Aggressive behaviours track transitions in seasonal phenotypes of female Siberian hamsters

Sex differences in responses to aggressive encounters among California mice

Despite how widespread female aggression is across the animal kingdom, there remains much unknown about its neuroendocrine mechanisms, especially in females that engage in aggression outside the peripartum period. Although the impact of aggressive experience on steroid hormone responses have been described, little is known about the impact of these experiences on female behavior or the subsequent neuropeptide responses to performing aggression. In this study, we compared behavioral responses in both male and female adult California mice based on if they had 0, 1, or 3 aggressive encounters using a resident intruder paradigm. We measured how arginine vasopressin and oxytocin cells in the paraventricular nucleus responded to aggression using c-fos immunohistochemistry. We saw that both sexes disengaged from intruders with repeated aggressive encounters, but that on the final day of testing females were more likely to freeze when they encountered intruders compared to no aggression controls - which was not significant in males. Finally, we saw that percent of arginine vasopressin and c-fos co-localizations in the posterior region of the paraventricular nucleus increased in males who fought compared to no aggression controls. No difference was observed in females. Overall, there is evidence that engaging in aggression induces stress responses in both sexes, and that females may be more sensitive to the effects of fighting.

A seasonal switch hypothesis for the neuroendocrine control of aggression

Aggression is a well-studied social behavior that is universally exhibited by animals across a wide range of contexts. Prevailing knowledge suggests gonadal steroids primarily mediate aggression; however, this is based mainly on studies of male-male aggression in laboratory rodents. When males and females of other species, including humans, are examined, a positive relationship between gonadal steroids and aggression is less substantiated. For instance, hamsters housed in short 'winter-like' days show increased aggression compared with long-day housed hamsters, despite relatively low circulating gonadal steroids. These results suggest alternative, non-gonadal mechanisms controlling aggression. Here, we propose the seasonal switch hypothesis, which employs a multidisciplinary approach to describe how seasonal variation in extra-gonadal steroids, orchestrated by melatonin, drives context-specific changes in aggression.

The Ancestral Modulation Hypothesis: Predicting Mechanistic Control of Sexually Heteromorphic Traits Using Evolutionary History

AbstractAcross the animal kingdom there are myriad forms within a sex across, and even within, species, rendering concepts of universal sex traits moot. The mechanisms that regulate the development of these trait differences are varied, although in vertebrates, common pathways involve gonadal steroid hormones. Gonadal steroids are often associated with heteromorphic trait development, where the steroid found at higher circulating levels is the one involved in trait development for that sex. Occasionally, there are situations in which a gonadal steroid associated with heteromorphic trait development in one sex is involved in heteromorphic or monomorphic trait development in another sex. We propose a verbal hypothesis, the ancestral modulation hypothesis (AMH), that uses the evolutionary history of the trait-particularly which sex ancestrally possessed higher trait values-to predict the regulatory pathway that governs trait expression. The AMH predicts that the genomic architecture appears first to resolve sexual conflict in an initially monomorphic trait. This architecture takes advantage of existing sex-biased signals, the gonadal steroid pathway, to generate trait heteromorphism. In cases where the other sex experiences evolutionary pressure for the new phenotype, that sex will co-opt the existing architecture by altering its signal to match that of the original high-trait-value sex. We describe the integrated levels needed to produce this pattern and what the expected outcomes will be given the evolutionary history of the trait. We present this framework as a testable hypothesis for the scientific community to investigate and to create further engagement and analysis of both ultimate and proximate approaches to sexual heteromorphism.

Sex and seasonal differences in neural steroid sensitivity predict territorial aggression in Siberian hamsters

Many animals display marked changes in physiology and behavior on a seasonal timescale, including non-reproductive social behaviors (e.g., aggression). Previous studies from our lab suggest that the pineal hormone melatonin acts via steroid hormones to regulate seasonal aggression in Siberian hamsters (Phodopus sungorus), a species in which both males and females display increased non-breeding aggression. The neural actions of melatonin on steroids and aggressive behavior, however, are relatively unexplored. Here, we housed male and female hamsters in long-day photoperiods (LDs, characteristic of breeding season) or short-day photoperiods (SDs, characteristic of non-breeding season) and administered timed melatonin (M) or control injections. Following 10 weeks of treatment, we quantified aggressive behavior and neural steroid sensitivity by measuring the relative mRNA expression of two steroidogenic enzymes (aromatase and 5α-reductase 3) and estrogen receptor 1 in brain regions associated with aggression or reproduction [medial preoptic area (MPOA), anterior hypothalamus (AH), arcuate nucleus (ARC), and periaqueductal gray (PAG)] via quantitative PCR. Although LD-M and SD males and females displayed increased aggression and similar changes in gene expression in the ARC, there were sex-specific effects of treatment with melatonin and SDs on gene expression in the MPOA, AH, and PAG. Furthermore, males and females exhibited different relationships between neural gene expression and aggression in response to melatonin and SDs. Collectively, these findings support a role for melatonin in regulating seasonal variation in neural steroid sensitivity and aggression and reveal how distinct neuroendocrine responses may modulate a similar behavioral phenotype in male and female hamsters.

