Agonistic behaviour and endogenous plasma hormones in male Japanese quail

The response to stressors in adulthood depends on the interaction between prenatal exposure to glucocorticoids and environmental context

Maternal stress during reproduction can influence how offspring respond to stress later in life. Greater lifetime exposure to glucocorticoid hormones released during stress is linked to greater risks of behavioral disorders, disease susceptibility, and mortality. The immense variation in individual's stress responses is explained, in part, by prenatal glucocorticoid exposure. To explore the long-term effects of embryonic glucocorticoid exposure, we injected Japanese quail (Coturnix japonica) eggs with corticosterone. We characterized the endocrine stress response in offspring and measured experienced aggression at three different ages. We found that prenatal glucocorticoid exposure affected (1) the speed at which the stress response was terminated suggesting dysregulated negative feedback, (2) baseline corticosterone levels in a manner dependent on current environmental conditions with higher levels of experienced aggression associated with higher levels of baseline corticosterone, (3) the magnitude of an acute stress response based on baseline concentrations. We finish by proposing a framework that can be used to test these findings in future work. Overall, our findings suggest that the potential adaptive nature of prenatal glucocorticoid exposure is likely dependent on environmental context and may also be tempered by the negative effects of longer exposure to glucocorticoids each time an animal faces a stressor.

Whither the challenge hypothesis?

Almost fifty years ago the advent of assay methods to measure circulating levels of hormones revolutionized endocrinology in relation to investigations of free-living and captive animals. This new field "environmental endocrinology" revealed that endocrine profiles in animals in their natural habitat were not only different from captive animals, but often deviated from predictions. It quickly became apparent that the organization and analysis of data from the field should be sorted by life history stages such as for reproductive processes, migration, molt etc. and spaced in time according to natural duration of those processes. Presentation of data by calendar date alone gives much simpler, even misleading, patterns. Stage-organized analyses revealed species-specific patterns of hormone secretion and dramatic inter-individual differences. The "Challenge Hypothesis" sparked exploration of these results, which diverged from expectations of hormone-behavior interactions. The hypothesis led to specific predictions about how the hypothalamo-pituitary-gonad axis, and particularly circulating patterns of testosterone, might respond to social challenges such as simulated territorial intrusions. Initially, a group of studies on free-living and captive birds played a key role in the formulation of the hypothesis. Over the decades since, the effects of social challenge and environmental context on hormonal responses have been tested in all vertebrate taxa, including humans, as well as in insects. Although it is now clear that the Challenge Hypothesis in its original form is simplistic, field and laboratory tests of the hypothesis have led to other concepts that have become seminal to the development of environmental endocrinology as a field. In this special issue these developments are addressed and examples from many different taxa enrich the emerging concepts, paving the way for investigations using recent technologies for genetic and transcriptome analyses.

Aggressive dominance can decrease behavioral complexity on subordinates through synchronization of locomotor activities

Social environments are known to influence behavior. Moreover, within small social groups, dominant/subordinate relationships frequently emerge. Dominants can display aggressive behaviors towards subordinates and sustain priority access to resources. Herein, Japanese quail (Coturnix japonica) were used, given that they establish hierarchies through frequent aggressive interactions. We apply a combination of different mathematical tools to provide a precise quantification of the effect of social environments and the consequence of dominance at an individual level on the temporal dynamics of behavior. Main results show that subordinates performed locomotion dynamics with stronger long-range positive correlations in comparison to birds that receive few or no aggressions from conspecifics (more random dynamics). Dominant birds and their subordinates also showed a high level of synchronization in the locomotor pattern, likely emerging from the lack of environmental opportunities to engage in independent behavior. Findings suggest that dominance can potentially modulate behavioral dynamics through synchronization of locomotor activities.

High-resolution behavioral time series of Japanese quail within their social environment

The behavioral dynamics within a social group not only could depend on individual traits and social-experience of each member, but more importantly, emerges from inter-individual interactions over time. Herein, we first present a dataset, as well as the corresponding original video recordings, of the results of 4 behavioral tests associated with fear and aggressive response performed on 106 Japanese quail. In a second stage, birds were housed with conspecifics that performed similarly in the behavioral tests in groups of 2 females and 1 male. By continuously monitoring each bird in these small social groups, we obtained time series of social and reproductive behavior, and high-resolution locomotor time series. This approach provides the opportunity to perform precise quantification of the temporal dynamics of behavior at an individual level within different social scenarios including when an individual showing continued aggressive behaviors is present. These unique datasets and videos are publicly available in Figshare and can be used in further analysis, or for comparison with existing or future data sets or mathematical models across different taxa.

