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In this study, we describe an approach based on an individual's hormonal reactive scope to study short-term effects of hormones on behavior. The control of territorial aggression has been traditionally linked to testosterone. Males of some vertebrate species show an increase in testosterone during territorial interactions and implantation studies suggest that such an increase in testosterone enhances the intensity and persistence of aggression. Here, we tested whether a short-term maximum release of testosterone – based on an individual's hormonal reactive scope – affects the intensity or persistence of territorial aggression in male black redstarts, a bird species in which testosterone does not increase during territorial encounters. An injection with gonadotropin-releasing-hormone (GnRH) induced a physiological peak in plasma testosterone that was specific for each individual (= individual reactive scope). However, such short-term surges in an individual's testosterone concentration did not affect the intensity or persistence of aggression. In conclusion, this study demonstrated (1) that a species that naturally does not increase testosterone during male–male encounters would not benefit from such an increase in terms of being more aggressive, (2) that behavioral studies using GnRH-injections represent a promising approach to study species differences in androgen responsiveness, and (3) that injections of releasing or tropic hormones in general may be a suitable approach to study short-term influences of hormones on behavior. These injections effectively mimic the potential short-term changes in hormones that can occur in the real life of individuals and enable us to study the effects of hormonal changes on behavior or other traits within an ecological and evolutionary framework.

This study evaluated the effect of game venue and starting status on precompetitive psychophysiological measures in elite rugby union. Saliva samples were taken from players (starting XV, n = 15, and nonstarters, n = 9) on a control day and 90 min before 4 games played consecutively at home and away venues against local rivals and league leaders. Precompetition psychological states were assessed using the Competitive State Anxiety Inventory−2. The squad recorded 2 wins (home) and 2 losses (away) over the study period. Calculated effect sizes (ESs) showed higher pregame cortisol- (C) and testosterone- (T) difference values before all games than on a baseline control day (ES 0.7−1.5). Similar findings were observed for cognitive and somatic anxiety. Small between-venues C differences were observed in starting XV players (ES 0.2−0.25). Conversely, lower home T- (ES 0.95) and higher away C- (ES 0.6) difference values were observed in nonstarters. Lower T-difference values were apparent in nonstarters (vs starting XV) before home games, providing evidence of a between-groups effect (ES 0.92). Findings show an anticipatory rise in psychophysiological variables before competition. Knowledge of starting status appears a moderating factor in the magnitude of player endocrine response between home and away games.

Little is known about the role of the endocrine system in financial decision-making. Here, we survey research on steroid hormones and their cognitive effects, and examine potential links to trader performance in the financial markets. Preliminary findings suggest that cortisol codes for risk and testosterone for reward. A key finding of this endocrine research is the different cognitive effects of acute versus chronic exposure to hormones: acutely elevated steroids may optimize performance on a range of tasks; but chronically elevated steroids may promote irrational risk-reward choices. We present a hypothesis suggesting that the irrational exuberance and pessimism observed during market bubbles and crashes may be mediated by steroid hormones. If hormones can exaggerate market moves, then perhaps the age and sex composition among traders and asset managers may affect the level of instability witnessed in the financial markets.

© 2016 A contribution to a special issue on Hormones and Human Competition. The relationship between androgenic hormones, like testosterone (T), and aggression is extensively studied in human populations. Yet, while this work has illuminated a variety of principals regarding the behavioral and phenotypic effects of T, it is also hindered by inherent limitations of performing research on people. In these instances, animal research can be used to gain further insight into the complex mechanisms by which T influences aggression. Here, we explore recent studies on T and aggression in numerous vertebrate species, although we focus primarily on males and on a New World rodent called the California mouse (Peromyscus californicus). This species is highly territorial and monogamous, resembling the modern human social disposition. We review (i) how baseline and dynamic T levels predict and/or impact aggressive behavior and disposition; (ii) how factors related to social and physical context influence T and aggression; (iii) the reinforcing or “rewarding” aspects of aggressive behavior; and (iv) the function of T on aggression before and during a combative encounter. Included are areas that may need further research. We argue that animal studies investigating these topics fill in gaps to help paint a more complete picture of how androgenic steroids drive the output of aggressive behavior in all animals, including humans.

Urban songbirds of several species more vigorously defend their territories in response to conspecific song playback than do their rural counterparts, but the hormonal basis of this behavioral difference is unclear. It is well established in vertebrates that both testosterone and corticosterone affect the intensity of territoriality. Previous studies have found no evidence that initial (i.e., immediately following territorial challenge, but prior to restraint) plasma testosterone accounts for the elevated territorial aggression of urban birds. Determining if testosterone still contributes to urban-rural differences in territoriality requires also assessing males' abilities to transiently increase plasma testosterone (in response to an injection of gonadotropin-releasing hormone). We tested whether these hormones are correlated with the territorial response to conspecific song playback in urban and rural male Song Sparrows (Melospiza melodia) in Montgomery County, Virginia. We found that the elevated territorial aggression of urban sparrows was not related to variation in either initial plasma testosterone or the ability to transiently increase testosterone. In contrast, despite no overall habitat difference in initial corticosterone, levels of this hormone were positively correlated with territoriality in urban and rural sparrows. Furthermore, for a given level of corticosterone, urban sparrows were more territorially aggressive. Our findings suggest that initial corticosterone may either play a role in the regulation of persistent differences in territorial behavior between free-ranging urban and rural male Song Sparrows or be affected by the intensity of behavioral response to territorial challenge.

