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Aggression is a crucial behavior that impacts access to limited resources in different environmental contexts. Androgens synthesized by the gonads promote aggression during the breeding season. However, aggression can be expressed during the non-breeding season, despite low androgen synthesis by the gonads. The brain can also synthesize steroids ("neurosteroids"), including androgens, which might promote aggression during the non-breeding season. Male song sparrows, Melospiza melodia, are territorial year-round and allow the study of seasonal changes in the steroid modulation of aggression. Here, we quantified steroids following a simulated territorial intrusion (STI) for 10 min in wild adult male song sparrows during the breeding and non-breeding seasons. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we examined 11 steroids: pregnenolone, progesterone, corticosterone, dehydroepiandrosterone, androstenedione, testosterone, 5α-dihydrotestosterone, 17β-estradiol, 17α-estradiol, estriol, and estrone. Steroids were measured in blood and 10 microdissected brain regions that regulate social behavior. In both seasons, STI increased corticosterone in the blood and brain. In the breeding season, STI had no rapid effects on androgens or estrogens. Intriguingly, in the non-breeding season, STI increased testosterone and androstenedione in several behaviorally relevant regions, but not in the blood, where androgens remained non-detectable. Also in the non-breeding season, STI increased progesterone in the blood and specific brain regions. Overall, rapid socially modulated changes in brain steroid levels are more prominent during the non-breeding season. Brain steroid levels vary with season and social context in a region-specific manner and suggest a role for neuroandrogens in aggression during the non-breeding season. [ABSTRACT FROM AUTHOR]

Simple Summary: In most primates, the "challenge hypothesis" predicts increases in male–male aggressive behavior and testosterone during reproductive challenges, i.e., mating competition, or during periods of social instability, especially in seasonal breeders. However, few studies have investigated the role of stress hormones, such as glucocorticoids, and social rank in non-seasonal primates. In this study, we tested some predictors of the "challenge hypothesis" during a period of social stability in a non-seasonal primate. We collected data on aggressive behavior (male-to-male and male-to-female), copulation, and fecal samples (n = 700) to quantify testosterone and glucocorticoid metabolites (fTm and fGCm, respectively) in seven adults of stumptail macaques (Macaca arctoides) living in semi-captivity. We found that fTm and fGCm increased during times of copulation, although neither of the two hormones were associated with aggression between males. Besides, fGCm levels were positively associated with male-to-female aggression, indicating a possible social stress challenge. Both fTm and fGCm levels were higher in higher- and middle-ranking males. Our findings partially support the "challenge hypothesis" in stumptail males and highlight the importance of glucocorticoid secretion during challenging situations; our results also indicate that during periods of social stability, testosterone is not associated with aggression, at least in a non-seasonal primate. The "challenge hypothesis" predicts higher male–male aggressive behavior along with increases in testosterone levels during times of reproductive challenges and social instability. In addition, in some primate species, higher glucocorticoid levels can be observed as well, but this is usually modulated by dominance rank. We studied rank-related aggressive behavior, mating activity, and fecal testosterone and glucocorticoid metabolites (fTm and fGCm) in male stumptail macaques (Macaca arctoides) in order to test some predictions of the "challenge hypothesis". Over a 20-month period, we collected data on aggressive behavior and copulation, as well as fecal samples (n = 700) to quantify fTm and fGCm in seven adult stumptail males living in captivity. During periods of mating activity, male-to-male aggression increased in higher- and middle-ranking males. Neither fTm nor fGCm levels predicted male-to-male aggression. fGCm levels (but not fTm) were positively associated with male-to-female aggression; however, this association was pronounced during periods of mating activity. fGCm levels differed according to social rank, with middle-ranking males having the highest levels. Both hormones were higher during periods of mating activity, but only in higher- and middle-ranking males. Taken together, our findings partially support the challenge hypothesis in a non-seasonal primate and shed some light on the unique social and mating system of the stumptail macaque. [ABSTRACT FROM AUTHOR]

