The relationship between circulating testosterone levels and male sexual behavior in rats

The relationships between plasma testosterone (T) and various parameters of male sexual behavior were examined in intact and castrated T-treated male rats. Repeated blood sampling and behavioral testing revealed no correlation between any measure of sexual behavior and plasma T in normal untreated sexually active males. T-Filled Silastic capsules, implanted subcutaneously at the time of castration, were found to produce plasma T levels proportional to capsule size. Plasma T titers less than 10% of normal (0.2 ng/ml) maintained ejaculatory behavior near normal levels for the 58 days of the experiment. Measures of sexual behavior which showed androgen dependence were intromission latency, postejaculatory interval, and intromission frequency. The plasma T concentration required to maintain these parameters within the intact range was 0.7 ng/ml, which is less than one-third of the mean intact level (2.6 ng/ml). No significant improvement in the sex behavior measures was seen with plasma T levels between 0.7 and 3.1 ng/ml. It was concluded that the absence of relationships between circulating T and sexual behavior in the normal rat population is due to the androgen requirement for this behavior being less than the amount normally present. Findings on T levels and T treatment in noncopulator males are also presented.     

Comparing the relative amount of testosterone required to restore sexual arousal, motivation, and performance in male rats

It is well established that male rat reproductive behaviors including sexual arousal, motivation, and performance are dependent on circulating levels of testosterone (T). The present study was designed to (1) compare the relative amount of T required to restore these different aspects of behavior in castrated rats, and (2) create an animal model for clinical populations with sexual impairments. Twenty-nine male Long–Evans rats were tested before and after castration for sexual performance (copulation), motivation (partner preference), and arousal (50 kHz ultrasonic vocalizations; measured together with scent marking). Sexual arousal was also inferred from copulation data. Rats were then assigned to one of four groups, and T was re-introduced via Silastic capsule implants varying in length and content: No T (empty capsules), Low T (2 mm capsules), Medium T (5 mm capsules), or High T (two 10 mm capsules). The highest dose was intended to restore physiological levels. Results indicate that High T is required for 50 kHz vocalizations, while Medium T was sufficient for the restoration of copulation, partner preference, and scent marking. These data suggest that sexual arousal may be most sensitive to reductions in testosterone. The role of T levels in measures of generalized and specific (sexual) arousal is discussed in the context of other reproductive behaviors. Furthermore, because the Low T group showed impairments across all behaviors during post-implant tests, we propose that these animals may provide a good animal model for studying clinical conditions marked by reduced motivation and arousal, including Hypoactive Sexual Desire Disorder.     

Influence of environmental events immediately after birth on postnatal testosterone secretion and adult sexual behavior in the male rat

A rise in plasma testosterone (T) levels occurs in male rats during the first 2 hr after birth which is of importance for the process of sexual differentiation. To study the influence of environmental factors on the postnatal T surge and sexual development, newborn male rats were subjected to various treatments immediately after cesarean delivery including cooling, ether anesthesia, and mother-infant separation. In adulthood, the animals were observed for masculine and feminine sexual behavior. Males anesthetized at 0 hr showed elevated levels of feminine sexual behavior and impaired masculine sexual behavior. Pups subjected to cooling or mother-infant separation showed slightly prolonged intromission latencies, but otherwise normal levels of feminine sexual behavior. Significantly elevated plasma T levels were found in intact pups 2 hr after birth but not in pups subjected to cooling or ether anesthesia. Significantly higher levels of T were observed in pups subjected to cooling 4 hr after birth, suggesting a delay of the T surge. The most pronounced impairing effects were seen in the defeminization process, but the masculinization process also is affected by ether anesthesia. It was concluded that ether anesthesia immediately after birth may permanently interfere with the sexual development by suppressing the neonatal T surge.     

