Hormonal and genetic influences underlying arousal as it drives sex and aggression in animal and human brains

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

Impacts of endocrine disrupting chemicals on reproduction in wildlife and humans

Endocrine disrupting chemicals (EDCs) are ubiquitous in aquatic and terrestrial environments. The main objective of this review was to summarize the current knowledge of the impacts of EDCs on reproductive success in wildlife and humans. The examples selected often include a retrospective assessment of the knowledge of reproductive impacts over time to discern how the effects of EDCs have changed over the last several decades. Collectively, the evidence summarized here within reinforce the concept that reproduction in wildlife and humans is negatively impacted by anthropogenic chemicals, with several altering endocrine system function. These observations of chemicals interfering with different aspects of the reproductive endocrine axis are particularly pronounced for aquatic species and are often corroborated by laboratory-based experiments (i.e. fish, amphibians, birds). Noteworthy, many of these same indicators are also observed in epidemiological studies in mammalian wildlife and humans. Given the vast array of reproductive strategies used by animals, it is perhaps not surprising that no single disrupted target is predictive of reproductive effects. Nevertheless, there are some general features of the endocrine control of reproduction, and in particular, the critical role that steroid hormones play in these processes that confer a high degree of susceptibility to environmental chemicals. New research is needed on the implications of chemical exposures during development and the potential for long-term reproductive effects. Future emphasis on field-based observations that can form the basis of more deliberate, extensive, and long-term population level studies to monitor contaminant effects, including adverse effects on the endocrine system, are key to addressing these knowledge gaps.

Relationship between Thyroid Hormone Levels and Crime Type: A Controlled Study in Prisoners

Various factors cause aggression, which can be related to imbalance of T3 and T4 hormones, which can act as neurotransmitters and are reported to be elevated during aggression. This indicates changes in the hypothalamic-pituitary-thyroid axis that cause long-term changes in aggressive behaviour, especially in criminals. Moreover, mental and behavioural disorders possibly occur in individuals with impairment in thyroid hormone balance. The main rationale for this study was to asses if high T3, high T4, and low TSH hormones may have an effect on aggression-related crime tendency. Furthermore, the study aimed to measure levels of thyroid hormones in prisoners and to examine relationships of the hormone levels with crime rates. Our study was conducted in Ankara Sincan Closed Prisons. The study group consisted of 208 male volunteers who were imprisoned and the control group included 82 male volunteers who were not imprisoned. Prisoners in the study group were divided into two groups: those who committed aggression-related crime (Group A, n = 96) and prisoners convicted of other crimes (Group B, n = 112). Pulse rates, T3, T4, and thyroid-stimulating hormone (TSH) levels, and theT3/T4 ratio were measured in these prisoners. Data were analysed using the Wilcoxon rank sum test and chi-square Fisher's exact test to test for any statistically significant differences. Results showed that toxic goitre rates, T3 and T4 values, and pulse rates were significantly higher in Group A than in the control group. Significant increase in T3 and T4 levels and the presence of toxic goitre were associated with aggression-related crime. These examinations should be performed on prisoners in general, especially those convicted of violent crimes. Additional rehabilitation and research programs should also be developed for such patients.

Circadian Control of Neuroendocrine Function: Implications for Health and Disease

The circadian timing system orchestrates daily rhythms in physiology and behavior via the suprachiasmatic nucleus (SCN), the master brain clock. Because endocrine secretions have far-reaching influence on the brain and periphery, circadian regulation of hormones is essential for normal functioning and disruptions to circadian timing (e.g., irregular sleep patterns, limited exposure to sunlight, jet lag, nighttime light exposure) have detrimental health consequences. Herein, we provide an overview of circadian timing in three major endocrine axes, the hypothalamo-pituitary-gonadal (HPG), hypothalamo-pituitary-adrenal (HPA) and hypothalamo-pituitary-thyroid (HPT) axes, and then consider the negative health consequences of circadian disruptions in each of these systems. For example, disruptions to HPG axis circadian timing lead to a host of negative reproductive outcomes such as irregular menstrual cycles, low sperm density and increased rates of miscarriages and infertility. Dysregulation of HPA axis timing is associated with obesity and metabolic disease, whereas disruptions to the HPT axis are associated with dysregulated metabolic gene rhythms in the heart. Together, this overview underscores the significance of circadian endocrine rhythms in normal health and disease prevention.