Winter madness: Melatonin as a neuroendocrine regulator of seasonal aggression

Individuals of virtually all vertebrate species are exposed to annual fluctuations in the deterioration and renewal of their environments. As such, organisms have evolved to restrict energetically expensive processes and activities to a specific time of the year. Thus, the precise timing of physiology and behavior is critical for individual reproductive success and subsequent fitness. Although the majority of research on seasonality has focused on seasonal reproduction, pronounced fluctuations in other non-reproductive social behaviors, including agonistic behaviors (e.g., aggression), also occur. To date, most studies that have investigated the neuroendocrine mechanisms underlying seasonal aggression have focused on the role of photoperiod (i.e., day length); prior findings have demonstrated that some seasonally breeding species housed in short "winter-like" photoperiods display increased aggression compared with those housed in long "summer-like" photoperiods, despite inhibited reproduction and low gonadal steroid levels. While fewer studies have examined how the hormonal correlates of environmental cues regulate seasonal aggression, our previous work suggests that the pineal hormone melatonin acts to increase non-breeding aggression in Siberian hamsters (Phodopus sungorus) by altering steroid hormone secretion. This review addresses the physiological and cellular mechanisms underlying seasonal plasticity in aggressive and non-aggressive social behaviors, including a key role for melatonin in facilitating a "neuroendocrine switch" to alternative physiological mechanisms of aggression across the annual cycle. Collectively, these studies highlight novel and important mechanisms by which melatonin regulates aggressive behavior in vertebrates and provide a more comprehensive understanding of the neuroendocrine bases of seasonal social behaviors broadly.

Uncovering the seasonal brain: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a biochemical approach for studying seasonal social behaviors

Many animals show pronounced changes in physiology and behavior across the annual cycle, and these adaptations enable individuals to prioritize investing in the neuroendocrine mechanisms underlying reproduction and/or survival based on the time of year. While prior research has offered valuable insight into how seasonal variation in neuroendocrine processes regulates social behavior, the majority of these studies have investigated how a single hormone influences a single behavioral phenotype. Given that hormones are synthesized and metabolized via complex biochemical pathways and often act in concert to control social behavior, these approaches provide a limited view of how hormones regulate seasonal changes in behavior. In this review, we discuss how seasonal influences on hormones, the brain, and social behavior can be studied using liquid chromatography-tandem mass spectrometry (LC-MS/MS), an analytical chemistry technique that enables researchers to simultaneously quantify the concentrations of multiple hormones and the activities of their synthetic enzymes. First, we examine studies that have investigated seasonal plasticity in brain-behavior interactions, specifically by focusing on how two groups of hormones, sex steroids and nonapeptides, regulate sexual and aggressive behavior. Then, we explain the operations of LC-MS/MS, highlight studies that have used LC-MS/MS to study the neuroendocrine mechanisms underlying social behavior, both within and outside of a seasonal context, and discuss potential applications for LC-MS/MS in the field of behavioral neuroendocrinology. We propose that this cutting-edge technology will provide a more comprehensive understanding of how the multitude of hormones that comprise complex neuroendocrine networks affect seasonal variation in the brain and behavior.

Electrocommunication signals and aggressive behavior vary among male morphs in an apteronotid fish, Compsaraia samueli

Within-species variation in male morphology is common among vertebrates and is often characterized by dramatic differences in behavior and hormonal profiles. Males with divergent morphs also often use communication signals in a status-dependent way. Weakly electric knifefish are an excellent system for studying variation in male morphology and communication and its hormonal control. Knifefish transiently modulate the frequency of their electric organ discharge (EOD) during social encounters to produce chirps and rises. In the knifefish Compsaraia samueli, males vary extensively in jaw length. EODs and their modulations (chirps and rises) have never been investigated in this species, so it is unclear whether jaw length is related to the function of these signals. We used three behavioral assays to analyze EOD modulations in male C. samueli: (1) artificial playbacks, (2) relatively brief, live agonistic dyadic encounters, and (3) long-term overnight recordings. We also measured circulating levels of two androgens, 11-ketotestosterone and testosterone. Chirp structure varied within and across individuals in response to artificial playback, but was unrelated to jaw length. Males with longer jaws were more often dominant in dyadic interactions. Chirps and rises were correlated with and preceded attacks regardless of status, suggesting these signals function in aggression. In longer-term interactions, chirp rate declined after one week of pairing, but was unrelated to male morphology. Levels of circulating androgens were low and not predictive of jaw length or EOD signal parameters. These results suggest that communication signals and variation in male morphology are linked to outcomes of non-breeding agonistic contests.

