A mechanism for virilization of female spotted hyenas in utero

The Biology and Evolution of Fierce Females (Moles and Hyenas)

Talpid moles and spotted hyenas have become the paradigms of anatomical and behavioral female masculinization. Females of many mole species develop ovotestes that produce testosterone, show external genitalia that resemble that of males, and close their vaginal orifice after every estrus, and female spotted hyenas lack an external vaginal orifice and develop a pseudoscrotum and a large pseudopenis through which they urinate, mate, and give birth. We review current knowledge about several significant aspects of the biology and evolution of these females, including (a) their specific study methods; (b) their unique anatomical features, and how these peculiarities influence certain physiological functions; and (c) the role that steroid hormones as well as genetic and environmental factors may have in urogenital system development, aggressive behavior, and social dominance. Nevertheless, both mole and hyena females are exceptionally efficient mothers, so their peculiar genitalia should not call into question their femininity.

Meerkat manners: Endocrine mediation of female dominance and reproductive control in a cooperative breeder

This article is part of a Special Issue (Hormones and Hierarchies). To gain more balanced understanding of sexual selection and mammalian sexual differentiation processes, this review addresses behavioral sex differences and hormonal mediators of intrasexual competition in the meerkat (Suricata suricatta) - a cooperative breeder unusual among vertebrates in its female aggression, degree of reproductive skew, and phenotypic divergence. Focused on the evolution, function, mechanism, and development of female dominance, the male remains a key reference point throughout. Integrated review of endocrine function does not support routine physiological suppression in subordinates of either sex, but instead a ramp up of weight, reproduction, aggression, and sex steroids, particularly androgens, in dominant females. Important and timely questions about female competition are thus addressed by shifting emphasis from mediators of reproductive suppression to mediators of reproductive control, and from organizational and activational roles of androgens in males to their roles in females. Unusually, we ask not only how inequity is maintained, but how dominance is acquired within a lifetime and across generations. Antiandrogens administered in the field to males and pregnant dominant females confirm the importance of androgen-mediated food competition. Moreover, effects of maternal endocrine milieu on offspring development reveal a heritable, androgenic route to female aggression, likely promoting reproductive priority along dominant matrilines. Integrating endocrine measures with long-term behavioral, ecological, morphological, and life-history data on normative and experimental individuals, across life stages and generations, provides better appreciation of the role of naturally circulating androgens in regulating the female phenotype, and sheds new light on the evolution of female dominance, reproductive inequity, and cooperative breeding.

Integrating the female masculinization and challenge hypotheses: Female dominance, male deference, and seasonal hormone fluctuations in adult blue-eyed black lemurs (Eulemur flavifrons)

In the decades since female social dominance was first described in strepsirrhine primates, researchers have sought to uncover the proximate and ultimate explanations for its development. In the females of various female-dominant species, androgens have been implicated as regulators of behavior and/or predictors of seasonal fluctuations in aggression (the 'Female Masculinization Hypothesis'). Males, more generally, respond to changing social demands via seasonal fluctuations in androgen-mediated behavior (the 'Challenge Hypothesis'), that may also entail changes in activation of the hypothalamic-pituitary-adrenal axis. Here, we explore if androgens, glucocorticoids, and intersexual behavior fluctuate seasonally in the female-dominant, blue-eyed black lemur (Eulemur flavifrons), with potential consequences for understanding female aggression and male deference. Across two studies conducted during the breeding and nonbreeding seasons, we assessed rates of mixed-sex, dyadic social behavior (aggression and affiliation) and concentrations of fecal glucocorticoid metabolites (Study 1) and serum sex hormones (androstenedione, testosterone, and estradiol; Study 2). Our results align with several predictions inspired by the Female Masculinization and Challenge Hypotheses for intersexual relations: During the breeding season, specifically, both aggression and androstenedione peaked in females, while female-initiated affiliation decreased, potentially to facilitate female resource access and reproductive control. By comparison, all target hormones (androgens, estrogen, and glucocorticoids) peaked in males, with glucocorticoid concentrations potentially increasing in response to the surge in female aggression, and unusually high estrogen concentrations year-round potentially facilitating male deference via male-initiated affiliation. These results suggest complex, seasonally and hormonally mediated behavior in Eulemur flavifrons.

An intergenerational androgenic mechanism of female intrasexual competition in the cooperatively breeding meerkat

Female intrasexual competition can be intense in cooperatively breeding species, with some dominant breeders (matriarchs) limiting reproduction in subordinates via aggression, eviction or infanticide. In males, such tendencies bidirectionally link to testosterone, but in females, there has been little systematic investigation of androgen-mediated behaviour within and across generations. In 22 clans of wild meerkats (Suricata suricatta), we show that matriarchs 1) express peak androgen concentrations during late gestation, 2) when displaying peak feeding competition, dominance behaviour, and evictions, and 3) relative to subordinates, produce offspring that are more aggressive in early development. Late-gestation antiandrogen treatment of matriarchs 4) specifically reduces dominance behaviour, is associated with infrequent evictions, decreases social centrality within the clan, 5) increases aggression in cohabiting subordinate dams, and 6) reduces offspring aggression. These effects implicate androgen-mediated aggression in the operation of female sexual selection, and intergenerational transmission of masculinised phenotypes in the evolution of meerkat cooperative breeding.

Maternal effects in mammals: Broadening our understanding of offspring programming

The perinatal period is a sensitive time in mammalian development that can have long-lasting consequences on offspring phenotype via maternal effects. Maternal effects have been most intensively studied with respect to two major conditions: maternal diet and maternal stress. In this review, we shift the focus by discussing five major additional maternal cues and their influence on offspring phenotype: maternal androgen levels, photoperiod (melatonin), microbiome, immune regulation, and milk composition. We present the key findings for each of these topics in mammals, their mechanisms of action, and how they interact with each other and with the maternal influences of diet and stress. We explore their impacts in the contexts of both predictive adaptive responses and the developmental origins of disease, identify knowledge gaps and research opportunities in the field, and place a particular emphasis on the application and consideration of these effects in non-model species and natural ecological systems.

Organizational and activational androgens, lemur social play, and the ontogeny of female dominance

The role of androgens in shaping "masculine" traits in males is a core focus in behavioral endocrinology, but relatively little is known about an androgenic role in female aggression and social dominance. In mammalian models of female dominance, including the ring-tailed lemur (Lemur catta), links to androgens in adulthood are variable. We studied the development of ring-tailed lemurs to address the behavioral basis and ontogenetic mechanisms of female dominance. We measured behavior and serum androgen concentrations in 24 lemurs (8 males, 16 females) from infancy to early adulthood, and assessed their 'prenatal' androgen milieu using serum samples obtained from their mothers during gestation. Because logistical constraints limited the frequency of infant blood sampling, we accounted for asynchrony between behavioral and postnatal hormone measurements via imputation procedures. Imputation was unnecessary for prenatal hormone measurements. The typical sex difference in androgen concentrations in young lemurs was consistent with adult conspecifics and most other mammals; however, we found no significant sex differences in rough-and-tumble play. Female (but not male) aggression increased beginning at approximately 15 months, coincident with female puberty. In our analyses relating sexually differentiated behavior to androgens, we found no relationship with activational hormones, but several significant relationships with organizational hormones. Notably, associations of prenatal androstenedione and testosterone with behavior were differentiated, both by offspring sex and by type of behavior within offspring sexes. We discuss the importance of considering (1) missing data in behavioral endocrinology research, and (2) organizational androgens other than testosterone in studies of female dominance.