The Role of Sensory Neurons in Cervical Ripening: Effects of Estrogen and Neuropeptides

Central nervous system nuclei and circuits, such as the medial preoptic, ventromedial and paraventricular nuclei of the hypothalamus, play important roles in reproduction and parturition, and are influenced by estrogen. Peripheral autonomic and sensory neurons also play important roles in pregnancy and parturition. Moreover, the steroid hormone estrogen acts directly, not only on the reproductive tract organs (uterus and cervix), but also on the central and peripheral nerves by regulating expression of various neuronal genes. The peripheral primary afferent neurons innervating the uterine cervix relay mechanical and biochemical sensory information induced by local cervical events and by passage of fetuses, to the spinal cord and supraspinal centers. Consequently, the birth process in mammals is influenced by the combined action of neurons and hormones. Peripheral sensory stimuli, induced physiologically by fetal expulsion or mechanically by vaginocervical stimulation, alter behavior, as well as autonomic and neuroendocrine systems. Recent evidence indicates that primary afferent neurons innervating the cervix, in addition to their sensory effects, likely exert local “efferent” actions on the ripening cervix near term. These efferent effects may involve estrogen-regulated production of such neuropeptides as substance P and calcitonin gene-related peptide in lumbosacral dorsal root ganglia, and their release in the cervix. Collectively, these findings suggest an interrelationship among estrogen, cervix-related sensory neurons, and local cervical events near term.

Gender differences in sleep in older men and women

Sleep disturbances increase with increasing age in both males and females and become fairly common in the older community when compared to their younger counterparts. Even though these sleep disturbances increase with advancing age, there are nevertheless inherent differences in sleep disturbances between males and females. When compared to older men, older women will have a longer sleep latency (number of minutes it takes to fall asleep), more daytime sleepiness, will sleep about 20 min less per day, have less NREM stages 1 and 2 sleep, have more slow-wave sleep, and are more predisposed to REM sleep. Women have at least a 40% increased risk for developing insomnia, are at twice the risk for restless legs syndrome, will have different obstructive sleep apnea symptoms and more partial obstructions during sleep compared to men. They are also less likely to use antidepressants but will metabolize zolpidem 50% slower than men.

Removal of reproductive suppression reveals latent sex differences in brain steroid hormone receptors in naked mole-rats, Heterocephalus glaber

BACKGROUND

Naked mole-rats are eusocial mammals, living in large colonies with a single breeding female and 1-3 breeding males. Breeders are socially dominant, and only the breeders exhibit traditional sex differences in circulating gonadal steroid hormones and reproductive behaviors. Non-reproductive subordinates also fail to show sex differences in overall body size, external genital morphology, and non-reproductive behaviors. However, subordinates can transition to breeding status if removed from their colony and housed with an opposite-sex conspecific, suggesting the presence of latent sex differences. Here, we assessed the expression of steroid hormone receptor and aromatase messenger RNA (mRNA) in the brains of males and females as they transitioned in social and reproductive status.

METHODS

We compared in-colony subordinates to opposite-sex subordinate pairs that were removed from their colony for either 1 day, 1 week, 1 month, or until they became breeders (i.e., produced a litter). Diencephalic tissue was collected and mRNA of androgen receptor (Ar), estrogen receptor alpha (Esr1), progesterone receptor (Pgr), and aromatase (Cyp19a1) was measured using qPCR. Testosterone, 17β-estradiol, and progesterone from serum were also measured.

RESULTS

As early as 1 week post-removal, males exhibited increased diencephalic Ar mRNA and circulating testosterone, whereas females had increased Cyp19a1 mRNA in the diencephalon. At 1 month post-removal, females exhibited increased 17β-estradiol and progesterone. The largest changes in steroid hormone receptors were observed in breeders. Breeding females had a threefold increase in Cyp19a1 and fivefold increases in Esr1 and Pgr, whereas breeding males had reduced Pgr and increased Ar.

