The effects of castration and Silastic implants of testosterone on intermale aggression in the mouse

Castration and testosterone (T) replacement were used to study developmental changes in aggressive behavioral responsiveness to androgenic stimulation. Male mice castrated at birth were less sensitive to circulating T than were prepubertal or adult castrates, but fighting was induced in neonatal castrates with a dose of androgen that produced hypertrophy of the accessory organ system in adult castrates. Gonadectomy shortly prior to pubertal increases in serum T concentration also reduced behavioral responsiveness to androgen administration. Intermale aggression was induced in prepubertal castrates only with T treatment that maintained accessory organ growth in adult castrates. The aggressive behavior of males castrated after the pubertal surge in serum T was supported with circulating levels of androgen that failed to stimulate the accessory organ system above that of oil-treated castrates. It was concluded that T stimulation during neonatal or pubertal life is not totally crucial for organization of neural substrates that mediate the ultimate expression of intermale aggression, but exposure to androgen from birth throughout pubertal development is normally required to produce maximal aggressive behavioral responsiveness to circulating T encountered in adulthood.     

Variation in fecal testosterone levels, inter-male aggression, dominance rank and age during mating and post-mating periods in wild adult male ring-tailed lemurs (Lemur catta)

In primate species exhibiting seasonal reproduction, patterns of testosterone excretion in adult males are variable: in some species, peaks correlate with female receptivity periods and heightened male-male aggression over access to estrous females, in others, neither heightened aggression nor marked elevations in testosterone have been noted. In this study, we examined mean fecal testosterone ( f T) levels and intermale aggression in wild adult male ring-tailed lemurs residing in three groups at Beza Mahafaly Reserve, Madagascar. Results obtained from mating and post-mating season 2003 were compared to test Wingfield et al. [1990. Am Nat 136:829-846] "challenge hypothesis", which predicts a strong positive relationship between male testosterone levels and male-male competition for access to receptive females during breeding season. f T levels and rates of intermale aggression were significantly higher during mating season compared to the post-mating period. Mean f T levels and aggression rates were also higher in the first half of the mating season compared with the second half. Number of males in a group affected rates of intermale agonism, but not mean f T levels. The highest-ranking males in two of the groups exhibited higher mean f T levels than did lower-ranking males, and young males exhibited lower f T levels compared to prime-aged and old males. In the post-mating period, mean male f T levels did not differ between groups, nor were there rank or age effects. Thus, although male testosterone levels rose in relation to mating and heightened male-male aggression, f T levels fell to baseline breeding levels shortly after the early mating period, and to baseline non-breeding levels immediately after mating season had ended, offsetting the high cost of maintaining both high testosterone and high levels of male-male aggression in the early breeding period.     

Correlated effects of the Y-chromosome of mice on developmental changes in testosterone levels and intermale aggression.

Serum testosterone (T) was studied developmentally in DBA/1/Bg and C57BL/10/Bg inbred mice, as well as in their reciprocal F1 hybrids. Testosterone determinations were made using a radioimmunoassay. At 35 and 40 days $\text{post partum}$, DBA/1/Bg mice had higher levels of T than C57BL/10/Bg males. Comparison of the regression coefficients for the serum T values over days 30, 35, 40, and 50 also indicated statistical differences in the two curves of developmental changes in T. The pubertal rise in B10D1F1 but not D1B10F1 males was found to be steeper in slope over days 30, 35, 40, and 50 than that of C57BL/10/Bg males. These data suggest that during the pubertal period there may be a more rapid increase in serum T titer associated with the DBA/1 Y-chromosome. However, the developmental curves of T for the two reciprocal hybrids were not statistically different from each other or from that of the DBA/1/Bg males. These findings are suggestive of a Y-chromosome effect on developmental changes in T and of a genetic correlation with intermale aggression.     

Testosterone-induced aggression in adult female mice

Silastic implants of testosterone (T) and injections of testosterone propionate (TP) were used to study the effects of ovariectomy and androgen administration on the fighting behavior of adult female mice. A dose of T previously shown to hyperstimulate accessory organ growth in adult male castrates was sufficient to induce the complete behavioral repertoire of male-like aggression in females never before treated with exogenous androgen. As determined by radioimmunoassay, blood levels of T produced by implants containing an aggression-inducing dose of T (10-mg implant) were within the range of T concentrations observed in intact males. Following treatment with a 10-mg T implant, the aggressive behavior of ovariectomized females could be fully maintained with a dose of T (0.3-mg implant) that failed to maintain weight of the accessory organs in adult male castrates. In fact, females “androgenized” were subsequently more responsive to the aggression-activating properties of T than were males castrated after prenatal and perinatal androgen exposure.     

