Seasonal variation in male testosterone levels in a tropical bird with year‐round territoriality

Testosterone is important in mediating investment in competing activities such as territoriality, parental care, and maintenance behavior. Most studies of testosterone function have focused on temperate species and less is known about the role of testosterone in territoriality or variation in mating systems of tropical species. Results of studies of tropical species with year‐round territoriality indicate that territorial aggression during the non‐breeding season is maintained with low levels of testosterone, and increased levels of testosterone in males during the breeding season may increase mating opportunities or aid in competition for mates. We studied seasonal variation in testosterone levels of male Red‐throated Ant‐tanagers (Habia fuscicauda), a socially monogamous species with year‐round territoriality and with high levels of extra‐pair matings (41% of young), to determine if testosterone levels increased during the breeding season. We captured males during the non‐breeding and breeding seasons and collected blood samples for hormone analysis. We found that mean testosterone concentrations were low during the non‐breeding season (0.18 ± 0.05 [SD] ng/ml, range = 0.11–0.31 ng/ml), and significantly higher during the breeding season (2.37 ± 2.47 ng/ml, range = 0.14–6.28 ng/ml). Testosterone levels of breeding males were not related to aggression levels as measured by attack rates toward a stuffed decoy or singing rates during simulated territorial intrusions. These results suggest that the higher testosterone levels of breeding male Red‐throated Ant‐tanagers may be important in an extra‐pair mating context, possibly in display behavior or mate attraction, but additional study is needed to clarify the role of testosterone during the breeding season.     

The effect of season on the induction of sexual behavior by estradiol in female rhesus monkeys

Rhesus monkeys housed in an outdoor environment are seasonal breeders, with ovulations and concomitant sexual behavior limited to the fall and winter months. To determine if there is a seasonal difference in the capacity of physiological levels of estradiol (E2) to induce sexual behavior in ovariectomized rhesus monkeys housed outdoors, subjects living in a social group were treated with subcutaneous E2 implants in a counter-balanced design during the nonbreeding season (May-July) and during the breeding season (September-November). Serum E2 levels were monitored by obtaining blood samples twice a week. Three levels of E2 were studied: baseline (less than 30 pg/ml), follicular (100 pg/ml), and periovulatory (200 pg/ml). Two of five adult males in the group were injected with human chorionic gonadotropin (hCG) twice a week to insure that males with high testosterone levels were present during each season. Focal observations of behavior of 15 minutes' duration on each subject were conducted 5 days per week, with frequencies and durations of social, sexual, and solitary behaviors recorded. Concomitant 2-h group scans were made to record all occurrences of mounting behavior. Neither heterosexual serial mounting nor seminal plugs were ever observed in E2-treated females during the summer months. In contrast, copulation and seminal plugs were observed in subjects at both treatment levels during the fall. While E2-treated females engaged in homosexual mounting behavior during both summer and fall, E2 treatment resulted in heterosexual copulation only during the fall. Changes in patterns of social behavior paralleled changes in sexual behavior and were significantly affected by treatment and season, with more male-female interactions during the fall months.(ABSTRACT TRUNCATED AT 250 WORDS)     

A longitudinal study of leptin during development in the male rhesus monkey: the effect of body composition and season on circulating leptin levels

The objective of this study was to examine longitudinal changes in serum leptin concentrations during development and to correlate those changes with sexual development in male rhesus monkeys housed under natural environmental conditions. Blood samples were drawn from 8 control animals approximately every other month from 10 to 30 mo of age and thereafter monthly through 80 mo of age. Leptin levels declined through the juvenile period until the onset of puberty and were negatively correlated with body weight. Seven of the eight animals became sexually mature during the breeding season of their fourth year of life. Puberty was delayed in the other animal until the subsequent breeding season. There were no significant fluctuations in leptin levels prior to or in association with the pubertal rise in LH and testosterone (T) secretion. During the peripubertal period, levels of leptin varied between 2 and 3 ng/ml. The animal that exhibited delayed puberty had the lowest body weight and highest leptin levels during this period. With the achievement of sexual maturity, leptin levels varied seasonally, with peak levels in the late winter (Jan-Mar) and a nadir in the late summer (Aug-Sept). A late winter rise in leptin was also evident in most of the animals during Years 2 and 3, but not during Year 4. In the fall of Years 5 and 6, the seasonal rise in leptin concentrations lagged 3-4 mo behind the seasonal increase in LH and T. In the fall of Year 5, but not thereafter, leptin levels were positively related to percent body fat and negatively correlated with lean body mass. The data do not support the hypothesis that increasing leptin concentrations trigger the onset of puberty in the male rhesus monkey. During the juvenile period and after sexual maturation, but not during the peripubertal period, leptin secretion varied with season in the animals; but the environmental factors that cue or drive this rhythm remain to be determined.     