Expression of aggressiveness modulates mesencephalic c-fos activation during a social interaction test in Japanese quail (Coturnix japonica)

It is well known that during a social conflict, interactions are dependent on the animal's propensity to behave aggressively as well as the behavior of the opponent. However, discriminating between these two confounding factors was difficult. Recently, a Social Interaction (SI) test using photocastrated males as non-aggressive stimuli was proposed as a useful tool to evaluate aggressiveness. The avian Intercollicular- Griseum centralis complex (comparable to mammalian periaqueductal gray) has been reported as a crucial node in the descending pathways that organize behavioral and autonomic aspects of defensive responses and aggressiveness. Herein, using the SI test, we evaluated whether mesencephalic areas are activated (expressed c-fos) when photostimulated adult males are confronted with non-responsive (non-aggressive) opponents. Furthermore, we also examined whether mesencephalic activation is related to male performance during the SI test (i.e., aggressive vs. non-aggressive males) in birds reared in enriched or in standard environments. Five mesencephalic areas at two anatomic levels (intermediate and rostral) and locomotion during SI testing were studied. Aggressive males showed increased c-fos expression in all areas studied, and moved at faster speeds in comparison to their non-aggressive and control counterparts. Non-aggressive males and the test controls showed similar c-fos labeling. In general, rearing condition did not appear to influence c-fos expression nor behavior during the SI test. Findings suggest that mesencephalic activation is involved when males are actively expressing aggressive behaviors. This overall phenomenon is shown regardless of both the environmental stimuli provided during the birds´ rearing and the potentially stressful stimuli during the SI trial.

Unexpected results when assessing underlying aggressiveness in Japanese quail using photocastrated stimulus birds

Aggressive behaviors can affect both animal welfare and productivity. Because the expression of aggressive behaviors is dependent on the quality of the opponent, they reflect relative rather than absolute levels of underlying aggressiveness. This study was aimed to characterize the aggressive responsiveness of photostimulated (14:10 h light:dark photoperiod) adult Japanese quail when interacting with a photocastrated (6:18 h light:dark photoperiod) counterpart in a novel test environment and to assesses interindividual variations. This was based on the assumption that photocastrated birds will not actively provoke an aggressive confrontation. Birds were reared in male-female pairs. Frequencies of behaviors (i.e., pecks, threats, chases, grabs, mounts) were determined during 10 min social interactions in a novel environment. A first experiment evaluated 78 encounters between a photostimulated male or female with either a photocastrated male or female (photocastration of sexually mature birds started at 11 wk of age). High interindividual variability was observed and in general, highly aggressive birds (performing 20 or more aggressive interactions) received little or no aggression from their test counterpart. However, unexpectedly, we also found that 37% and 32% of photocastrated males and females, respectively, performed aggressions toward their photostimulated counterparts, and initiated the aggressive interactions in a similar proportion than photostimulated males. Aggressive photocastrated males did not perform reproductive-type behaviors (i.e., grabs, mounts). Aggressiveness in the photocastrated birds was attributed to their social experience prior to photocastration. Therefore, a second experiment evaluated 106 encounters between a photostimulated male or female and a naive photocastrated male (photocastration started at 4 wk of age, prior to sexual development). Photocastrated males performed no aggressions toward their photostimulated counterparts. Consistently with previous studies, our findings show that naive photocastrated males can be used as a non-aggressive stimulus during a social interaction aimed to assess expression of aggressiveness in photostimulated birds. However, caution should be taken when applying the photocastration protocol considering that prior fighting and sexual experience or other physiological changes related with maturation can interfere during subsequent aggressive testing.

