Serotonin promotes feminization of the sexually dimorphic nucleus of the preoptic area,but not the calbindin cell group

Testosterone and its metabolites masculinize the brain during a critical perinatal window, including the relative volume of sexually dimorphic brain areas such as the sexually dimorphic nucleus of the preoptic area (SDN), which is larger in males than females. Serotonin (5HT) may mediate this hormone action, since 5HT given during the second week of life decreases (i.e., feminizes) SDN volume in males and testosterone‐treated females. Although previous work indicates that the 5HT_2A/2C receptor is sufficient to induce feminization, it is unclear whether other serotonin receptors are required and which subpopulation(s) of SDN cells are specifically organized by 5HT. Therefore, we injected male and female Sprague‐Dawley rat pups with saline, a nonselective 5HTR agonist, a 5HT_2A/2C agonist, or a 5HT_2A/2C antagonist over several timecourses in early life, and measured the Nissl‐SDN as well as a calbindin+ subdivision of the SDN, the CALB‐SDN. When examined on postnatal day 18 or early adulthood, the size of the Nissl‐SDN was feminized in males treated with any of the serotonergic drugs, eliminating the typical sex difference. In contrast, the sex difference in CALB‐SDN size was maintained regardless of serotoninergic drug treatment. This pattern suggests that although gonadal hormones shape the whole SDN, individual cellular phenotypes respond to different intermediary signals to become sexually dimorphic. Specifically, 5HT mediates sexual differentiation of non‐calbindin population(s) within the SDN. The results also caution against using measurement of the CALB‐SDN in isolation, as the absence of an effect on the CALB‐SDN does not preclude an effect on the overall nucleus. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1241–1253, 2016     

Sex differences in the level of Bcl‐2 family proteins and caspase‐3 activation in the sexually dimorphic nuclei of the preoptic area in postnatal rats

In developing rats, sex differences in the number of apoptotic cells are found in the central division of the medial preoptic nucleus (MPNc), which is a significant component of the sexually dimorphic nucleus of the preoptic area, and in the anteroventral periventricular nucleus (AVPV). Specifically, male rats have more apoptotic cells in the developing AVPV, whereas females have more apoptotic cells in the developing MPNc. To determine the mechanisms for the sex differences in apoptosis in these nuclei, we compared the expression of the Bcl‐2 family members and active caspase‐3 in postnatal female and male rats. Western blot analyses for the Bcl‐2 family proteins were performed using preoptic tissues isolated from the brain on postnatal day (PD) 1 (day of birth) or on PD8. In the AVPV‐containing tissues of PD1 rats, there were significant sex differences in the level of Bcl‐2 (female > male) and Bax (female < male) proteins, but not of Bcl‐xL or Bad proteins. In the MPNc‐containing tissues of PD8 rats, there were significant sex differences in the protein levels for Bcl‐2 (female < male), Bax (female > male), and Bad (female < male), but not for Bcl‐xL. Immunohistochemical analyses showed significant sex differences in the number of active caspase‐3‐immunoreactive cells in the AVPV on PD1 (female < male) and in the MPNc on PD8 (female > male). We further found that active caspase‐3‐immunoreactive cells of the AVPV and MPNc were immunoreactive for NeuN, a neuronal marker. These results suggest that there are sex differences in the induction of apoptosis via the mitochondrial pathway during development of the AVPV and MPNc. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Projection from the ventral bed nucleus of the stria terminalis to the retrorubral field in rats and the effects of cells in these areas on mating in male rats versus gerbils

This research identified the rat counterpart of the lateral cell group of the sexually dimorphic area (SDA) found in medial preoptic area (MPOA) gerbil of gerbils. The lateral SDA (lSDA) is critical for mating in male gerbils and contains most of the SDA cells projecting to the retrorubral field (RRF), a projection that is also important for mating. Therefore, to locate the counterpart of the lateral SDA, we traced the inputs to the rat RRF, which were dense in the ventral part of the bed nucleus of the stria terminalis (BST). To determine if the ventral BST or its projection to the RRF affects mating in male rats, we disrupted them bilaterally by placing cell-body lesions bilaterally in the ventral BST or unilaterally there and in the contralateral RRF. We also studied the effects of RRF lesions in both rats and gerbils. Bilateral ventral BST lesions, which left the medial preoptic nucleus intact, produced persistent and severe mating deficits. Disconnecting the ventral BST from the RRF also had long-lasting, but less severe, consequences. RRF lesions produced only temporary mating deficits in rats, but virtually eliminated mating in gerbils. The recovery of mating in rats after RRF, but not ventral BST, lesions, and the intermediate effects of disconnecting these areas from each other suggest that the ventral BST may contain mating-related projection neurons other than those projecting to the RRF or that its RRF-projecting cells send collaterals to another site. In either case, the pedunculopontine tegmental nucleus or raphe nuclei may be involved.     