Morphology of the genital organs of the female red-rumped agouti (Dasyprocta leporina, Linnaeus, 1758) during estrous cycle phases and in advanced pregnancy

The study investigated the gross and microscopic anatomy of the genital organs of 20 agoutis at different stages of the estrous cycle and four in the final trimester of pregnancy. Specimens were euthanized and their reproductive organs were fixed in a 4% paraformaldehyde or 2.5% glutaraldehyde solution and submitted to routine histological techniques for light and scanning electron microscopy. In the ovary, during the proestrus phase, we observed developing follicles and corpus luteum (CL) in regression; during estrus, there were Graafian follicles; during metestrus, there was a hemorrhagic corpus, whereas in diestrus, there was a mature CL. The uterus was partially double because the cervix was cranially septate but caudally, the septum disappeared, forming a single ostium that opened into the vagina. Changes occurred along the estrous cycle in the uterine and vaginal epithelia, that is, an increase in the uterine epithelium height accompanied by an increase of thickness of the vaginal epithelium during the follicular phase and a decrease of thickness of both epithelia during the luteal phase. The endometrial lining was composed of a simple cuboidal epithelium to simple columnar epithelium with basal nuclei. The vaginal mucosa consisted of epithelium that varied from nonkeratinized stratified squamous (luteal phase) to keratinized stratified squamous (follicular phase). The clitoris was external to the vagina. It presented two protruding lateral keratinized spicules and a centralized urethra, with no common parts between the urinary and genital tracts. Anatomical and histological changes were observed mainly in the cervix, vagina and spicules of the clitoris during the EC.

Development of urogenital system in the Spix cavy: A model for studies on sexual differentiation

This study documented, for the first time, the morphological patterns of differentiation of male and female genital organs of Spix cavy (Galea spixii) using histological and ultrastructural analyses, with immuno-localization of steroidogenic enzymes, cytochromes P450 aromatase (P450arom) and 17α-hydroxylase/17, 20-lyase (P450c17), involved in the synthesis of estrogens and androgens respectively throughout fetal sexual development. Undifferentiated gonads of Spix cavy develop into ovaries in females after 25 days of gestation (DG), exhibiting P450arom immunoreactivity. After 25 DG, paramesonephric ducts develop and form oviducts, uterine horns and cranial portion of the vagina. The caudal portion of the vagina originates from the urogenital sinus, and a vaginal closure membrane is present at the end of gestation. Partial channeling of the urethra into the clitoris occurs after 40 DG, but complete channeling never occurs. A preputial meatus emerges near the tip of organ. In males, undifferentiated gonads develop into testes at 25 DG and develop immunoreactivity for P450c17, which is required for androgens synthesis and likely maintenance of mesonephric ducts. Mesonephric ducts develop subsequently, forming the epididymis and ductus deferens. The pelvic urethra develops after 25 DG with channeling into the penis occurring around 30 DG. This is the first morphological study describing the process of sexual differentiation during gestation in a hystricomorph rodent and one of the most comprehensive analyses conducted in any mammal. Male genital organ development follows the general pattern described in other domestic mammals, but does not include formation of the baculum as occurs in mice and rats. In females, clitoral development includes partial canalization by the urethra and development of a preputial meatus. Further studies are required to clarify the mechanisms involved in the differentiative processes described.

Exceptional endocrine profiles characterise the meerkat: sex, status, and reproductive patterns

In vertebrates, reproductive endocrine concentrations are strongly differentiated by sex, with androgen biases typifying males and estrogen biases typifying females. These sex differences can be reduced in female-dominant species; however, even the most masculinised of females have less testosterone (T) than do conspecific males. To test if aggressively dominant, female meerkats (Suricata suricatta) may be hormonally masculinised, we measured serum androstenedione (A₄), T and estradiol (E₂) in both sexes and social classes, during both ‘baseline’ and reproductive events. Relative to resident males, dominant females had greater A₄, equivalent T and greater E₂ concentrations. Males, whose endocrine values did not vary by social status, experienced increased T during reproductive forays, linking T to sexual behaviour, but not social status. Moreover, substantial E₂ concentrations in male meerkats may facilitate their role as helpers. In females, dominance status and pregnancy magnified the unusual concentrations of measured sex steroids. Lastly, faecal androgen metabolites replicated the findings derived from serum, highlighting the female bias in total androgens. Female meerkats are thus strongly hormonally masculinised, possibly via A₄’s bioavailability for conversion to T. These raised androgen concentrations may explain female aggressiveness in this species and give dominant breeders a heritable mechanism for their daughters’ competitive edge.

Adrenal glands of Spix's yellow-toothed cavy (Galea spixii, Wagler, 1831): morphological and morphometric aspects

Abstract Considering the physiological importance and need of greater morphophysiological knowledge of adrenal glands, the aims of present study were compare the morphometric data between left and right adrenal of male and female; perform a histological, scanning and transmission electron microscopy study showing tissue constitution of glands; finally, in order to define the presence and correct site of the cytochrome P450c17 expression in adrenal glands, immunohistochemical study of this enzyme was performed in 18 adrenal glands (right n=9 and left n=9) of nine adult Galea spixii (four males and five females). Right adrenal was more cranially positioned than left adrenal; dimensions (weight, length and width) of right adrenal was larger than left adrenal; no differences between male and female body and adrenal measurements were found; the morphology of cells and different amounts of lipid droplets may be related to the different demands of steroid hormones production, related to each zone of the adrenal cortex; and, the cytochrome P450c17 immunolocalization in fasciculate and reticular zone may be related with synthesis of 17-hydroxy-pregnenolone, 17-hydroxy-progesterone, dehydroepiandrosterone or androstenedione.

Female rule in lemurs is ancestral and hormonally mediated

Female social dominance (FSD) over males is unusual in mammals, yet characterizes most Malagasy lemurs, which represent almost 30% of all primates. Despite its prevalence in this suborder, both the evolutionary trajectory and proximate mechanism of FSD remain unclear. Potentially associated with FSD is a suite of behavioural, physiological and morphological traits in females that implicates (as a putative mechanism) 'masculinization' via androgen exposure; however, relative to conspecific males, female lemurs curiously show little evidence of raised androgen concentrations. By observing mixed-sex pairs of related Eulemur species, we identified two key study groups--one comprised of species expressing FSD and increased female scent marking, the other comprised of species (from a recently evolved clade) showing equal status between the sexes and the more traditional pattern of sexually dimorphic behaviour. Comparing females from these two groups, we show that FSD is associated with more masculine androgen profiles. Based on the widespread prevalence of male-like features in female lemurs and a current phylogeny, we suggest that relaxation of hormonally mediated FSD emerged only recently and that female masculinization may be the ancestral lemur condition, an idea that could revolutionize our understanding of the ancient socioecology and evolution of primate social systems.