Testosterone metabolic clearance rate and production rate in the male infant rhesus monkey

The male infant rhesus monkey (Macaca mulatta) undergoes a period of testicular activation similar to that seen in the human infant. Plasma testosterone (T) concentrations rise after birth, reaching levels of about 500 ng/dl at 1-3 mo of age and then fall to approximately 50 ng/dl at 60 mo. The plasma T metabolic clearance rates (MCRT) and production rates (PRT) were measured in two rhesus infants at 1 and 6 mo of age to determine the mechanism of the observed increase in plasma T. While there was little change in the MCRT between 1 and 6 mo, PRT was much higher at 1 mo than at 60 mo of age. These observations are consistent with the hypothesis that the increased plasma testosterone levels in infant rhesus monkeys reflect an increased production of testosterone rather than an altered metabolic disposition of the hormone.     

Plasma Testosterone in Bulls

Levels of peripheral plasma testosterone and LH were studied in 4 bulls hourly during a 12 hr. sampling period at 5 times of the year. The average plasma testosterone levels were significantly lower in October (1.8 ng/ml, Ρ < 0.001) and December (2.5 ng/ml, Ρ < 0.05) than in February, June and August (3.5, 3.7 and 3.7 ng/ml respectively). LH showed a slight fluctuation during the day, with values ranging between 0.8 and 3.8 ng/ml, but underwent no significant seasonal variation. A significant increase in average plasma testosterone was observed 1 hr. after the LH peaks (P < 0.001).     

Effects of testosterone on the behavior of male cynomolgus monkeys (Macaca fascicularis)

To determine the threshold doses of testosterone propionate (TP) that cause clear-cut behavioral changes in the sexual behavior of castrated male cynomolgus monkeys, observations were made on three males during successive 5-week treatment periods while they received daily subcutaneous doses of 100 μg TP increasing in octaves to 25.6 mg TP. Males were tested with each of the same two ovariectomized, estrogen-treated females (6 pairs, 330 1-hr behavior tests). To mimic the diurnal plasma testosterone rhythm, TP injections were given at 1600 hr and blood samples were obtained at 0800 hr (141 samples). Male ejaculatory activity increased at the threshold dose of 200 μg TP per day giving plasma testosterone levels of 830 ng/100 ml, which is in the physiological range of 600–1600 ng/100 ml for intact males. This threshold dose was eight times higher than in rhesus monkeys on a dose per kilogram body weight basis. There was a further marked increase in ejaculatory performance at higher doses (6.4 to 25.6 mg) giving supraphysiological plasma levels of 4000–9000 ng/100 ml. There were individual differences in the behavioral changes occurring with TP treatment, and the female partner modulated the effects. These findings were generally similar to those obtained with male rhesus monkeys, but certain species differences were noted.     

Relationship between seasonal plasma estradiol-17 beta and testosterone levels and in vitro production by ovarian follicles of amago salmon (Oncorhynchus rhodurus)

Plasma estradiol-17 beta and testosterone levels were assessed by radioimmunoassay during the sexual maturation of female amago salmon (Oncorhynchus rhodurus). Estradiol-17 beta levels gradually increased during vitellogenesis (June to September), reached a peak in September (about 16 ng/ml) and rapidly decreased in mature and ovulated fish (about 3-4 ng/ml) in October. The seasonal pattern of plasma testosterone levels lagged behind and followed that of estradiol-17 beta during vitellogenesis, but levels remained high in mature and ovulated fish (90-110 ng/ml). Estradiol-17 beta levels and the gonadosomatic index (GSI) values correlated well during vitellogenesis: GSI values showed a linear increase, and reached a peak (29.9 +/- 1.4) in October. Values were extremely low in ovulated fish (1.2 +/- 0.2). In vitro production of estradiol-17 beta and testosterone by ovarian follicles in response to partially purified chinook salmon gonadotropin (SG-G100) was examined monthly using 18-h incubations. Throughout the vitellogenic period SG-G100 stimulated both estradiol-17 beta and testosterone production: the steroidogenic response of follicles increased from June (about 2 ng/ml estradiol-17 beta; 0.1 ng/ml testosterone) to September (about 10 and 14 ng/ml, respectively). In October full-grown immature follicles which could be induced to mature in vitro by hormone treatment produced large amounts of testosterone (about 130 ng/ml) but not estradiol-17 beta. Postovulatory follicles also produced testosterone but the values were low (10 ng/ml) compared with full-grown immature follicles. Very low levels of estradiol-17 beta were produced by postovulatory follicles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma testosterone and male dominance in a Japanese macaque (Macaca fuscata) troop compared with repeated measures of testosterone in laboratory males