A circuit logic for sexually shared and dimorphic aggressive behaviors in Drosophila

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Neurogenesis of the sexually dimorphic vasopressin cells of the bed nucleus of the stria terminalis and amygdala of rats

The bed nucleus of the stria terminalis (BNST) and centromedial amygdala share many neuroanatomical and neurochemical characteristics, suggesting similarities in their development. Here we compare the neurogenesis of a group of cells for which already several common characteristics have been documented, that is, the sexually dimorphic arginine vasopressin-immunoreactive (AVP-ir) cells of the BNST and amygdala. To determine when these cells are born, pregnant rats received intraperitoneal injections of the thymidine analogue bromo-2-deoxy-5-uridine (BrdU) on one of nine embryonic days, E10 to E18; E1 being the day that a copulatory plug was found. At 3 months of age, the offsprings of these females were killed and their brains stained immunocytochemically for BrdU and AVP. Most AVP-ir cells were labeled with BrdU by injections on E12 and E13. Although BrdU labeling of AVP-ir cells did not differ between the BNST and amygdala, it differed between males and females. From E12 to E13, the percentage of BrdU-labeled AVP-ir cells decreased more in males than in females. AVP-ir cells appeared to be born earlier than most other cells in the same area, the majority of which were labeled with BrdU by injections on E14, E15, and E16. The similarities in the birthdates of AVP-ir cells in the BNST and amygdala may help to explain why these cells take on so many similar characteristics. The sex difference in birthdates of AVP-ir cells may help to explain which cellular processes underlie the sexual differentiation of these cells. © 1996 John Wiley & Sons, Inc.     

Intralocus sexual conflict and the genetic architecture of sexually dimorphic traits in Prochyliza xanthostoma (Diptera: Piophilidae)

Because homologous traits of males and females are likely to have a common genetic basis, sex-specific selection (often resulting from sexual selection on one sex) may generate an evolutionary tug-of-war known as intralocus sexual conflict, which will constrain the adaptive divergence of the sexes. Theory suggests that intralocus sexual conflict can be mitigated through reduction of the intersexual genetic correlation (rMF), predicting negative covariation between rMF and sexual dimorphism. In addition, recent work showed that selection should favor reduced expression of alleles inherited from the opposite-sex parent (intersexual inheritance) in traits subject to intralocus sexual conflict. For traits under sexual selection in males, this should be manifested either in reduced maternal heritability or, when conflict is severe, in reduced heritability through the opposite-sex parent in offspring of both sexes. However, because we do not know how far these hypothesized evolutionary responses can actually proceed, the importance of intralocus sexual conflict as a long-term constraint on adaptive evolution remains unclear. In this study, we investigated the genetic architecture of sexual and nonsexual morphological traits in Prochyliza xanthostoma. The lowest rMF and greatest dimorphism were exhibited by two sexual traits (head length and antenna length) and, among all traits, the degree of sexual dimorphism was correlated negatively with rMF. Moreover, sexual traits exhibited reduced maternal heritabilities, and the most strongly dimorphic sexual trait (antenna length) was heritable only through the same-sex parent in offspring of both sexes. Our results support theory and suggest that intralocus sexual conflict can be resolved substantially by genomic adaptation. Further work is required to identify the proximate mechanisms underlying these patterns.     