Steroid metabolism in the brain: From bird watching to molecular biology, a personal journey

Since Arnold Adolph Berthold established in 1849 the critical role of the testes in the activation of male sexual behavior, intensive research has identified many sophisticated neurochemical and molecular mechanisms mediating this action. Studies in Japanese quail demonstrated the critical role of testosterone action and of testosterone aromatization in the sexually dimorphic medial preoptic nucleus in the activation of male copulatory behavior. The development of an immunohistochemical visualization of brain aromatase in quail then allowed further refinement in the localization of the sites of neuroestrogens production. Testosterone aromatization is required for the activation of both appetitive and consummatory aspects of male sexual behavior. Brain aromatase activity is modulated by steroid-induced changes in the transcription of the corresponding gene but also more rapidly by phosphorylation processes. Sexual interactions with a female also rapidly regulate brain aromatase activity in an anatomically specific manner presumably via the release and action of endogenous glutamate. These rapid changes in estrogen production modulate sexual behavior and in particular its motivational component with latencies ranging between 15 and 30min. Brain estrogens seem to act in a manner akin to a neurotransmitter or at least a neuromodulator. More recently, assays of brain estradiol concentrations in micropunched samples or in dialysis samples obtained from behaviorally active males suggested that aromatase activity measured ex vivo might not be an accurate proxy to the rapid changes in local neuroestrogens production and concentrations. Studies of brain testosterone metabolism are thus not over and will keep scientists busy for a little longer. Elsevier SBN Keynote Address, Montreal.

Male-male sexual behavior in Japanese quail: being "on top" reduces mating and fertilization with females

Male Japanese quail (Coturnix japonica) engage in vigorous same-sex sexual interactions that have been interpreted as aggressive behavior reflecting dominance relationships. The consequences of this behavior for reproductive success, and whether it is a form of competition over mating and fertilization, are unclear. Three experiments were conducted to determine the effect of seeing or interacting with another male on a male's subsequent mating and fertilization success with females. A vigorous interaction with another male in which the subject performed more cloacal contact movements (movements to try to make contact with the other bird's cloacal opening) reduced subsequent mating and fertilization success with a female to a similar extent as a prior mating with a different female. Receiving one or more cloacal contacts from another male was less detrimental for subsequent success. The mere presence of another (stimulus) male delayed mating initiation in those male subjects that approached the stimulus first instead of the female. These results do not support the idea that the male "on top" in male-male sexual interactions is the dominant bird who goes on to achieve greater reproductive success. Instead, the results are consistent with male-male sexual behavior as an occasionally costly by-product of strong mating motivation.

Contextual modulation of androgen effects on agonistic interactions

Seasonal changes in steroid hormones are known to have a major impact on social behavior, but often are quite sensitive to environmental context. In the bi-directionally sex changing fish, Lythrypnus dalli, stable haremic groups exhibit baseline levels of interaction. Status instability follows immediately after male removal, causing transiently elevated agonistic interactions and increase in brain and systemic levels of a potent fish androgen, 11-ketotestosterone (KT). Coupling KT implants with a socially inhibitory environment for protogynous sex change induces rapid transition to male morphology, but no significant change in social behavior and status, which could result from systemically administered steroids not effectively penetrating into brain or other tissues. Here, we first determined the degree to which exogenously administered steroids affect the steroid load within tissues. Second, we examined whether coupling a social environment permissive to sex change would influence KT effects on agonistic behavior. We implanted cholesterol (Chol, control) or KT in the dominant individual (alpha) undergoing sex change (on d0) and determined the effects on behavior and the degree to which administered steroids altered the steroid load within tissues. During the period of social instability, there were rapid (within 2 h), but transient effects of KT on agonistic behavior in alphas, and secondary effects on betas. On d3 and d5, all KT, but no Chol, treated females had male typical genital papillae. Despite elevated brain and systemic KT 5 days after implant, overall rates of aggressive behavior remained unaffected. These data highlight the importance of social context in mediating complex hormone-behavior relationships.