The Challenge Hypothesis, designed originally to explain the patterning of competitive behavior and androgen levels in seasonally breeding birds, predicts that males will increase their androgen levels in order to become more competitive in reproductive contexts. Here we test predictions derived from the Challenge Hypothesis in white-faced capuchin monkeys (Cebus capucinus), a species that has somewhat seasonal reproduction. We analyzed demographic and hormonal data collected over a 5.25-year period, from 18 males in nine social groups living in or near Lomas Barbudal Biological Reserve, Costa Rica. Alpha males had higher androgen levels than subordinates. Contrary to our predictions, neither the number of breeding-age males nor the number of potentially fertile females was obviously associated with androgen levels. Furthermore, male androgen levels were not significantly linked to social stability, as measured by stability of male group membership or recency of change in the alpha male position. Androgen levels changed seasonally, but not in a manner that had an obvious relationship to predictions from the Challenge Hypothesis: levels were generally at their lowest near the beginning of the conception season, but instead of peaking when reproductive opportunities were greatest, they were at their highest near the end of the conception season or shortly thereafter. This lack of correspondence to the timing of conceptions suggests that there may be ecological factors not yet identified that influence ifA levels. We expected that the presence of offspring who were young enough to be vulnerable to infanticide during an alpha male takeover might influence androgen levels, at least in the alpha male, but this variable did not significantly impact results.

How and why individuals differ from each other is a central question in behavioral and evolutionary ecology, because selection particularly acts on this among-individual variation. It is therefore important to accurately partition phenotypic variances into their within- and between-individual components. Partitioning covariances into both components can also inform about underlying mechanistic pathways that potentially interlink trait expression. In the current study, we applied such a (co)variance partitioning approach to test key predictions of two central hypotheses in behavioral ecology, namely the immunocompetence handicap hypothesis and the challenge hypothesis. To this end, we assessed potential pleiotropic effects of testosterone on male sexual signaling, immune function, and parental care. We here repeatedly measured a set of relevant traits in 47 breeding pairs of captive canaries (Serinus canaria). We found that a within-individual increase in female testosterone level suppressed immune function. Furthermore, testosterone levels were positively related to male song repertoire size as an important component of sexual signaling at the between-male level. These were, however, the only relevant significant correlations. Overall, our data do therefore not convincingly support the hypotheses tested and suggest rather limited hormonal pleiotropic effects of testosterone on immune function, parental care, and male sexual signaling, at least in our study system. Phenotypic variances and covariances can nowadays be partitioned easily into within- and between-individual components. These components inform about trait repeatability and the extent to which multiple traits form phenotypic suites of traits, as well as about their joint underlying mechanistic pathways. Testosterone for example, may be key to mediate the expression of suites of traits in many vertebrate species. We here used captive canaries and repeatedly measured male and female traits, relevant in the context of endocrinology, sexual signaling, immune function, and parental investment. For both sexes, we report particularly weak correlations between testosterone and all other measured traits at both within- and between-individual levels. Our case study thus questions the pleiotropic effects of testosterone, but exemplifies the applicability and relevance of (co)variance partitioning in behavioral ecological research.

The study of testosterone and aggression originated in experimental studies of animals, showing a direct causal link in some species. Human studies showed an overall weak correlation between testosterone and aggression. A theoretical framework ('the challenge hypothesis') enabled testosterone-behavior interactions in humans to be framed within a theory that emphasized hormonal responses to competition influencing subsequent aggressive behavior. The short-term administrations of testosterone to young women and to young men showed influences on behavioral and neural processes associated with aggression. Other findings are that testosterone influences aggression in high dominance men, and in those with low cortisol levels; and that testosterone can affect both aggressive and prosocial behavior, within the context of an experimental game.

Investigating causes and consequences of variation in hormonal expression is a key focus in behavioral ecology. Many studies have explored patterns of secretion of the androgen testosterone in male vertebrates, using the challenge hypothesis (Wingfield, Hegner, Dufty, & Ball, 1990; The American Naturalist, 136(6), 829–846) as a theoretical framework. Rather than the classic association of testosterone with male sexual behavior, this hypothesis predicts that high levels of testosterone are associated with male–male reproductive competition but also inhibit paternal care. The hypothesis was originally developed for birds, and subsequently tested in other vertebrate taxa, including primates. Such studies have explored the link between testosterone and reproductive aggression as well as other measures of mating competition, or between testosterone and aspects of male behavior related to the presence of infants. Very few studies have simultaneously investigated the links between testosterone and male aggression, other aspects of mating competition and infant‐related behavior. We tested predictions derived from the challenge hypothesis in wild male Barbary macaques (Macaca sylvanus), a species with marked breeding seasonality and high levels of male‐infant affiliation, providing a powerful test of this theoretical framework. Over 11 months, 251 hr of behavioral observations and 296 fecal samples were collected from seven adult males in the Middle Atlas Mountains, Morocco. Fecal androgen levels rose before the onset of the mating season, during a period of rank instability, and were positively related to group mating activity across the mating season. Androgen levels were unrelated to rates of male–male aggression in any period, but higher ranked males had higher levels in both the mating season and in the period of rank instability. Lower androgen levels were associated with increased rates of male‐infant grooming during the mating and unstable periods. Our results generally support the challenge hypothesis and highlight the importance of considering individual species’ behavioral ecology when testing this framework.

Testosterone levels show large variations over time and among individuals. Testosterone levels are determined by three factors: the physiological baseline, cyclical (often seasonal) reproductive patterns, and responses to social environment or challenges. Understanding the regulation of testosterone in this manner helps to explain its highly variable expression.

Testosterone and cortisol are hormones expected to play a major role in competitive behaviours (i.e. aggression), and are related to rank and hierarchical stability. Through a non-invasive technique, we analyzed faecal testosterone (FTM 1 ) and cortisol (FCM 2 ) metabolites of dominant and subordinate males from two wild groups of bearded capuchin monkeys. One group had a stable dominance hierarchy while the other had an unstable hierarchy, with a marked conflict period related to a male take-over. In the unstable hierarchy group (1) the dominant male had higher FTM peaks than subordinates, and (2) basal FTM levels were higher than in the stable group. These findings are in accordance with the Challenge Hypothesis and rank-based predictions, and confirm that in Sapajus libidinosus hierarchy stability, social status, aggression rates and testosterone are closely related. Dominants of both groups had higher basal and peak FCM levels, suggesting that in S. libidinosus the dominant male has a higher allostatic load than subordinates, related to his role in protection against predators, intragroup appeasement, and control of food sources. Finally, we suggest that males of S. libidinosus are resistant to testosterone suppression by cortisol, because in the unstable group in spite of an increase in FCM there was also an increase in FTM during the conflict period. This article is part of a Special Issue entitled: Neotropical Behaviour.