CONCLUSIONS

These data demonstrate that sex differences in circulating gonadal steroids and hypothalamic gene expression emerge weeks to months after subordinate animals are removed from reproductive suppression in their home colony.

A framework for analyzing the relationship between gene expression and morphological, topological, and dynamical patterns in neuronal networks

BACKGROUND

A key point in developmental biology is to understand how gene expression influences the morphological and dynamical patterns that are observed in living beings.

NEW METHOD

In this work we propose a methodology capable of addressing this problem that is based on estimating the mutual information and Pearson correlation between the intensity of gene expression and measurements of several morphological properties of the cells. A similar approach is applied in order to identify effects of gene expression over the system dynamics. Neuronal networks were artificially grown over a lattice by considering a reference model used to generate artificial neurons. The input parameters of the artificial neurons were determined according to two distinct patterns of gene expression and the dynamical response was assessed by considering the integrate-and-fire model.

RESULTS

As far as single gene dependence is concerned, we found that the interaction between the gene expression and the network topology, as well as between the former and the dynamics response, is strongly affected by the gene expression pattern. In addition, we observed a high correlation between the gene expression and some topological measurements of the neuronal network for particular patterns of gene expression.

COMPARISON WITH EXISTING METHODS

To our best understanding, there are no similar analyses to compare with.

CONCLUSIONS

A proper understanding of gene expression influence requires jointly studying the morphology, topology, and dynamics of neurons. The proposed framework represents a first step towards predicting gene expression patterns from morphology and connectivity.

Neurobiological correlates in forensic assessment: a systematic review

BACKGROUND

With the increased knowledge of biological risk factors, interest in including this information in forensic assessments is growing. Currently, forensic assessments are predominantly focused on psychosocial factors. A better understanding of the neurobiology of violent criminal behaviour and biological risk factors could improve forensic assessments.

OBJECTIVE

To provide an overview of the current evidence about biological risk factors that predispose people to antisocial and violent behaviour, and determine its usefulness in forensic assessment.

METHODS

A systematic literature search was conducted using articles from PsycINFO, Embase and Pubmed published between 2000 and 2013.

RESULTS

This review shows that much research on the relationship between genetic predisposition and neurobiological alterations with aggression is performed on psychiatric patients or normal populations. However, the number of studies comparing offenders is limited. There is still a great need to understand how genetic and neurobiological alterations and/or deficits are related to violent behaviour, specifically criminality. Most studies focus on only one of the genetic or neurobiological fields related to antisocial and/or violent behaviour. To reliably correlate the findings of these fields, a standardization of methodology is urgently needed.

CONCLUSION

Findings from the current review suggest that violent aggression, like all forms of human behaviour, both develops under specific genetic and environmental conditions, and requires interplay between these conditions. Violence should be considered as the end product of a chain of life events, during which risks accumulate and potentially reinforce each other, displaying or triggering a specific situation. This systematic review did not find evidence of predispositions or neurobiological alterations that solely explain antisocial or violent behaviour. With better designed studies, more correlation between diverse fields, and more standardisation, it might be possible to elucidate underlying mechanisms. Thus, we advocate maintaining the current case-by-case differentiated approach to evidence-based forensic assessment.

Sleep deprivation lowers reactive aggression and testosterone in men

The role of sleep deprivation in aggressive behavior has not been systematically investigated, despite a great deal of evidence to suggest a relationship. We investigated the impact of 33 h of sleep loss on endocrine function and reactive aggression using the Point Subtraction Aggression Paradigm (PSAP) task. PSAP performance was assessed in 24 young men and 25 women who were randomly assigned to a sleep deprivation or control condition. Sleep deprivation lowered reactive aggression and testosterone (but not cortisol) in men, and disrupted the positive relationship between a pre-post PSAP increase in testosterone and aggression that was evident in rested control men. While women increased aggression following provocation as expected, no influence of sleep deprivation was found. This is the first experimental study to demonstrate that sleep deprivation lowers reactive aggression in men. Testosterone, but not cortisol, played a role in the relationship between sleep and reactive aggression in men.