Aromatase and estrogen receptor α deficiency

Studies on the phenotypes of women and men with mutations disrupting estrogen biosynthesis and action have significantly advanced our knowledge of the physiologic roles of estrogen in humans. Aromatase deficiency results from autosomal recessive inheritance of mutations in the CYP19A1 gene. It gives rise to ambiguous genitalia in 46,XX fetuses. At puberty, affected girls have hypergonadotropic hypogonadism, do not develop secondary sexual characteristics, and exhibit progressive virilization. The affected 46,XY men have normal male sexual differentiation and pubertal maturation. These men, however, are extremely tall and have eunucoid proportions with continued linear growth into adulthood, severely delayed epiphyseal closure, and osteoporosis due to estrogen deficiency. Although estrogen has been shown to be essential for normal sperm production and function in mice, its role in fertility is not clear in men. Thus far, one man and an unrelated woman with estrogen resistance due to mutations in the estrogen receptor α (ESR1) gene have been described. Their clinical presentations are similar to that of aromatase-deficient men and women.

Development of the external genitalia: perspectives from the spotted hyena (Crocuta crocuta)

This review/research paper summarizes data on development of the external genitalia of the spotted hyena, a fascinating mammal noted for extreme masculinization of the female external genitalia. The female spotted hyena is the only extant mammal that mates and gives birth through a pendulous penis-like clitoris. Our studies indicate that early formation of the phallus in both males and females is independent of androgens; indeed the phallus forms before the fetal testes or ovaries are capable of synthesizing androgens. Likewise, pre- and postnatal growth in length of the penis and clitoris is minimally affected by "androgen status". Nonetheless, several internal morphologies, as well as external surface features of the phallus, are androgen-dependent and thus account for dimorphism between the penis and clitoris. Finally, estrogens play a critical role in penile and clitoral development, specifying the position of the urethral orifice, determining elasticity of the urethral meatus, and facilitating epithelial-epithelial fusion events required for proper formation of the distal urethra/urogenital sinus and prepuce. Accordingly, prenatal inhibition of estrogen synthesis via administration of letrozole (an aromatase inhibitor) leads to malformations of the glans as well as the prepuce (hypospadias). The effects of prenatal androgens, anti-androgens and impaired estrogen synthesis correlated with the tissue expression of androgen and estrogen receptors.

The influence of androgenic steroid hormones on female aggression in 'atypical' mammals

Dimorphism on dominance and agonistic behaviour in mammals tends to be strongly biased toward males. In this review, we focus on a select few species of mammals in which females are as or more aggressive than males, and/or are dominant to males, and explore the role of androgenic hormones in mediating this important difference. While the data are not as clear-cut as those published on traditional laboratory mammals, our review highlights important endocrine substrates for both organizational and activational influences of steroids on female aggressive behaviour. We highlight areas in which further observations and experiments are crucial, especially the potential facilitative effects of androgens on female aggression. Finally, new and innovative techniques, including molecular genetics and receptor pharmacology, portend important insights into the ways in which androgenic hormones regulate aggressive behaviour in 'atypical' female mammals.

Placental contribution to the origins of sexual dimorphism in health and diseases: sex chromosomes and epigenetics

Sex differences occur in most non-communicable diseases, including metabolic diseases, hypertension, cardiovascular disease, psychiatric and neurological disorders and cancer. In many cases, the susceptibility to these diseases begins early in development. The observed differences between the sexes may result from genetic and hormonal differences and from differences in responses to and interactions with environmental factors, including infection, diet, drugs and stress. The placenta plays a key role in fetal growth and development and, as such, affects the fetal programming underlying subsequent adult health and accounts, in part for the developmental origin of health and disease (DOHaD). There is accumulating evidence to demonstrate the sex-specific relationships between diverse environmental influences on placental functions and the risk of disease later in life. As one of the few tissues easily collectable in humans, this organ may therefore be seen as an ideal system for studying how male and female placenta sense nutritional and other stresses, such as endocrine disruptors. Sex-specific regulatory pathways controlling sexually dimorphic characteristics in the various organs and the consequences of lifelong differences in sex hormone expression largely account for such responses. However, sex-specific changes in epigenetic marks are generated early after fertilization, thus before adrenal and gonad differentiation in the absence of sex hormones and in response to environmental conditions. Given the abundance of X-linked genes involved in placentogenesis, and the early unequal gene expression by the sex chromosomes between males and females, the role of X- and Y-chromosome-linked genes, and especially those involved in the peculiar placenta-specific epigenetics processes, giving rise to the unusual placenta epigenetic landscapes deserve particular attention. However, even with recent developments in this field, we still know little about the mechanisms underlying the early sex-specific epigenetic marks resulting in sex-biased gene expression of pathways and networks. As a critical messenger between the maternal environment and the fetus, the placenta may play a key role not only in buffering environmental effects transmitted by the mother but also in expressing and modulating effects due to preconceptional exposure of both the mother and the father to stressful conditions.

Maternal gestational androgens are associated with decreased juvenile play in white-faced marmosets (Callithrix geoffroyi)

Exposure to androgens during prenatal development shapes both physiological and behavioral developmental trajectories. Notably, in rhesus macaques, prenatal androgen exposure has been shown to increase rough-and-tumble play, a prominent behavioral feature in males during the juvenile period in primates. While macaques are an Old World, polygamous species with marked sexually dimorphic behavior, New World callitrichine primates (marmosets and tamarins) live in cooperative breeding groups and are considered to be socially monogamous and exhibit minimal sexual dimorphism in social play, which suggests that androgen may affect this species in different ways compared to macaques. In addition, we previously described considerable variation in maternal androgen production during gestation in marmosets. Here we tested the association between this variation and variation in offspring rough-and-tumble play patterns in both males and females. We measured testosterone and androstenedione levels in urine samples collected from pregnant marmoset mothers and then observed their offspring's play behavior as juveniles (5-10 months of age). In contrast to findings in rhesus macaques, hierarchical regression analyses showed that higher gestational testosterone levels, primarily in the second semester, were associated with decreased rough-and-tumble play in juveniles, and this relationship appears to be driven more so by males than females. We found no reliable associations between gestational androstenedione and juvenile play behavior. Our findings provide evidence to suggest that normative variation in levels of maternal androgen during gestation may influence developmental behavioral trajectories in marmosets in a way that contradicts previous findings in Old World primates.

Morphology of the external genitalia of the adult male and female mice as an endpoint of sex differentiation

Adult external genitalia (ExG) are the endpoints of normal sex differentiation. Detailed morphometric analysis and comparison of adult mouse ExG has revealed 10 homologous features distinguishing the penis and clitoris that define masculine vs. feminine sex differentiation. These features have enabled the construction of a simple metric to evaluate various intersex conditions in mutant or hormonally manipulated mice. This review focuses on the morphology of the adult mouse penis and clitoris through detailed analysis of histologic sections, scanning electron microscopy, and three-dimensional reconstruction. We also present previous results from evaluation of "non-traditional" mammals, such as the spotted hyena and wallaby to demonstrate the complex process of sex differentiation that involves not only androgen-dependent processes, but also estrogen-dependent and hormone-independent mechanisms.