Plasma testosterone levels measured by radioimmunoassay did not correlate with dominance rank or aggressive behavior in 21 adult males of a natural troop of Japanese macaques (Macaca fuscata). The data were replicated during two consecutive breeding seasons. Levels of male dominance and aggressive behavior were highly correlated from year to year, but testosterone levels of individual males were not. Nor were individual levels of testosterone correlated in five laboratory males sampled every 15 min over a 2-hr period. These laboratory males showed multiple, apparently random peaks but the mean testosterone level of 11.73 ng/ml did not vary significantly throughout the 120 min that were studied. By demonstrating that the stress of blood collection did not depress plasma testosterone titers in laboratory males, these data validated the method used in the study of the natural troop.     

Seasonal changes in serum dehydroepiandrosterone,androstenedione, and testosterone levels in the squirrel monkey (Saimiri boliviensis boliviensis)

The squirrel monkey (Saimiri boliviensis boliviensis) has a well-defined breeding season during which adult males undergo androgen-dependent morphological changes, with acquisition of active spermatogenesis. To assess the hormonal events of this annual cycle, blood samples were obtained weekly from ten adult males, and serum was assayed for testosterone (T), androstenedione (ΔA), and dehydroepiandrosterone (DHEA). A significant seasonal variation was noted in mean serum T (P < 0.02), ΔA (P < 0.02), and DHEA (P < 0.001) concentrations. Mean ΔA concentrations increased from a nonbreeding season nadir of 91.4 ± 12.9 ng/ml (mean ± standard error) to a prebreeding concentration of 139 ± 10.5 ng/ml and breeding season peak of 167.5 ± 15.4 ng/ml (P < 0.05). Mean DHEA concentrations increased from a nonbreeding season nadir of 8.3 ± 0.8 to a breeding season peak of 14.3 ± 1.2 (P < 0.001). Mean T levels in the nonbreeding (52.2 ± 11.6 ng/ ml) and prebreeding season (48.6 ± 7.4) were similar. However, T significantly increased during the breeding season to 103.5 ± 12.8 ng/ml (P < 0.05). Progressive changes in body weight and morphology paralleled the rise in serum ΔA levels. The pattern of peripheral serum androgen concentrations throughout the year would suggest annual activation of the hypothalamic-pituitary-adrenal and/or hypothalamic-pituitary-gonadal axes.     

Impact of season and social challenge on testosterone and corticosterone levels in a year-round territorial bird

Plasma testosterone increases during breeding in many male vertebrates and has long been implicated in the promotion of aggressive behaviors relating to territory and mate defense. Males of some species also defend territories outside of the breeding period. For example, the European nuthatch (Sitta europaea) defends an all-purpose territory throughout the year. To contribute to the growing literature regarding the hormonal correlates of non-breeding territoriality, we investigated the seasonal testosterone and corticosterone profile of male (and female) nuthatches and determined how observed hormone patterns relate to expression of territorial aggression. Given that non-breeding territoriality in the nuthatch relates to the reproductive context (i.e., defense of a future breeding site), we predicted that males would exhibit surges in plasma testosterone throughout the year. However, we found that males showed elevated testosterone levels only during breeding. Thus, testosterone of gonadal origin does not appear to be involved in the expression of non-breeding territoriality. Interestingly, territorial behaviors of male nuthatches were stronger in spring than in autumn, suggesting that in year-round territorial species, breeding-related testosterone elevations may upregulate male-male aggression above non-breeding levels. In females, plasma testosterone was largely undetectable. We also examined effects of simulated territorial intrusions (STIs) on testosterone and corticosterone levels of breeding males. We found that STIs did not elicit a testosterone response, but caused a dramatic increase in plasma corticosterone. These data support the hypothesis that corticosterone rather than testosterone may play a role in the support of behavior and/or physiology during acute territorial encounters in single-brooded species.     