Estrous behavior in dairy cows: identification of underlying mechanisms and gene functions

Selection in dairy cattle for a higher milk yield has coincided with declined fertility. One of the factors is reduced expression of estrous behavior. Changes in systems that regulate the estrous behavior could be manifested by altered gene expression. This literature review describes the current knowledge on mechanisms and genes involved in the regulation of estrous behavior. The endocrinological regulation of the estrous cycle in dairy cows is well described. Estradiol (E2) is assumed to be the key regulator that synchronizes endocrine and behavioral events. Other pivotal hormones are, for example, progesterone, gonadotropin releasing hormone and insulin-like growth factor-1. Interactions between the latter and E2 may play a role in the unfavorable effects of milk yield-related metabolic stress on fertility in high milk-producing dairy cows. However, a clear understanding of how endocrine mechanisms are tied to estrous behavior in cows is only starting to emerge. Recent studies on gene expression and signaling pathways in rodents and other animals contribute to our understanding of genes and mechanisms involved in estrous behavior. Studies in rodents, for example, show that estrogen-induced gene expression in specific brain areas such as the hypothalamus play an important role. Through these estrogen-induced gene expressions, E2 alters the functioning of neuronal networks that underlie estrous behavior, by affecting dendritic connections between cells, receptor populations and neurotransmitter releases. To improve the understanding of complex biological networks, like estrus regulation, and to deal with the increasing amount of genomic information that becomes available, mathematical models can be helpful. Systems biology combines physiological and genomic data with mathematical modeling. Possible applications of systems biology approaches in the field of female fertility and estrous behavior are discussed.

Time-of-day-dependent changes in GnRH1 neuronal activities and gonadotropin mRNA expression in a daily spawning fish, medaka

GnRH neurons in the preoptic area and hypothalamus control the secretion of GnRH and form the final common pathway for hypothalamic-pituitary-gonadal axis regulation in vertebrates. Temporal regulation of reproduction by coordinating endogenous physiological conditions and behaviors is important for successful reproduction. Here, we examined the temporal regulation of reproduction by measuring time-of-day-dependent changes in the electrical activity of GnRH1 neurons and in levels of expression of pituitary gonadotropin mRNA using a daily spawning teleost, medaka (Oryzias latipes). First, we performed on-cell patch-clamp recordings from GnRH1 neurons that directly project to the pituitary, using gnrh1-green fluorescent protein transgenic medaka. The spontaneous firing activity of GnRH1 neurons showed time-of-day-dependent changes: overall, the firing activity in the afternoon was higher than in the morning. Next, we examined the daily changes in the pituitary gonadotropin transcription level. The expression levels of lhb and fshb mRNA also showed changes related to time of day, peaking during the lights-off period. Finally, we analyzed effects of GnRH on the pituitary. We demonstrated that incubation of isolated pituitary with GnRH increases lhb mRNA transcription several hours after GnRH stimulation, unlike the well-known immediate LH releasing effect of GnRH. From these results, we propose a working hypothesis concerning the temporal regulation of the ovulatory cycle in the brain and pituitary of female medaka.

The gad2 promoter is a transcriptional target of estrogen receptor (ER)alpha and ER beta: a unifying hypothesis to explain diverse effects of estradiol

Estradiol (E(2)) regulates a wide range of neural functions, many of which require activation of estrogen receptor alpha (ERalpha) and/or ERbeta, ligand-gated transcriptional regulators. Surprisingly, very few neural gene targets of ERs have been identified, and these cannot easily explain the myriad effects of E(2). GABA regulates most of the same neural functions as E(2), and GABAergic neurons throughout the brain contain ER. Therefore, we examined whether E(2) directly regulates expression of glutamic acid decarboxylase 2 (gad2), the enzyme primarily responsible for GABA synthesis for synaptic release. Using dual luciferase assays, we found that E(2), but not other gonadal steroids, stimulated the activity of a 2691 bp rat gad2 promoter reporter construct. Activation required either ERalpha or ERbeta, and ERbeta did not repress ERalpha-mediated transactivation. Site-directed mutagenesis studies identified three estrogen response elements (EREs) with cell-specific functions. An ERE at -711 upstream of the gad2 translational start site was essential for transactivation in both MCF-7 breast cancer cells and SN56.B5.G4 neural cells, but an ERE at -546 enhanced transcription only in neural cells. A third ERE at -1958 was inactive in neural cells but exerted potent transcriptional repression in E(2)-treated MCF-7 cells. Chromatin immunoprecipitation assays in mouse GABAergic N42 cells confirmed that E(2) induced ERalpha binding to a DNA fragment containing sequences corresponding to the -546 and -711 EREs of the rat promoter. Based on these data, we propose that direct transcriptional regulation of gad2 may explain, at least in part, the ability of E(2) to impact such a diverse array of neural functions.