Phylogenetic comparisons implicate sex hormone-binding globulin in "masculinization" of the female spotted hyena (Crocuta crocuta)

Exposures to sex steroids during fetal development are thought to contribute to the unique urogenital anatomy and social dominance of the female spotted hyena: overt phenotypes not shared by other hyenids (i.e. striped hyena, brown hyena, and aardwolf). Because both androgens and estrogens influence development of genitalia and behavior, and because plasma SHBG regulates their access to tissues, we compared the Shbg gene sequences, structures, and steroid-binding properties in the four extant hyenids. We found the hyenid Shbg genes (>95% identical) and mature protein sequences (98% identical) are highly conserved. As in other mammals, the hyenid SHBG all bind 5α-dihydrotestosterone with high affinity (K(d) = 0.62-1.47 nm), but they also bind estrone and dehydroepiandrosterone with similarly high affinity, and this unusual property was attributed to specific amino acids within their SHBG steroid-binding sites. Phylogenetic comparisons also indicated that the spotted hyena SHBG precursor uniquely lacks two leucine residues and has a L15W substitution within its secretion signal polypeptide, the reduced size and hydrophobicity of which markedly decreases the production of SHBG and may therefore explain why serum SHBG concentrations in male and female spotted hyenas are approximately five times lower than in other hyenids. This is important because low plasma SHBG concentrations in spotted hyenas will increase exposure to biologically active androgens and estrogen as well as to their precursors (dehydroepiandrosterone and estrone), which may contribute to the masculinized external genitalia of female spotted hyenas and to female social dominance over males.

Endocrine correlates of pregnancy in the ring-tailed lemur (Lemur catta): implications for the masculinization of daughters

Female ring-tailed lemurs (Lemur catta) are Malagasy primates that are size monomorphic with males, socially dominate males, and exhibit a long, pendulous clitoris, channeled by the urethra. These masculine traits evoke certain attributes of female spotted hyenas (Crocuta crocuta) and draw attention to the potential role of androgens in lemur sexual differentiation. Here, hormonal correlates of prenatal development were assessed to explore the possibility that maternal androgens may shape the masculine morphological and behavioral features of developing female lemurs. Maternal serum 17α-hydroxyprogesterone, dehydroepiandrosterone sulphate (DHEA-S), ∆⁴ androstenedione (androst-4-ene-3,17,dione), testosterone, and 17β-estradiol were charted throughout the 19 pregnancies of 11 ring-tailed lemurs. As in spotted hyenas, lemur pregnancies were associated with an immediate increase in androgen concentrations (implicating early maternal derivation), followed by continued increases across stages of gestation. Pregnancies that produced singleton males, twin males, or mixed-sex twins were marked by greater androgen and estrogen concentrations than were pregnancies that produced singleton or twin females, especially in the third trimester, implicating the fetal testes in late-term steroid profiles. Concentrations of DHEA-S were mostly below detectable limits, suggesting a minor role for the adrenals in androgen biosynthesis. Androgen concentrations of pregnant lemurs bearing female fetuses, although less than those of pregnant hyenas, exceeded preconception and postpartum values and peaked in the third trimester. Although a maternal (and, on occasion, fraternal) source of androgen may exist for fetal lemurs, further research is required to confirm that these steroids would reach the developing female and contribute to her masculinization.

The anti-androgen combination, flutamide plus finasteride, paradoxically suppressed LH and androgen concentrations in pregnant spotted hyenas, but not in males

The androgen receptor blocker flutamide and the 5α-reductase inhibitor finasteride have been used in a variety of species to investigate the ontogeny of sexual dimorphisms by treating pregnant females or neonates at critical periods of sexual differentiation. Likewise, we have used these drugs to study the profound masculinization of the external genitalia in female spotted hyenas. However, a potential pitfall of administering flutamide, either alone or in combination with finasteride, is that it maintains or even raises plasma concentrations of luteinizing hormone (LH) and testosterone (T), because negative feedback of the hypothalamic-pituitary-gonadal axis is disrupted. Contrary to expectations, when pregnant spotted hyenas were treated with flutamide and finasteride (F&F), the concentrations of T during late gestation were suppressed relative to values in untreated dams. Herein, we further investigate the paradoxical effects of F&F treatment on a battery of sex hormones in spotted hyenas. Beyond the effects on T, we found plasma concentrations of LH, estradiol, progesterone and androstenedione (A4) were also significantly lower in F&F-treated pregnant hyenas than in controls. Flutamide and finasteride did not have similar effects on LH, T, and A4 concentrations in male hyenas. The paradoxical effect of F&F treatment on LH and T concentrations in the maternal circulation suggests that negative feedback control of gonadotropin and androgen secretion may be modified in spotted hyenas during pregnancy.

Sexual selection in males and females

Research on sexual selection shows that the evolution of secondary sexual characters in males and the distribution of sex differences are more complex than was initially suggested but does not undermine our understanding of the evolutionary mechanisms involved. However, the operation of sexual selection in females has still received relatively little attention. Recent studies show that both intrasexual competition between females and male choice of mating partners are common, leading to strong sexual selection in females and, in extreme cases, to reversals in the usual pattern of sex differences in behavior and morphology.

Placental Expression and Molecular Characterization of Aromatase Cytochrome P450 in the Spotted Hyena (Crocuta crocuta)

At birth, the external genitalia of female spotted hyenas (Crocuta crocuta) are the most masculinized of any known mammal, but are still sexually differentiated. Placental aromatase cytochrome P450 (P450arom) is an important route of androgen metabolism protecting human female fetuses from virilization in utero. Therefore, placental P450arom expression was examined in spotted hyenas to determine levels during genital differentiation, and to compare molecular characteristics between the hyena and human placental enzymes. Hyena placental P450arom activity was determined at gestational days (GD) 31, 35, 45, 65 and 95 (term, 110), and the relative sensitivity of hyena and human placental enzyme to inhibition by the specific inhibitor, Letrozole, was also examined. Expression of hyena P450arom in placenta was localized by immuno-histochemistry, and a full-length cDNA was cloned for phylogenetic analysis. Aromatase activity increased from GD31 to a peak at 45 and 65, apparently decreasing later in gestation. This activity was more sensitive to inhibition by Letrozole than was human placental aromatase activity. Expression of P450arom was localized to syncytiotrophoblast and giant cells of mid-gestation placentas. The coding sequence of hyena P450arom was 94% and 86% identical to the canine and human enzymes respectively, as reflected by phylogenetic analyses. These data demonstrate for the first time that hyena placental aromatase activity is comparable to that of human placentas when genital differentiation is in progress. This suggests that even in female spotted hyenas clitoral differentiation is likely protected from virilization by placental androgen metabolism. Decreased placental aromatase activity in late gestation may be equally important in allowing androgen to program behaviors at birth. Although hyena P450arom is closely related to the canine enzyme, both placental anatomy and P450arom expression differ. Other hyaenids and carnivores must be investigated to determine the morphological and functional ancestral state of their placentas, as it relates to evolutionary relationships among species in this important taxonomic group.