Developmental effects of testosterone on behavior in male and female green anoles (Anolis carolinensis)

This study addressed the role of testosterone (T) in the development of sexually dimorphic behavior in the green anole lizard, Anolis carolinensis. We documented the pattern of endogenous T concentrations during ontogeny and we determined the behavioral effects of experimentally elevated T in juvenile males and females. T concentrations were measured in the plasma of hatchlings from eggs incubated in the laboratory, in juveniles of all sizes sampled in the field, and in the yolks of freshly laid eggs in the laboratory and were compared to plasma T in adult females (measured in this study) and adult males. There were no sex differences in plasma T in hatchling and small juvenile (<26-mm snout-vent length, SVL; <14 days old) males and females, concentrations of which in both sexes tended to decline over the 14-day posthatching period. Plasma T sharply increased in juvenile males, but not females, after approximately 14 days posthatching (>25-mm SVL), and it became significantly higher after approximately 38 days posthatching (>30-mm SVL). Plasma T for juvenile males was within the range detected in breeding adult females, but it was 20- to 45-fold lower than that of adult males, breeding or postbreeding. All eggs contained detectable yolk T, but eggs that gave rise to males contained nearly twice as much yolk T as those that gave rise to females. We do not know whether this yolk T comes from the mother, embryo, or both. In behavior trials conducted in the laboratory, juveniles (36- to 42-mm SVL) with T implants, regardless of whether they were male or female, had increased activity levels compared to juveniles with blank implants, due to increased rates of nearly every behavior monitored. These results are discussed in the context of the organization-activation theory of sexual differentiation and the particular life history of A. carolinensis.     

Sexual behavior in developing male rats

During normal development, the onset of reproductive behavior in male rats was not preceded by any change in plasma testosterone (T) levels. Implantation of Silastic capsules containing T in 14-day-old male rats advanced the onset of all parameters of sexual behavior by 20 days. Implantation of Silastic capsules containing estradiol in 14-day-old male rats stimulated precocious mounting and intromitting, but not ejaculation. Implantation of dihydrotestosterone-filled Silastic capsules in 14-day-old male rats completely inhibited the development of sexual behavior. All hormones suppressed plasma LH levels. These findings in immature male rats are similar to previous findings in adult males. Immature male rats were behaviorally less responsive to T than adult males, and it was suggested that, during development, male rats become progressively more sensitive to the behavior-stimulating effects of circulating T. No effects of copulatory experience on plasma concentration of T or on the weights of testes, penes, or accessory sexual glands were detected.     

Adrenal contribution to the induction of sexual behavior in the female musk shrew.

Sexually experienced female musk shrews (Suncus murinus) lack an ovarian, vaginal, and behavioral estrous cycle. Females, once induced by their initial contact with a male, are able to display copulatory behavior whenever a male is present (Rissman, Silveira, and Bronson, 1988). Based on plasma levels of steroids, and on hormone replacement studies conducted after ovariectomy (OVX), we have shown that testosterone (T) plays an essential role in the regulation of female sexual behavior (Rissman and Crews, 1988; Rissman, Clendenon, and Krohmer, 1990a; Rissman, 1991). To date we have only examined the potential contribution of adrenal steroids to female sexual behavior in a preliminary manner. After adrenalectomy, gonadally intact females display significantly lower levels of sexual behavior than controls (Rissman and Bronson, 1987). The following experiments were conducted to examine the role the adrenal steroids (in contrast to the medullary hormones) play in the induction of female sexual behavior in the musk shrew. In the first experiment gonadally intact females were treated with dexamethasone (DEX) to reduce the secretion of adrenal steroids. Significantly fewer females receiving DEX demonstrated sexual behavior as compared with controls. In the second study, OVX females received T-filled Silastic implants. When DEX was administered to OVX + T females at a dose that dropped circulating T levels to those found in ovary and adrenal intact females, no effect on sexual behavior was noted. The data show that the adrenals are a behaviorally important source of T and contribute toward the hormonal control of sexual behavior in these female mammals.     