The challenge hypothesis posits a correlation between male androgen levels and mating system, male–male aggression in a sexual context, and parental effort. This model has received support across a variety of taxa, including primates. Most primate studies have focused on multi-male societies characterized by relatively high levels of male–male aggression and limited paternal care. To expand this dataset, predictions of the challenge hypothesis were tested in a population of wild siamangs (Symphalangus syndactylus), small apes characterized by intense territoriality, monogamous/polyandrous grouping patterns, and varying amounts of paternal behavior. Behavioral data were collected on 11 study groups (five two-male groups and six one-male groups). Seven hundred thirty-four fecal samples were collected from 18 adult males to quantify concentrations of fecal immunoreactive androgens (fiA) by radioimmunoassay. As predicted by the challenge hypothesis, males involved in the aggressive replacement of a resident male had significantly higher fiA concentrations than control males, while males displaying active parental care had significantly lower fiA concentrations than average. Results showed no association between male androgen concentration and group composition, male rank, or rate of intragroup male–male aggression. These latter findings, seemingly inconsistent with the hypothesis, could be explained by the clear and stable dominance relationships between co-resident males: given the very low rates of physical aggression, a low baseline testosterone is to be expected. Furthermore, the effects of increased intragroup conflict experienced by males in two-male groups might be offset by the higher frequency of intergroup aggression experienced by males in one-male groups.

Male courtship signals often stimulate the production of sex steroids in both female and male receivers. Such effects benefit signallers by increasing receptivity in females, but impose costs on signallers by promoting sexual behaviour and aggression in male competitors. To resolve this androgen-based conflict, males should use strategies that suppress sex steroid production in rival males. In green treefrogs, Hyla cinerea, chorus sounds (i.e. advertisement calls from aggregates of males) are known to stimulate androgen production in receiver males. Here, I examined whether males of this species counter these effects by eliciting an endocrine stress response in male conspecifics during close-range vocal interactions. I show that corticosterone (CORT) levels were higher in males that lost vocal contests in natural choruses compared to contest winners and nonaggressive males. Testosterone levels were also lower in contest losers compared to nonaggressive males, but not contest winners; dihydrotesterone levels did not differ among the three groups. Aggressive and advertisement calls were then broadcast to males in an experiment that simulated close-range vocal communication. Aggressive calls rapidly (45 min) elicited an increase in CORT and a reduction in androgens in receivers. Advertisement calls did not elicit an increase in CORT, but CORT levels were sustained relative to controls exposed to silence and were accompanied by a reduction in androgens in small males. Endocrine responses to acoustic signals in this species thus vary depending upon context, call type and size of signal receivers. Signallers benefit from eliciting CORT production in competitors because elevated CORT suppresses vocalization.

Photoperiod is a major environmental cue that signals breeding conditions in animals living in temperate climates. Therefore, the activity of the reproductive (i.e. hypothalamic–pituitary–gonadal, HPG) axis and of the expression of reproductive behaviors, including territoriality, is responsive to changes in day length. However, at low latitudes the seasonal variation in day length decreases dramatically and photoperiod becomes less reliable as a breeding entraining cue in tropical species. In spite of this, some tropical mammals and birds have been found to still respond to small amplitude changes in photoperiod (e.g. 17 min). Here we tested the effect of 2 photoperiod regimes, referred to as long-day (LD: 16L:08D) and short-day (SD: 08L:16D), on the activity of the HPG axis, on aggressive behavior and in the androgen response to social challenges in males of the tropical cichlid fish Tilapia rendalli. For each treatment, fish were transferred from a pre-treatment photoperiod of 12L:12D to their treatment photoperiod (either LD or SD) in which they were kept for 20 days on stock tanks. Afterwards, males were isolated for 4 days in glass aquaria in order to establish territories and initial androgen levels (testosterone, T; 11-ketotestosterone, KT) were assessed. On the 4th day, territorial intrusions were promoted such that 1/3 of the isolated males acted as residents and another 1/3 as intruders. Territorial intrusions lasted for 1 h to test the effects of a social challenge under different photoperiod regimes. Photoperiod treatment (either SD or LD) failed to induce significant changes in the HPG activity, as measured by androgen levels and gonadosomatic index. However, SD increased the intensity of aggressive behaviors and shortened the time to settle a dominance hierarchy in an androgen-independent manner. The androgen responsiveness to the simulated territorial intrusion was only present in KT but not for T. The percent change in KT levels in response to the social challenge was different between treatments (SD > LD) and between male types (resident > intruder). The higher androgen response to a social challenge in residents under SD may be explained by the time course of the androgen response that due to the long time it takes to fight resolution under LD, might have been delayed. This result illustrates the importance of incorporating time response data in social endocrinology studies. Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Brazil, PDSE fellowship; Fundação de Amparo à Pesquisa no Estado de São Paulo (FAPESP, Brazil; Fundação para a Ciência e a Tecnologia (FCT, Portugal)