Maternal stress and the maternal microbiome have sex-specific effects on offspring development and aggressive behavior in Siberian hamsters (Phodopus sungorus)

The gut microbiome, a community of commensal, symbiotic and pathogenic bacteria, fungi, and viruses, interacts with many physiological systems to affect behavior. Prenatal experiences, including exposure to maternal stress and different maternal microbiomes, are important sources of organismal variation that can affect offspring development. These physiological systems do not act in isolation and can have long-term effects on offspring development and behavior. Here we investigated the interactive effects of maternal stress and manipulations of the maternal microbiome on offspring development and social behavior using Siberian hamsters, Phodopus sungorus. We exposed pregnant females to either a social stressor, antibiotics, both the social stressor and antibiotics, or no treatment (i.e., control) over the duration of their pregnancy and quantified male and female offspring growth, gut microbiome composition and diversity, stress-induced cortisol concentrations, and social behavior. Maternal antibiotic exposure altered the gut microbial communities of male and female offspring. Maternal treatment also had sex-specific effects on aspects of offspring development and aggressive behavior. Female offspring produced by stressed mothers were more aggressive than other female offspring. Female, but not male, offspring produced by mothers exposed to the combined treatment displayed low levels of aggression, suggesting that alteration of the maternal microbiome attenuated the effects of prenatal stress in a sex-specific manner. Maternal treatment did not affect non-aggressive behavior in offspring. Collectively, our study offers insight into how maternal systems can interact to affect offspring in sex-specific ways and highlights the important role of the maternal microbiome in mediating offspring development and behavior.

Adrenal MT1 melatonin receptor expression is linked with seasonal variation in social behavior in male Siberian hamsters

Many animals exhibit pronounced changes in physiology and behavior on a seasonal basis, and these adaptations have evolved to promote survival and reproductive success. While the neuroendocrine pathways mediating seasonal reproduction are well-studied, far less is known about the mechanisms underlying seasonal changes in social behavior, particularly outside of the context of the breeding season. Our previous work suggests that seasonal changes in melatonin secretion are important in regulating aggression in Siberian hamsters (Phodopus sungorus); it is unclear, however, how melatonin acts via its receptors to modulate seasonal variation in social behavior. In this study, we infused a MT₁ melatonin receptor-expressing (MT₁) or control (CON) lentivirus into the adrenal glands of male Siberian hamsters. We then housed hamsters in long-day (LD) or short-day (SD) photoperiods, administered timed melatonin or control injections, and quantified aggressive and non-aggressive social behaviors (e.g., investigation, self-grooming) following 10 weeks of treatment. LD hamsters infused with the MT₁ lentivirus had significantly higher adrenal mt1 expression than LD CON hamsters, as determined via quantitative PCR. While melatonin administration was necessary to induce SD-like reductions in body and relative reproductive mass, only LD hamsters infused with the MT₁ lentivirus displayed SD-like changes in social behavior, including increased aggression and decreased investigation and grooming. In addition, SD CON and LD hamsters infused with the MT₁ lentivirus exhibited similar relationships between adrenal mt1 expression and aggressive behavior. Together, our findings suggest a role for adrenal MT₁ receptor signaling in regulating behavior, but not energetics or reproduction in seasonally breeding species.

Territorial aggression by dwarf hamster females (Phodopus campbelli): A hormonal approach

Females of many vertebrate species are territorial. However, few studies have been performed on territorial aggression and its hormonal bases in females. The present study determined whether dwarf hamster females (Phodopus campbelli) display territorial aggression and the role that estradiol (E₂ ) and progesterone (P₄ ) play in this behavior. Two experiments were carried out. First, 28 virgin females were mated with sexually inexperienced males. The females were submitted to oviduct ligation 2 weeks before mating to prevent pregnancy. After 15 days of mating, 16 out of 28 females were submitted to resident-intruder tests, whereas the remaining 12 females were nonconfronted. Blood samples were collected 30 min after the aggression tests to determine E₂ and P₄ by ELISA. In the second experiment, 40 females were mated with vasectomized males. Thirty days after mating, 30 dwarf hamsters were subjected to ovariectomy; 10 were nontreated, 10 were treated with E₂ , and 10 with P₄ . The remaining 10 were sham-operated. All females were submitted to resident-intruder tests, and blood samples were taken to quantify E₂ and P₄ . In the first experiment, 87.5% of dwarf hamster females displayed territorial aggression, and in the second, 100% of females were aggressive. Ovariectomy and ovariectomy plus E₂ or P₄ replacement did not affect aggressive territorial behavior. These results showed that this species' females are territorial, and this behavior is not dependent on ovarian steroid hormones.