For decades, avian endocrinology has been informed bymale perspectives andmalefocused research, leaving significant gaps in our understanding of female birds. Male birds have been favored as research subjects because their reproductive behaviors are considered more conspicuous and their reproductive physiology is presumably less complex than female birds. However, female birds should not be ignored, as female reproductive behavior and physiology are essential for the propagation of all avian species. Endocrine research in female birds has made much progress in the last 20 years, but a substantial disparity in knowledge between male and female endocrinology persists. In this perspective piece, we provide examples of why ornithology has neglected female endocrinology, and we propose considerations for field and laboratory techniques to facilitate future studies. We highlight recent advances that showcase the importance of female avian endocrinology, and we challenge historic applications of an oversimplified, male-biased lens. We further provide examples of species for which avian behavior differs from the stereotypically described behaviors of male and female birds, warning investigators of the pitfalls in approaching endocrinology with a binary bias. We hope this piece will inspire investigators to engage in more comprehensive studies with female birds, to close the knowledge gap between the sexes, and to look beyond the binary when drawing conclusions about what is 'male' versus 'female' biology. [ABSTRACT FROM AUTHOR]

Testosterone (T) is a sexual hormone capable of modulating several traits in birds, including aggressiveness and reproductive behavior. Although variation in T-related traits is well-known for temperate zone birds, this variation has not been extensively studied in tropical species. The campo miner (Geositta poeciloptera) is a threatened bird endemic to the grasslands of the South American Cerrado. We investigated the seasonal variation in plasma T levels and associated behavior in the campo miner, addressing the following questions: (1) Does the species exhibit seasonal variation in T profile? (2) Do males have higher plasma T levels than females, irrespective of season? (3) Are males with higher plasma T levels more aggressive than males with lower T levels? (4) Do males' plasma T levels decrease after females lay eggs? We found that T levels are higher during the breeding season than during the nonbreeding season and that males present higher T levels than females throughout the year. Such high T levels are associated with a higher probability to engage in aggressive behavior; however, T levels decline toward the egg-laying date and keep decreasing afterward. Higher T levels before egg laying are apparently related to territorial defense against invaders and extrapair copulations. With the beginning of parental care, T levels decrease, which is in line with previous observations that the species becomes less aggressive after egg laying. This study contributes to the understanding of environmental endocrinology of tropical birds, filling some knowledge gaps about the diverse Neotropical avifauna. [ABSTRACT FROM AUTHOR]

The association between androgens and competition in women has been understudied compared with men. The current study examined the link between testosterone (T) and competition in elite female athletes, using a sample of female wrestlers that included athletes competing at both the national and international level. In a repeated-measures design, saliva samples were collected before and after wrestling bouts, with comparable samples of wins and losses, and subsequently analyzed for T. Study results showed a 22% increase in circulating bioavailable T from pre- to postbout, F(1, 12) = 9.71, P = .009. There was no significant difference in T between win or loss outcomes. These findings—showing a link between individual head-to-head competition and T in women—demonstrate that women's androgenic responses to environmental contexts are dynamic and may be an important factor to address in research on competitive performance. [ABSTRACT FROM AUTHOR]

Objectives This study examined potentially bidirectional connections of older U.S. women's salivary testosterone with their social network connections. Methods Data were from the 2005–2006 and 2010–2011 waves of the National Social Life, Health and Aging Project (NSHAP), a national probability sample of older U.S. adults. Autoregressive cross-lagged panel models tested linkages of women's testosterone with their social networks. Results Consistent with recent biological theory suggesting social modulation of hormones, a higher kin proportion in one's egocentric (person-centered) network, arguably a stable compositional feature, negatively predicted women's testosterone levels. In contrast, findings for tie strength were consistent with hormonal regulation of women's sociality—with both perceived support from friends and family, and closeness to network members, negatively influenced by testosterone. Discussion Rather than being a static and exogenous biological factor, older women's testosterone levels seem partly an outcome of their social context. Implications for sexual health and hormone therapy are discussed. However, this androgen also influences dimensions of their intimate networks critical to successful aging. Findings suggest the need for social scientists to engage with the neuroendocrine literature, which offers suggestions on linkages of hormones with specific network patterns. [ABSTRACT FROM AUTHOR]