In many species testosterone fluctuates in concert with outcome-dependent changes in social status, such that winning a competition leads to an increase in circulating testosterone (i.e., competition effect). Although this phenomenon has been well studied in humans, the cumulative endocrine impact of multiple successive competitions is poorly understood. Moreover, although changes in testosterone after a competition seem to predict immediate aggressive behavior, competitive motivation, risk-taking, and affiliation, whether this endocrine response also has long-term behavioral effects, as suggested by studies in non-human animals, has not been examined. In this study, salivary testosterone was collected from pairs of male participants engaging, on two consecutive days, in head-to-head competitions on a previously validated laboratory task. We found that testosterone reactivity on the first day, which was congruent with the competition effect (i.e., net testosterone increase in randomly assigned winners), predicted the task performance on the second day. Further, when looking at testosterone reactivity on the second day, those individuals that lost both competitions experienced the steepest decline in testosterone compared to those individuals who lost on the second day but won on the first day. Testosterone fluctuations on the second day were also analyzed considering the type of status hierarchy (stable vs. unstable) that emerged as a result of the combined outcomes of the two competitions. In accordance with the challenge hypothesis, men in unstable hierarchies (first day winners/second day losers and first day losers/second day winners) experienced an increase in testosterone compared to men in the stable hierarchies (double winners and double losers). Results are discussed within a comparative perspective, drawing parallels with the winner effect and the challenge hypothesis observed in non-human animals.

In many male vertebrates, the secretion of reproductive (gonadal androgens) and adrenocortical (glucocorticoids) hormones varies seasonally and in response to environmental stimuli, and these hormones exert numerous behavioral and metabolic effects. We performed two field studies on adult male Rufous-winged Sparrows, Peucaea carpalis, a Sonoran Desert rain-dependent sedentary species, to (a) determine seasonal changes in initial (baseline) and acute stress-induced plasma testosterone (T), corticosterone (CORT), and two metabolites (uric acid and glucose) and (b) compare the effects of two types of social challenge (song playback or simulated territorial intrusion consisting of song playback plus exposure to a live decoy bird) on plasma T, CORT, these metabolites, and territorial behavior. Initial plasma T was higher during the summer breeding period than during post-breeding molt. Acute stress resulting from capture and restraint for 30 min decreased plasma T in breeding condition birds but not in the fall, revealing that this decrease is seasonally regulated. Initial plasma CORT did not change seasonally, but plasma CORT increased in response to acute stress. This increase was likewise seasonally regulated, being relatively smaller during autumnal molt than in the summer. We found no evidence that acute stress levels of CORT are functionally related to stress-depressed plasma T and, therefore, that plasma T decreases during stress as a result of elevated plasma CORT. Thirty minutes of exposure to simulated territorial intrusion resulted in different behavior than 30 min of exposure to song playback, with increased time spent near the decoy and decreased number of overhead flights. Neither type of social challenge influenced plasma T, thus offering no support for the hypothesis that plasma T either responds to or mediates the behavioral effects of social challenge. Exposure to both social challenges elevated plasma CORT, but simulated territorial intrusion was more effective in this respect than song playback. Plasma uric acid and glucose decreased during acute stress, but only plasma uric acid decreased during social challenge. Thus, an elevation in plasma CORT was consistently associated with a decrease in plasma uric acid, but not with a change in glycemia. These results enhance our understanding of the short-term relationships between T, CORT, and avian territorial behavior. They provide novel information on the endocrine effects of acute stress, in particular on plasma T, in free-ranging birds, and are among the first in these birds to link these effects to metabolic changes.

Many animals adapt to rapidly changing social environments by using social information to modulate their hormone titres. Modulation of hormone titres, as proposed by the challenge hypothesis, provides a mechanism by which individuals can match their behaviour to their current social environment and thereby avoid costs associated with prolonged high hormone titres. Thus far, most work on social responsiveness of hormone titres has focused on androgens in vertebrates. However, there is mounting evidence that insect hormones, especially juvenile hormone, may respond to social stimuli in ways that parallel androgens in vertebrates. This review will integrate work on social modulation of hormone titres in vertebrates and insects. First we review how the hormone-mediated trade-off between fecundity and life span may be a key selective force favouring socially responsive hormone titres in both systems. Then we review theoretical and empirical work in vertebrates and insects that address how factors such as social instability, dominance rank and mating system influence hormone titre responsiveness. These studies illustrate that (1) juvenile hormone (JH) is responsive to social stimuli in a range of insect taxa and (2) JH responses match key predictions of the challenge hypothesis. We conclude that there are strong similarities in endocrine responsiveness across vertebrate and insect taxa. The challenge hypothesis provides a useful conceptual framework for hypothesis-driven research in insect endocrinology. In addition, exploring areas of convergence and divergence across vertebrates and insects may help clarify how selection has shaped patterns of endocrine responsiveness.

The challenge hypothesis posits that male testosterone levels increase in the presence of fertile females to facilitate mating and increase further in the presence of male rivals to facilitate male–male competition. This hypothesis has been supported in a number of nonhuman animal species. We conducted an experiment to test the challenge hypothesis in men. Thirty-four men were randomly assigned to view high-competitive or low-competitive male rivals at high and low fertility within their partner's ovulatory cycle (confirmed by luteinizing hormone tests). Testosterone was measured upon arrival to the lab and before and after the manipulation. Based on the challenge hypothesis, we predicted that a) men's baseline testosterone would be higher at high relative to low fertility within their partner's cycle, and b) men's testosterone would be higher in response to high-competitive rivals, but not in response to low-competitive rivals, at high relative to low fertility within their partner's cycle. Contrary to the first prediction, men's baseline testosterone levels did not differ across high and low fertility. However, consistent with the second prediction, men exposed to high-competitive rivals showed significantly higher post-test testosterone levels at high relative to low fertility, controlling for pre-test testosterone levels. Men exposed to low-competitive rivals showed no such pattern (though the fertility by competition condition interaction fell short of statistical significance). This preliminary support for the challenge hypothesis in men builds on a growing empirical literature suggesting that men possess mating adaptations sensitive to fertility cues emitted by their female partners.