Seasonal weight changes in male rhesus monkeys (Macaca mulatta)

Adult male rhesus monkeys lose weight during the breeding season and regain it during the nonbreeding season. The annual pattern of maximum weight gain just prior to the onset of breeding resembles the seasonal “fattening” seen in squirrel monkeys, but the period of weight gain is less discrete. The magnitude of weight change is less in younger males, in that sexually immature males gain weight in both seasons, but significantly less during the breeding season. Females do not lose weight during the breeding season. Post hoc analyses revealed no significant correlations between male testosterone levels, dominance ranks, weights, or weight changes. The heaviest animals as juveniles were predictably the heaviest as adolescents. The timing of seasonal changes in testosterone did not correlate with the timing of changes in weight; weight losses followed the rise in testosterone, and weight gains continued until early in the breeding season after testosterone levels had already begun to rise. It is suggested that seasonal hormonal changes may influence activities in individuals and that changes in the activities of particular group members may alter the activity patterns of other group members. This alteration of activity patterns due to group influences on individuals as well as individual influences on the group may explain why hormonal regulation of seasonal weight appears to be indirect and why individuals (juveniles) experiencing no seasonal hormonal changes nonetheless show differences in activity patterns and seasonal weight changes.     

Seasonal variations of LH, prolactin, androstenedione, testosterone and testicular FSH binding in the male blue fox (Alopex lagopus)

The seasonal changes in testicular weight in the blue fox were associated with considerable variations in plasma concentrations of LH, prolactin, androstenedione and testosterone and in FSH-binding capacity of the testis. An increase in LH secretion and a 5-fold increase in FSH-binding capacity were observed during December and January, as testis weight increased rapidly. LH levels fell during March when testicular weight was maximal. Plasma androgen concentrations reached their peak values in the second half of March (androstenedione: 0.9 +/- 0.1 ng/ml: testosterone: 3.6 +/- 0.6 ng/ml). A small temporary increase in LH was seen in May and June after the breeding season as testicular weight declined rapidly before levels returned to the basal state (0.5-7 ng/ml) that lasted until December. There were clear seasonal variations in the androgenic response of the testis to LH challenge. Plasma prolactin concentrations (2-3 ng/ml) were basal from August until the end of March when levels rose steadily to reach peak values (up to 13 ng/ml) in May and June just before maximum daylength and temperature. The circannual variations in plasma prolactin after castration were indistinguishable from those in intact animals, but LH concentrations were higher than normal for at least 1 year after castration.     

季节性变化对雌性恒河猴生殖功能的影响

目的　研究季节性变化对雌性恒河猴生殖功能的影响。方法　采用随机抽样法和放射免疫测定法 ,分析了不同时期雌性恒河猴性皮肤变化、月经周期和生殖激素变化的特点。结果　 ( 1)性征的季节性变化 :在生殖季节雌性恒河猴几乎都出现性皮肤反应 ,出现比较规则月经周期 ,在非生殖季节只有部分雌性恒河猴出现性皮肤反应 ,月经周期不规则 ,行经频率低 ,有的出现长时间的闭经 ;( 2 )生殖激素的季节性变化 :在生殖季节促性腺激素和性类固醇激素的分泌水平都出现周期性的变化 ,而非生殖季节促性腺激素和性类固醇激素的分泌水平没有显著的差异。结论　雌性恒河猴性皮肤变化、月经周期和生殖激素存在明显的季节性差异 ,这种差异导致了雌性恒河猴生殖功能的季节性变化     

Seasonal variation in the feedback of sex steroid hormones on serum LH concentrations in the male horse