Social living animals have to adjust their behavior to rapid changes in the social environment. It has been hypothesized that the expression of social behavior is better explained by the activity pattern of a diffuse social decision-making network (SDMN) in the brain than by the activity of a single brain region. In this study, we tested the hypothesis that it is the assessment that individuals make of the outcome of the fights, rather than the expression of aggressive behavior per se , that triggers changes in the pattern of activation of the SDMN which are reflected in socially driven behavioral profiles (e.g., dominant vs. subordinate specific behaviors). For this purpose, we manipulated the perception of the outcome of an agonistic interaction in an African cichlid fish (Oreochromis mossambicus) and assessed if either the perception of outcome or fighting by itself was sufficient to trigger rapid changes in the activity of the SDMN. We have used the expression of immediate early genes (c-fos and egr-1) as a proxy to measure the neuronal activity in the brain. Fish fought their own image on a mirror for 15 min after which they were allocated to one of three conditions for the two last minutes of the trial: (1) they remained fighting the mirror image (no outcome treatment); (2) the mirror was lifted and a dominant male that had just won a fight was presented behind a transparent partition (perception of defeat treatment); and (3) the mirror was lifted and a subordinate male that had just lost a fight was presented behind a transparent partition (perception of victory treatment). Results show that these short-term social interactions elicit distinct patterns in the SDMN and that the perception of the outcome was not a necessary condition to trigger a SDMN response as evidenced in the second treatment (perception of defeat treatment). We suggest that the mutual assessment of relative fighting behavior drives these acute changes in the state of the SDMN. [ABSTRACT FROM AUTHOR]

Objective: Cross‐culturally, men's T declines in response to pairbonding and fatherhood, but less is known about what happens to T during and after life history transitions that theoretically lead to renewed mating effort. We tested whether men's T rises (or declines less with age) as their children age, or when pairbonds end, independent of changes in fatherhood‐related variables such as co‐residence with children. Methods: We used demographic, behavioral, and salivary hormone data (waking and pre‐bed T) collected in 2009 and 2014 for the Cebu Longitudinal Health and Nutrition Survey (n = 571 men). Results: Fathers with older children tended to have attenuated decline in pre‐bedtime T between 2009 and 2014 compared to men with younger children, after controlling for pairbonding (ß = 1.58, SE = 0.88, P = 0.074). Separated men had higher pre‐bedtime T than pairbonded men, controlling for fatherhood‐related variables (ß = 11.74, SE = 4.33, P = 0.007). Change in T did not significantly differ for men who separated between the two surveys, relative to men who remained pairbonded throughout. Conclusion: We found modest support for the prediction that men experience less of an age‐related drop in T as their youngest child ages, a trend that might strengthen as children age further. We also replicate the finding that separated men have higher T, although longitudinal changes in the hormone were not significantly different in these men. Our data suggest that, of two life history transitions that may predict renewed mating effort, pair bond loss is more strongly endocrine mediated than potential mating effort shifts related to the aging of children. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. Significance statement: 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. [ABSTRACT FROM AUTHOR]

This study used population-representative longitudinal data from the 2005-2006 and 2010-2011 waves of the National Social Life, Health and Aging Project-a probability sample of US adults aged 57-85 at baseline (N = 650 women and 620 men)-to examine the causal direction in linkages of endogenous testosterone (T) with sexual activity and relationship quality. For both genders, our autoregressive effects indicated a large amount of temporal stability, not just in individual-level attributes (T, masturbation) but also dyadic ones (partnered sex, relationship quality)-indicating that a need for more nuanced theories of relational processes. Cross-lagged results suggested gender-specific effects-generally more consistent with sexual or relational modulation of T than with hormonal causation. Specifically, men's findings indicated their T might be elevated by their sexual (masturbatory) activity but not vice versa, although androgen levels did lower men's subsequent relationship quality. Women's T, in contrast, was negatively influenced not just by their higher relationship quality but also by their more frequent partnered sex-perhaps reflecting a changing function of sexual activity in late life. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It has been proposed in the literature that the testosterone (T) response to competition in humans may be modulated by cognitive variables. In a previous experiment with a female sample we have reported that opponent familiarity and threat appraisal moderated the T response to competition in women. With this experiment we aim to investigate if these variables have the same impact on males T response to competition, extending the previous findings in our lab. Forty male participants (20 dyads) were recruited to engage in a same sex, face to face competition using the Number Tracking Test as a competitive task. Levels of T, cortisol (C) and dehydroepiandrosterone (DHEA) were measured before and 20 min after the competition. Results show that losers report higher levels of threat than winners and increased their T levels after the competition, however this T change was not predicted by opponent familiarity or threat appraisal. No variation was detected for C and DHEA levels. These findings suggest that there could be sex differences for the moderators/mediators of the T response to competition in humans. [ABSTRACT FROM AUTHOR]