The effect of pituitary homografts on the accessory sex organs and serum hormonal levels with or without androgen in mature male rats

The effect of pituitary homografts on the accessory sex organs and hormonal levels were studied in Wistar mature male rats. Grafted rats were further divided into four experiments: rats were bled once daily via a jugular vein cannula for seven days to investigate when serum prolactin began to rise after transplantation. rats were decapitated on the seventh day after transplantation to test whether 7 days were long enough to show the effect of pituitary grafts on the weight of prostate and seminal vesicles. rats were orchiectomized or orchiectomized and adrenalectomized on the seventh day after transplantation and then decapitated 4 weeks later to test a long term action of pituitary grafts and hormonal levels on the accessory sex organs without androgen. Rats grafted with several pieces of muscle were used as controls in each experiment. The initial rise in serum prolactin level was observed on the fourth day after pituitary transplantation, and then a higher serum prolactin level was maintained thereafter. Despite the higher prolactin level in the pituitary-grafted rat than in the control, no significant differences from the control in the weight of prostates and seminal vesicles and adrenal gland and the concentrations of serum luteinizing hormone (LH) and follicular stimulating hormone (FSH) were measured. This result showed that the weight of accessory sex organs was not affected by a higher serum prolactin within seven days.(ABSTRACT TRUNCATED AT 250 WORDS)     

The breeding behaviour of a northern temperate orconectid crayfish, Orconectes rusticus

Observations of breeding activities of male and female crayfish (Orconectes rusticus) in a natural stream population during the explosive spring breeding period, and supplemental laboratory observations, indicate a promiscuous mating system characterized by intense intermale aggression. Males wandered extensively over the substrate, fought frequently with each other, and attempted to disrupt copulating pairs. Larger males (as distinguished by large chela size) more frequently initiated and won aggressive encounters and interrupted copulating pairs more successfully than smaller males, but were subjected to greater predation. Intense intermale aggression ceased when females sequestered themselves under rocks in the stream or were removed from the laboratory population. Brief resistance by females to approaching males, along with sequestering behaviour, suggests that female choice may also occur.     

Serum testosterone, male dominance, and aggression in captive groups of vervet monkeys (Cercopithecus aethiops sabaeus)

The relationship of serum testosterone concentration to male dominance rank and frequency of aggression was investigated in stable vervet monkey social groups, each containing two or three adult males, several adult females, and their offspring. Dominance relationships were determined by noting an animal's success in intermale aggressive encounters. A striking finding was the marked within-subject variation in testosterone concentration: 5- to 10-fold fluctuations were often observed on successive days. When all 15 groups were considered together, testosterone concentration was unrelated to dominance rank. Although mean testosterone concentration for all dominant males was higher than the mean for all subordinate males, this difference was not significant. In a subset of 4 groups, the rate of aggression initiated was significantly correlated with same-day testosterone in dominant but not in subordinate males.     

Testosterone and dihydrotestosterone levels in epididymis, vas deferens, seminal vesicle and preputial gland of mice after hCG injection

Temporal changes of testosterone (T) and dihydrotestosterone (DHT) levels were measured by RIA in epididymis, vas deferens, seminal vesicle and preputial gland of adult male mice after a single injection of hCG. The response of circulating T to hCG stimulation was rapid and persisted over a period of 48 h. The temporal changes of androgen content of target organs paralleled the modifications of circulating T. In all organs the high androgen levels attained at 1 or 4 h plateaued until 24 h, decreased thereafter and returned to basal values at 72 h. The concentration of T by sex accessory organs was more accelerated by hCG injection than its conversion into DHT.     