Intra- and interspecific aggression do not modulate androgen levels in dusky gregories, yet male aggression is reduced by an androgen blocker

Discussions about social behavior are generally limited to fitness effects of interactions occurring between conspecifics. However, many fitness relevant interactions take place between individuals belonging to different species. Our detailed knowledge about the role of hormones in intraspecific interactions provides a starting point to investigate how far interspecific interactions are governed by the same physiological mechanisms. Here, we carried out standardized resident-intruder (sRI) tests in the laboratory to investigate the relationship between androgens and both intra- and interspecific aggression in a year-round territorial coral reef fish, the dusky gregory, Stegastes nigricans. This damselfish species fiercely defend cultivated algal crops, used as a food source, against a broad array of species, mainly food competitors, and thus represent an ideal model system for comparisons of intra-and interspecific territorial aggression. In a first experiment, resident S. nigricans showed elevated territorial aggression against intra- and interspecific intruders, yet neither elicited a significant increase in androgen levels. However, in a second experiment where we treated residents with flutamide, an androgen receptor blocker, males but not females showed decreased aggression, both towards intra- and interspecific intruders. Thus androgens appear to affect aggression in a broader territorial context where species identity of the intruder appears to play no role. This supports the idea that the same hormonal mechanism may be relevant in intra- and interspecific interactions. We further propose that in such a case, where physiological mechanisms of behavioral responses are found to be context dependent, interspecific territorial aggression should be considered a social behavior.

Long-lasting and sex-specific consequences of elevated egg yolk testosterone for social behavior in Japanese quail

In birds, early exposure to steroid hormones deposited in egg yolks is hypothesized to result in long-lasting effects on brain and behavior. However, the long-term effects of maternal androgens on the development of social behavior, and whether these could interfere with the effects of the endogenous gonadal hormones that mediate sexual differentiation, remain poorly known. To answer these questions, we enhanced yolk testosterone by injecting testosterone (T) in oil into Japanese quail (Coturnix japonica) eggs prior to incubation. Vehicle-injected (V) eggs served as controls. From age 3 weeks to 8 weeks, sexual development was measured using morphological and physiological traits, and social behavior was measured, including male-typical sexual behavior. In females, treatment with testosterone boosted growth. Males from T-injected eggs developed an affiliative preference for familiar females and differed from V-injected males in the acoustic features of their crows, whereas sexual interest (looking behavior) and copulatory behavior were not affected. These long-lasting and sex-specific yolk testosterone effects on the development of dimorphic traits, but without disrupting sexual differentiation of reproductive behavior suggest potential organizational effects of maternal testosterone, but acting through separate processes than the endocrine mechanisms previously shown to control sexual differentiation. Separate processes could reflect the action of androgens at different times or on multiple targets that are differentially sensitive to steroids or develop at different rates.

Neurogenomic mechanisms of aggression in songbirds

Our understanding of the biological basis of aggression in all vertebrates, including humans, has been built largely upon discoveries first made in birds. A voluminous literature now indicates that hormonal mechanisms are shared between humans and a number of avian species. Research on genetics mechanisms in birds has lagged behind the more typical laboratory species because the necessary tools have been lacking until recently. Over the past 30 years, three major technical advances have propelled forward our understanding of the hormonal, neural, and genetic bases of aggression in birds: (1) the development of assays to measure plasma levels of hormones in free-living individuals, or "field endocrinology"; (2) the immunohistochemical labeling of immediate early gene products to map neural responses to social stimuli; and (3) the sequencing of the zebra finch genome, which makes available a tremendous set of genomic tools for studying gene sequences, expression, and chromosomal structure in species for which we already have large datasets on aggressive behavior. This combination of hormonal, neuroendocrine, and genetic tools has established songbirds as powerful models for understanding the neural basis and evolution of aggression in vertebrates. In this chapter, we discuss the contributions of field endocrinology toward a theoretical framework linking aggression with sex steroids, explore evidence that the neural substrates of aggression are conserved across vertebrate species, and describe a promising new songbird model for studying the molecular genetic mechanisms underlying aggression.

Individual variation and the endocrine regulation of behaviour and physiology in birds: a cellular/molecular perspective

Investigations of the cellular and molecular mechanisms of physiology and behaviour have generally avoided attempts to explain individual differences. The goal has rather been to discover general processes. However, understanding the causes of individual variation in many phenomena of interest to avian eco-physiologists will require a consideration of such mechanisms. For example, in birds, changes in plasma concentrations of steroid hormones are important in the activation of social behaviours related to reproduction and aggression. Attempts to explain individual variation in these behaviours as a function of variation in plasma hormone concentrations have generally failed. Cellular variables related to the effectiveness of steroid hormone have been useful in some cases. Steroid hormone target sensitivity can be affected by variables such as metabolizing enzyme activity, hormone receptor expression as well as receptor cofactor expression. At present, no general theory has emerged that might provide a clear guidance when trying to explain individual variability in birds or in any other group of vertebrates. One strategy is to learn from studies of large units of intraspecific variation such as population or sex differences to provide ideas about variables that might be important in explaining individual variation. This approach along with the use of newly developed molecular genetic tools represents a promising avenue for avian eco-physiologists to pursue.