The Challenge Hypothesis was developed to explain why and how regulatory mechanisms underlying patterns of testosterone secretion vary so much across species and populations as well as among and within individuals. The hypothesis has been tested many times over the past 30years in all vertebrate groups as well as some invertebrates. Some experimental tests supported the hypothesis but many did not. However, the emerging concepts and methods extend and widen the Challenge Hypothesis to potentially all endocrine systems, and not only control of secretion, but also transport mechanisms and how target cells are able to adjust their responsiveness to circulating levels of hormones independently of other tissues. The latter concept may be particularly important in explaining how tissues respond differently to the same hormone concentration. Responsiveness of the hypothalamo-pituitary-gonad (HPG) axis to environmental and social cues regulating reproductive functions may all be driven by gonadotropin-releasing hormone (GnRH) or gonadotropin-inhibiting hormone (GnIH), but the question remains as to how different contexts and social interactions result in stimulation of GnRH or GnIH release. These concepts, although suspected for many decades, continue to be explored as integral components of environmental endocrinology and underlie fundamental mechanisms by which animals, including ourselves, cope with a changing environment. Emerging mass spectrometry techniques will have a tremendous impact enabling measurement of multiple steroids in specific brain regions. Such data will provide greater spatial resolution for studying how social challenges impact multiple steroids within the brain. Potentially the Challenge Hypothesis will continue to stimulate new ways to explore hormone-behavior interactions and generate future hypotheses.

Few studies have investigated the influence of changes in testosterone on subsequent competitive, antagonistic behavior in humans. Further, little is known about the extent to which such effects are moderated by personality traits. Here, we collected salivary measures of testosterone before and after a rigged competition. After the competition, participants were given the opportunity to act antagonistically against the competitor (allocate a low honorarium). We hypothesized that changes in testosterone throughout the competition would predict antagonistic behavior such that greater increases would be associated with the allocation of lower honorariums. Further, we investigated the extent to which personality traits related to psychopathy (fearless dominance, FD; self-centered impulsivity, SCI; and coldheartedness) moderated this relationship. In men (n=104), greater increases in testosterone and greater FD were associated with more antagonistic behavior, but testosterone concentrations did not interact with personality measures. In women (n=97), greater FD and SCI predicted greater antagonistic behavior, but there were no significant endocrine predictors or interactions with personality measures. In a secondary set of analyses, we found no support for the dual-hormone hypothesis that the relationship between baseline testosterone concentrations and behavior is moderated by cortisol concentrations. Thus, results are consistent with previous findings that in men, situation-specific testosterone reactivity rather than baseline endocrine function is a better predictor of future antagonistic behavior. The results are discussed with respect to the Challenge Hypothesis and the Biosocial Model of Status, and the possible mechanisms underlying the independent relations of testosterone and personality factors with antagonistic behavior.

Adaptation to insular environments often arises from changes and innovations in feeding behaviour allowing expanded foraging habits and an increased niche breadth. These shifts and innovations have traditionally been thought to be related to community-wide processes, but could also be the direct result of environmental constraints determining the abundance, availability and suitability of a particular food providing specific nutrients for survival, growth and reproduction. The link between environmental constraints on nutrients and life-history of insular organisms can help in understanding the convergent set of adaptations sustaining the ‘island syndrome’. We tested whether a potential insular nutrient shortage can drive diet shifts, nutritional biochemistry and growth stress, thus contributing to the modulation of life-history traits in a large passerine bird, the red-billed chough (Pyrrhocorax pyrrhocorax). Results supported the insular nutritional challenge hypothesis, linked to an insular insect shortage. An insect shortage may in turn have determined the reduced consumption of this source of protein but increased consumption of other arthropods, and notably fruits, by insular nestlings and fully-grown individuals. Island birds showed comparatively low circulating levels of nutrients and metabolites associated with the consumption of protein-rich animal matter as opposed to carbohydrate-rich vegetal matter, as well as high growth stress reflected in poor feather quality. We propose that feeding shifts derived from an insular insect shortage may exert a strong influence on the allocation of limited time, energy and nutrients among competing functions associated with physiological changes and investment in reproduction and self-maintenance. Traits and patterns generally defining the insular syndrome could thus be linked to particular insular nutrient constraints forcing feeding shifts and nutritional challenges with physiological, demographic and life-history consequences.

To compete for parental food deliveries nestling birds have evolved diverse behaviors such as begging displays and sibling aggression. Testosterone has been suggested to be an important mechanism orchestrating such competitive behaviors, but evidence is scarce and often indirect. Siblicidal species provide an interesting case in which a clear dominance hierarchy is established and the dominant chicks lethally attack siblings. We experimentally elevated testosterone in chicks of a facultatively siblicidal species, the black-legged kittiwake, Rissa tridactyla, and showed that testosterone-treated chicks were more aggressive toward their sibling than were control chicks. In such facultatively siblicidal species, chicks normally exhibit intense aggression only when threatened by starvation. Indeed, we found that chicks in relatively poorer condition were more aggressive than were chicks in better condition, even among testosterone-treated chicks, suggesting the action of an additional signal modulating aggression. Relatively larger siblings were also more aggressive than were relatively smaller siblings, confirming the importance of size advantage in determining dominance hierarchies within the brood. In addition, testosterone increased aggression toward a simulated predator, indicating that in kittiwakes testosterone can increase aggression in contexts other than siblicide. Testosterone promoted aggression-mediated dominance, which increased begging although testosterone treatment did not have a significant separate effect on begging. Therefore, testosterone production in the kittiwake and most likely other siblicidal species seems an important fitness mediator already early in life, outside the sexual context and not only manifesting itself in aggressive behavior but also in dominance-mediated effects on food solicitation displays toward parents.