Development of the human hypothalamus

The hypothalamus has been claimed to be involved in a great number of physiological functions in development, such as sexual differentiation (gender, sexual orientation) and birth, as well as in various developmental disorders including mental retardation, sudden infant death syndrome (SIDS), Kallman's syndrome and Prader-Willi syndrome. In this review a number of hypothalamic nuclei have therefore been discussed with respect to their development in health and disease. The suprachiasmatic nucleus (SCN) is the clock of the brain and shows circadian, and seasonal fluctuations in vasopressin-expressing cell numbers. The SCN also seems to be involved in reproduction, adding interest to the sex differences in shape of the vasopressin-containing SCN subnucleus and in its VIP cell number. In addition, differences in relation to sexual orientation can be seen in this perspective. The vasopressin and VIP, neurons of the SCN develop mainly postnatally, but as premature children may have circadian temperature rhythms, a different SCN cell type is probably more mature at birth.Thesexually dimorphic nucleus (SDN, intermediate nucleus, INAH-1), is twice as large in young male adults as in young females. At the moment of birth only 20% of the SDN cell number is present. From birth until two to four years of age cell numbers increase equally rapidly in both sexes. After this age cell numbers start to decrease in girls, creating the sex difference. The size of the SDN does not show any relationship to sexual orientation in men. The large neurosecretory cells of thesupraoptic (SON) andparaventricular nucleus (PVN) project to the neurohypophysis, where they release vasopressin and oxytocin into the blood circulation. In the fetus these hormones play an active role in the birth process. Fetal oxytocin may initiate or accelerate the course of labor. Fetal vasopressin plays a role in the adaptation to stress—caused by the birth process—by redistribution of the fetal blood flow.Corticotropin-releasing hormone (CRH) neurons of the PVN play a central role in stress response. Thus fetal CRH neurons may play a role in the timing of the moment of birth. Recently, alterations have been described in peptidergic, aminergic and cholinergic transmitters in the hypothalamus in SIDS. Future research will have to establish whether these changes are part of the course of SIDS. A large proportion of the SON and PVN neurons also produce tyrosine hydroxylase (TH). In neonates the majority of TH-immunoreactive neurons colocalizes vasopressin, while in the adult the majority of TH-positive neurons colocalizes oxytocin. TH-expression might be a sign of hyperactivation, for example from perinatal hypoxia.Oxytocin neurons also project to the brain stem. These neurons have an inhibitory effect on eating. Interestingly, in the Prader-Willi syndrome, characterized for example by insatiable hunger, we have found that the number of oxytocin-expressing neurons is about half the normal value. It can be concluded that the various hypothalamic nuclei are involved in a great number of functions and show clear and differential changes in development with respect to sexual differentiation, birth and a number of diseases. I believe that only a small proportion of such changes has at present been revealed.Special issue dedicated to Dr. Robert Balázs     

Ultrastructural characterization of the sexually dimorphic medial preoptic nucleus of male Japanese quail

The medial preoptic nucleus is a sexually dimorphic structure whose cytoarchitecture, afferent and efferent connections, and functions have been previously described. No detailed ultrastructural study has, however, been perfomed to date. Here we describe the ultrastructural organization of this important preoptic structure of the male quail. Neuronal cell bodies of the medial preoptic nucleus generally show extensive development of protein-synthesis-related organelles (rough endoplasmic reticulum, polysomes), and of secretory structures (Golgi complexes, secretory vesicles, dense bodies). Previous morphometrical studies at the light-microscopical level have demonstrated the presence of a medial and a lateral neuronal population distinguished by the size of their cell bodies (the medial neurons are smaller than the lateral neurons). The present ultrastructural investigation confirms the difference in size, but no difference has been observed in the ultrastructural organization of the neurons. In both the medial and the lateral part, the nucleus is characterized by a large variety of cell bodies, including some that, on the basis of their ultrastructure, can be considered as putative peptidergic neurons. Close contacts are frequently observed between adjacent cell bodies that are normally arranged in clusters. Various types of synaptic endings are also present, suggesting a rich supply of nerve fibers. A few glial cells are scattered within the nucleus. In view of the crucial role of this region in regulating quail sexual behavior, the large heterogeneity of neurons and of afferent nervous fibers suggest that this region might have an important role in the integration of information arriving from different brain regions.     

The postnatal ontogeny of the sexually dimorphic vocal apparatus in goitred gazelles (Gazella subgutturosa)