Afternoon T: testosterone level is higher in red than yellow male polychromatic lizards

Recent work on within-species polymorphism across a broad range of taxa has renewed and considerably increased the attention to this classic evolutionary area, notably in lizard species where colors covary with reproductive strategies. We demonstrate elsewhere that red-headed males beat yellow-headed males in staged contests for females in the Australian painted dragon lizard Ctenophorus pictus. This morph difference in behaviour is linked to what appears to be a convention of red dominance in male-male interactions set very early in ontogeny, long before coloration has developed. In the current note, we investigate the relationship between time of day, which is directly linked to vigilance time in territorial males, and plasma levels of testosterone and corticosterone. We show that red males have higher testosterone levels in late afternoon following a day of territory patrolling and a non-significant trend in plasma corticosterone levels that decline with time of day. In conclusion, there are significant differences in testosterone profile between the two color morphs, providing a potential proximate link to the behavioural differences between them.

Steroid hormones and aggression in female Galápagos marine iguanas

We studied steroid hormone patterns and aggression during breeding in female Galápagos marine iguanas (Amblyrhynchus cristatus). Females display vigorously towards courting males after copulating (female-male aggression), as well as fight for and defend nest sites against other females (female-female aggression). To understand the neuroendocrine basis of this aggressive behavior, we examined changes in testosterone (T), estradiol (E2), corticosterone (CORT), and progesterone (P4) during the mating and nesting periods, and then measured levels in nesting females captured during aggressive interactions. Testosterone reached maximal levels during the mating stage when female-male aggression was most common, and increased slightly, but significantly, during the nesting stage when female-female aggression was most common. However, fighting females had significantly lower T, but higher E2 and P4, than non-fighting females. It remains unclear whether these changes in hormone levels during aggressive interactions are a cause or a consequence of a change in behavior. Our results support the "challenge hypothesis", but suggest that E2 and/or P4 may increase in response to aggressive challenges in females just as T does in males. Females may be rapidly aromatizing T to elevate circulating levels of E2 during aggressive interactions. This hypothesis could explain why non-fighting females had slightly elevated baseline T, but extremely low E2, during stages when aggressive interactions were most common. Although P4 increased rapidly during aggressive encounters, it is unclear whether it acts directly to affect behavior, or indirectly via conversion to E2. The rapid production and conversion of E2 and P4 may be an important mechanism underlying female aggression in vertebrates.

The metabolic cost of egg production is repeatable

The metabolic cost of egg production in birds (passerines) has been measured as a 16–27% increase in basal or resting metabolic rate (BMR and RMR, respectively) when comparing non-breeding values with those in egg-producing individuals. However, available data to date have been obtained in free-living birds and may thus be confounded by the effect of variable ecological conditions on non-reproductive physiological machinery (organ mass or metabolic activity) which might contribute to measured variation in RMR. Here, we show that in captive, controlled conditions, the process of egg formation induces a 22% increase in RMR in female zebra finches Taeniopygia guttata. Among individuals, variation in laying RMR is independent of egg mass, clutch size or total clutch mass. Importantly, we show that individual variation in both non-breeding and laying RMR is repeatable over periods of at least 8-10 months, i.e. individual variation in RMR remained constant over time for any given physiological state. This suggests that the metabolic cost of egg formation should respond to selection. However, we also show that in males, but not females, repeatability of RMR declines over time even when birds are kept in constant controlled conditions.