article i nfo Article history: Initial receipt 5 March 2013 Final revision received 19 June 2013 Testosterone plays an important role in mediating male reproductive trade-offs in many vertebrate species, augmenting muscle and influencing behavior necessary for male-male competition and mating-effort. Among humans, testosterone may also play a key role in facilitating male provisioning of offspring as muscular and neuromuscular performance is deeply influenced by acute changes in testosterone. This study examines acute changes in salivary testosterone among 63 Tsimane men ranging in age from 16 to 80 (mean 38.2) years during one-hour bouts of tree-chopping while clearing horticultural plots. The Tsimane forager- horticulturalists living in the Bolivian Amazon experience high energy expenditure associated with food production, have high levels of parasites and pathogens, and display significantly lower baseline salivary testosterone than age-matched US males. Mixed-effects models controlling for BMI and time of specimen collection reveal increased salivary testosterone (p b 0.001) equivalent to a 48.6% rise, after one hour of tree chopping. Age had no effect on baseline (p = 0.656) or change in testosterone (p = 0.530); self-reported illness did not modify testosterone change (p = 0.488). A comparison of these results to the relative change in testosterone during a competitive soccer tournament in the same population reveals larger relative changes in testosterone following resource production (tree chopping), compared to competition (soccer). These findings highlight the importance of moving beyond a unidimensional focus on changes in testosterone and male-male aggression to investigate the importance of testosterone-behavior interactions across additional male fitness-related activities. Acutely increased testosterone during muscularly intensive horticultural food production may facilitate male productivity and provisioning.

An accumulation of evidence suggests that increased exposure to androgens is associated with prostate cancer risk. The unrestricted energy budget that is typical of Western diets represents a novel departure from the conditions in which men's steroid physiology evolved and is capable of supporting distinctly elevated testosterone levels. Although nutritional constraints likely underlie divergent patterns of testosterone secretion between Westernized and non-Western men, considerable variability exists in men's testosterone levels and prostate cancer rates within Westernized populations. Here, I use evolutionary life history theory as a framework to examine prostate cancer risk. Life history theory posits trade-offs between investment in early reproduction and long-term survival. One corollary of life history theory is the 'challenge hypothesis', which predicts that males augment testosterone levels in response to intrasexual competition occurring within reproductive contexts. Understanding men's evolved steroid physiology may contribute toward understanding susceptibility to prostate cancer. Among well-nourished populations of Westerners, men's testosterone levels already represent an outlier of cross-cultural variation. I hypothesize that Westernized men in aggressive social environments, characterized by intense male-male competition, will further augment testosterone production aggravating prostate cancer risk.

Although testosterone (T) has well known organizational and activational effects on aggression, the relationship between the two is not always clear. The challenge hypothesis addresses this problem by proposing that T will affect aggression only in fitness-enhancing situations. One way to test the challenge hypothesis is to examine the relationship between T and different types of aggression. Chimpanzees, Pan troglodytes, show aggressive behaviours in several contexts and provide an opportunity for such a test. Here we show that urinary T influences a form of male chimpanzee reproductive aggression, territorial boundary patrols. In contrast, T does not affect predatory behaviour, a form of aggression that has no immediate link to male reproduction. While these results are consistent with the challenge hypothesis, our results indicate that male chimpanzees experience a significant drop in urinary T during hunts. Additional analyses reveal that males who share meat with others display this decrease. The reason for this decrement is unclear, but we hypothesize that the relative lack of aggression that results from voluntary sharing episodes and the tolerance engendered by such acts may be contributory factors.

Social challenges from rival conspecifics are common in the lives of animals, and changes in an animal’s social environment can influence physiology and behavior in ways that appear to be adaptive in the face of continued social instability (i.e. social priming). Recently, it has become clear that testosterone, long thought to be the primary mediator of these effects, may not always change in response to social challenges, an observation that highlights gaps in our understanding of the proximate mechanisms by which animals respond to their social environment. Here, our goal is to address the degree to which testosterone mediates organismal responses to social cues. To this end, we review the behavioral and physiological consequences of social challenges, as well as their underlying hormonal and gene regulatory mechanisms. We also present a new case study from a wild songbird, the dark-eyed junco Junco hyemalis, in which we find largely divergent genome-wide transcriptional changes induced by social challenges and testosterone, respectively, in muscle and liver tissue. Our review underscores the diversity of mechanisms that link the dynamic social environment with an organisms’ genomic, hormonal, and behavioral state. This diversity among species, and even among tissues within an organism, reveals new insights into the pattern and process by which evolution may alter proximate mechanisms of social priming.

Decades of behavioral endocrinology research have shown that hormones and behavior have a bidirectional relationship; hormones both influence and respond to social behavior. In contrast, hormones are often thought to have a unidirectional relationship with ornaments. Hormones influence ornament development, but little empirical work has tested how ornaments influence hormones throughout life. Here, we experimentally alter a visual signal of fighting ability in Polistes dominulus paper wasps and measure the behavioral and hormonal consequences of signal alteration in signalers and receivers. We find wasps that signal inaccurately high fighting ability receive more aggression than controls and receiving aggression reduces juvenile hormone (JH) titers. As a result, immediately after contests, inaccurate signalers have lower JH titers than controls. Ornaments also directly influence rival JH titers. Three hours after contests, wasps who interacted with rivals signaling high fighting ability have higher JH titers than wasps who interacted with rivals signaling low fighting ability. Therefore, ornaments influence hormone titers of both signalers and receivers. We demonstrate that relationships between hormones and ornaments are flexible and bidirectional rather than static and unidirectional. Dynamic relationships among ornaments, behavior, and physiology may be an important, but overlooked factor in the evolution of honest communication.

