as adults to have low activity scores, more typical of adult males, when compared to the high scores of oil-treated females. In Experiment 2 the adult open-field behavior of female rats treated neonatally with testosterone or the metabolites was compared to that of male rats treated from Day 1 to 10 of life with the aromatizing enzyme inhibitor, androst-1,4,6-triene-3,17-dione (ATD). These same animals were later tested for lordotic behavior after gonadectomy and priming with EB and progesterone. All male animals and female animals exposed neonatally to testosterone or to either of the metabolites had suppressed open-field activity scores compared to oil-treated females. However, the lordotic behavior of females exposed to DHT and of males exposed to ATD was not defeminized and was comparable to that of oil-treated females. These observations were discussed in terms of a role for the androgenic actions of testosterone in establishing sex differences in nonreproductive behavior in the rat.     

Plasma steroid hormones in relation to behavioral sex role reversal in the spotted sandpiper, Actitis macularia

Plasma samples collected from spotted sandpipers during the reproductive season were analyzed for testosterone, dihydrotestosterone (DHT), estradiol-17 beta and progesterone. Prior to incubation, plasma testosterone and DHT levels were significantly greater in males than in females. Estradiol levels of paired females were significantly greater than those of paired males. Testosterone and DHT levels of unpaired resident and paired males were significantly greater than those of incubating and brooding males. A 25-fold decline in testosterone occurred in males from the 1- or 2-egg stage to the 3-egg stage, when incubation is initiated. In females, testosterone values were low in unpaired, brooding, and transient birds. Paired females had levels 7-fold greater than unpaired birds. In both sexes, there was a strong correlation between testosterone and DHT levels. Prolactin values were negatively correlated with testosterone and DHT in males. These results indicate that the high level of intrasexual competition for mates among female spotted sandpipers is not based upon a total reversal of the normal male/female levels of androgens and estradiol. Territoriality and intense competition for mates in females may be based upon enhanced receptivity of neural centers to moderate hormone levels. Relative changes in testosterone between unpaired and paired females indicates that this hormone may play a role in mate acquisition and territoriality of these sex role-reversed females.     

Sex Steroids and the Differentiation of Avian Reproductive Behavior

Experiments in which avian embryos are treated with sex steroidsor steroid antagonists suggest that sexual differentiation ofreproductive behavior (and thus differentiation of the brainmechanisms for such behavior) is controlled by steroids producedby the embryonic gonads. In chickens and Japanese quail, maleshatched from eggs treated with estradiol or testosterone duringincubation are feminized (demasculinized); they fail to exhibitmasculine sexual behavior as adults, and no longer are behaviorallydistinguishable from females. Some evidence suggests that testosteronemay mimic the feminizing action of estradiol by being convertedto an estrogen in the embryonic brain. Genetic female quailexposed to an antiestrogen during embryonic development aremasculinized; they exhibit an increased ability to display themasculine copulatory pattern. Thus the behavior of these speciesis feminized by embryonic exposure to sex steroids, the anhormonal(neutral) sex for behavioral differentiation appears to be themale, and females appear to result from estrogen produced bythe embryonic ovaries. In contrast, sex steroid treatment ofmammals early in development masculinizes behavior, the femaleis the neutral sex, and males result from fetal androgen secretion.These opposite patterns of psychosexual differentiation in birdsand mammals are correlated with a major difference between theavian and mammalian sex-determining mechanism. Implicationsfor other vertebrates are discussed.     

Aromatization: Important for sexual differentiation of the neonatal rat brain

This paper examines the hypothesis that testosterone (T) produces its differ-entiative effect on the neonatal rat brain after undergoing conversion in situ to estradiol-17β (E₂). We examined the abilities of an aromatizing enzyme inhibitor, (1,4,6-androstatriene-3,17-dione) (ATD), and an anti-estrogen, CI628, to inhibit sexual differentiation. Male and female rats were treated during the first few days of postnatal life with ATD or CI628, and females were treated on the following day with T in Silastic capsules or its propionate (TP) in oil. ATD and ATD+T females were normal with respect to time of vaginal opening, ovarian weight, ability to demonstrate an LH surge, and lordosis behavior. T and TP females were masculinized with respect to all these measures. CI628 and CI628+TP females had impaired ovarian function and intermediate lordosis quotients (LQs) compared to controls, though they were higher in both measures than T females. ATD males demonstrated high LQs in response to estradiol benzoate (EB) + progesterone, comparable to those of control females. CI628 males had intermediate LQs, which were significantly higher than those of control males. These results indicate that ATD can substantially protect the neonatal rat brain from the differentiative effects of exogenous or endogenous T, probably by blocking aromatization. CI628 affords only partial protection against T and produces a weak differentiative effect due to its own weak estrogenicity. Thus, aromatization of T in newborn rat brains appears to be essential if sexual differentiation is to occur.     