Juvenile hormone and the ontogeny of cockroach aggression

Our previous study [Kou et al., 2008. Juvenile hormone levels are increased in winners of cockroach fights. Horm. Behav. 52, 252–260] showed that the basic principle of the challenge hypothesis (hormone levels can respond to social stimuli to modulate aggression in vertebrates] could be applied to juvenile hormone (JH) levels and aggression in the lobster cockroach Nauphoeta cinerea. In that study, 80- to 85-day-old socially naïve males were used, as fighting is much more easily initiated in these older animals than in younger males, and JH III levels in the dominant were found to be significantly increased after an encounter compared to before the encounter and were significantly higher than those in the subordinates. In N. cinerea, newly emerged males usually show no aggressiveness towards each other and aggression is only initiated after several days of close contact. To investigate the development of aggression from an early age, in the present study, newly emerged males were paired to investigate the relationship between JH levels and aggression. The results showed that injection of JH III significantly increased the probability of the young males being fight winners. In each age group in which aggression was initiated, the dominants had significantly higher JH levels than either the subordinates or the same aged non-fighters. JH injection of subordinates on the day of rank establishment had no effect on the probability of rank switch. These results indicate that, (i) in newly emerged male pairs, JH plays a decisive role in rank establishment and the fact that dominant status is significantly associated with a higher JH titer and subordinate status with a lower JH titer is consistent with the basic principle of the challenge hypothesis, and (ii) after rank establishment, the lack of effect of JH treatment on rank change is consistent with the idea of “social inertia” in vertebrates.     

Effects of testosterone on sexual behavior and morphology in adult female leopard geckos, Eublepharis macularius.

The leopard gecko, Eublepharis macularius, is a species in which testosterone (T) is the primary circulating sex hormone in adults of both sexes. There are, however, sex differences in T physiology. Whereas males have prolonged periods with high T levels, T levels cycle in accord with follicular development in females. Specifically, T concentration increases during vitellogenesis, drops after ovulation, and then remains at previtellogenic levels until eggs are laid and the next follicular cycle begins. To determine the function of T in females, we manipulated both the level and the duration of T elevation using Silastic implants in intact, adult female leopard geckos. Females had low ( approximately 1 ng/ml), medium ( approximately 100 ng/ml), or high ( approximately 200 ng/ml) T levels for either a short (8 days) or a long (35 days) duration. Behavior tests with males were conducted on days 1-5 in the short-duration group or on days 29-33 in the long-duration group. For both short- and long-duration groups, T treatment decreased attractivity in females with medium and high T levels compared to females with low T levels. In contrast, females with a medium T level were more receptive than females with a low T level in the short-duration group. Females in the long-duration group were unreceptive regardless of T level. Females treated for a long duration also displayed more aggression toward and evoked more aggression from males than short duration females. Short-duration T treatment had no masculinizing effect on female morphology, whereas medium and high T levels for a long duration induced development of hemipenes. Overall, these results suggest that T can both increase and decrease sexual behaviors in the female leopard gecko.     

Winning fights elevates testosterone levels in California mice and enhances future ability to win fights

The 'winner effect' has been studied in a variety of species, but only rarely in mammals. We compared effects of winning three, two, one, or zero resident-intruder encounters on the likelihood of winning a subsequent aggressive encounter in the California mouse (Peromyscus californicus). During the training phase, we ensured that resident males won all encounters by staging contests with mildly sedated, smaller intruders. During the test phase, the resident male encountered an unfamiliar, more evenly matched intruder that had experience winning an encounter and was larger than the resident. Testosterone (T) plasma levels significantly increased after the final test when they had experienced two prior winning encounters, and the probability of winning a future encounter increased significantly after three prior wins independent of intrinsic fighting ability. We hypothesize a 'winner-challenge' effect in which increased T levels serve to reinforce the winner effect in male California mice.     

Photoperiod alters central distribution of estrogen receptor alpha in brain regions that regulate aggression

Testosterone or its metabolite, estrogen, regulates aggression in males of many mammalian species. Because plasma testosterone levels are typically positively correlated with both aggression and reproduction, aggression is expected to be higher when males are in reproductive condition. However, in some photoperiodic species such as Siberian hamsters (Phodopus sungorus), males are significantly more aggressive in short day lengths when the testes are regressed and circulating testosterone concentrations are reduced. These results led to the formation of the hypothesis that aggression is modulated independently of circulating steroids in Siberian hamsters. Thus, recent studies have been designed to characterize the role of other neuroendocrine factors in modulating aggression. However, aggression may be mediated by testosterone or estrogen despite basal concentrations of these steroids by increasing sensitivity to steroids in specific brain regions. Consistent with this hypothesis, we found that males housed under short days have increased expression of estrogen receptor alpha in the bed nucleus of the stria terminalis, medial amygdala, and central amygdala. Neural activation in response to an aggressive encounter was also examined across photoperiod.     