Endocrine differentiation of fetal ovaries and testes of the spotted hyena (Crocuta crocuta): timing of androgen-independent versus androgen-driven genital development

Female spotted hyenas (Crocuta crocuta) have an erectile peniform clitoris and a pseudoscrotum but no external vagina, all established by day 35 of a 110-day gestation. Recent studies indicate that these events are androgen-independent, although androgen secretion by fetal ovaries and testis was hypothesized previously to induce phallic development in both sexes. We present the first data relating to the capacity of the ovaries and testes of the spotted hyena to synthesize androgens at different stages of fetal life. Specifically, spotted hyena fetal gonads were examined by immunohistochemistry at GD 30, 45, 48, 65, and 95 for androgen-synthesizing enzymes, as related to the morphological development. Enzymes included 17alpha-hydroxylase/17,20-lyase cytochrome P450 (P450c17), cytochrome b5, 3beta-hydroxysteroid dehydrogenase (3betaHSD), and cholesterol side-chain cleavage cytochrome P450 (P450scc). Anti-Müllerian-hormone (AMH) expression was also examined. AMH was strongly expressed in fetal Sertoli cells from GD 30 and after. P450c17 expression was detected in Leydig cells of developing testes and surprisingly in Müllerian duct epithelium. Fetal ovaries began to organize and differentiate by GD 45, and medullary cells expressed P450c17, cytochrome b5, 3betaHSD, and P450scc. The findings support the hypothesis that external genital morphology is probably androgen-independent initially, but that fetal testicular androgens modify the secondary, male-specific phallic form and accessory organs. Fetal ovaries appear to develop substantial androgen-synthesizing capacity but not until phallic differentiation is complete, i.e. after GD 45 based on circulating androstenedione concentrations. During late gestation, fetal ovaries and testes synthesize androgens, possibly organizing the neural substrates of aggressive behaviors observed at birth in spotted hyenas. These data provide an endocrine rationale for sexual dimorphisms in phallic structure and reveal a potential source of androgenic support for neonatal aggression in female and male C. crocuta.

Transient genital abnormalities in striped hyenas (Hyaena hyaena)

The highly masculinized genitalia of female spotted hyenas Crocuta crocuta is unique among mammals: Crocuta have no external vagina so urination, penile intromission and parturition take place through the clitoris, which mimics a fully erectile male penis. Among hyenids, virilization of external female genitalia has previously been observed only in Crocuta, so functional explanations of masculinization have focused on aspects of social ecology unique to the species. Here we first show that the striped hyena Hyaena hyaena exhibits both unusual similarity in male and female androgen concentrations and transient genital anomalies characterized by a convergence in genital appearance among young males and females. We then evaluate hypotheses regarding the evolution of genital masculinization in the Hyaenidae and other taxa. Hyaena are behaviorally solitary, so discovery of unusual genital development patterns in this species does not support any current evolutionary models for masculinization in Crocuta, which all rely on the trait originating within a highly social species. Some hypotheses can be modified so that masculinization in Crocuta represents an extreme elaboration of a preexisting trait, shared as a homology with Hyaena.

Sex and seasonal differences in aggression and steroid secretion in Lemur catta: are socially dominant females hormonally 'masculinized'?

Female social dominance characterizes many strepsirrhine primates endemic to Madagascar, but currently there is no comprehensive explanation for how or why female lemurs routinely dominate males. Reconstructing the evolutionary pressures that may have shaped female dominance depends on better understanding the mechanism of inheritance, variation in trait expression, and correlating variables. Indeed, relative to males, many female lemurs also display delayed puberty, size monomorphism, and 'masculinized' external genitalia. As in the spotted hyena (Crocuta crocuta), a species characterized by extreme masculinization of the female, this array of traits focuses attention on the role of androgens in female development. Consequently, I examined endocrine profiles and social interaction in the ringtailed lemur (Lemur catta) to search for a potential source of circulating androgen in adult females and an endocrine correlate of female dominance or its proxy, aggression. I measured serum androstenedione (A(4)), testosterone (T), and estradiol (E(2)) in reproductively intact, adult lemurs (10 females; 12 males) over four annual cycles. Whereas T concentrations in males far exceeded those in females, A(4) concentrations were only slightly greater in males than in females. In both sexes, A(4) and T were positively correlated, implicating the Delta(4)-biosynthetic pathway. Moreover, seasonal changes in reproductive function in both sexes coincided with seasonal changes in behavior, with A(4) and T in males versus A(4) and E(2) in females increasing during periods marked by heightened aggression. Therefore, A(4) and/or E(2) may be potentially important steroidal sources in female lemurs that could modulate aggression and underlie a suite of masculinized features.

Regulation of male traits by testosterone: implications for the evolution of vertebrate life histories

The negative co-variation of life-history traits such as fecundity and lifespan across species suggests the existence of ubiquitous trade-offs. Mechanistically, trade-offs result from the need to differentially allocate limited resources to traits like reproduction versus self-maintenance, with selection favoring the evolution of optimal allocation mechanism. Here I discuss the physiological (endocrine) mechanisms that underlie optimal allocation rules and how such rules evolve. The hormone testosterone may mediate life-history trade-offs due to its pleiotropic actions in male vertebrates. Conservation in the actions of testosterone in vertebrates has prompted the 'evolutionary constraint hypothesis,' which assumes that testosterone signaling mechanisms and male traits evolve as a unit. This hypothesis implies that the actions of testosterone are similar across sexes and species, and only the levels of circulating testosterone concentrations change during evolution. In contrast, the 'evolutionary potential hypothesis' proposes that testosterone signaling mechanisms and male traits evolve independently. In the latter scenario, the linkage between hormone and traits itself can be shaped by selection, leading to variation in trade-off functions. I will review recent case studies supporting the evolutionary potential hypothesis and suggest micro-evolutionary experiments to unravel the mechanistic basis of life-history evolution.

Mammalian sexual differentiation: lessons from the spotted hyena

Female spotted hyenas (Crocuta crocuta) are the only female mammals that lack an external vaginal opening. Mating and birth take place through a urogenital canal that exits at the tip of a hypertrophied clitoris. This 'masculine' phenotype spurred a search for an alternate source of fetal androgens. Although androstenedione from the maternal ovary is readily metabolized to testosterone by the hyena placenta, formation of the penile clitoris and scrotum appear to be largely androgen independent. However, secretions from the fetal testes underlie sex differences in the genitalia and central nervous system that are essential for male reproduction. Naturally circulating androgens, acting prenatally, reduce reproductive success in adult female spotted hyenas. Effects on aggression and dominance might offset these reproductive 'costs' of female androgenization in utero.

Distribution of vasopressin in the forebrain of spotted hyenas

The extreme virilization of the female spotted hyena raises interesting questions with respect to sexual differentiation of the brain and behavior. Females are larger and more aggressive than adult, non-natal males and dominate them in social encounters; their external genitalia also are highly masculinized. In many vertebrates, the arginine vasopressin (VP) innervation of the forebrain, particularly that of the lateral septum, is associated with social behaviors such as aggression and dominance. Here, we used immunohistochemistry to examine the distribution of VP cells and fibers in the forebrains of adult spotted hyenas. We find the expected densely staining VP immunoreactive (VP-ir) neurons in the paraventricular and supraoptic nuclei, as well as an unusually extensive distribution of magnocelluar VP-ir neurons in accessory regions. A small number of VP-ir cell bodies are present in the suprachiasmatic nucleus and bed nucleus of the stria terminalis; however, there are extensive VP-ir fiber networks in presumed projection areas of these nuclei, for example, the subparaventricular zone and lateral septum, respectively. No significant sex differences were detected in the density of VP-ir fibers in any area examined. In the lateral septum, however, marked variability was observed. Intact females exhibited a dense fiber network, as did two of the four males examined; the two other males had almost no VP-ir septal fibers. This contrasts with findings in many other vertebrate species, in which VP innervation of the lateral septum is consistently greater in males than in females.