Immediate early gene expression associated with induction of brooding behavior in Japanese quail

Certain species can be induced to foster infant or neonatal animals through the process of sensitization. We induced brooding behavior in adult Japanese quail through repeated exposure to foster chicks across five 20-min trials. Brooding behavior was characterized by a bird allowing chicks to approach and remain underneath its wings while assuming a distinctive stationary crouching posture, preening, and feather fluffing. Birds who did not show brooding behavior actively avoided chicks. Among the birds that brooded chicks, females brooded chicks for longer durations compared to males. Brooding females continued a regular daily egg laying pattern; males showed no significant changes in testosterone levels after exposure to chicks. In a second experiment, we measured expression of two immediate early gene (IEG) protein products, ZENK and Fos, to identify the brain regions activated or inhibited by brooding behavior in females. ZENK and Fos expression in brooding or sensitized females (SF) were compared with expression in nonmaternal females with chicks (NMF) and with females without chicks and with blocks as control objects (BL). There was a reduced density of ZENK-like immunoreactive (ZENK-lir) cells in the medial preoptic nucleus (POM) in NMF birds. In SF birds, the density of Fos-like immunoreactive (Fos-lir) cells was elevated in the bed nucleus stria terminalis, medial portion (BSTm), and ectostriatum (E). These experiments begin to define the neural circuitry underlying brooding behavior in Japanese quail, and establish a model for future studies of the neural mechanisms of avian parental behavior.

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.

Comparison of social ranks based on worm-running and aggressive behaviour in young domestic fowl

Worm-running is behaviour in which a chick runs carrying a worm-like object while flock mates follow and attempt to grab the object from its beak. We hypothesised that social ranks based on worm-running frequency are stable over time and are positively correlated with social ranks based on success in aggressive interactions when older. At 8-12 days of age, we scored worm-running in 17 groups of 12 female White Leghorn chicks during three 10-min tests. Based on instantaneous scans at 5-s intervals, the bird carrying the 'worm' most often was placed in rank one and so on down the rank order. These tests were repeated at 68-70 days of age. An aggression index for each bird was calculated as the number of aggressive acts given, divided by the number given and received, during three 1-h observation periods when the birds were 68-70 days. Ranks obtained in worm-running tests were positively correlated over the two age periods (P < 0.05) but were not correlated with ranks based on the aggression index (P > 0.05). Our results indicate that worm-running ranks are not predictive of success in aggressive interactions. Instead, worm-running fits some criteria for play.

Predatory aggression, but not maternal or intermale aggression, is associated with high voluntary wheel-running behavior in mice

Predatory (towards crickets), intermale, and maternal aggression were examined in four replicate lines of mice that had been selectively bred for high wheel-running (S) and in four random-bred control lines (C). In generation 18, individual differences in both predatory and intermale aggression were significantly consistent across four trial days, but predatory and intermale aggression were uncorrelated both at the individual level and among the eight line means. Latencies to attack crickets were significantly lower in S lines as a group. Intermale aggression, however, did not differ between S and C lines. S lines were significantly smaller in body mass, but did not differ in either testes mass or plasma testosterone. In generations 28 and 30, respectively, S and C lines did not differ in either maternal or intermale aggression. However, significant differences among the individual lines were found for maternal aggression, and one S line exhibited an extremely high mean time of aggression (>120 sec for a 5-min test). Maternal and intermale aggression were not correlated among the eight line means or at the level of individual variation. Overall, our results suggest: (1) predatory aggression and voluntary wheel-running are positively related at the genetic level; (2) predatory and intermale aggression are unrelated at a genetic level; and (3) maternal and intermale aggression are not tightly related at the genetic level. Possible relationships between predatory aggression, dopamine, and wheel-running behavior are discussed.

Testosterone is correlated with courtship but not aggression in the tropical buff-banded rail, Gallirallus philippensis

Past studies into the roles of testosterone in birds have focused on species that occur in temperate regions. In such species, plasma testosterone levels are high during the establishment of territories in the spring and are associated with increased aggression. In contrast to most temperate species, tropical birds frequently defend territories year-round, during which time territoriality often occurs in a nonsexual context. The few studies that have been carried out on tropical birds show lower levels of circulating testosterone than occur in their temperate counterparts. In some year-round territorial tropical species, testosterone and aggression are dissociated, while in other species testosterone still plays a role in regulating aggression. This study examined the relationship between aggression and plasma testosterone levels in a year-round territorial, subtropical population of the buff-banded rail with characteristics typical of tropical species. Peak testosterone levels were substantially lower than those found in temperate species. Males displayed a seasonal peak in plasma testosterone level when their partners were most likely to be fertile. At other times, testosterone levels were mostly undetectable, despite year-round territoriality. We found that T levels increased with courtship behavior but showed no relationship with aggression, supporting the hypothesis that dissociation between testosterone and territoriality may be widespread among tropical avian taxa.