Exposure to developing females induces polyuria, polydipsia, and altered urinary levels of creatinine, 17β-estradiol, and testosterone in adult male mice (Mus musculus)

Novel male mice can accelerate reproductive maturation in proximal developing females, an effect mediated by the chemistry of the males' urine. Exogenous estrogens can similarly accelerate female sexual development. In Experiment 1, adult male mice were housed across wire grid from either empty compartments or those containing post-weanling females. Proximity of females caused males to urinate more, progressively over days of exposure, with most urination directed towards females' compartments. Male urine collected after 5 days in these conditions was analyzed by enzyme immunoassay for 17β-estradiol, testosterone, and creatinine. Urinary creatinine of isolated males significantly exceeded that of female-exposed males. Unadjusted urinary steroids also trended toward higher levels in isolates, but creatinine-adjusted estradiol and testosterone of female-exposed males significantly exceeded that of isolated males. In Experiment 2, measurement of water consumption indicated significantly greater drinking by female-exposed as opposed to isolated males. In Experiment 3, males were housed in isolation or beside post-weanling intact (sham-operated) females, ovariectomized females, or intact (sham-operated) males. Male water consumption was elevated in all conditions involving social contact. Urinary creatinine was significantly lower in female-exposed males compared to isolated controls, while unadjusted testosterone was significantly lower in males in all social conditions. Again, creatinine-adjusted estradiol in female-exposed males significantly exceeded that of isolates. These data indicate that adult males drink and urinate more, have more dilute urine, and have a higher ratio of estradiol to creatinine when they are near developing females. These dynamics increase females' exposure to urinary steroids and other urinary constituents that can hasten sexual maturity.     

Persistence of sexual behavior in ovariectomized stumptail macaques following dexamethasone treatment or adrenalectomy

Daily administration over a period of 6 weeks of increasing doses of dexamethasone sodium phosphate (DEX) to seven long-term ovariectomized female stumptail monkeys significantly lowered circulating levels of testosterone without reducing any aspect of the females' sexual behavior or that of their male partners. Since treatment with DEX failed to suppress serum testosterone levels completely an additional experiment was performed in which the sexual behavior of five ovariectomized stumptails was compared before and after bilateral adrenalectomy, combined with chronic administration of both gluco- and mineralocorticoids. Serum levels of both testosterone and estradiol were reduced to very low levels in females after ovariectomy and adrenalectomy, yet no significant depression of females' sexual performance or that of their male partners occurred. Subsequent sc administration of estradiol or estradiol + testosterone in Silastic capsules to ovariectomized, adrenalectomized stumptails had little effect on sexual interaction. In a third experiment five ovariectomized stumptails which initially were relatively unreceptive and unattractive to males were given first testosterone and then testosterone + estradiol sc in Silastic capsules. One of the three indexes of females' receptivity increased significantly after testosterone; however, no other essential aspect of sexual interaction was affected. These findings suggest that sex steroids are normally not required in the female stumptail macaque for activation of preceptive and receptive sexual behaviors or for maintenance of sexual attractivity.     

Faecal sexual steroids in sex typing and endocrine status of great bustards

Faecal sexual steroids have been used in field studies evaluating the relationships between gender and the multiple factors influencing endocrine status of individuals. The determination of faecal steroids has been also proposed as an alternative, non-invasive sexing method when other methods were deemed impractical or risky for the health of birds. In this study, we quantified sexual steroid hormones in faeces of the great bustard (Otis tarda), a large and sexually dimorphic polyginic bird species that it is threatened and subjected to intense wildlife management. We evaluated differences between captivity and wild conditions, flocks and sexes, and used faecal steroids to develop sex determination procedures. We found similar steroid levels in captive and wild bustards, no differences between unisexual wild flocks and clear between-sexes differences in testosterone but not estradiol. Faecal steroids accurately discriminated gender in both captive and wild known-sex great bustards. Total testosterone concentration was always higher than estradiol concentration in faecal samples from males, but estradiol was not always higher than testosterone in females. Faecal steroids failed to reveal the presence of young males in female flocks during winter, despite faecal testosterone levels increased with age in a small sample of captive males. Our results show that faecal steroid measurement for both sexing and characterizing the endocrine status of great bustards is feasible, and therefore it should be valuable in wildlife management, especially in combination with additional information obtained from faeces as diet.     