Masculinized otoacoustic emissions in female spotted hyenas (Crocuta crocuta)

In humans and rhesus monkeys, click-evoked otoacoustic emissions (CEOAEs) are stronger in females than in males, and there is considerable circumstantial evidence that this sex difference is attributable to the greater exposure to androgens prenatally in males. Because female spotted hyenas are highly androgenized beginning early in prenatal development, we expected an absence of sexual dimorphism in the CEOAEs of this species. The CEOAEs obtained from 9 male and 7 female spotted hyenas confirmed that expectation. The implication is that the marked androgenization to which female spotted hyenas are exposed masculinizes the cochlear mechanism responsible for CEOAEs. The CEOAEs measured in 3 male and 3 female hyenas that had been treated with anti-androgenic agents during prenatal development were stronger than the CEOAEs of the untreated animals, in accord with the implied inverse relationship between prenatal androgen exposure and the strength of the cochlear mechanisms producing CEOAEs. The CEOAEs of three ovariectomized females and two castrated males were essentially the same as those for the untreated females and males, suggesting that there is little or no activational effect of hormones on CEOAE strength in spotted hyenas. Distortion product OAEs (DPOAEs) also were measured. Those sex differences also were generally small (as they are in humans), and the effects of the anti-androgen agents were inconsistent. Thus, prenatal androgen exposure apparently does affect OAEs, but the effects appear to be greater for the reflection-based cochlear mechanism that underlies CEOAEs than for the nonlinear cochlear mechanism underlying DPOAEs.

Rank-related maternal effects of androgens on behaviour in wild spotted hyaenas

Within any hierarchical society, an individual's social rank can have profound effects on its health and reproductive success, and rank-related variation in these traits is often mediated by variation in endocrine function. Maternal effects mediated by prenatal hormone exposure are potentially important for non-genetic inheritance of phenotypic traits related to social rank, and thus for shaping individual variation in behaviour and social structure. Here we show that androgen concentrations in wild female spotted hyaenas (Crocuta crocuta) are higher during late gestation in dominant females than in subordinate females. Furthermore, both male and female cubs born to mothers with high concentrations of androgens in late pregnancy exhibit higher rates of aggression and mounting behaviour than cubs born to mothers with lower androgen concentrations. Both behaviours are strongly affected in other mammals by organizational effects of androgens, and both have important effects on fitness in hyaenas. Therefore, our results suggest that rank-related maternal effects of prenatal androgen exposure can adaptively influence offspring phenotype in mammals, as has previously been shown to occur in birds. They also suggest an organizational mechanism for the development of female dominance and aggressiveness in spotted hyaenas, traits that may offset the costs of extreme virilization.

Female testosterone, dominance rank, and aggression in an Ethiopian population of hybrid baboons

Studies on the relationship between female testosterone (T) measures and behavior, particularly in free-ranging primate populations, remain scant. In this study we used fecal steroid analysis to examine the effects of seasonal, reproductive, and social factors on female T in a group of free-ranging hybrid baboons (Papio sp.) in the Awash National Park of Ethiopia. We collected behavioral and hormonal data from 25 adult females across an 11-month period. Solid phase extraction and radioimmunoassay (RIA) techniques were used to quantify T in 776 fecal samples collected weekly from each female. The results indicate that 1) the females had elevated T during pregnancy and during the wet season relative to other periods, 2) female dominance rank was positively related to T measures, and 3) female T and aggression were positively related within subjects but not across subjects. Higher T concentrations during pregnancy are consistent with other published profiles of pregnancy in primates. In combination with data on foraging, wet season increases in T may indicate contest competition for females. The rank-T relationship may be mediated by supplants or aggression. Finally, we discuss the different interpretations of the hormone-behavior relationship based on within- and across-subject analyses.

Sexual differentiation in three unconventional mammals: spotted hyenas, elephants and tammar wallabies

The present review explores sexual differentiation in three non-conventional species: the spotted hyena, the elephant and the tammar wallaby, selected because of the natural challenges they present for contemporary understanding of sexual differentiation. According to the prevailing view of mammalian sexual differentiation, originally proposed by Alfred Jost, secretion of androgen and anti-Mullerian hormone (AMH) by the fetal testes during critical stages of development accounts for the full range of sexually dimorphic urogenital traits observed at birth. Jost's concept was subsequently expanded to encompass sexual differentiation of the brain and behavior. Although the central focus of this review involves urogenital development, we assume that the novel mechanisms described in this article have potentially significant implications for sexual differentiation of brain and behavior, a transposition with precedent in the history of this field. Contrary to the "specific" requirements of Jost's formulation, female spotted hyenas and elephants initially develop male-type external genitalia prior to gonadal differentiation. In addition, the administration of anti-androgens to pregnant female spotted hyenas does not prevent the formation of a scrotum, pseudoscrotum, penis or penile clitoris in the offspring of treated females, although it is not yet clear whether the creation of masculine genitalia involves other steroids or whether there is a genetic mechanism bypassing a hormonal mediator. Wallabies, where sexual differentiation occurs in the pouch after birth, provide the most conclusive evidence for direct genetic control of sexual dimorphism, with the scrotum developing only in males and the pouch and mammary glands only in females, before differentiation of the gonads. The development of the pouch and mammary gland in females and the scrotum in males is controlled by genes on the X chromosome. In keeping with the "expanded" version of Jost's formulation, secretion of androgens by the fetal testes provides the best current account of a broad array of sex differences in reproductive morphology and endocrinology of the spotted hyena, and androgens are essential for development of the prostate and penis of the wallaby. But the essential circulating androgen in the male wallaby is 5alpha androstanediol, locally converted in target tissues to DHT, while in the pregnant female hyena, androstenedione, secreted by the maternal ovary, is converted by the placenta to testosterone (and estradiol) and transferred to the developing fetus. Testicular testosterone certainly seems to be responsible for the behavioral phenomenon of musth in male elephants. Both spotted hyenas and elephants display matrilineal social organization, and, in both species, female genital morphology requires feminine cooperation for successful copulation. We conclude that not all aspects of sexual differentiation have been delegated to testicular hormones in these mammals. In addition, we suggest that research on urogenital development in these non-traditional species directs attention to processes that may well be operating during the sexual differentiation of morphology and behavior in more common laboratory mammals, albeit in less dramatic fashion.