Endocrine control of sexual behavior in sneaker males of the peacock blenny Salaria pavo: effects of castration, aromatase inhibition, testosterone and estradiol

The effects of castration and sex steroid manipulations on the expression of sexual behavior were investigated in a small fish, the peacock blenny, Salaria pavo. In this species, large males defend nests and attract females while small "sneaker" males reproduce by imitating the female morphology and courtship behavior in order to approach nests during spawning events and parasitically fertilize eggs. Sneakers switch into nest holders in their second breeding season, thus displaying both male and female-like sexual behavior during their lifetime. We tested the effects of castration and of an aromatase inhibitor (Fadrozole, F), testosterone (T) or 17beta-estradiol (E(2)) implants on the expression of male and female-like behavior in sneakers. Sneakers were either sham-operated, castrated or castrated and implanted with vehicle, F, T+F or E(2)+F. Seven days after the treatment, sneakers were placed in a tank with a nesting male, two ripe females and an available nest. Castrated fish had lower levels of circulating T and increased the time spent displaying female typical nuptial coloration. T implants had the opposite effect, inhibiting the expression of female-like behavior and coloration. E(2) implants had no significant effect on the display of sexual behavior but the frequency of aggressive displays decreased. The results agree with previous findings in sneakers of S. pavo that demonstrated an inhibition of female-like behavior by 11-ketotestosterone (11-KT). The reported increase in T and 11-KT production when sneakers change into nest holders may thus contribute to behaviorally defeminize sneakers. Contrarily, both T and E(2) failed to promote male-like behavior, suggesting that behavioral masculization during tactic switching depends on other neuroendocrine mechanisms or that the time length of the experiment was insufficient to induce male-like behavioral changes in sneakers.     

Social influences on plasma testosterone levels in male talapoin monkeys

Three social groups of laboratory-housed talapoin monkeys (Miopithecus talapoin) consisting of four adult males and four or five adult females, were observed over a 4-year period. All females were ovariectomized and given estradiol implants at intervals to render them sexually attractive; except for two castrated males with testosterone implants, all males were intact. Sexual and aggressive interactions were recorded, and testosterone levels were measured in plasma taken from males twice weekly. In each group males formed a linear dominance order, defined in terms of the direction of aggression between animals. The hormonal responses of intact males were monitored with respect to the presence of attractive females, access to these females, and transfer from the social group to isolation. In all groups the behavioral and endocrine responses of males to these treatments were rank related. In some instances, rising in rank was associated with elevated testosterone and falling in rank with decreased testosterone; these hormonal changes were associated with changes in sexual and aggressive interactions. The effects of sexual and aggressive behavior on plasma testosterone titers are discussed.     

Sexual deprivation and its influence on testosterone levels and sexual behavior of old and middle-aged rhesus males

After approximately 6 mo without access to a receptive female, eight old (19 to 25 yr) and eight middle-aged (9 to 15 yr) males were given nine daily 10-min tests of sexual behavior with receptive females. Three ovariectomized, estrogen-primed females served as partners. Blood samples were taken from the males at 0900 and 2100 h on 2 consecutive days before and after the series of nine behavioral tests. Serum levels of testosterone (T) were measured by means of radioimmunoassay. There were no significant differences between serum T levels after 6 mo of sexual deprivation and after the nine tests of sexual behavior, nor did serum T levels differ between middle-aged and old males. Serum T levels were significantly higher in the 2100-h samples than in the 0900-h samples for both groups. The nocturnal increases in serum T, both absolute amounts and percentages of increase, did not differ between middle-aged and old males. Repeated daily testing had little effect on the major components of male sexual behavior. Middle-aged males outperformed old males; middle-aged males had higher rates of contracting the female, mounting and intromitting. They ejaculated earlier and had a greater percentage of tests with ejaculations than did old males.     

Influence of progesterone on sexual behavior in castrated male rhesus monkeys treated with testosterone

The effect of progesterone (P) on sexual behavior was tested in five long-term castrated rhesus monkeys treated simultaneously with testosterone (T) and P. A control group consisting of three long-term castrates was treated only with T. Plasma levels of T and P were measured at weekly intervals, and behavioral data were collected for each animal twice a week throughout the study. Progesterone has been reported to inhibit sexual behavior in male guinea pigs and mice, but the levels of behavior in our two groups of monkeys did not differ from each other over 8 weeks of treatment and testing.     