A contribution to a special issue on Hormones and Human Competition. Since Archer's (2006) influential meta-analysis, there has been a major increase in the number of studies investigating the effect of competition outcome on testosterone reactivity patterns in humans. Despite this increased research output, there remains debate as to whether competition outcome modulates testosterone concentrations. The present paper examines this question using a meta-analytic approach including papers published over the last 35years. Moreover, it provides the first meta-analytic estimate of the effect of competition outcome on testosterone concentrations in women. Results from a meta-analysis involving 60 effect sizes and >2500 participants indicated that winners of a competition demonstrated a larger increase in testosterone concentrations relative to losers (D=0.20)-an effect that was highly heterogeneous. This 'winner-loser' effect was most robust in studies conducted outside the lab (e.g., in sport venues) (D=0.43); for studies conducted in the lab, the effect of competition outcome on testosterone reactivity patterns was relatively weak (D=0.08), and only found in studies of men (D=0.15; in women: D=-0.04). Further, the 'winner-loser' effect was stronger among studies in which pre-competition testosterone was sampled earlier than (D=0.38, after trim and fill correction) rather than within (D=0.09) 10min of the start of the competition. Therefore, these results also provide important insight regarding study design and methodology, and will be a valuable resource for researchers conducting subsequent studies on the 'winner loser' effect.

Within an evolutionary framework, in recent years some questions have been raised about whether women have a pattern of psychobiological response to social stress similar to that described in men. The main objective of this study was to analyze women's patterns of neuroendocrine, cardiovascular and mood responses to an individual competitive task, taking into account the outcome obtained. For this purpose, we measured salivary testosterone (T) and cortisol (C) levels, heart rate (HR) and blood pressure (BP), in addition to mood changes, in 40 healthy young women before, during and after a face-to-face laboratory competition. We also assessed some relevant psychological traits. Our results indicate that women who became winners presented greater T and positive mood increases, together with higher cardiovascular (CV) responses, than those who lost and did not show significant changes during the period studied. These results suggest a biological and psychological pattern of response to a laboratory competition differentially associated with outcome. Furthermore, these findings suggest that women who are involved in competitive situations, use both passive and active coping strategies, which can be explained by integrating the existing hypotheses.

Social status primarily determines male mammalian reproductive success, and hypotheses on the endocrinology of dominance have stimulated unprecedented investigation of its costs and benefits. Under the challenge hypothesis, male testosterone levels rise according to competitive need, while the social stress hypothesis predicts glucocorticoid (GC) rises in high-ranking individuals during social unrest. Periods of social instability in group-living primates, primarily in baboons, provide evidence for both hypotheses, but data on social instability in seasonally breeding species with marked social despotism but lower reproductive skew are lacking. We tested these hypotheses in seasonally breeding rhesus macaques on Cayo Santiago, Puerto Rico. We documented male fecal GC and androgen levels over a 10-month period in relation to rank, age, natal status, and group tenure length, including during a socially unstable period in which coalitions of lower ranked males attacked higher ranked males. Androgen, but not GC, levels rose during the mating season; older males had lower birth season levels but underwent a greater inter-season rise than younger males. Neither endocrine measure was related to rank except during social instability, when higher ranked individuals had higher and more variable levels of both. High-ranking male targets had the highest GC levels of all males when targeted and also had high and variable GC and androgen levels across the instability period. Our results provide evidence for both the challenge and social stress hypotheses.

Recent research points to decreasing testosterone (T) levels as well as decreasing relationship quality during the transition to fatherhood, and it has been suggested that T reflects and affects motivation and behavior with respect to mating or paternal effort. Accordingly, we hypothesized that decreases in T are associated with decreasing relationship quality in new fathers. Thirty-seven fathers and 38 men in committed romantic relationships without children (controls) were recruited. All subjects participated actively by collecting saliva samples for T assessment three times a day on two assessment days, four weeks prior to birth (day 1) and eight weeks after birth (day 2) for fathers, and three months after the first assessment for controls and by filling out questionnaires on relationship quality. Results revealed significantly lower T levels (AUCg-T) in fathers than in controls at day 2 and significant decreases in relationship quality from day 1 to day 2 in fathers, but not in controls. In particular, the new fathers reported tenderness in their relationship to have significantly decreased from pre to post birth in comparison to the controls. These results were partially moderated by T levels at day 1. We interpret our results as being in line with the "challenge hypothesis" in humans, according to which T levels are positively associated with mating effort and negatively related to paternal activities.

Male song sparrows (Melospiza melodia) are territorial year-round. However, neuroendocrine responses to simulated territorial intrusions (STI) differ between breeding (spring) and non-breeding seasons (autumn). In spring, exposure to STI leads to increases in plasma levels of luteinizing hormone and testosterone (consistent with the challenge hypothesis), but not in autumn. This suggests that there are fundamental differences in the mechanisms driving neuroendocrine responses to STI between seasons despite apparently identical behavioral responses. Recent studies have also shown that areas of the telencephalon and diencephalon involved with singing behavior and aggression express the enzymes necessary to synthesize sex steroids de novo from cholesterol. Of these, aromatase (that regulates the conversion of testosterone to estradiol) and 3β-hydroxysteroid dehydrogenase (that regulates the synthesis of biologically active steroids) are regulated seasonally, whereas receptors for sex steroids such as androgen receptor and estrogen receptor alpha and beta are not. Functional analyses indicate specific genes that may be involved in the mechanisms of differential neuroendocrine responses to aggressive interactions in different life-history stages. Microarrays were used to test the hypothesis that gene expression profiles in the hypothalamus after territorial aggression differ between the seasons. Over 150 genes were differentially expressed between spring and autumn in the control birds and 59 genes were significantly affected by STI in autumn, but only 14 genes in spring. Real-time PCR was performed for validation, and it indicated that STI drives differential genomic responses in the hypothalamus in the breeding versus non-breeding seasons. The results suggest major underlying seasonal effects in the hypothalamus that determine the differential response upon social interaction.