The possibility of seasonal variation in the feedback effect of testosterone or oestradiol was investigated by giving replacement treatment to geldings for 2-3 weeks during breeding and non-breeding seasons. In the non-breeding season, testosterone suppressed LH values (mean +/- s.e.m., ng/ml) in all geldings (before treatment, 7.5 +/- 2.3; final treatment week, 1.8 +/- 0.2; P less than 0.05), whereas early in the breeding season, testosterone caused a prolonged rise in LH (before, 6.8 +/- 2.3; final week, 18.9 +/- 6.4; P less than 0.05). In all testosterone experiments, LH returned to pretreatment levels within 2 weeks after treatment. Oestradiol treatment caused a prolonged increase (P less than 0.05) in LH concentrations (mean +/- s.e.m., ng/ml) in both seasons (breeding: before 5.2 +/- 1.1; final week, 16.2 +/- 4.8; non-breeding before, 10.9, 20.1 +/- 5.2). We conclude that in geldings the feedback effect of testosterone varies with season and, further, that testosterone replacement may be able to restore to geldings the stallion's seasonal pattern of LH secretion. The results suggest that, in male horses, testosterone and possibly oestradiol, are important components in the neuroendocrine pathway controlling seasonal breeding and, moreover, are essential for the generation of a positive signal for LH secretion in the breeding season.     

Seasonal testosterone pattern in Hawaiian monk seals (Monachus schauinslandi).

Blood samples from four captive male Hawaiian monk seals were collected at intervals of one month for one year for testosterone assay. Plasma testosterone concentrations, measured by radioimmunoassay, revealed a clear seasonal pattern. The lowest mean testosterone concentration (0.09 +/- 0.04 ng ml-1) occurred in January, and the highest (1.78 +/- 0.40 ng ml-1) in June. The seasonal occurrence of births and of injuries related to mating in wild populations of Hawaiian monk seals showed a distinct association with the period of high testosterone. This study supports other data that indicate that the Hawaiian monk seal is a seasonal breeder and is reproductively active for longer than monachine seals that live in higher latitudes.     

Seasonality of serum testosterone levels and sperm density in Tursiops truncatus

We trained a mature male bottlenose dolphin, Tursiops truncatus, to provide semen samples on command. After completion of the 10-week training period, semen was collected twice weekly and blood was sampled twice monthly for a period of 28 months. Total sperm per ejaculate ranged from near 0 to 54.6 x 10(9) (n = 1332). Sperm densities from each session ranged from no sperm to 1,587 x 10(6)/ml (n = 241). Testosterone levels ranged from 1.1 to 54.4 ng/ml (n = 79). Seasonal variations were observed in total sperm per ejaculate, sperm density per ml of ejaculate, and in serum testosterone levels. Peak sperm densities were detected during September and October of three consecutive breeding seasons. Serum testosterone levels peaked in June, decreased during July and August, and were lowest in September and October, the period of greatest sperm density. Peak sperm production and density were coincident with the peak period of breeding activity but at a time when serum testosterone levels were lowest.     

Morphological and hormonal parameters in two species of macaques: impact of seasonal breeding

To compare physiological and developmental differences between two cogeneric species that differ by seasonal vs. aseasonal breeding, values for morphological measurements, testicular volume, serum testosterone, estradiol, and dehydroepiandrosterone-sulfate levels were obtained from 53 rhesus during the early breeding season, as well as 41 pig-tailed macaque males maintained at the Tulane Primate Center. The two species exhibited similar body size, testosterone, and estradiol levels, but differed substantially in testicular volume (3.00 +/- 1.7 vs. 1.72 +/- 1.3 cc), abdominal skinfold measures (15.7 +/- 9.2 vs. 9.0 +/- 7.7 mm), and DHEA-S levels (18.0 +/- 11.7 vs. 7.6 +/- 5.4 microg/dl). Significant interaction effects for species by age group were found for weight, tricep circumference, length, and estradiol level. In addition, length was more closely related to testicular volume among rhesus compared to pig-tailed macaques, suggesting different developmental patterns between the species. Predictors of hormonal levels differed between the two species. In the rhesus, estradiol levels were related to testicular volume and testosterone levels while there were no anthropometric predictors of testosterone or DHEA-S. For the pig-tailed macaques, testicular volume was related to tricep circumference, testosterone to triceps skinfold and testicular volume, and estradiol to weight. It is argued that rhesus have larger testes for body size and more abdominal fat deposits during the early breeding season relative to pig-tailed macaques reflecting the increased demands of sperm competition in a seasonally breeding species. Hormonal differences associated with the difference in breeding system appear to be primarily related to adrenal rather than testicular activity.     