Apart from their role in reproduction androgens also respond to social challenges and this response has been seen as a way to regulate the expression of behavior according to the perceived social environment (Challenge hypothesis, Wingfield et al., 1990). This hypothesis implies that social decision-making mechanisms localized in the central nervous system (CNS) are open to the influence of peripheral hormones that ultimately are under the control of the CNS through the hypothalamic-pituitary-gonadal axis. Therefore, two puzzling questions emerge at two different levels of biological analysis: (1) Why does the brain, which perceives the social environment and regulates androgen production in the gonad, need feedback information from the gonad to adjust its social decision-making processes? (2) How does the brain regulate gonadal androgen responses to social challenges and how do these feedback into the brain? In this paper, we will address these two questions using the integrative approach proposed by Niko Tinbergen, who proposed that a full understanding of behavior requires its analysis at both proximate (physiology, ontogeny) and ultimate (ecology, evolution) levels. [ABSTRACT FROM AUTHOR]

In the evolution of caste-based societies in Hymenoptera, the classical insect hormones juvenile hormone (JH) and ecdysteroids were co-opted into new functions. Social wasps, which show all levels of sociality and lifestyles, are an ideal group in which to study such functional changes. Virtually all studies on the physiological mechanisms underlying reproductive division of labor and caste functions in wasps have been done on independent-founding paper wasps, and the majority of these studies have focused on species specially adapted for overwintering. The relatively little-studied tropical swarm-founding wasps of the Epiponini (Vespidae) are a diverse group of permanently social wasps, with some species maintaining caste flexibility well into the adult phase. We investigated the behavior, reproductive status, JH and ecdysteroid titers in hemolymph, ecdysteroid content of the ovary and cuticular hydrocarbon (CHC) profiles in the caste-monomorphic, epiponine wasp Polybia micans Ducke. We found that the JH titer was not elevated in competing queens from established multiple-queen nests, but increased in lone queens that lack direct competition. In queenless colonies, JH titer rose transiently in young potential reproductives upon challenge by nestmates, suggesting that JH may prime the ovaries for further development. Ovarian ecdysteroids were very low in workers but higher and correlated with the number of vitellogenic oocytes in the queens. Hemolymph ecdysteroid levels were low and variable in both workers and queens. Profiles of P. micans CHCs reflected caste, age and reproductive status, but were not tightly linked to either hormone. These findings show a significant divergence in hormone function in swarm-founding wasps compared with independently founding ones. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Challenge Hypothesis proposes that testosterone mediates aggression during periods of heightened conflict between males, especially episodes that have important fitness consequences. Considerable evidence from seasonally breeding species provides support for this hypothesis, but few data exist in animals that mate year-round. We tested predictions generated by the Challenge Hypothesis in chimpanzees, a non-seasonally breeding primate, through a study of individuals living in an exceptionally large community at Ngogo, Kibale National Park, Uganda. Results indicated that dominance rank had no influence on testosterone levels. Instead of rank influencing testosterone production, additional analyses revealed an important role for reproductive competition. Male chimpanzees displayed more aggression when they were in the same party as parous estrous females than when reproductively active females were unavailable. Male chimpanzees competed more intensely for mating opportunities with parous females than with nulliparas, and as a consequence, males displayed more aggression around the former than the latter. When males accompanied parous estrous females, their urinary testosterone concentrations were significantly higher than baseline concentrations. In contrast, urinary testosterone concentrations did not exceed baseline when males associated with nulliparous estrous females. These differences in testosterone levels could not be attributed to mating per se because males copulated equally often with parous and nulliparous females. Furthermore, variation in testosterone concentrations were not due to males gathering together in large parties, as their levels in these situations did not exceed baseline. Taken together, these findings, derived from a relatively large sample of males and estrous females, replicate those from a prior study and furnish additional support for the Challenge Hypothesis. Our results suggest that the Challenge Hypothesis is likely to be broadly applicable to chimpanzees and increase our understanding of the physiological costs to males who compete for estrous females. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The challenge hypothesis posits that acute increases in testosterone (T) during male-male competition enhance performance and survivability while limiting the physiological costs of consistently high T. Human challenge hypothesis research focuses on young men in industrial populations, who have higher baseline T levels than men in subsistence populations. We tested whether the Tsimane, pathogenically stressed forager-horticulturalists of the Bolivian Amazon, would express acute T increases in response to physical competition. Saliva was collected from 88 Tsimane men (aged 16-59 years) before and after a competitive soccer match. Tsimane men had significantly lower baseline levels of T (β = -0.41, p < 0.001) when compared with age-matched United States (US) males. Linear mixed-effects models were used to establish that T increased significantly immediately following competition (β = 0.23, p < 0.001), remaining high 1 h later (β = 0.09, p = 0.007); equivalent to 30.1 and 15.5 per cent increases in T, respectively. We did not find larger increases in Tamong winners (p = 0.412), although T increases were positively associated with self-rated performance (β = 9.07, p = 0.004). These results suggest that despite lower levels of T than US males, Tsimane males exhibit acute increases in Tat the same relative magnitude reported by studies in industrialized settings, with larger increases in T for those who report better individual performance. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Competition elevates plasma testosterone in a wide variety of vertebrates, including humans. The 'challenge hypothesis' proposes that seasonal peaks in testosterone during breeding are caused by social challenges from other males. However, during experimentally induced male-male conflicts, testosterone increases only in a minority of songbird species tested so far. Why is this so? Comparative evidence suggests that species with a short breeding season may not elevate testosterone levels during territory defence. These species may even be limited in their physiological capability to increase testosterone levels, which can be tested by injecting birds with gonadotropin-releasing hormone (GnRH). We studied two populations of black redstarts that differ in breeding altitude, morphology and the length of their breeding season. Unexpectedly, males of neither population increased testosterone in response to a simulated territorial intrusion, but injections with GnRH resulted in a major elevation of testosterone. Thus, black redstarts would have been capable of mounting a testosterone response during the male-male challenge. Our data show, for the first time, that the absence of an androgen response to male-male challenges is not owing to physiological limitations to increase testosterone. Furthermore, in contrast to comparative evidence between species, populations of black redstarts with a long breeding season do not show the expected elevation in testosterone during male-male challenges. [ABSTRACT FROM AUTHOR]