Hormone release during computer-monitored sexual behavior in mature and aged male rats

Sexual behavior and the increase in plasma hormone levels of LH, prolactin, and testosterone associated with sexual behavior were examined in three age groups of sexually naive male rats. The two younger groups (5- and 11-month-old) mated normally and their behavioral latencies decreased significantly following sexual experience. Both plasma testosterone and LH concentrations increased significantly following entrance of a receptive female into the mating arena. Plasma prolactin levels rose but not significantly. However, the 27-month-old rats neither mated nor showed an increase in the three plasma hormone concentrations during exposure to a receptive female. Only basal testosterone levels were significantly lower than those of the younger animals. Low testosterone levels possibly contributed to deficiencies in both behavior and its associated hormone release. The monitoring of sexual behavior was facilitated by a computer, programmed to record, store, and analyze behavioral events.     

Changes in serum corticosterone and testosterone during induced maternal behavior in rats

Serum corticosterone (Cpd B) and testosterone (T) concentrations were studied during the avoidance phase of artificially induced parental behavior in male rats. Adult rats were exposed to the presence of standard size foster pups for 60 min daily. On day 1 the avoidance behavior characterized by typical burying reaction was accompanied by an elevation of Cpd B and T. The behavioral responses and hormonal changes diminished during the days of repeated exposure. It was found that neither pup-killing nor spontaneous retrieval were dependent on circulating hormones in the male rat. Subcutaneous injection of ACTH 4-10 inhibited extinction of the avoidance and the humoral responses in both female and male animals. Oxytocin failed to exert any influence on the behavioral and hormonal components of the aversive reaction.     

The role of AT1 receptor-mediated reproductive function in renovascular hypertension in male rats

There is an association between hypertension and reproductive dysfunction. Angiotensin II (Ang II) is involved in the pathogenesis of hypertension and the regulation of reproduction. The present study aimed to determine whether the angiotensinergic system mediates the effects of hypertension on reproductive function in male rats subjected to a two-kidney, one-clip (2K1C) model. Sexual behavior parameters, gametogenesis and plasma concentrations of Ang II, testosterone, prolactin and corticosterone were evaluated in male rats 28days after 2K1C or sham surgery and losartan (Los) treatment (a type 1 angiotensin II (AT1) receptor antagonist) or vehicle (V) treatment. The animals were divided into Sham+V, 2K1C+V, Sham+Los and 2K1C+Los groups. The 2K1C+V group showed a hypertensive response, inhibition of sexual behavior, spermatogenesis dysfunction, and increases in plasma Ang II and prolactin. Conversely, plasma testosterone decreased, and plasma corticosterone remained constant. Losartan treatment normalized blood pressure and prevented the changes in plasma testosterone and prolactin, sexual behavior and spermatogenesis in the 2K1C+Los group. In addition, losartan treatment caused an additional increase in circulating Ang ll in both groups (Sham+Los and 2K1C+Los). Together, these results suggest that Ang ll, acting through the AT1 receptor, modulates behavioral and endocrine parameters of reproductive function during renovascular hypertension. In addition, the effects of circulating Ang II on plasma testosterone and prolactin seem to contribute to the spermatogenic and sexual dysfunctions in hypertensive rats.     

Behavioral characteristics of freemartins administered estradiol, estrone, testosterone, and dihydrotestosterone

Eighteen genetic females born co-twin with males and diagnosed as being sterile intersexes (freemartins) were studied from birth to 79 weeks of age. Testosterone (T) and estrone (EI) were administered in Silastic capsules of two groups from birth to 50 weeks of age and other animals were left untreated. At 50 weeks the two treated groups had larger implants installed and the untreated animals were assigned to a new estrone (EII) and estradiol (E2) treatment. Later a dihydrotestosterone (DHT) group was formed in comparison with new E2 and testosterone propionate-enanthate (TP-TE) groups, plus untreated controls. Vulvar interest, Flehmen lip curl, mounting, and agonistic behavior were recorded daily for 30 min while animals were allowed social interaction. Agonistic behavior, interest in the genital area, and mounting were induced or stimulated by T, TP-TE, and E2, but not by DHT or estrone (EI or EII). Also, only animals in the T, TP-TE, and E2 groups induced to mount displayed the standing type of behavioral estrus. Flehmen lip curl was stimulated only by T or TP-TE. The evidence is interpreted to indicate that T, per se, evokes the lip curl, but it probably stimulates other responses at the neural level by conversion to E2. Also, the freemartin response, the response of castrates to steroid hormones, and current knowledge of circulating steroid hormones in male and female cattle could be interpreted to indicate that the neural tissue responsible for sexual behavior in both sexes of this species may respond similarly in several respects.     