Dispersal Status Influences Hormones and Behavior in the Male Spotted Hyena

Male spotted hyenas (Crocuta crocuta) reach puberty at 24 months of age and then invariably emigrate from their natal clans 1 to 38 months later. Thus there are two classes of reproductively mature males in everyCrocutaclan: adult natal males born in the clan and adult immigrant males born elsewhere. In one free-living hyena population in Kenya, these two groups of males were compared with respect to measures of aggression, social dominance, sexual behavior, and circulating hormone levels. Adult natal males engaged in higher hourly rates of aggression than did immigrants, won all fights with immigrants, and were socially dominant to immigrants. In addition, adult natal males engaged in far lower hourly rates of sexual behavior with resident females than did immigrants, and natal males were never observed to copulate with natal females. Mean basal plasma cortisol values did not differ between the two groups of adult males, but cortisol concentrations in immigrants were positively correlated with tenure in the clan and with immigrant male social rank. Adult natal males had plasma testosterone levels significantly lower than those of immigrants. Social rank and plasma testosterone values were positively correlated among immigrant males. Thus two different relationships appear to exist between circulating testosterone and social rank in maleCrocuta:one apparent in immigrants and the other in natal adult males. Our results suggest that dispersal might disinhibit testosterone secretion in postpubertal male hyenas.     

Repeated Recent Aggressive Encounters Do Not Affect Behavioral Consistency in Male Siamese Fighting Fish

Consistent individual differences in behavior suggest that individuals respond in a predictable and repeatable manner in a specific situation while differing from other individuals. Male Siamese fighting fish exhibit consistent individual differences in decision‐making strategies when they encounter a receptive female and a rival male simultaneously. However, whether these differences are altered by recent experience is unknown. We examined the influence of repeated aggressive encounters on behavioral consistency and decision‐making. Males were presented with paired female–male dummies prior to any aggressive experiences to obtain a baseline measure. Next, males either won or lost three consecutive contests against rivals and then received the paired female–male dummies after each of these encounters. Overall levels of highly aggressive behaviors were affected by contest outcome, while levels of female‐directed were not. Not surprisingly, winning a fight led to an increase in male‐directed bites, an overtly aggressive behavior that only occurs after fights have escalated. Fighting a male before encountering the dummies caused males to perform more tail beats to the dummy male, perhaps as a result of increased motivation. Males exhibited similar levels of repeatability and used the same strategies when faced with conflicting stimuli regardless of fighting experience. Thus, while winning or losing a fight impacts overall aggression, it does not influence behavioral consistency. This study demonstrates that consistent individual differences and decision‐making strategies may be resistant to recent aggressive experiences, even over a period of days.     

Manipulation of aggressive behavior in adult DBA/2/Bg and C57BL/10/Bg male mice implanted with testosterone in silastic tubing

Adult DBA/2/Bg and C57BL/10/Bg male mice were orchidectomized and implanted subcutaneously with one of two doses of testosterone (T) in Silastic tubing. Steroid doses which resulted in significant atrophy and hypertrophy of accessory sex tissues were determined for each strain. Relative to sham-castrated vehicle-implanted controls, both hypophysiological and hyperphysiological implants caused augmentation of aggression in C57BL/10/Bg males. In DBA/2/Bg mice, the hypophysiological dose reduced and the hyperphysiological implant increased aggression scores. The results demonstrate that agonistic behavior in male mice is influenced by the titer of circulating T and emphasize the importance of genetic variance in the study of hormones and behavior.     

No barriers to gene flow among sympatric polychromatic 'small'Telmatherina antoniae from Lake Matano, Indonesia

Genetic divergence, assortative courtship and intermale aggression were assessed between sympatric colour morphs of the sailfin silverside Telmatherina antoniae , endemic to Lake Matano, Indonesia. Genetic analysis using microsatellite markers showed no barriers to gene flow among T. antoniae primary colour morphs (blue and yellow) within sampling sites, sympatric populations or at the lake-wide level. Low but significant genetic differentiation was found between yellow morphs and mixed (blue–yellow) morphs. Behavioural surveys indicated assortative courtship does occur along primary colour lines; however, intermale aggression among paired and intruding male morphs appeared equal with respect to male colour. These observations support the hypothesis that males view other males as threats to their courtship regardless of their colour. This study supports recent work suggesting that assortative mating is present in T. antoniae despite a lack of reproductive isolation among colour morphs.     