Vertebrate males commonly experience elevations in testosterone levels in response to sexual stimuli, such as presentation of a novel mating partner. Some previous human studies have shown that watching erotic movies increases testosterone levels in males although studies measuring testosterone changes during actual sexual intercourse or masturbation have yielded mixed results. Small sample sizes, 'unnatural' lab-based settings, and invasive techniques may help account for mixed human findings. Here, we investigated salivary testosterone levels in men watching ( n = 26) versus participating ( n = 18) in sexual activity at a large U.S. sex club. The present study entailed minimally invasive sample collection (measuring testosterone in saliva), a naturalistic setting, and a larger number of subjects than previous work to test three hypotheses related to men's testosterone responses to sexual stimuli. Subjects averaged 40 years of age and participated between 11:00 pm and 2:10 am. Consistent with expectations, results revealed that testosterone levels increased 36% among men during a visit to the sex club, with the magnitude of testosterone change significantly greater among participants (72%) compared with observers (11%). Contrary to expectation, men's testosterone changes were unrelated to their age. These findings were generally consistent with vertebrate studies indicating elevated male testosterone in response to sexual stimuli, but also point out the importance of study context since participation in sexual behavior had a stronger effect on testosterone increases in this study but unlike some previous human lab-based studies. [ABSTRACT FROM AUTHOR]