Influence of sex on sleep regulatory mechanisms

The ability of biological sex and sex-driven characteristics to alter sleep states may contribute to gender disparities in sleep disorders. Sex influences sleep-wake amount, the daily timing of the sleep-wake cycle, and the ability to restore sleep after extended wakefulness. Several lines of evidence suggest that in mammals, reproductive hormones are responsible for the effects of sex on sleep and may have organizational and activational influences on sleep regulatory mechanisms. In humans, exogenously administered estrogens and progestins generally enhance sleep amount and continuity, whereas androgens appear to have a positive impact on rapid eye movement (REM) sleep but disrupt sleep consolidation. In rodent studies, however, female reproductive hormones appear to enhance wakefulness, and male gonadal hormones reinforce sleep. Rodent studies have also revealed that neonatal exposure to reproductive hormones organizes adult sleep-wake architecture. This paper reviews how sex and reproductive hormones interact with circadian and homeostatic sleep regulatory mechanisms in humans and animal models. We examine the organizational and activational nature of these interactions and also review how these interactions change with advancing age. Finally, we discuss the potential for genetic sex to influence sleep states. It is our hope that a better understanding of the mechanisms through which sex influences sleep-wake states will lead to improvements in the design of studies that examine gender disparities in sleep-wake disorders.

The neurophysiology of sexual arousal

Our understanding of the process and initiation of sexual arousal is being enhanced by both animal and human studies, inclusive of basic science principles and research on clinical outcomes. Sexual arousal is dependent on neural (sensory and cognitive) factors, hormonal factors, genetic factors and, in the human case, the complex influences of culture and context. Sexual arousal activates the cognitive and physiologic processes that can eventually lead to sexual behavior. Sexual arousal comprises a particular subset of central nervous system arousal functions which depend on primitive, fundamental arousal mechanisms that cause generalized brain activity, but are manifest in a sociosexual context. The neurophysiology of sexual arousal is seen as a bidirectional system universal to all vertebrates. The following review includes known neural and genomic mechanisms of a hormone-dependent circuit for simple sex behavior. New information about hormone effects on causal steps related to sex hormones' nuclear receptor isoforms expressed by hypothalamic neurons continues to enrich our understanding of this neurophysiology.

Behavioral Aggression Is Associated with the 2D:4D Ratio in Men but Not in Women

Abstract. Numerous studies have investigated the relationship between testosterone (T) and aggression but yielded inconsistent findings in healthy subjects. One possible reason for this might be the measurement of actual T-levels, ignoring that its role in neurodevelopment seems to be of predominant importance. The aim of our study was to further elucidate the effects of T availability during early phases of gestation, operationalized by the measurement of the 2nd to 4th digit ratio (2D:4D), on behavioral aggression in healthy volunteers. A low 2D:4D (indicating a long ring finger relative to the index finger) relates to high levels of prenatal T. A total number of 171 healthy subjects (98 men, 73 women) ranging in age from 20 to 30 years were tested. Participants were subjected to a modified version of a competitive reaction-time task, a commonly used and well-established tool to elicit and measure aggression (Taylor paradigm). They also completed self-report scales on trait aggression. Ventral surface scans of both hands were used to determine the 2D:4D ratio. As expected, the 2D:4D was lower in men as compared to women for both hands. Moreover, in contrast to questionnaire data, men reveal higher levels of aggression in the computer task compared to women. Finally, a negative correlation between 2D:4D and aggression was found in males but not in females. Results are discussed with respect to the usefulness of the 2D:4D construct as well as to sex differences in organizational effects of T.