Postparturitional testosterone surge in male offspring of rats stressed and/or fed ethanol during late pregnancy

Male offspring of rats exposed to restraint stress and/or alcohol during late pregnancy show aberrant patterns of sexual behavior masculinization and defeminization that vary as a function of treatment. The impact of these treatments on the postparturitional testosterone (T) surge that contributes to sexual behavior differentiation was investigated. Plasma T was measured using radioimmunoassay in individual males sampled on day 21 of gestation within 10 min of cesarean delivery or 1, 2, or 4 h thereafter. Neonatal T in the group exposed only to stress did not differ from that in the control group. T was lower than control levels at birth in both alcohol groups. The magnitude of the T surge that occurred during the first hour of birth in the control group was diminished by 50% in both alcohol groups, whose T pattern was very similar. There was no common alteration in postparturitional T associated with the increased lordotic behavior potential that males in all three treatment groups typically share, nor were there idiosyncratic endocrine abnormalities linked to the very different male copulatory pattern each exhibits. Exposure to an abnormal T milieu during fetal as well as neonatal ontogeny may underlie the etiology of the different sexual behavior patterns exhibited by males exposed to stress and/or alcohol. Possible unique effects each treatment exerts on perinatal plasma T and it's aromatization to estradiol in hypothalamic targets are discussed.     

Territorial aggression, circulating levels of testosterone, and brain aromatase activity in free-living pied flycatchers

Testosterone (T) is a critical endocrine factor for the activation of many aspects of reproductive behavior in vertebrates. Castration completely eliminates the display of aggressive and sexual behaviors that are restored to intact level by a treatment with exogenous T. There is usually a tight correlation between the temporal changes in plasma T and the frequency of reproductive behaviors during the annual cycle. In contrast, individual levels of behavioral activity are often not related to plasma T concentration at the peak of the reproductive season suggesting that T is available in quantities larger than necessary to activate behavior and that other factors limit the expression of behavior. There is some indication from work in rodents that individual levels of brain aromatase activity (AA) may be a key factor that limits the expression of aggressive behavior, and in agreement with this idea, many studies indicate that estrogens produced in the brain by the aromatization of T may contribute to the activation of reproductive behavior, including aggression. We investigated here in pied flycatcher (Ficedula hypoleuca) the relationships among territorial aggression, plasma T, and brain AA at the peak of the reproductive season. In a first experiment, blood samples were collected from unpaired males holding a primary territory and, 1 or 2 days later, their aggressive behavior was quantified during standardized simulated territorial intrusions. No relationship was found between individual differences in aggressive behavior and plasma T or dihydrotestosterone levels but a significant negative correlation was observed between number of attacks and plasma corticosterone. In a second experiment, aggressive behavior was measured during a simulated territorial intrusion in 22 unpaired males holding primary territories. They were then immediately captured and AA was measured in their anterior and posterior diencephalon and in the entire telencephalon. Five males that had attracted a female (who had started egg-laying) were also studied. The paired males were less aggressive and correlatively had a lower AA in the anterior diencephalon but not in the posterior diencephalon and telencephalon than the 22 birds holding a territory before arrival of a female. In these 22 birds, a significant correlation was observed between number of attacks/min displayed during the simulated territorial intrusion and AA in the anterior diencephalon but no correlation was found between these variables in the two other brain areas. Taken together, these data indicate that the level of aggression displayed by males defending their primary territory may be limited by the activity of the preoptic aromatase, but plasma T is not playing an important role in establishing individual differences in aggression. Alternatively, it is also possible that brain AA is rapidly affected by agonistic interactions and additional work should be carried out to determine whether the correlation observed between brain AA and aggressive behavior is the result of an effect of the enzyme on behavior or vice versa. In any case, the present data show that preoptic AA can change quite rapidly during the reproductive cycle (within a few days after arrival of the female) indicating that this enzymatic activity is able to regulate rapid behavioral transitions during the reproductive cycle in this species.     