Simultaneous treatment with an aromatase inhibitor and an anti-androgen decreases the likelihood of dawn song in free-living male great tits, Parus major

Gonadal steroids, most importantly testosterone (T), are considered to be a major factor in the expression of adult song behavior in temperate-zone songbirds. The action of T within specific brain regions involved in the regulation of song may occur either directly, or through its androgenic or estrogenic metabolites. In the present study, we tested steroid-dependence of great tit dawn song by blocking both known pathways of T action by simultaneous implantation of flutamide, an anti-androgen, and ATD, an aromatase inhibitor. By our knowledge, this is the first study investigating the effects of androgen inhibitors on dawn song in free-living birds. Male great tits were implanted during their mate's egg laying stage, being the time of maximal male song activity at dawn. Treatment with ATD and flutamide significantly increased plasma T levels, probably because feedback mechanisms on T secretion were inhibited. The treatment decreased the likelihood of showing dawn song, which is in line with the hypothesis that sex steroids are involved in the endocrine control of song behavior. In males that did show dawn song, we found no evidence for a treatment effect on song quality. Although the implants were present for the larger part of the breeding season, males were able to maintain control of a territory and mate and to complete their brood cycle as successful as control males.     

Testosterone and autumn territorial behavior in male red grouse Lagopus lagopus scoticus

In many bird species, males exhibit territorial aggression outside the breeding season, when testosterone concentrations are low and may not regulate territorial behaviors. The hormonal regulation of aggression at this time of year has only been studied in passerine birds. Here, we investigated the role of testosterone in the regulation of aggression in a non-passerine bird, the red grouse Lagopus lagopus scoticus. Male red grouse are aggressive in early spring when breeding starts, in autumn when they establish territories, and sporadically through much of the winter. We first describe seasonal variations in plasma testosterone concentrations and in the size of males' sexual ornaments, their red combs, which relates to aggressiveness. Testosterone concentrations and comb size were correlated. Both increased in autumn to a peak in October, and then increased again in spring, to a greater peak in early April. Secondly, we experimentally investigated the effects of testosterone, and of an anti-androgen (flutamide) used in combination with an aromatase inhibitor (ATD), on autumn territorial behavior. Males were treated with either empty implants, as controls (C-males), testosterone implants (T-males), or with flutamide and ATD implants (FA-males). One month after implanting, both T- and FA-males had higher concentrations of testosterone than C-males. Comb size, aggressive call rate, and response to playbacks of territorial call all significantly increased in T-males. However, the increase in testosterone in FA-males did not increase comb size or aggressive behavior. In the following spring, after the content of implants was used, FA-males had significantly lower testosterone than C-males, and had a reduced seasonal increase in comb size. The results suggest that testosterone plays a significant role in regulating red grouse aggressive behavior in autumn. However, the observation that flutamide and ATD treatment did not reduce territorial behavior, suggests that estradiol may also be involved in the regulation of non-breeding aggression.     

Sex differences in otoacoustic emissions measured in rhesus monkeys (Macaca mulatta)

Click-evoked otoacoustic emissions (CEOAEs) and distortion-product OAEs (DPOAEs) were measured in about 60 rhesus monkeys. CEOAE strength was substantially greater in females than in males, just as in humans. DPOAE strength was generally slightly stronger in females, just as in humans. In males, CEOAEs were weaker (more masculine) in the fall breeding season and in winter than in the summer. In females, CEOAEs were slightly stronger (more feminine) in the fall, when sex steroids are elevated in females (and males), than in the summer when rhesus monkeys are reproductively quiescent. Thus, the sex differences in CEOAEs were greater in the fall than in the summer. We presume that the seasonal fluctuations in OAEs reflect activational hormonal effects, while the basic sex differences in OAEs likely reflect organizational effects of prenatal androgen exposure. Some monkeys of both sexes had been treated with additional testosterone or the anti-androgen flutamide during prenatal development. In accord with expectations, prenatal androgen treatment weakened CEOAEs in females, and prenatal flutamide treatment strengthened CEOAEs in males. For DPOAEs, the differences between treated and untreated groups were mostly small and often inconsistent. Taken as a whole, the data from both rhesus monkeys and humans suggest that the linear, reflection-based mechanism of OAE production that underlies CEOAEs is more sensitive to prenatal androgenic processes than is the nonlinear distortion mechanism that underlies DPOAEs.     