This study quantitatively documents the progressive development of sexual dimorphism of the vocal organs along the ontogeny of the goitred gazelle (Gazella subgutturosa). The major, male‐specific secondary sexual features, of vocal anatomy in goitred gazelle are an enlarged larynx and a marked laryngeal descent. These features appear to have evolved by sexual selection and may serve as a model for similar events in male humans. Sexual dimorphism of larynx size and larynx position in adult goitred gazelles is more pronounced than in humans, whereas the vocal anatomy of neonate goitred gazelles does not differ between sexes. This study examines the vocal anatomy of 19 (11 male, 8 female) goitred gazelle specimens across three age‐classes, that is, neonates, subadults and mature adults. The postnatal ontogenetic development of the vocal organs up to their respective end states takes considerably longer in males than in females. Both sexes share the same features of vocal morphology but differences emerge in the course of ontogeny, ultimately resulting in the pronounced sexual dimorphism of the vocal apparatus in adults. The main differences comprise larynx size, vocal fold length, vocal tract length, and mobility of the larynx. The resilience of the thyrohyoid ligament and the pharynx, including the soft palate, and the length changes during contraction and relaxation of the extrinsic laryngeal muscles play a decisive role in the mobility of the larynx in both sexes but to substantially different degrees in adult females and males. Goitred gazelles are born with an undescended larynx and, therefore, larynx descent has to develop in the course of ontogeny. This might result from a trade‐off between natural selection and sexual selection requiring a temporal separation of different laryngeal functions at birth and shortly after from those later in life. J. Morphol. 277:826–844, 2016. © 2016 Wiley Periodicals, Inc.     

The heritability of sexually dimorphic traits in the yellow dung fly Scathophaga stercoraria (L.)

A precise method was used for estimating the proportion of heritable variation in two life history parameters of the yellow dung fly, whereby environmental components of variance were minimized. Significant heritable variation for body size was revealed for father to son and mother to daughter relationships. Variation in development time was not significantly heritable. There is a marked sexual dimorphism in body size in this species which is discussed in the light of the observed sex-genotype interaction in heritabilities and low genetic correlation for size between the sexes. It is suggested that opposing pressures of sexual and natural selection and/or genetic pleotropy may be responsible for the maintenance of heritable variation, and the evolution of sexual dimorphism in these two traits.     

Developmental changes in synaptic formation in the hypothalamic arcuate nucleus of female rats

Summary The hypothalamic arcuate nucleus (ARCN) of female rats at 5, 20, 45 and 90 days of age was examined ultrastructurally. Axodendritic and axosomatic synapses were counted in 18,000 m² area of the ARCN in each brain. Axodendritic and axosomatic synapses in the ARCN of day 5 rats were very small in number. Axon terminals contained small spherical vesicles (SSVs, 40–60 nm in diameter). Occasionally large granular vesicles (LGVs, 75–130 nm in diameter) were found to coexist with SSVs in the endings. Pre- and postsynaptic membranes were thin. The ARCN at this age exhibited a large extracellular space which decreased with advancing age. In day 20 rats, axodendritic and axosomatic synapses increased in number up to about one-half of those of day 45 or day 90 animals. Synaptic vesicles increased in number and mitochondria were frequently encountered in the axon terminals. Pre- and postsynaptic membranes became thicker than those of day 5 rats. Further increase in the number of axodendritic and axosomatic synapses in the ARCN of day 45 rats was observed, and there were no significant difference in the morphology and incidence of synapses between day 45 and day 90 rats. Synaptic vesicles were numerous and pre- and postsynaptic membranes were thick. In tissue incubated with 5-hydroxydopamine (5-OH-DA) before fixation, small granular vesicles (SGVs, about 50 nm in diameter) which were labeled with 5-OH-DA were detected in a certain number of endings in all material taken from each age group, but the incidence of synapses containing SGVs was usually low. From these results, it can be proposed that an increase in the number of synapses in the ARCN is correlated with functional maturation of the ARC neurons. Acknowledgements. The authors wish to thank Prof. T. Kojima, Nihon University, for valuable suggestions during the initial stage of this study. This study was supported by grants from the Ministry of Education of Japan     

Onset of the hormone-sensitive perinatal period for sexual differentiation of the sexually dimorphic nucleus of the preoptic area in female rats

The volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) of the rat brain is severalfold larger in males than in females. The volume of the SDN-POA can be influenced significantly by the hormonal milieu during the perinatal "critical period" of sexual differentiation of the brain. The purpose of the present study was to determine the onset of this period of sexual differentiation of the SDN-POA. Pregnant rats received no treatment or were injected subcutaneously with oil on day 17, 18, or 20, or testosterone (T;5 mg) on days 16-22 of gestation. On postnatal day 15, unilateral SDN-POA volumes from female offspring prenatally exposed to testosterone on day 16 or 17 were not different from values of control (untreated or oil-injected) offspring. Female offspring from mothers treated with testosterone on day 18, 19, or 20 of gestation showed a significant and similar increase in SDN-POA volume over values from control animals. SDN-POA volumes from female offspring exposed to testosterone on day 21 or 22, although larger than those of controls, were not different statistically. We conclude that with the specific paradigm used in this study SDN-POA development is insensitive prior to day 18 of gestation, the day on which the onset of the hormone-sensitive period occurs.     