The Challenge Hypothesis postulates that male vertebrates can respond to social challenges, such as simulated territorial intrusions, by rapidly increasing their concentrations of plasma androgens, such as testosterone (T). This increase may facilitate the expression of aggressive behavior and lead to persistence of this behavior even after withdrawal of the challenge, thus potentially promoting territoriality and the probability of winning future challenges. The scope of the Challenge Hypothesis was tested by exposing free-ranging male Cassin's Sparrows, Peucaea cassinii, to conspecific song playback (SPB) at the beginning of the vernal nesting season. Exposure to SPB stimulated aggressive behavior but did not influence plasma T. Furthermore, plasma T did not correlate with the duration of exposure to SPB, and the behavioral response to SPB did not differ in males that were challenged a second time shortly after the first challenge. As birds were investigated at a stage of their reproductive cycle when plasma T is presumably seasonally high due to photostimulation, the lack of hormonal response to SPB may have been due to the hypothalamus-pituitary-gonadal axis secreting hormones at maximum rates. This was not the case, however, because administration of gonadotropin-releasing hormone I rapidly stimulated the secretion of luteinizing hormone (LH) and T, and treatment with ovine LH rapidly stimulated T secretion.

The giant panda (Ailuropoda melanoleuca) is notoriously difficult to study in the wild, but its interesting reproductive ecology makes the effort worthwhile. Perhaps more than most species, the panda is energy-limited, which alters the cost/benefit analysis of its reproductive ecology. Using global positioning system/very high frequency radiocollars to locate mating aggregations, we used behavioral observations and fecal testosterone assays to gain insight into male panda reproductive effort and strategies, and test theories relating to reproductive competition. Male pandas initially competed fiercely for access to females that were about to be fertile, but once male competitive status was determined, aggression rates declined. Contact aggression was only observed during the first 2 days of mating aggregations; thereafter, it was replaced with noncontact aggression and avoidance. Agonistic interactions were highly asymmetrical, with contest losers (subordinates) showing less aggression and more avoidance than contest winners (dominants), both before and after contest outcome was established. The competitively superior male displayed mate-guarding tactics and secured all observed copulations. Contrary to theoretical predictions, testosterone levels did not predict aggression levels or contest winners and also were not affected by winning or losing a contest. Body size appeared to be the primary determinant of contest outcome. We discuss our findings in light of theoretical predictions, such as those arising from the “challenge hypothesis,” in the context of the giant panda’s foraging and nutritional ecology.

Previous winning experience increases the probability of winning a subsequent contest. However, it is not clear whether winning probability is affected only by the outcome of the contest (winning or losing) or whether fighting experience itself is also sufficient to induce this effect. We investigated this question in the East African cichlid fish Pundamilia spec. To create an unresolved conflict we allowed males to fight their own mirror image prior to a real fight against a size-matched non-mirror-stimulated control male. When males fight their own mirror image, the image's response corresponds to the action of the focal animal, creating symmetrical fighting conditions without the experience of losing or winning. We found that mirror-stimulated males were more likely to win an ensuing contest than control males. Interestingly, in this species mirror stimulation also induced an increase in circulating androgens, which is consistent with the hypothesis that stimulation of these sex steroids during aggressive encounters may prepare the animal for subsequent encounters. Our results suggest that fighting experience alone coupled with an androgen response, increases the likelihood of winning, even in the absence of a winning experience. (C) 2011 Elsevier Inc. All rights reserved.

Evolutionary theories (e.g., the challenge hypothesis) suggest that testosterone plays an important role in intrasexual competition. In addition, those theories suggest that testosterone responses during competition should depend upon the presence of potential, immediate mating opportunities associated with the competition. The current research tested the hypothesis that the sex composition of individuals at a competition (ratio of opposite-sex, potential mates to same-sex individuals) would influence changes in competitors' testosterone levels. Consistent with our hypotheses, higher ratios of opposite- to same-sex individuals at an ultimate frisbee tournament were associated with greater increases in salivary testosterone among competitors. The relationship between sex ratio and increased salivary testosterone was observed for both male and female competitors and occurred regardless of whether competitors won or lost. Findings are consistent with the hypothesis that testosterone responses during competition are influenced by cues of potential, immediate mating opportunities.

The steroid hormones, testosterone and cortisol, have some common characteristics, but they are related to generally antagonic processes at both the physiological and psychological levels. In addition, they are the product of the activation of two axes, the hypothalamic–pituitary–gonadal (HPG) and hypothalamic–pituitary–adrenal (HPA), which are very sensitive to a wide range of stressors. Our review focuses on the role of testosterone and cortisol in some social situations, such as competition and others related to the challenge hypothesis, that are evolutionary-relevant and have a component of social stress. Research findings are presented on these points, especially emphasizing the relevance of how the individual interprets social stimuli and attributes of the other participant in the interaction, producing consequences in the response pattern to the social situation. This paper presents empirical support for the role of the interaction between the reproductive HPG and stress HPA axes in several social behaviors with important adaptive significance.

Summary 1. Testosterone (T) is a key mediator in the expression of numerous morphological and behavioural traits in mammals, but the factors underlying individual variation in circulating T levels are poorly understood. 2. The intimate structural integration of sperm and T production within the testes, alongside the dependency of sperm production on high levels of T, suggests that T requirements for spermatogenesis could be an important driver of individual differences in T. 3. To test this hypothesis, we examine how male capacity for sperm production (as indicated by their testes size) is associated with T levels in a feral population of Soay sheep, resident on St. Kilda, Scotland, during their rutting season. 4. We found a strong positive relationship between an individual’s testes size (as measured before their seasonal enlargement) and the levels of circulating T during their rut, suggesting that T requirements for spermatogenesis has a prominent influence on the production of this androgen. 5. In contrast, body condition and competitive ability did not independently predict T levels, findings that are inconsistent with conventional ‘condition-dependent’ and ‘challenge’ hypotheses of T production. 6. This influence of male’s capacity for sperm production on T appeared to be substantial enough to be biologically relevant, as testes size also predicted male aggression and mate-seeking behaviour. 7. Our results suggest that a male’s inherent capacity for sperm and T production is tightly phenotypically integrated, with potential consequences for a wide range of other T-mediated reproductive traits.