Testosterone increases singing and aggression but not male-typical sexual partner preference in early estrogen treated female zebra finches

Female zebra finches given estradiol benzoate (EB) as nestlings and testosterone propionate (TP) as adults show masculinized sexual partner preference, preferring females instead of males. This suggests an organizational effect of EB on sexual partner preference in a socially monogamous species that pairs for life. It is not known whether there is an activational hormone effect on sexual partner preference in this species, or whether adult testosterone treatment is necessary for masculinized preference to be expressed. In this experiment females were injected with EB daily for the first 2 weeks posthatching. As adults they were given TP filled or empty implants. Subjects were then given two-choice preference tests with male vs female stimuli, in which singing as well as proximity to the stimuli was recorded, followed by tests in a group aviary for social behavior and pairing preference. Females with TP implants sang more than females with empty implants and were more aggressive toward other females. They did not, however, differ from females with empty implants in any measure of sexual partner preference. Neither group showed a marked preference for males; instead both groups were equally interested in males and females. Thus adult testosterone treatment is not necessary for early estrogen treated females to show a shift in sexual partner preference in the male-typical direction.     

Sex-specific effects of gonadal steroids on conspecific odor preference in the rat

Effects of gonadal steroids on conspecific odor preference for either (1) sexually active male or active female, (2) sexually active or gonadectomized (gdx) males, (3) sexually active or gdx females, and (4) gdx males or gdx females were determined in male and female rats in a three-chamber apparatus. For the first test, gdx females were made sexually active by treatments with estradiol benzoate (EB) and progesterone (P), and sexually active males were selected by prior screening. Sexually active males and females preferred opposite-sex odor over same-sex odor. Odor of sexually active opposite-sex conspecifics was preferred over that of inactive ones. Immediately after the completion of the first test, sexually active males were gdx and females were left without hormonal treatment. Second and third tests were carried out 2 and 5 weeks after the first test. In the second test, gdx males preferred odor of sexually active males rather than that of receptive females (male-directed preference); in the third test, both males and females showed no preference when tested with four stimulus pairs. The final tests were carried out in gdx males with EB and P, and gdx females with 2-week exposure to testosterone (T). Males with EB and P showed a male-directed preference again, whereas T-treated females kept their own female preference. Injection of EB alone to gdx males did not induce any preference. The present study clearly demonstrated sex difference in conspecific odor preference. Although both male and female preferences depend on their circulating sex steroids, the direction of male preference is more susceptible to their hormonal states, compared to that of females.     

The effects of sex steroids and vasotocin on behavioral responses to visual and olfactory sexual stimuli in ovariectomized female roughskin newts

Previous studies have found that vasotocin (AVT) administration to male roughskin newts (Taricha granulosa) enhances courtship clasping as well as appetitive responses to specific sexual stimuli and that treating female newts with androgens plus AVT induces the expression of male-typical courtship clasping (the selective clasping of females). However, the unique and/or interactive effects of sex steroids and AVT on appetitive responses to specific sexual stimuli have not yet been determined. To first identify male-typical, sexually dimorphic appetitive responses to female sexual stimuli, we tested intact newts during the breeding season and found that males, but not females, are attracted to female visual and pheromonal sexual stimuli. We then used ovariectomized (ovx) females implanted with empty silastic capsules (Blk) or with capsules containing testosterone (T), dihydrotestosterone (DHT), or estradiol (E2) and then injected with either saline or AVT to determine the effects of steroids and AVT, alone or in combination with each other, on male-typical behavioral responses to those stimuli. E2 treatment depressed responses toward female visual stimuli independently of AVT. On the other hand, only T-implanted, AVT-injected females displayed male-typical behavioral responses toward female olfactory stimuli, preferring to spend more time in proximity to female-scented than unscented newt models and selectively clasping the female-scented models. Together, these results support the conclusion that sex steroids and AVT influence behavioral responses to sexual stimuli via sensory-specific mechanisms. Furthermore, they suggest that T and AVT interact within the brain to influence sensorimotor processing in the pathways that integrate olfactory sexual stimuli into male-typical courtship behaviors.