Melatonin-dependent changes in neurosteroids are associated with increased aggression in a seasonally breeding rodent

Aggression is a complex social behaviour that allows individuals to compete for access to limited resources (eg, mates, food and territories). Excessive or inappropriate aggression, however, has become problematic in modern societies, and current treatments are largely ineffective. Although previous work in mammals suggests that aggressive behaviour varies seasonally, seasonality is largely overlooked when developing clinical treatments for inappropriate aggression. Here, we investigated how the hormone melatonin regulates seasonal changes in neurosteroid levels and aggressive behaviour in Siberian hamsters, a rodent model of seasonal aggression. Specifically, we housed males in long-day (LD) or short-day (SD) photoperiods, administered timed s.c. melatonin injections (which mimic a SD-like signal) or control injections, and measured aggression using a resident-intruder paradigm after 9 weeks of treatment. Moreover, we quantified five steroid hormones in circulation and in brain regions associated with aggressive behaviour (lateral septum, anterior hypothalamus, medial amygdala and periaqueductal gray) using liquid chromatography-tandem mass spectrometry. SD hamsters and LD hamsters administered timed melatonin injections (LD-M) displayed increased aggression and exhibited region-specific decreases in neural dehydroepiandrosterone, testosterone and oestradiol, but showed no changes in progesterone or cortisol. Male hamsters also showed distinct associations between neurosteroids and aggressive behaviour, in which neural progesterone and dehydroepiandrosterone were positively correlated with aggression in all treatment groups, whereas neural testosterone, oestradiol and cortisol were negatively correlated with aggression only in LD-M and SD hamsters. Collectively, these results provide insight into a novel neuroendocrine mechanism of mammalian aggression, in which melatonin reduces neurosteroid levels and elevates aggressive behaviour.

Seasonal patterns of melatonin alter aggressive phenotypes of female Siberian hamsters

Many animal species exhibit year-round aggression, a behaviour that allows individuals to compete for limited resources in their environment (eg, food and mates). Interestingly, this high degree of territoriality persists during the non-breeding season, despite low levels of circulating gonadal steroids (ie, testosterone [T] and oestradiol [E₂ ]). Our previous work suggests that the pineal hormone melatonin mediates a 'seasonal switch' from gonadal to adrenal regulation of aggression in Siberian hamsters (Phodopus sungorus); solitary, seasonally breeding mammals that display increased aggression during the short, 'winter-like' days (SDs) of the non-breeding season. To test the hypothesis that melatonin elevates non-breeding aggression by increasing circulating and neural steroid metabolism, we housed female hamsters in long days (LDs) or SDs, administered them timed or mis-timed melatonin injections (mimic or do not mimic a SD-like signal, respectively), and measured aggression, circulating hormone profiles and aromatase (ARO) immunoreactivity in brain regions associated with aggressive or reproductive behaviours (paraventricular hypothalamic nucleus [PVN], periaqueductal gray [PAG] and ventral tegmental area [VTA]). Females that were responsive to SD photoperiods (SD-R) and LD females given timed melatonin injections (Mel-T) exhibited gonadal regression and reduced circulating E₂ , but increased aggression and circulating dehydroepiandrosterone (DHEA). Furthermore, aggressive challenges differentially altered circulating hormone profiles across seasonal phenotypes; reproductively inactive females (ie, SD-R and Mel-T females) reduced circulating DHEA and T, but increased E₂ after an aggressive interaction, whereas reproductively active females (ie, LD females, SD non-responder females and LD females given mis-timed melatonin injections) solely increased circulating E₂ . Although no differences in neural ARO abundance were observed, LD and SD-R females showed distinct associations between ARO cell density and aggressive behaviour in the PVN, PAG and VTA. Taken together, these results suggest that melatonin increases non-breeding aggression by elevating circulating steroid metabolism after an aggressive encounter and by regulating behaviourally relevant neural circuits in a region-specific manner.

Seasonal regulation of behaviour: what role do hormone receptors play?

Many animals differentially express behaviours across the annual cycle as life stages are coordinated with seasonal environmental conditions. Understanding of the mechanistic basis of such seasonal changes in behaviour has traditionally focused on the role of changes in circulating hormone levels. However, it is increasingly apparent that other endocrine regulation mechanisms such as changes in local hormone synthesis and receptor abundance also play a role. Here I review what is known about seasonal changes in steroid hormone receptor abundance in relation to seasonal behaviour in vertebrates. I find that there is widespread, though not ubiquitous, seasonal variation in the expression of steroid hormone receptors in the brain, with such variation being best documented in association with courtship, mating and aggression. The most common pattern of seasonal variation is for there to be upregulation of sex steroid receptors with the expression of courtship and mating behaviours, when circulating hormone levels are also high. Less well-documented are cases in which seasonal increases in receptor expression could compensate for low circulating hormone levels or seasonal downregulation that could serve a protective function. I conclude by identifying important directions for future research.