The Ontogeny of the Urogenital System of the Spotted Hyena (Crocuta crocuta Erxleben)1

Studies were conducted to elucidate the importance of androgen-mediated induction of the extreme masculinization of the external genitalia in female spotted hyenas. Phallic size and shape; androgen receptor (AR) and alpha-actin expression; and sex-specific differences in phallic retractor musculature, erectile tissue, tunica albuginea, and urethra/urogenital sinus were examined in male and female fetuses from Day 30 of gestation to term. Similar outcomes were assessed in fetuses from dams treated with an AR blocker and a 5alpha-reductase inhibitor (antiandrogen treatment). Clitoral and penile development were already advanced at Day 30 of gestation and grossly indistinguishable between male and female fetuses throughout pregnancy. Sex-specific differences in internal phallic organization were evident at Gestational Day 45, coincident with AR expression and testicular differentiation. Antiandrogen treatment inhibited prostatic development in males and effectively feminized internal penile anatomy. We conclude that gross masculinization of phallic size and shape of male and female fetuses is androgen-independent, but that sexual dimorphism of internal phallic structure is dependent on fetal testicular androgens acting via AR in the relevant cells/tissues. Androgens secreted by the maternal ovaries and metabolized by the placenta do not appear to be involved in gross masculinization or in most of the sex differences in internal phallic structure.

Masculinization of Female Mammals: Lessons from Nature

Although varying degrees of genital masculinization are a reasonably common phenomenon in the world of female mammals, the majority of such variation has not been investigated. In this chapter we have described research on the "masculinized" genitalia of moles and hyenas. Such research raises intriguing possibilities regarding the coordinated role that androgens, estrogens and peptide hormones (e.g., relaxin) might play, at different stages of sexual differentiation and development, in preparing genital tissues for their functional roles in reproduction. Such studies also suggest that non-androgenic mechanisms need to be considered. Arnold (1996) and Carruth et al. (2002) have recently presented the argument for broadening our view of sexual differentiation of brain and behavior, emphasizing direct genetic effects. A similar view has been presented for the Tammar wallaby, where formation of a scrotum, or a pouch, is a direct consequence of the presence/absence of two X chromosomes (Pask and Renfree, 2001). Although our research on moles and hyenas has not yet yielded such definitive results, the research reviewed in this chapter calls attention to processes that could well operate in other mammals, including humans.

Intersexual dominance, masculinized genitals and prenatal steroids: comparative data from lemurid primates

Masculinization of female genitalia and female intersexual dominance distinguish spotted hyenas (Crocuta crocuta) and Malagasy primates (Lemuriformes) from most other mammals. An unusual prenatal endocrine environment has been proposed to proximately underlie the development of these traits in hyenas. To examine whether female dominance and genital masculinization are similarly enhanced by the prenatal environment in lemurid primates, we measured androgen and estrogen excretion in pregnant wild redfronted lemurs (Eulemur fulvus rufus). Our results showed that estrogen levels during the second phase of gestation were much higher in females carrying a male fetus than in female-carrying mothers. This may indicate the onset of testicular activity in male fetuses, because androgens of fetal origin are aromatized to maternal estrogens. Levels of androgen excretion were similar in all mothers regardless of the fetus' sex, which may suggest that androgen-independent mechanisms also contribute to female masculinization. The much higher androgen/estrogen ratio in female-carrying mothers indicates that relative, rather than absolute, prenatal steroid concentrations may play a role in female masculinization.

Effects of prenatal treatment with antiandrogens on luteinizing hormone secretion and sex steroid concentrations in adult spotted hyenas, Crocuta crocuta

Prenatal androgen treatment can alter LH secretion in female offspring, often with adverse effects on ovulatory function. However, female spotted hyenas (Crocuta crocuta), renowned for their highly masculinized genitalia, are naturally exposed to high androgen levels in utero. To determine whether LH secretion in spotted hyenas is affected by prenatal androgens, we treated pregnant hyenas with antiandrogens (flutamide and finasteride). Later, adult offspring of the antiandrogen-treated (AA) mothers underwent a GnRH challenge to identify sex differences in the LH response and to assess the effects of prenatal antiandrogen treatment. We further considered the effects of blocking prenatal androgens on plasma sex steroid concentrations. To account for potential differences in the reproductive state of females, we suppressed endogenous hormone levels with a long-acting GnRH agonist (GnRHa) and then measured plasma androgens after an hCG challenge. Plasma concentrations of LH were sexually dimorphic in spotted hyenas, with females displaying higher levels than males. Prenatal antiandrogen treatment also significantly altered the LH response to GnRH. Plasma estradiol concentration was higher in AA-females, whereas testosterone and androstenedione levels tended to be lower. This trend toward lower androgen levels disappeared after GnRHa suppression and hCG challenge. In males, prenatal antiandrogen treatment had long-lasting effects on circulating androgens: AA-males had lower T levels than control males. The sex differences and effects of prenatal antiandrogens on LH secretion suggest that the anterior pituitary gland of the female spotted hyena is partially masculinized by the high androgen levels that normally occur during development, without adverse effects on ovulatory function.

Exposure to naturally circulating androgens during foetal life incurs direct reproductive costs in female spotted hyenas, but is prerequisite for male mating

Among all extant mammals, only the female spotted hyena (Crocuta crocuta) mates and gives birth through the tip of a peniform clitoris. Clitoral morphology is modulated by foetal exposure to endogenous, maternal androgens. First births through this organ are prolonged and remarkably difficult, often causing death in neonates. Additionally, mating poses a mechanical challenge for males, as they must reach an anterior position on the female's abdomen and then achieve entry at the site of the retracted clitoris. Here, we report that interfering with the actions of androgens prenatally permanently modifies hyena urogenital anatomy, facilitating subsequent parturition in nulliparous females who, thereby, produce live cubs. By contrast, comparable, permanent anatomical changes in males probably preclude reproduction, as exposure to prenatal anti-androgens produces a penis that is too short and has the wrong shape necessary for insertion during copulation. These data demonstrate that the reproductive costs of clitoral delivery result from exposure of the female foetus to naturally circulating androgens. Moreover, the same androgens that render an extremely unusual and laborious process even more reproductively costly in the female are apparently essential to the male's physical ability to reproduce with a normally masculinized female.

Virilization of the female spotted hyena cannot be explained by alterations in the amino acid sequence of the androgen receptor (AR)

The external genitalia of the female spotted hyena are male in character, consistent with virilization by androgens during embryogenesis that results in the fusion of the vaginal labia to form a pseudo scrotum and enlargement of the clitoris to form a phallus. Explanations advanced to account for these anatomic differences have centered on the production or metabolism of androgens in utero or on abnormalities of the androgen receptor (such as a constitutively active AR). The structure of the spotted hyena AR was examined at the level of genomic DNA and cDNA. Southern analysis detected two Eco RI endonuclease cleavage fragments (4.4 and 4.7 kb) that encode the bulk of the AR hormone-binding domain. Isolation of the smaller fragment from a size fractionated genomic library revealed that it contained exons 6, 7 and 8. The remaining portions of the coding sequence were cloned by RT-PCR and RACE analyses. The spotted hyena cDNA sequence predicts protein 912 amino acids in length, which is most closely related to the sequence of the dog AR. Although a number of differences in the predicted amino acid sequence are identified, particularly within the amino terminus, only single amino acid substitutions are present in the DNA- and ligand-binding domains compared to the human AR. In transfection assays, the spotted hyena AR does not exhibit constitutive activity and responds normally to a range of androgenic and non-androgenic ligands. These findings suggest that the structural changes in the AR do not account for the abnormal virilization in the female spotted hyena. These results serve to focus attention on processes proximal (an abnormality of hormone formation in situ) or distal (activation by other mechanisms of processes normally regulated by androgen) to the AR as the cause of the virilization.