The endocrine control mechanisms for female mammalian aggression have been largely unstudied. Although it has been proposed that androgens may modulate female aggressive behavior in a similar manner to males, very little conclusive evidence exists. Previous work in male marmosets found that post-encounter increases in testosterone (T) were dependent on the intensity of aggression displayed during the aggressive encounter. We exposed female marmosets ( Callithrix kuhlii), a monogamous and biparental primate, to aggressive interactions with unfamiliar intruders. Individual female marmosets exhibited changes in T and estradiol (E2) that are associated with aggressiveness dependent on the intensity of aggression displayed as well as their role during the encounter. Resident females exhibited increased E2 immediately following an encounter in which they displayed high rates of aggression. If resident females received high rates of aggression from the intruder, the resident displayed increased T 24 hr following the encounter. Interestingly, if the female was an intruder in the encounter, the intensity of her aggression was associated with increased cortisol immediately following the trials, whereas received aggression was associated with increased T and E2 immediately following the trial. Female primates do exhibit situation-dependent changes in gonadal steroids in association with aggression that may serve to prime them for future aggressive interactions. Am. J. Primatol. 73:1072-1081, 2011. © 2011 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

In species in which males care for young, testosterone (T) is often high during mating periods but then declines to allow for caregiving of resulting offspring. This model may apply to human males, but past human studies of T and fatherhood have been cross-sectional, making it unclear whether fatherhood suppresses T or if men with lower T are more likely to become fathers. Here, we use a large representative study in the Philippines (n = 624) to show that among single nonfathers at baseline (2005) (21.5 ± 0.3 y), men with high waking T were more likely to become partnered fathers by the time of follow-up 4.5 y later (P < 0.05). Men who became partnered fathers then experienced large declines in waking (median: -26%) and evening (median: -34%) T, which were significantly greater than declines in single nonfathers (P < 0.001). Consistent with the hypothesis that child interaction suppresses T, fathers reporting 3 h or more of daily childcare had lower T at follow-up compared with fathers not involved in care (P < 0.05). Using longitudinal data, these findings show that T and reproductive strategy have bidirectional relationships in human males, with high T predicting subsequent mating success but then declining rapidly after men become fathers. Our findings suggest that T mediates tradeoffs between mating and parenting in humans, as seen in other species in which fathers care for young. They also highlight one likely explanation for previously observed health disparities between partnered fathers and single men. [ABSTRACT FROM AUTHOR]

While social interactions influence vertebrate endocrine regulation, the dynamics of regulation in relation to specific behaviors have not been clearly elucidated. In the current study, we investigated whether androgens (testosterone) or glucocorticoids (cortisol) play a functional role in aggressive offspring defense behavior in wild smallmouth bass ( Micropterus dolomieu), a teleost fish with sole paternal care. We measured circulating testosterone and cortisol concentrations in plasma samples taken from parental males following a simulated nest intrusion by a common nest predator, the bluegill sunfish ( Lepomis macrochirus). To understand whether endocrine regulation changes across the parental care period, we looked both at males guarding fresh eggs and at males guarding hatched embryos. Plasma testosterone levels increased in males subjected to a simulated nest intrusion when compared to sham controls. Testosterone concentrations in males guarding embryos were lower than in males guarding fresh eggs, but circulating testosterone was positively correlated with the level of aggression towards the nest predator at both offspring development stages. However, there was no increase in cortisol levels following a simulated nest intrusion, and no relationship between cortisol and any measured parameter. These results suggest that androgens play an important role in promoting aggressive nest defense behavior in teleost fish. [ABSTRACT FROM AUTHOR]

The article presents the results of a study which investigated the neuroendocrine responses in wrestlers which correlated with success or failure in competitive wrestling matches. An overview of related previous research is provided, along with details of the experimental protocol, which involved analysis of testosterone and cortisol levels in serum samples.

The challenge hypothesis was proposed as a mechanism for vertebrates to optimize their testosterone titers by upregulating testosterone during periods of aggressive competition. Here we test key predictions of the challenge hypothesis in an independently evolved endocrine system: juvenile hormone (JH) in a social insect. We assess how social conflict influences JH titers in Polistes dominulus wasps. Aggressive conflict was produced by removing the queen from some colonies (experimental) and a low-ranked worker from other colonies (control). Queen removal produced competition among workers to fill the reproductive vacancy. Workers upregulated their JH titers in response to this social conflict; worker JH titers were higher in queenless than in queenright colonies. Furthermore, JH titers were associated with an individual's ability to dominate rivals; the worker that took over as the replacement queen had a substantially higher JH titer than did other workers. Finally, JH titers were positively associated with aggression in queenless colonies, but there was no relationship between JH and aggression in stable, queenright colonies. Overall, these results match key predictions of the challenge hypothesis and parallel much of the work on testosterone in vertebrates. Social modulation of hormone titers is not confined to a particular endocrine system but is likely to be an adaptive feature of endocrine systems across diverse taxa. [ABSTRACT FROM AUTHOR]