How research on female vertebrates contributes to an expanded challenge hypothesis

The bi-directional links between hormones and behavior have been a rich area of research for decades. Theory on the evolution of testosterone (T) was greatly advanced by the challenge hypothesis, which presented a framework for understanding interspecific, seasonal, and social variation in T levels in males, and how they are shaped by the competing demands of parental care and male-male competition. Female competition is also widespread in nature, although it is less clear whether or how the challenge hypothesis applies to females. Here, we evaluate this issue in four parts: (1) We summarize and update prior analyses of seasonal plasticity and interspecific variation in T in females. (2) We evaluate experimental links between T and female aggression on shorter timescales, asking how T manipulations affect aggression and conversely, how social manipulations affect T levels in female mammals, birds, lizards, and fishes. (3) We examine alternative mechanisms that may link aggression to the social environment independently of T levels in circulation. (4) We present a case study, including new data analyses, in an aggressive female bird (the tree swallow, Tachycineta bicolor) to explore how variation in tissue-level processing of T may bridge the gap between circulating T and variation in behavior that is visible to natural selection. We close by connecting these multivariate levels of sex steroid signaling systems alongside different temporal scales (social, seasonal, and evolutionary) to generate broadly applicable insights into how animals respond to their social environment, regardless of whether they are male or female.

Seasonal and social factors associated with spacing in a wild territorial electric fish

In this study, we focused on the seasonal variation of the determinants of territory size in the weakly electric fish Gymnotus omarorum. This species is a seasonal breeder that displays year-round territorial aggression. Female and male dyads exhibit indistinguishable non-breeding territorial agonistic behavior and body size is the only significant predictor of contest outcome. We conducted field surveys across seasons that included the identification of individual location, measurements of water physico-chemical variables, characterization of individual morphometric and physiological traits, and their correlation to spatial distribution. G. omarorum tolerates a wide range of dissolved oxygen concentration, and territory size correlated positively with dissolved oxygen in both seasons. In the non-breeding season, territory size was sexually monomorphic and correlated only with body size. In the breeding season, territory size no longer correlated with body size but differed between sexes: (i) the overall spatial arrangement was sexually biased, (ii) territory size depended on gonadal hormones in both sexes, which was expected for males, but not previously reported in females, (iii) female territory size showed a positive relationship with gonadal size, and (iv) females showed relatively larger territories than males. This study demonstrates seasonal changes in the determinants of territory size and thus contributes to the understanding of the mechanisms underlying the behavioral plasticity natural territorial behavior.

Photoperiod Affects Harderian Gland Morphology and Secretion in Female Cricetulus barabensis: Autophagy, Apoptosis, and Mitochondria

Photoperiod is an important factor of mammalian seasonal rhythm. The Harderian gland (HG) appears to act as a “standby” structure of the retinal-pineal axis, mediating light signals in vitro and neuroendocrine regulation in vivo; however, the effect of photoperiod on the HG is not clear. Here, we studied morphological differences in the HG of female striped dwarf hamsters (Cricetulus barabensis), a small mammal that experiences an annual rhythm, under different photoperiods (i.e., SP, short photoperiod; MP, moderate photoperiod; LP, long photoperiod), and further investigated the molecular mechanisms related to these morphological differences. Results showed that body weight, carcass weight, and HG weight were higher in the SP and LP groups than that in the MP group. Protein expression of hydroxyindole-o-methyltransferase, a key enzyme in melatonin synthesis, was higher in the SP group than in the other two groups. Somatostatin showed highest expression in the LP group. Furthermore, comparison of changes in the HG ultrastructure demonstrated autolysosome formation in the SP group. Protein aggregation and mRNA expression of LC3 and protein expression of LC3II/LC3I were higher in the SP group than in the MP group, indicating elevated autophagy under SP. Chromatin agglutination and mitochondrial damage were observed and bax/bcl2 and cytochrome C expression increased at the protein and mRNA levels in the SP and LP groups, suggesting increased apoptosis. Protein expression of dynamin-related protein 1 and mitochondrial fission factor (Mff) were highest in the SP group, suggesting elevated mitochondrial fission. Protein expression levels of adenosine triphosphate (ATP) synthase and citrate synthase were lower in the LP group than in the SP and MP groups. These results indicated that autophagy and apoptosis imbalance under SP and LP conditions may have led to HG weight loss and up-regulation of mitochondrial apoptosis may have weakened mitochondrial function under LP conditions. Finally, melatonin synthesis appeared to be positively correlated with the time hamsters entered darkness.