Placental buffering of maternal steroid hormone effects on fetal and yolk hormone levels: a comparative study of a viviparous lizard, Sceloporus jarrovi, and an oviparous lizard, Sceloporus graciosus

We investigated maternal-fetal hormone transfer in the mountain spiny lizard, Sceloporus jarrovi, a viviparous species with a simple chorioallantoic placenta. In one experiment, we examined the effect of elevated maternal progesterone on fetal and yolk hormone levels. Progesterone implants increased maternal progesterone nearly 100-fold; however, the resulting increase in fetal and yolk progesterone was only about 2% of that seen in mothers, providing evidence that the placenta buffers hormone diffusion. In addition, some effects of progesterone treatment differed between male and female fetuses, suggesting that this buffering may differ between the sexes. In a second experiment, we examined the relationship between maternal and fetal hormone levels in viviparous versus oviparous species. We measured endogenous progesterone, testosterone, estradiol, and corticosterone levels in pregnant S. jarrovi and their fetuses and neonates, and in gravid S. graciosus (an oviparous congener) and their fetuses and hatchlings. No clear relationship was identified between maternal and fetal or hatchling S. graciosus hormone levels. However, the data for S. jarrovi suggest that maternal hormones may inhibit perinatal hormone secretion. These findings indicate that, despite the relatively recent evolutionary origin and simple structure of the S. jarrovi placenta, mechanisms for placental mediation of the maternal-fetal endocrine relationship have evolved. Although the placenta appears to buffer hormone transport, maternal hormones can affect fetal and yolk hormone levels, suggesting that disruption of endocrine regulation could be a physiological cost of the evolution of viviparity.

Embryonic development of mouse external genitalia: insights into a unique mode of organogenesis

The mammalian external genitalia are specialized appendages for efficient copulation, internal fertilization and display marked morphological variation among species. In this paper, we described the embryonic development of mouse genital tubercle (GT), an anlage of the external genitalia utilizing the scanning electron microscope (SEM) analysis. It has been shown that the Distal Urethral Epithelium (DUE) may fulfill an essential role in the outgrowth control of the GT. Our present SEM analysis revealed a small distal protrusion at the tip of the GT of normal embryos as well as some morphological differences between male and female embryonic external genitalia. Previous analysis shows that the teratogenic dose of Retinoic Acid (RA) induces a drastic marformation of the urethral plate, but not gross abnormalities for GT outgrowth. Interestingly, a small distal protrusion at the tip of GT was clearly observed also after RA treatement. Furthermore, we showed that treatment with anti-androgen flutamide resulted in the demasculinization of the GT in males. The unique character of GT development and the sexual dimorphism are discussed.

Sexual mimicry in hyenas

Genital masculinization in female spotted hyenas has been widely explained as an incidental consequence of high androgen levels. High androgen levels, in turn, were supposed to be favored because they led to adaptive aggressive behavior. Incidental androgenization is no longer a tenable hypothesis, however, because genital masculinization has been shown to proceed in the absence of androgenic steroids. Thus, an alternative hypothesis is required. The genitals of spotted hyena females are not simply masculinized, but exhibit a detailed physical resemblance to the male genitalia. In the absence of satisfactory alternative explanations, we propose that selection may have favored sexual mimicry in females because they are more likely than males to be targets of aggression from other females. Male-like camouflage could theoretically be protective in three contexts: neonate sibling aggression, infanticide by conspecific females, and interclan territoriality. Current data suggest that if sexual mimicry is important, its effects are strongest among infants.

Pregnancy outcomes in women with congenital virilizing adrenal hyperplasia

Although low fertility rates have traditionally been reported among women with classic CAH and especially among women with the salt-wasting variant, more recent data suggest that fertility rates are significantly improved, largely owing to earlier treatment of CAH, improved compliance with therapy, and surgical advances in genital reconstruction. Furthermore, ovulation induction and assisted reproductive techniques are now available to women who remain infertile despite effective adrenal androgen suppression. Although the pregnancy experience in women with classic CAH remains limited, it is apparent that, once pregnant, these women have a high probability of successful outcome. Key issues should be emphasized in the management of CAH during gestation, including the need for assessing adrenal steroid replacement and adrenal androgen suppression, particularly in light of the interplay between maternal hyperandrogenism and the protective effect of placental aromatase activity, which provides a relatively large margin of safety for the female fetus. Maternal hormone levels should be evaluated in the context of laboratory-specific reference ranges for pregnancy. The infant should be examined for ambiguous genitalia and monitored for evidence of adrenal insufficiency. Although an affected female infant with classic CAH has not been reported as a pregnancy outcome of a mother with classic virilizing CAH, these concerns should be discussed during preconception counseling. Patients should also be aware of the importance of medication compliance and careful hormonal monitoring during the entire pregnancy. In most cases, successful gestational management requires the close coordination of care between the obstetrician and endocrinologist.

Androgens and the role of female "hyperaggressiveness" in spotted hyenas (Crocuta crocuta)

It has been suggested that female dominance in spotted hyenas evolved due to selection for "hyperaggressive" androgenized females. According to this view, virilized external genitalia of female hyenas developed as a byproduct of selection for "androgen-facilitated social dominance." The evidence that female hyenas have higher androgen levels than males or other female mammals is inconclusive. We compared concentrations of testosterone (T), 5alpha-dihydrotestosterone (DHT), and androstenedione (AE) from a population in the Serengeti. Females had significantly lower T and DHT levels than predispersal and postdispersal males. AE levels did not significantly differ between females and postdispersal males, but were significantly lower in predispersal males. Our results, and those from studies that have demonstrated that male and female hyenas experience similar levels of maternal androgens during fetal development, provide little support for a theory of selection for female dominance and hyperaggressiveness through enhanced secretion of androgens. Our data are consistent with an alternative view that female virilization is a byproduct of selection for precocial aggressive cubs of both sexes. According to this view, high investment in lactation favored selection for accelerated fetal development, high neonatal aggression, and facultative siblicide.

Androgen levels and female social dominance in Lemur catta

Morphological and behavioural traits which improve agonistic power are subject to intrasexual selection and, at the proximate level, are influenced by circulating androgens. Because intrasexual selection in mammals is more intense among males, they typically dominate females. Female social dominance is therefore unexpected and, indeed, rare. Ring-tailed lemurs (Lemur catta) are sexually monomorphic primates in which all adult females dominate all males. The goal of our study was to test the prediction that female dominance in this species is associated with high androgen levels. Using two captive groups, we collected data on agonistic behaviour and non-invasively assessed their androgen concentrations in faeces and saliva by enzyme immunoassay. We found that adult female L. catta do not have higher androgen levels than males. However, during the mating season there was a twofold increase in both the androgen levels and conflict rates among females. This seasonal increase in their androgen levels was probably not due to a general increase in ovarian hormone production because those females showing the strongest signs of follicular development tended to have low androgen concentrations. At the individual level neither the individual aggression rates nor the proportion of same-sexed individuals dominated were correlated with their androgen levels. We conclude that female dominance in ring-tailed lemurs is neither based on physical superiority nor on high androgen levels and that it is equally important to study male subordination and prenatal brain priming effects for a complete understanding of this phenomenon.