The challenge hypothesis (Wingfield et al. in Am Nat 136:829–846, ) predicts that circulating testosterone increases when socially monogamous male birds are challenged during breeding. Although the challenge hypothesis has been confirmed in large-scale interspecific comparisons of seasonal hormone profiles, experimental tests of the challenge hypothesis are still uncommon and the results equivocal. We tested one of the predictions of the challenge hypothesis by investigating the behavioural and hormonal response of free-living European robins during simulated territorial intrusions (STIs) in the breeding season. We conducted STIs by placing a stuffed decoy in a territory and playing robin song. After the behaviour of the focal male had been recorded for at least 10 min, it was captured and a blood sample was taken immediately. Controls were caught within 10 min of the first response of the territory owner. Hormone concentrations were measured by radio-immunoassay. Although previous studies have shown that testosterone has an impact on aggression, European robins do not respond to STIs by increasing circulating levels of testosterone. [ABSTRACT FROM AUTHOR]

Measurement of steroids that are released into the water via the gills has previously been shown to be an effective way of studying the reproductive endocrinology of the male three-spined stickleback Gasterosteus aculeatus without having to kill the fish. In the present paper, a previous observation on the existence of a compound other than 11-ketotestosterone (11-KT) in water, which cross-reacted in the 11-KT radioimmunoassay was repeated. The amounts of this compound, however, were not sufficient to warrant a separation step prior to carrying out assay. The lack of association between androstenedione levels in water and those in plasma was also confirmed. For the first time, the amounts of testosterone released into the water were shown to be positively correlated with the amounts in plasma, the sampling procedure (placing the fish for 30 min in 50 ml water) had no effect on the rate of release of cortisol but caused a rapid drop in the rate of release of 11-KT (which means that the fish should not be sampled twice in short succession), physical interaction between two nesting males (which was accompanied by aggression) significantly increased the rate of release of 11-KT, androstenedione and testosterone (but not of cortisol) and the rate of release of 11-KT was at its maximum between 2 and 4 h after exposure. [ABSTRACT FROM AUTHOR]

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In primate species exhibiting seasonal reproduction, patterns of testosterone excretion in adult males are variable: in some species, peaks correlate with female receptivity periods and heightened male–male aggression over access to estrous females, in others, neither heightened aggression nor marked elevations in testosterone have been noted. In this study, we examined mean fecal testosterone ( f T) levels and intermale aggression in wild adult male ring-tailed lemurs residing in three groups at Beza Mahafaly Reserve, Madagascar. Results obtained from mating and post-mating season 2003 were compared to test Wingfield et al. [1990. Am Nat 136:829–846] “challenge hypothesis”, which predicts a strong positive relationship between male testosterone levels and male–male competition for access to receptive females during breeding season. f T levels and rates of intermale aggression were significantly higher during mating season compared to the post-mating period. Mean f T levels and aggression rates were also higher in the first half of the mating season compared with the second half. Number of males in a group affected rates of intermale agonism, but not mean f T levels. The highest-ranking males in two of the groups exhibited higher mean f T levels than did lower-ranking males, and young males exhibited lower f T levels compared to prime-aged and old males. In the post-mating period, mean male f T levels did not differ between groups, nor were there rank or age effects. Thus, although male testosterone levels rose in relation to mating and heightened male–male aggression, f T levels fell to baseline breeding levels shortly after the early mating period, and to baseline non-breeding levels immediately after mating season had ended, offsetting the high cost of maintaining both high testosterone and high levels of male–male aggression in the early breeding period. Am. J. Primatol. 69:1325–1339, 2007. © 2007 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Despite the many benefits that testosterone has on male reproduction, sustaining high levels of testosterone for long periods can be costly. The challenge hypothesis predicts that males will show temporarily sustained elevations of testosterone at critical periods, counterbalanced by decreased levels during noncritical periods. We investigated male testosterone measures extracted from fecal samples in a group of chacma baboons ( Papio hamadryas ursinus) living in the Okavango Delta, Botswana. Because rank serves as a proxy for competition for mates, we examined how male testosterone was related to dominance rank, age, aggression, and mating activity. Males showed an elevation in testosterone at maturity; young adult males had the highest testosterone levels followed by a steady decline with age. Among dispersing males, testosterone was temporarily elevated in the month following dispersal. After controlling for age, testosterone and rank were unrelated, but testosterone and changes in rank were positively correlated, such that males rising in rank had higher testosterone than males falling in rank. Thus, for males in this group, testosterone was predictive of a male's rank trajectory, or future rank. Similarly, male testosterone levels predicted future, rather than current, mating activity. Finally, male testosterone and aggression rates were unrelated during stable periods in the dominance hierarchy but positively related during unstable periods when high ranks were being contested. In general, our results support the challenge hypothesis with males exhibiting elevated testosterone in association with the acquisition of high rank (ensuring access to mates), rather than with mating itself. [ABSTRACT FROM AUTHOR]