The estrogenic pathway modulates non-breeding female aggression in a teleost fish

Aggressive behaviors are widespread among animals and are critical in the competition for resources. The physiological mechanisms underlying aggression have mostly been examined in breeding males, in which gonadal androgens, acting in part through their aromatization to estrogens, have a key role. There are two alternative models that contribute to further understanding hormonal mechanisms underlying aggression: aggression displayed in the non-breeding season, when gonadal steroids are low, and female aggression. In this study we approach, for the first time, the modulatory role of estrogens and androgens upon non-breeding aggression in a wild female teleost fish. We characterized female aggression in the weakly electric fish Gymnotus omarorum and carried out acute treatments 1 h prior to agonistic encounters in dyads treated with either an aromatase inhibitor or an antagonist of androgen receptors. Anti-androgen treatment had no effect on behavior whereas acute aromatase inhibition caused a strong distortion of aggressive behavior. Territorial non-breeding aggression was robust and depended on rapid estrogen actions to maintain high levels of aggression, and ultimately reach conflict resolution from which dominant/subordinate status emerged. Our results, taken together with our own reports in males and the contributions from non-breeding aggression in bird and mammal models, suggest a common strategy involving fast-acting estrogens in the control of this behavior across species. In addition, further analysis of female non-breeding aggression may shed light on potential sexual differences in the fine tuning of social behaviors.

Photoperiod modulates the gut microbiome and aggressive behavior in Siberian hamsters

Seasonally breeding animals undergo shifts in physiology and behavior in response to changes in photoperiod (day length). Interestingly, some species, such as Siberian hamsters (Phodopus sungorus), are more aggressive during the short-day photoperiods of the non-breeding season, despite gonadal regression. While our previous data suggest that Siberian hamsters employ a 'seasonal switch' from gonadal to adrenal regulation of aggression during short-day photoperiods, there is emerging evidence that the gut microbiome, an environment of symbiotic bacteria within the gastrointestinal tract, may also change seasonally and modulate social behaviors. The goal of this study was to compare seasonal shifts in the gut microbiome, circulating levels of adrenal dehydroepiandrosterone (DHEA) and aggression in male and female Siberian hamsters. Hamsters were housed in either long-day (LD) or short-day (SD) photoperiods for 9 weeks. Fecal samples were collected and behaviors were recorded following 3, 6 and 9 weeks of housing, and circulating DHEA was measured at week 9. SD females that were responsive to changes in photoperiod (SD-R), but not SD-R males, displayed increased aggression following 9 weeks of treatment. SD-R males and females also exhibited distinct changes in the relative abundance of gut bacterial phyla and families, yet showed no change in circulating DHEA. The relative abundance of some bacterial families (e.g. Anaeroplasmataceae in females) was associated with aggression in SD-R but not LD or SD non-responder (SD-NR) hamsters after 9 weeks of treatment. Collectively, this study provides insight into the complex role of the microbiome in regulating social behavior in seasonally breeding species.

Melatonin mediates seasonal transitions in aggressive behavior and circulating androgen profiles in male Siberian hamsters

Some seasonally-breeding animals are more aggressive during the short, "winter-like" days (SD) of the non-breeding season, despite gonadal regression and reduced circulating androgen levels. While the mechanisms underlying SD increases in aggression are not well understood, previous work from our lab suggests that pineal melatonin (MEL) and the adrenal androgen dehydroepiandrosterone (DHEA) are important in facilitating non-breeding aggression in Siberian hamsters (Phodopus sungorus). To characterize the role of MEL in modulating seasonal transitions in aggressive behavior, we housed male hamsters in long days (LD) or SD, treated them with timed MEL (M) or saline injections, and measured aggression after 3, 6, and 9 weeks. Furthermore, to assess whether MEL mediates seasonal shifts in gonadal and adrenal androgen synthesis, serum testosterone (T) and DHEA concentrations were quantified 36 h before and immediately following an aggressive encounter. LD-M and SD males exhibited similar physiological and behavioral responses to treatment. Specifically, both LD-M and SD males displayed higher levels of aggression than LD males and reduced circulating DHEA and T in response to an aggressive encounter, whereas LD males elevated circulating androgens. Interestingly, LD and SD males exhibited distinct relationships between circulating androgens and aggressive behavior, in which changes in serum T following an aggressive interaction (∆T) were negatively correlated with aggression in LD males, while ∆DHEA was positively correlated with aggression in SD males. Collectively, these findings suggest that SD males transition from synthesis to metabolism of circulating androgens following an aggressive encounter, a mechanism that is modulated by MEL.