Action by and sensitivity to neuroactive steroids in menstrual cycle related CNS disorders

Neuroactive steroids are a large group of substances having effect in the brain and on brain function. The steroids most studied are allopregnanolone (ALLO), tetrahydrodesoxycorticosterone (THDOC), pregnenolone sulfate (PS) dihydroepiandrosteronesulfate (DHEAS), and estradiol (E2). ALLO and THDOC are called gamma-aminobutyric acid (GABA) steroids as they are positive modulators of the GABAA receptor in a similar way as benzodiazepines, barbiturates, and alcohol. GABA steroids not only have similar behavioral effects as benzodiazepines and barbiturates but, possibly, also similar adverse effects as well. This review aims to elucidate the possible role that neuroactive steroids play in the development of mood disorders in women. One of the most clear-cut examples of the interaction between mood, neuroactive steroids, and the GABA system is premenstrual dysphoric disorder (PMDD), which is a cluster of negative mood symptoms occurring during the luteal phase of the menstrual cycle in 2-6% of reproductive women. Furthermore, certain women also experience adverse mood effects during sequential progestin addition to postmenopausal estrogen treatment, which is why the role of neuroactive steroids in postmenopausal women is also addressed in this review.

Reproductive behavior of small animals

Normal and abnormal reproductive behavior of dogs and cats, and known and proposed hormonal bases for these behaviors are addressed. Emerging information includes use of oxytocin to promote pair-bonding between dam and offspring and the possible effect of prolactin in inhibiting sexual behaviors.

Neurotropic action of androgens: principles, mechanisms and novel targets

The importance of androgen signaling is well recognized for numerous aspects of central nervous system (CNS) function, ranging from sex-specific organization of neuroendocrine and behavioral circuits to adaptive capacity, resistance and repair. Nonetheless, concepts for the therapeutic use of androgens in neurological and mental disorders are far from being established. This review outlines some critical issues which interfere with decisions on the suitability of androgens as therapeutic agents for CNS conditions. Among these, sex-specific organization of neural substrates and resulting differential responsiveness to endogenous gonadal steroids, convergence of steroid hormone actions on common molecular targets, co-presence of different sex steroid receptors in target neuronal populations, and in situ biotransformation of natural androgens apparently pose the principal obstacles for the characterization of specific neurotropic effects of androgens. Additional important, albeit less explored aspects consist in insufficient knowledge about molecular targets in the CNS which are under exclusive or predominant androgen control. Own experimental data illustrate the variability of pharmacological effects of natural and synthetic androgens on CNS functions of adaptive relevance, such as sexual behavior, anxiety and endocrine responsiveness to stress. Finally, we present results from an analysis of the consequences of aging for the rat brain transcriptome and examination of the influence of androgens on differentially expressed genes with presumable significance in neuropathology.

Gender-selective patterns of aggressive behavior in Drosophila melanogaster

Complex behaviors, such as aggression, are comprised of distinct stereospecific behavioral patterns (modules). How such patterns get wired into nervous systems remains unknown. Recently, we reported on a quantitative analysis of fighting behavior in male flies of the common Canton-S strain of Drosophila melanogaster. Here, we report a similar analysis of fighting behavior in females of the same species. Fights were carried out between pairs of virgin and pairs of mated females in competition for a yeast resource. Each fight was videotaped and analyzed by using transition matrices and Markov chain analyses. We observe only small difference in fighting intensity between virgin and mated females. In contrast to what is seen in male fights, however, no clear hierarchical relationship is formed in the female fights. A further comparison of the behavioral patterns making up male and female fights reveals that some modules are shared by both sexes, whereas others are highly selective. Within the shared components, transitions between the modules also show gender-selective differences. By using the powerful genetic methods available for examining behavior in fruit flies, it should be possible to use the gender-selective differences in fighting behavior to address the question of how these behavioral patterns get established in the brains of fruit flies.