Reproductive success in male primates can be influenced by testosterone (T) and cortisol (C). We examined them in wildSaguinus mystaxvia fecal hormone analysis. Firstly, we wanted to characterize male hormonal status over the course of the year. Further we tested the influence of the reproductive status of the breeding female, social instability, and intergroup encounter rates on T levels, comparing the results with predictions of the challenge hypothesis (Wingfieldet al., 1990). We also tested for interindividual differences in hormonal levels, possibly related to social or breeding status. We collected data during a 12-mo study on 2 groups of moustached tamarins at the Estación Biológica Quebrada Blanco in northeastern Peru. We found fairly similar T and C levels over the course of the year for all males. Yet an elevation of T shortly after the birth of infants, during the phase of ovarian inactivity of the group’s breeding female, was evident. Hormonal levels were not significantly elevated during a phase of social instability, did not correlate with intergroup encounter rates, and did not differ between breeding and nonbreeding males. Our results confirm the challenge hypothesis (Wingfieldet al., 1990). The data suggest that reproductive competition in moustached tamarins is not based on endocrinological, but instead on behavioral mechanisms, possibly combined with sperm competition. [ABSTRACT FROM AUTHOR]

This study investigated hormonal and demographic processes underlying unimale and multimale mating systems in primates. Reproductive skew and challenge models of male competition provide conflicting predictions of the relationship of male residence to group composition and androgen regulation. These predictions were tested using endocrine and socioecological data from Kenyan vervet monkeys (Cercopithecus aethiops). Serum samples from 57 adult male monkeys, drawn from 19 separate groups and 4 populations, were assayed for testosterone by radioimmunoassay. Male ability to respond to conspecific challenge was assessed by their testosterone response to the capture protocol.Analyses showed that reproductive skew models were useful predictors of intergroup and interpopulation variation in male residence and T profiles. The Limited Control model of male residence was supported by positive correlations of the number of males per group with the number of females without dependent offspring, demonstrating that monopolization potential was a key determinant of male residence. Testosterone concentrations under conditions that elevated serum levels were positively correlated with infanticide risk, supporting the Concession model. Population comparisons provided evidence for increased T responsiveness where groups were predominantly unimale. Unimale populations were from sites with higher rainfall, suggesting that ecological factors contributed to population differences in male residence and T regulation. [ABSTRACT FROM AUTHOR]

In an attempt to describe hormone–behavior interactions in a sample of wild male chimpanzees, we quantified testosterone in 67 fecal samples obtained from 22 adult male chimpanzees at Ngogo, Kibale National Park, Uganda. A mixed-model methodology that controlled for age-class identified a significant positive association between testosterone levels and dominance rank. The results are consistent with those reported from a separate, smaller chimpanzee community in the same population in a study that analyzed testosterone levels in urine [Muller & Wrangham, 2004]. As in that earlier study, our results held during a period of social stability, which is not consistent with predictions of the “challenge hypothesis.” We concur with Muller and Wrangham [2004] that the challenge hypothesis requires modification to explain the chimpanzee data, because fission-fusion sociality in chimpanzees makes challenges unpredictable. We also discuss the utility of fecal samples and a mixed-model statistical method for behavioral endocrinology studies. Am. J. Primatol. 64:71–82, 2004. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]