Seasonal aromatase activity in the brain of the male red-sided garter snake

We investigated regional and seasonal variations in neural aromatase activity (AA), the enzyme that converts androgens into estrogens, to examine a possible indirect role of testosterone (T) in mediating spring reproductive behavior of red-sided garter snakes, a species exhibiting a dissociated reproductive pattern. Neural AA in male snakes varied significantly among brain regions. Additionally, there were significant interactions between brain region and season. In the spring, actively courting males had greater AA in the olfactory region (O) compared to the septum/anterior-hypothalamus preoptic area (S/AHPOA), nucleus sphericus (NS) and midbrain (Mb). Fall animals collected as they returned to the den prior to winter dormancy had significantly greater AA in the S/AHPOA compared to all other regions. These findings were consistent using either regional (gross) dissection or punch microdissection, which allowed us to separate the S and AHPOA. There were no significant differences in AA production between the S and AHPOA. This study provides the first documentation of seasonal and regional variations in AA in a snake brain and suggests that aromatization of androgens may play a role in regulating reproduction in red-sided garter snakes. During spring mating, elevated AA in the O may activate pathways essential for detection of courtship pheromones, while increased AA in the S and AHPOA of fall animals suggests that circulating androgens play an indirect role in programming critical neural pathways involved in reproduction. Thus, as in many other vertebrates, estrogenic metabolites of testosterone may be a critical hormonal component regulating reproductive behavior in this dissociated breeder.     

Prostatic androgen receptor and plasma testosterone levels in streptozotocin-induced diabetic rats.

Diabetes was induced in male Sprague-Dawley (S-D) rats by streptozotocin (STZ) administration. Following STZ injection, plasma glucose levels in the treated rats were significantly elevated from values of untreated controls. Over the experimental period (140 days) plasma testosterone (T) levels, prostatic nuclear androgen receptor (AR) contents and prostatic weights declined with increasing age in the rats. The declines in both STZ-treated and untreated rats were similar in manner and no notable differences were discerned in the data obtained from the two groups. On the contrary, prostatic cytosolic AR contents in untreated rats remained unchanged with advancing age, but was reduced to 50% of normal control values in diabetic rats following STZ treatment. Correlation analyses revealed that prostatic nuclear AR contents correlated positively with plasma T levels while prostatic cytosolic AR contents correlated negatively with plasma glucose levels. These data support former claims that prostatic nuclear AR content is dependent on circulating T level and suggest a possible link between prostatic cytosolic AR content and plasma glucose concentrations.     

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.     

The influence of testosterone on territorial defence and parental behavior in male free-living rufous whistlers, Pachycephala rufiventris

We studied a population of rufous whistlers, Pachycephala rufiventris, throughout a single breeding season in central New South Wales, Australia. We evaluated the relation between plasma testosterone (T) and reproductive behaviors using both simulated territorial intrusions (STIs) and subcutaneous T implants. We compared circulating T values to aggression levels of males (using STI) during pair bond and territory establishment and again during incubation. Although plasma T levels were significantly lower in the latter period, male responsiveness to STI, in terms of proximity to decoy, call rate, and number of attacks on the decoy, was indistinguishable between the two breeding stages. T levels of males exposed to STI were not different from the levels of unexposed free-living males at the same breeding stage. The effect of exogenous T on parental behavior was examined by comparing duration of incubation bouts of males and their mates prior to and after T treatment. T males significantly reduced the amount of time they incubated following implantation, whereas Control males maintained their incubation effort. After cessation of breeding activities, T males displayed significantly higher call rates due to increased use of the primary intersexual advertisement call in this species. The reduction of incubation behavior following T implantation emphasises the functional significance of the rapid decline in T in free-living males during incubation. The results from both experiments suggest that intersexual advertisement, rather than territorial aggression, may be dependent on high T levels in this species.