A Teleost Fish Model to Understand Hormonal Mechanisms of Non-breeding Territorial Behavior

Aggressive behaviors occurring dissociated from the breeding season encourage the search of non-gonadal underlying regulatory mechanisms. Brain estrogen has been shown to be a key modulator of this behavior in bird and mammal species, and it remains to be understood if this is a common mechanism across vertebrates. This review focuses on the contributions of Gymnotus omarorum, the first teleost species in which estrogenic modulation of non-breeding aggression has been demonstrated. Gymnotus omarorum displays year-long aggression, which has been well characterized in the non-breeding season. In the natural habitat, territory size is independent of sex and determined by body size. During the breeding season, on the other hand, territory size no longer correlates to body size, but rather to circulating estrogens and gonadosomatic index in females, and 11-ketotestosterone in males. The hormonal mechanisms underlying non-breeding aggression have been explored in dyadic encounters in lab settings. Males and females display robust aggressive contests, whose outcome depends only on body size asymmetry. This agonistic behavior is independent of gonadal hormones and fast acting androgens. Nevertheless, it is dependent on fast acting estrogenic action, as acute aromatase blockers affect aggression engagement, intensity, and outcome. Transcriptomic profiling in the preoptic area region shows non-breeding individuals express aromatase and other steroidogenic enzyme transcripts. This teleost model reveals there is a role of brain estrogen in the control of non-breeding aggression which seems to be common among distant vertebrate species.

Prepubertal ovarian inhibition of Light/Dark Box exploration and novel object investigation in juvenile Siberian hamsters

The overwhelming majority of research on the role of gonadal hormones in behavioral development has focused on perinatal, pubertal, or adult life stages. The juvenile period has been overlooked because it is thought to be a time of gonadal quiescence. In the present study, we tested whether prepubertal gonadectomy impacts the behavior of male and female juvenile hamsters on the Light/Dark Box, Novel Object, and Social Approach tests (Experiment 1) and compared these findings to those obtained after adult gonadectomy (Experiment 2). Prepubertal ovariectomy increased exploration (i.e. time spent in the light zone of the Light/Dark Box) and novel object investigation of juveniles indicating an inhibitory role for the juvenile ovary; social approach was unaffected. In contrast, adult ovariectomy and castration (both prepubertal and adult) had no effect on any behavioral measure. Experiment 3 tested whether rearing hamsters in a short day length (SD), which delays puberty in this species, extends the interval of juvenile ovarian inhibition on exploration and novelty seeking. We also tested whether provision of estradiol reverses the effects of prepubertal ovariectomy. Hormonal manipulations and behavioral tests of Experiment 3 were conducted at ages when long day-reared hamsters are adult (as in Experiment 2), but SD-reared hamsters remain reproductively immature. Ovariectomy again increased exploration in the SD-reared juveniles despite the older age of surgery and testing. Estradiol treatment had no effect. These findings reveal a novel role for the juvenile ovary in exploration and novelty seeking that is unlikely to be mediated exclusively by estradiol.

Evolutionary diversity as a catalyst for biological discovery

The tremendous diversity of animal behaviors has inspired generations of scientists from an array of biological disciplines. To complement investigations of ecological and evolutionary factors contributing to behavioral evolution, modern sequencing, gene editing, computational and neuroscience tools now provide a means to discover the proximate mechanisms upon which natural selection acts to generate behavioral diversity. Social behaviors are motivated behaviors that can differ tremendously between closely related species, suggesting phylogenetic plasticity in their underlying biological mechanisms. In addition, convergent evolution has repeatedly given rise to similar forms of social behavior and mating systems in distantly related species. Social behavioral divergence and convergence provides an entry point for understanding the neurogenetic mechanisms contributing to behavioral diversity. We argue that the greatest strides in discovering mechanisms contributing to social behavioral diversity will be achieved through integration of interdisciplinary comparative approaches with modern tools in diverse species systems. We review recent advances and future potential for discovering mechanisms underlying social behavioral variation; highlighting patterns of social behavioral evolution, oxytocin and vasopressin neuropeptide systems, genetic/transcriptional "toolkits," modern experimental tools, and alternative species systems, with particular emphasis on Microtine rodents and Lake Malawi cichlid fishes.