Cell death in the sexually dimorphic dorsal preoptic area/anterior hypothalamus of perinatal male and female ferrets

A sexually dimorphic male nucleus (MN) is present in Nissl-stained sections through the dorsal (d) preoptic area/anterior hypothalamus (POA/AH) of male ferrets. The MN-POA/AH is composed of a cluster of large cells which is organized in males by the action of estradiol, formed via the neural aromatization of circulating testosterone (T), during the last quarter of a 41-day gestation. Several recent studies using rodent species have raised the possibility that the hormone-induced masculinization of POA/AH morphology is mediated at least in part by a perinatal modulation of cell death. We asked whether a perinatal reduction in cell death contributes to the differentiation of the MN-POA/AH in the male ferret, which is a carnivore species. The appearance of internucleosomal DNA fragmentation, detected by in situ end labeling (ISEL) using the ApopTag™ kit (Oncor Corp.) and of pyknotic cell nuclei in Nissl-stained sections were used to estimate the occurrence of cell death. Male and female ferret kits were killed at four different ages spanning the perinatal period during which the MN-POA/AH is organized and assumes an adult phenotype. A peak density of dying cells was present in both sexes at postnatal day (P) 2, which is nearly 1 week after the age, embryonic day (E) 37, when the MN-POA/AH is first visible in male ferrets using Nissl stains. The density of cells in the sexually dimorphic dPOA/AH which were either ISEL-positive or pyknotic was similar in males and females on E34, as well as on P2, 10, and 20. In the nondimorphic ventral POA/AH, the highest density of dying cells was present in both sexes at E34, and there were significantly more ISEL-positive cells present in males than females at this particular age. In contrast to previous studies using rodents, our results suggest that in fetal male ferrets a modulation of the incidence of cell death contributes little to estradiol's organizational action in the dPOA/AH. © 1998 John Wiley & Sons, Inc. J Neurobiol 34: 242–252, 1998     

Opposing selection on a sexually dimorphic trait through female choice and male competition in a water boatman

Female choice and male-male competition are traditionally considered to act in concert, with male competition facilitating female choice. This situation would enforce the strength of directional selection, which could reduce genetic variation and thus the benefits of choice. Here I show that in a water boatman, Sigara falleni, the direction of selection through female choice and male competition vary among traits under laboratory conditions. The two forces were mutually enforcive in acting on body size but exerted opposing selection on a sexually selected trait, male foreleg pala size. Female choice favored large palae, whereas male competition favored smaller palae, suggesting that large palae are costly in competition. This conflicting selection through female choice and male competition could be one of the forces that contribute to the maintenance of genetic variation in sexually selected traits.     

Neurogenesis and cell migration into the sexually dimorphic preoptic area/anterior hypothalamus of the fetal ferret

A sexually dimorphic male nucleus (MN) of the preoptic area/anterior hypothalamus (POA/AH), comprising large, estradiol-receptor containing neurons, is formed in male ferrets due to the action of estradiol, derived from the neural aromatization of circulating testosterone, during the last quarter of a 41-day gestation. Two experiments were conducted to compare the birthdates and the migration pattern of cells into the sexually dimorphic portion of the dorsomedial POA/AH as well as the nondimorphic ventral nucleus (VN) of the POA/AH of males and females. In experiment 1 the thymidine analog, bromodeoxyuridine (BrdU), was injected into the amniotic sacs of fetuses of different mothers between embryonic (E) days 18 and 30. Kits from all mothers were sacrificed on E38, and brains were processed to localize BrdU immunoreactivity (IR) for determining the birthdates of neurons in the POA/AH. Cells in the MN-POA/AH of males and in a comparable region of females were born between E22 and E28; cells in the nondimorphic VN-POA/AH of both sexes were born between these same ages. These results suggest that cells in the sexually dimorphic as well as the nondimorphic subdivision of the ferret POA/AH are born during the same embryonic period. This is well before the ages (E30–E41) when administering testosterone to females can stimulate, and blocking androgen aromatization in males can inhibit, MN-POA/AH differentiation. In experiment 2 BrdU was injected on E24, and kits from different litters were perfused on E30, E34, or E38. Brains were processed for BrdU-IR as well as glial fibrillary acidic protein (GFAP), which served as a marker for radial glial processes. The orientation of radial glial processes in fetal brains of both sexes suggested that cells migrate into the dorsomedial POA/AH from proliferative zones lining the lateral as well as the third ventricles. Quantitative, computer-assisted image analysis of BrdU-IR in groups of male and female brains supported this hypothesis. There were no significant sex differences in the distribution of BrdU-IR over the three ages studied, suggesting that formation of the MN-POA/AH in males cannot be attributed to an effect of estradiol on the migration of those cells born on E24 into this sexually dimorphic structure. Finally, total BrdU-IR did not change significantly in the POA/AH of male and female kits killed at E30, E34, or E38 while the area of the POA/AH increased more than 2.5-fold over this period, suggesting that few of the POA/AH cells born on E24 die during this period in either sex. In the absence of evidence that formation of the male ferret's MN-POA/AH depends on steroid-induced changes in neurogenesis, cell migration, or death, we suggest that the specification of a particular neuronal phenotype (e.g., large somal size; capacity to produce some undetermined neurotransmitter or neuropeptide) may be responsible. © 1996 John Wiley & Sons, Inc.     