Hormonal and behavioral responses of starlings during a confrontation with males or females at nest boxes during the reproductive season

We investigated in an aviary experiment the behavioral and hormonal responses of European starlings (Sturnus vulgaris) that were moved from a same sex group to an aviary containing either a nest box alone, a nest box and another male, or a nest box and a female. Luteinizing hormone (LH) and testosterone (T) levels increased significantly and independently of the situation, suggesting that nest boxes were the most important stimulus affecting the levels of these hormones. Some birds occupied more than two boxes (winners), and others a single or no box (losers). Levels of T increased less in males that did not acquire a nest box. However, the increase in LH was similar in all males after the test. Singing was positively correlated with T levels. Winners started singing earlier and sang more during a contest than losers. In the presence of females LH increased more in winners than in losers, while the increase in T was similar in both groups. In females, there was no increase in T but LH increased in the presence of males. Levels were higher in females paired with winners than in females paired with losers. Finally, winners advertised their nest boxes more frequently than losers. These results indicate that within a relatively short time frame levels of LH and T increase following the transfer from a flock to a territorial situation and can react independently from each other depending on reproductive circumstances. For males, the possession of a nest box and, for females, the qualities of the male seemed to be the most important factors stimulating reproduction.

Social stimuli, testosterone, and aggression in gull chicks: support for the challenge hypothesis

We tested the challenge hypothesis for the hormonal regulation of aggression in chicks of the black-headed gull, Larus ridibundus. Chicks of this species are highly aggressive toward conspecifics, but never to peers that hatched from the same clutch (modal clutch size is three). Therefore, in the first experiment small families were housed together in large groups (challenged condition) and compared to families kept isolated (nonchallenged condition). As expected, in the challenged condition during the initial stage of territory establishment basal levels of testosterone (T) were clearly higher than those in the nonchallenged condition. In the second experiment we tested the effect of a short social challenge on short-term T-fluctuations. The design was based on an earlier experiment, showing that after temporary T-treatment chicks become very sensitive to social challenges while having low basal T-levels. We now show that these social challenges induce brief elevations in plasma T-levels. These peaks are similar to those in previously untreated chicks but untreated chicks do not respond with aggression to a challenge. Therefore, we conclude that the initial exposure to elevated T-levels increases the sensitivity to brief changes in T induced by social challenges. In this way exposure to T, that may be detrimental for development, is minimized while birds remain able to defend territories. This is the first report showing that the challenge hypothesis as established for adult birds, is also applicable for aggressive behavior in young birds outside the sexual context. Furthermore we suggest that a phase of priming with T is necessary to obtain the high behavioral responsiveness to a challenge.

Social modulation of androgen levels in male teleost fish

Androgens are classically thought of as the sex steroids controlling male reproduction. However, in recent years evidence has accumulated showing that androgens can also be affected by the interactions between conspecifics, suggesting reciprocal interactions between androgens and behaviour. These results have been interpreted as an adaptation for individuals to adjust their agonistic motivation and to cope with changes in their social environment. Thus, male-male interactions would stimulate the production of androgens, and the levels of androgens would be a function of the stability of its social environment ['challenge hypothesis', Gen. Comp. Endocrinol. 56 (1984) 417]. Here the available data on social modulation of androgen levels in male teleosts are reviewed and some predictions of the challenge hypothesis are addressed using teleosts as a study model. We investigate the causal link between social status, territoriality and elevated androgen levels and the available evidence suggests that the social environment indeed modulates the endocrine axis of teleosts. The association between higher androgen levels and social rank emerges mainly in periods of social instability. As reported in the avian literature, in teleosts the trade-off between androgens and parental care is indicated by the fact that during the parental phase breeding males decreased their androgen levels. A comparison of androgen responsiveness between teleost species with different mating and parenting systems also reveals that parenting explains the variation observed in androgen responsiveness to a higher degree than the mating strategy. Finally, the adaptive value of social modulation of androgens and some of its evolutionary consequences are discussed.