Dissociation of Puberty and Adolescent Social Development in a Seasonally Breeding Species

Alongside the development of sexual characteristics and reproductive competence, adolescents undergo marked cognitive, social, and emotional development [1]. A fundamental question is whether these changes are triggered by activation of the hypothalamic-pituitary-gonadal (HPG) axis at puberty (puberty dependent) or whether they occur independently of HPG activation (puberty independent). Disentangling puberty-dependent from puberty-independent mechanisms is difficult because puberty and adolescence typically proceed concurrently. Here, we test a new approach that leverages natural adaptations of a seasonally breeding species to dissociate pubertal status from chronological age. Siberian hamsters (Phodopus sungorus) reared in a long, summer-like day length (LD) exhibit rapid pubertal development, whereas those reared in a short, winter-like day length (SD) delay puberty by several months to synchronize breeding with the following spring [2, 3]. We tested whether the SD-induced delay in puberty delays the peri-adolescent decline in juvenile social play and the rise in aggression that characterizes adolescent social development in many species [4-6] and compared the results to those obtained after prepubertal gonadectomy. Neither SD rearing nor prepubertal gonadectomy altered the age at which hamsters transitioned from play to aggression; SD-reared hamsters completed this transition prior to puberty. SD rearing and prepubertal gonadectomy, however, increased levels of play in male and female juveniles, implicating a previously unknown role for prepubertal gonadal hormones in juvenile social behavior. Levels of aggression were also impacted (decreased) in SD-reared and gonadectomized males. These data demonstrate that puberty-independent mechanisms regulate the timing of adolescent social development, while prepubertal and adult gonadal hormones modulate levels of age-appropriate social behaviors.

Neural Androgen Synthesis and Aggression: Insights From a Seasonally Breeding Rodent

Aggression is an essential social behavior that promotes survival and reproductive fitness across animal systems. While research on the neuroendocrine mechanisms underlying this complex behavior has traditionally focused on the classic neuroendocrine model, in which circulating gonadal steroids are transported to the brain and directly mediate neural circuits relevant to aggression, recent studies have suggested that this paradigm is oversimplified. Work on seasonal mammals that exhibit territorial aggression outside of the breeding season, such as Siberian hamsters (Phodopus sungorus), has been particularly useful in elucidating alternate mechanisms. These animals display elevated levels of aggression during the non-breeding season, in spite of gonadal regression and reduced levels of circulating androgens. Our laboratory has provided considerable evidence that the adrenal hormone precursor dehydroepiandrosterone (DHEA) is important in maintaining aggression in both male and female Siberian hamsters during the non-breeding season, a mechanism that appears to be evolutionarily-conserved in some seasonal rodent and avian species. This review will discuss research on the neuroendocrine mechanisms of aggression in Siberian hamsters, a species that displays robust neural, physiological, and behavioral changes on a seasonal basis. Furthermore, we will address how these findings support a novel neuroendocrine pathway for territorial aggression in seasonal animals, in which adrenal DHEA likely serves as an essential precursor for neural androgen synthesis during the non-breeding season.

Aggressive behaviours track transitions in seasonal phenotypes of female Siberian hamsters

Seasonally breeding animals exhibit profound physiological and behavioural responses to changes in ambient day length (photoperiod), including changes in reproductive function and territorial aggression.Species where aggression persists when gonads are regressed and circulating levels of gonadal hormones are low, such as Siberian hamsters (Phodopus sungorus) and song sparrows (Melospiza melodia), challenge the well-established framework that gonadal hormones are important mediators of aggression.A solution to this apparent paradox is that a season-specific increase in sensitivity to hormones in brain areas associated with aggression offsets low levels of gonadal hormones during periods of reproductive quiescence.To test this hypothesis, we manipulated photoperiod to induce natural fluctuations in seasonal phenotype across multiple stages of the annual reproductive cycle in female Siberian hamsters that display increased aggression during short-day reproductive quiescence, suggesting that behaviour persists independent of gonadal steroids.Females were housed in long "summer" days or short "winter" days for 10, 24 or 30 weeks to capture gonadal regression, transition back to a reproductively functional state and full gonadal recrudescence, respectively.Long-day animals maintained reproductive functionality and displayed low aggression across all time points. By week 10, short-day reproductively responsive females underwent gonadal regression and displayed increased aggression; non-responsive animals showed no such changes. At week 24, animals were in a transitional period and displayed an intermediate phenotype with respect to reproduction and aggression. By week 30, short-day females were fully recrudesced and returned to long-day-like levels of aggression.Consistent with our hypothesis, gonadally regressed females displayed decreases in 17β-oestradiol (oestradiol) levels, but site-specific increases in the abundance of brain oestrogen receptor-alpha (ERα) in regions associated with aggression, but not reproduction. Increased site-specific ERα may function as a compensatory mechanism to allow increased responsiveness to oestradiol in regulating aggression in lieu of high circulating concentrations of hormones.Collectively, these results broaden our understanding of how breeding phenology maps onto social behaviour and the mechanisms that have evolved to coordinate behaviours that occur in non-breeding contexts.