Condition dependence and the maintenance of genetic variance in a sexually dimorphic black scavenger fly

The maintenance of genetic variation in traits under strong sexual selection is a longstanding problem in evolutionary biology. The genic capture model proposes that this problem can be explained by the evolution of condition dependence in exaggerated male traits. We tested the predictions that condition dependence should be more pronounced in male sexual traits and that genetic variance in expression of these traits should increase under stress as among‐genotype variation in overall condition is exposed. Genetic variance in female and nonsexual traits should, by contrast, be similar across environments as a result of stabilizing selection on trait expression. The relationship between the degree of sexual dimorphism, condition dependence and additive genetic variance (V_a) was assessed for two morphological traits (body size and relative fore femur width) affecting male mating success in the black scavenger fly Sepsis punctum (Diptera: Sepsidae) and for development time (a nonsexual trait often correlated with body size). We compared trait expression between the sexes for two cross‐continental populations that differ in degree of sexual dimorphism (Ottawa and Zurich). Condition dependence was indeed most pronounced in males of the strongly dimorphic Zurich population (males larger), and V_a was similar for males and females unless the trait was strongly sex specific and condition dependent. Contrary to prediction, however, V_a primarily increased under food limitation in both sexes, and genetic variance in fore femur width was low to nil, perhaps depleted by putatively strong sexual selection. Solely for body size of Zurich males, V_a increased more in males than females at limited food, in accordance with the predictions of the genic capture model. Overall therefore, quantitative genetic evidence in support of the model was inconsistent and weak at best.     

The evolution of sexually dimorphic earwig forceps: social interactions among adults of the toothed earwig, Vostox apicedentatus

Earwigs (Insecta, Dermaptera) are characterized by uniquelyelaborated cerci, commonly called forceps, the function of whichremains unclear. We studied intrasexual and intersexual interactionsin the laboratory to examine the context and pattern of forcepsuse in the toothed earwig. Vostox apictdenlatus (Caudell). Interactionsbetween pairs of earwigs were recorded in four social situations:(1) two males, (2) two males plus a virgin female, (3) two females,and (4) one male and one female. Forceps were used as both weaponsand display structures by males and females in all of thesesocial contexts. During pairwise male-male interactions, onemale clearly dominated the other male. Dominant males were moreactive and more likely to use their forceps in intrasexual interactionsthan were subordinate males. In interactions where there weretwo males and one female present, the male that dominated male-maleinteractions was able to maintain exclusive access to the female.There was no indication of active female choice during or aftercourtship. During intersexual interactions, only males usedtheir forceps during courtship. The behavioral repertoire involvingforceps was greater for males than for females, especially inintrasexual contests. There was no clear outcome of intrasexualinteractions among females. These results suggest that forcepsfunction mainly as weapons in male-male interactions and mayhave evolved, at least in part, as a result of sexual selection.Further research is required to test for female mate choiceand to separate the various mechanisms of sexual selection ifmate choice exists. Comparative studies are needed to determineif sexual selection was the original evolutionary mechanismleading to the development of these unusual structures or ifsexual selection is relegated to a secondary effect, leadingto the elaboration and sexualdimorphism of these structuresin selected groups of earwigs.