Testosterone and year‐round territoriality in tropical and non‐tropical songbirds

Past studies have suggested a fundamental difference in testosterone concentrations between tropical and northern latitude male birds, with the convention being that males in the tropics express much lower levels of testosterone. However, recent comparative studies have shown that tropical males with a short and synchronous breeding season (i.e. a breeding season typical of northern species) express maximum testosterone levels similar to those of northern latitude birds. Here, we ask the converse: do northern latitude songbirds that express a defining life‐history characteristic typical of the tropics, i.e. year‐round territoriality, have an annual testosterone profile similar to that of tropical songbirds? For the few year‐round territorial species for which data are available, we found that seasonal testosterone profiles and seasonal maxima in plasma testosterone were similar between males of tropical and non‐tropical species. For example, males of both groups expressed seasonal maxima during the period when females were fertile, and testosterone levels at this time were similar. In contrast, this and other studies show that species with seasonal territories typically express maximum testosterone levels earlier in the breeding cycle, when territories are first being established. Taken together, we suggest that specific life‐history traits may play a more important role in determining testosterone profiles of tropical and non‐tropical birds than breeding latitude and encourage further studies to allow for more formal comparisons.     

Validation of radioimmunoassays for LH and FSH in the sooty mangabey (Cercocebus atys): Characterization of LH and the response to GnRH

Heterologous radioimmunoassays (RIA) for macaque LH and FSH were validated for the measurement of these hormones in the sooty mangabey and mangabey pituitary LH was characterized relative to rhesus monkey LH. Dilutions of a pituitary mangabey extract and a partially purified preparation of mangabey LH ran parallel to a rhesus monkey standard (LER 1909-2) in the ovine-ovine (o-o) LH assay but showed some deviation from parallelism in the rhesus monkey FSH assay. The LH potency of the mangabey extract and standard were six and 190 times more potent, respectively, than LER 1909-2 in the LH RIA. Mangabey LH was estimated to have a molecular weight of 40,000–42,000 daltons vs 35,000–38,000 daltons for rhesus LH on Sephadex G-100 chromatography. Plasma levels of radioimmunoreactive LH, FSH, and testosterone were assayed before and after a bolus administration of 25, 50, or 100 μg synthetic go-nadotropin releasing hormone (GnRH) to adult male mangabeys. A significant increase in serum levels of LH was seen within 30 min with levels more than fourfold higher than the basal level of LH after administration of 100 μg GnRH. However, no consistent increases in plasma FSH values were detected. The integrated mean LH response above preinjection levels following 25, 50, or 100 μg GnRH was dose related. Serum levels of testosterone were also elevated after administration of GnRH, but peak concentrations of testosterone lagged behind peak levels of LH by approximately 30 min. These studies indicate that the heterologous RIAs may be used for measuring gonadotropins in the mangabey and that the male mangabey is apparently more sensitive to GnRH than the rhesus monkey.     

Blood testosterone levels in Italian Mediterranean buffalo bulls managed in two different breeding conditions

In Italian buffalo cows the spontaneous cyclic ovarian activity is mainly high in autumn, while during spring and early summer it is very low. However many farmers separate males from females in the October-February period to obtain births in winter-spring. In order to verify if blood testosterone concentration in adult buffalo bulls is affected by season and by different management of the contact with females, 20 adult buffalo males, bred in central Italy were submitted to monthly blood sampling for 1 year, from September to August. The bulls were kept together with females all the time (group A; n=9) or were held separated from cows from October to February (group B; n=11). The mean (+/-S.E.M.) serum testosterone concentrations were higher in spring and summer than in autumn and winter in group B (2.07+/-0.1 ng/mL versus 0.99+/-0.08 ng/mL, P<0.01) but in group A the seasonal difference was not significant (2.09+/-0.13 versus 1.48+/-0.28). The management of the contact with females affected testosterone values (P<0.01): in the separation period (October-February) the mean serum concentration in group B was lower than in March-September, when the cows were together with the bulls (0.94+/-0.09 ng/mL versus 1.95+/-0.1 ng/mL, P<0.05). This is not true for group A (1.49+/-0.20 ng/mL versus 2.00+/-0.13 ng/mL, NS). It is concluded that contact with females exerted a major stimulus for the testicular androgen secretion in buffalo bulls, even if other seasonal factors (climate, food intake) may affect control of gonadal activity.