Interactions between behavior and plasma steroids within the scramble mating system of the promiscuous green turtle, Chelonia mydas.

We measured plasma androgen (combined testosterone and 5alpha-dihydrotestosterone) (A) and corticosterone (B) in the promiscuous green turtle (Chelonia mydas) during courtship in the southern Great Barrier Reef. This study examined if reproductive behaviors and intermale aggression induced behavioral androgen and adrenocortical responses in reproductively active male and female green turtles. Associations between reproductive behavior and plasma steroids were investigated in green turtles across the population and within individuals. Levels across a range of both asocial and social behaviors were compared including (a) free swimming behavior; (b) initial courtship interactions; (c) mounted behavior (male and female turtles involved in copulatory activities); (d) intermale aggression (rival males that physically competed with another male turtle or mounted males recipient to these aggressive interactions); and (e) extensive courtship damage (male turtles that had accumulated excessive courtship damage from rival males). Behavioral androgen responses were detected in male turtles, in that plasma A was observed to increase with both attendant and mounted behavior. Male turtles who had been subjected to intermale aggression or who had accumulated severe courtship damage exhibited significantly lower plasma A than their respective controls. No pronounced adrenocortical response was observed after either intermale aggression or accumulation of extensive courtship damage. Female turtles exhibited a significant increase in plasma B during swimming versus mounted behavior, but no change in plasma A. We discuss our results in terms of how scramble polygamy might influence behavioral androgen interactions differently from more typical combative and territorial forms of male polygamy.     

Thyroid gland responses to intermale aggression in an inherently aggressive wild rat

The aim of the present investigation was to ascertain whether intermale aggression (by fighting) affects thyroid gland activity in wild rat. Following intermale aggression for three hours, thyro-follicular epithelial height was significantly decreased and colloidal content in follicular lumen increased only in the subordinate rats whereas the thyroid gland of the dominant rats was not perceptibly changed. Thus, aggression inhibits thyroid gland activity only in the subordinate rats. It is suggested that this effect may be due to psychosomatic rather than physical stress during aggression.     

β-Nerve Growth Factor Measurements in Mouse Serum

Mouse serum beta-nerve growth factor (NGF) levels were measured using a newly developed competitive beta-NGF radioimmunoassay. The basal serum beta-NGF levels in male and female mice were consistently less than 2 ng/ml when these animals were maintained in individual cages for at least 7 days before they were killed. However, in male mice, serum beta-NGF levels were significantly elevated when they were housed 5 per cage. The rise in serum beta-NGF levels, presumably mediated by intermale aggression, was confirmed by grouping previously isolated mice together in one cage for 20 min before they were killed. In all aggressive male mice, serum beta-NGF levels were elevated by two orders of magnitude. The beta-NGF radioimmunoassay values were also validated by a neurite outgrowth bioassay system using the serum of aggressive male mice. In summary, both measurement techniques confirm that mouse serum beta-NGF levels undergo marked changes depending on animal handling conditions.     

Effects of cocaine hydrochloride on the male reproductive system

The reproductive system effects of cocaine were studied in male rats. The analysis included measurements of circulating levels of luteinizing hormone (LH) and testosterone (T) by radioimmunoassay (RIA). The weights of the testes and sex accessory organs were also assessed and compared with control animals. Dosage level, duration of treatment, and interval between injection and sacrifice were the parameters examined. Following a single intraperitoneal (IP) injection, LH levels decreased over a 3-hour period. At a high dosage (40 mg/kg), cocaine caused a significant elevation in serum T followed by a significant depression of T for at least 2 hours. When administered chronically for 15 days, the low dose group (10 mg/kg) did not vary significantly from the vehicle controls. However, the high dose group had lower LH and T levels, as well as correspondingly lighter weight seminal vesicles and epididymis. No changes were noted in the weights of the ventral prostate or testes. This research suggests that cocaine acts primarily at the hypothalamic-hypophyseal axis with a possible secondary action at the gonadal level.