Seasonal differences in the hormonal control of territorial aggression in free-living European stonechats

In birds, territorial aggression during the breeding season is regulated by testosterone (T). However, many bird species also express aggressive behavior during the nonbreeding season, when plasma levels of T are low. It has been suggested that during this period estrogens might play a major role in regulating territorial aggression. In the present study we compared the effects of simultaneous blockage of androgenic and estrogenic actions on territorial aggression during the breeding and nonbreeding seasons in free-living male European stonechats (Saxicola torquata rubicola). European stonechats are of particular interest since they establish territories and form pairs during both the breeding and the nonbreeding seasons. Thus territorial aggression and its endocrine control can be compared between reproductive and non-reproductive contexts. Inhibition of androgenic and estrogenic actions by simultaneous application of Flutamide and ATD reduced territorial aggression during the breeding season, but not during the nonbreeding season. Our results show that androgens and/or estrogens are involved in the endocrine control of territorial aggression in stonechats only in a reproductive context, but not in a non-reproductive one.

Individual variation in corticosterone secretion in laying hens

1. Plasma corticosterone responses to a handling and isolation stressor were measured in laying hens once, before two groups were mixed into a single larger group, and again after the groups were mixed. Hens were bled before the stressor was applied and within 3 min of removal from groups (0 min sample), and again 15 and 45 min later 2. Social interactions were recorded throughout the experiment using continuous, all-occurrences sampling during group observations and the social rank index of each hen was calculated. 3. Plasma corticosterone was low immediately after birds were first picked up and sampled on each occasion, and rose to a peak after repeated handling, 15 min later. It then declined and was almost back to basal levels 40 min after first handling. 4. Corticosterone responses were greater 6 d before mixing (first day of sampling) than they were 1, 7 or 21 d after mixing. Corticosterone concentrations and responses were not elevated 1 d after the two groups of hens were mixed. 5. Variation in corticosterone concentrations and responses within and between hens was quantified by calculating coefficients of variation of corticosterone concentrations and areas under corticosterone response curves. Variation tended to be less within than between hens. 6. There was a transient increase in aggressive interactions immediately after mixing. One bird was clearly dominant in each observed dyadic interaction and hierarchies tended to strong linearity in all groups. Social rank was not related to corticosterone concentrations or responses. 7. Area under the corticosterone response curve can be used to compare differences between and within laying hens in their responsiveness to a handling and isolation stressor.

Castration in Gambel's and Scaled Quail: ornate plumage and dominance persist, but courtship and threat behaviors do not

During the breeding season, testosterone in male birds is often linked to some secondary sexual ornaments, courtship behaviors, and intrasexual aggression. I examined the effect of castration on plumage expression in Gambel's Quail (Callipepla gambelii), a species in which males are highly ornate, and in Scaled Quail (C. squamata), an unornamented species. Using male pairs, each consisting of a castrate and a control, I also assessed whether castration affected (1) the behavior of males, (2) the mating decisions of females, or (3) the outcome of male-male competition. Castration did not alter the plumage of male Gambel's or Scaled Quail. In these species, and some other members of the avian order Galliformes, production of ornate plumage appears to be independent of testosterone. In contrast, castration reduced or eliminated courtship behaviors. Females almost never preferred castrated individuals. During male-male competition, castrates also exhibited lower rates of threat behaviors, which appear to be identical to those used during courtship. Castration did not, however, influence the outcome of male-male competition. Castrates of both species exhibited overt aggression (pecks, chases, displacement) and frequently won male contests. Such results suggest that certain types of aggressive behavior may be testosterone-independent. In both Gambel's and Scaled Quail, male body size correlated positively with dominant individuals.

Social status does not affect resting metabolic rate in wintering dark-eyed junco (Junco hyemalis)

Studies of wintering birds have demonstrated a correlation between social rank and energy expenditures. It is assumed that dominance is energetically costly because of increased activity, possibly caused by elevated androgen levels. As winter acclimatization leads to an increase in metabolic rate, maintaining dominance status in a cold climate can be a substantial challenge. We measured resting metabolic rates in dominant and subordinate dark-eyed juncos (Junco hyemalis) living in small groups in a controlled winter environment. We found no significant effect of social rank when controlling for body size. It has been shown previously that high testosterone levels during the nonbreeding season can lead to higher body conductance, fat loss, and higher nocturnal body temperature. A hypothesis explaining our result is that for juncos it is preferable to maintain low androgen levels during winter and to maintain social rank using a mechanism other than higher agonistic activity.