Androgens and masculinization of genitalia in the spotted hyaena (Crocuta crocuta). 3. Effects of juvenile gonadectomy

Applications of conceptual models from lifecourse epidemiology in ecology and evolutionary biology

In ecology and evolutionary biology (EEB), the study of developmental plasticity seeks to understand ontogenetic processes underlying the phenotypes upon which natural selection acts. A central challenge to this inquiry is ascertaining a causal effect of the exposure on the manifestation of later-life phenotype due to the time elapsed between the two events. The exposure is a potential cause of the outcome-i.e. an environmental stimulus or experience. The later phenotype might be a behaviour, physiological condition, morphology or life-history trait. The latency period between the exposure and outcome complicates causal inference due to the inevitable occurrence of additional events that may affect the relationship of interest. Here, we describe six distinct but non-mutually exclusive conceptual models from the field of lifecourse epidemiology and discuss their applications to EEB research. The models include Critical Period with No Later Modifiers, Critical Period with Later Modifiers, Accumulation of Risk with Independent Risk Exposures, Accumulation of Risk with Risk Clustering, Accumulation of Risk with Chains of Risk and Accumulation of Risk with Trigger Effect. These models, which have been widely used to test causal hypotheses regarding the early origins of adult-onset disease in humans, are directly relevant to research on developmental plasticity in EEB.

Development of the human penis and clitoris

The human penis and clitoris develop from the ambisexual genital tubercle. To compare and contrast the development of human penis and clitoris, we used macroscopic photography, optical projection tomography, light sheet microscopy, scanning electron microscopy, histology and immunohistochemistry. The human genital tubercle differentiates into a penis under the influence of androgens forming a tubular urethra that develops by canalization of the urethral plate to form a wide diamond-shaped urethral groove (opening zipper) whose edges (urethral folds) fuse in the midline (closing zipper). In contrast, in females, without the influence of androgens, the vestibular plate (homologue of the urethral plate) undergoes canalization to form a wide vestibular groove whose edges (vestibular folds) remain unfused, ultimately forming the labia minora defining the vaginal vestibule. The neurovascular anatomy is similar in both the developing human penis and clitoris and is the key to successful surgical reconstructions.

Comparative Morphology of the Penis and Clitoris in Four Species of Moles (Talpidae)

The penile and clitoral anatomy of four species of Talpid moles (broad-footed, star-nosed, hairy-tailed, and Japanese shrew moles) were investigated to define penile and clitoral anatomy and to examine the relationship of the clitoral anatomy with the presence or absence of ovotestes. The ovotestis contains ovarian tissue and glandular tissue resembling fetal testicular tissue and can produce androgens. The ovotestis is present in star-nosed and hairy-tailed moles, but not in broad-footed and Japanese shrew moles. Using histology, three-dimensional reconstruction, and morphometric analysis, sexual dimorphism was examined with regard to a nine feature masculine trait score that included perineal appendage length (prepuce), anogenital distance, and presence/absence of bone. The presence/absence of ovotestes was discordant in all four mole species for sex differentiation features. For many sex differentiation features, discordance with ovotestes was observed in at least one mole species. The degree of concordance with ovotestes was highest for hairy-tailed moles and lowest for broad-footed moles. In relationship to phylogenetic clade, sex differentiation features also did not correlate with the similarity/divergence of the features and presence/absence of ovotestes. Hairy-tailed and Japanese shrew moles reside in separated clades, but they exhibit a high degree of congruence. Broad-footed and hairy-tailed moles reside within the same clade but had one of the lowest correlations in features and presence/absence of ovotestes. Thus, phylogenetic affinity and the presence/absence of ovotestes are poor predictors for most sex differentiation features within mole external genitalia.

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.

Social competition and selection in males and females

During the latter half of the last century, evidence of reproductive competition between males and male selection by females led to the development of a stereotypical view of sex differences that characterized males as competitive and aggressive, and females as passive and choosy, which is currently being revised. Here, we compare social competition and its consequences for selection in males and females and argue that similar selection processes operate in both sexes and that contrasts between the sexes are quantitative rather than qualitative. We suggest that classifications of selection based on distinction between the form of competition or the components of fitness that are involved introduce unnecessary complexities and that the most useful approach in understanding the evolution and distribution of differences and similarities between the sexes is to compare the operation of selection in males and females in different reproductive systems.

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.

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.

External genital morphology of the ring-tailed lemur (Lemur catta): females are naturally "masculinized"

The extravagance and diversity of external genitalia have been well characterized in male primates; however, much less is known about sex differences or variation in female form. Our study represents a departure from traditional investigations of primate reproductive anatomy because we 1) focus on external rather than internal genitalia, 2) measure both male and female structures, and 3) examine a strepsirrhine rather than an anthropoid primate. The subjects for morphological study were 21 reproductively intact, adult ring-tailed lemurs (Lemur catta), including 10 females and 11 males, two of which (one per sex) subsequently died of natural causes and also served as specimens for gross anatomical dissection. Male external genitalia presented a typical masculine configuration, with a complex distal penile morphology. In contrast, females were unusual among mammals, presenting an enlarged, pendulous external clitoris, tunneled by the urethra. Females had a shorter anogenital distance and a larger urethral meatus than did males, but organ diameter and circumference showed no sex differences. Dissection confirmed these characterizations. Noteworthy in the male were the presence of a "levator penis" muscle and discontinuity in the corpus spongiosum along the penile shaft; noteworthy in the female were an elongated clitoral shaft and glans clitoridis. The female urethra, while incorporated within the clitoral body, was not surrounded by erectile tissue, as we detected no corpus spongiosum. The os clitoridis was 43% the length and 24% the height of the os penis. On the basis of these first detailed descriptions of strepsirrhine external genitalia (for either sex), we characterize those of the female ring-tailed lemur as moderately "masculinized." Our results highlight certain morphological similarities and differences between ring-tailed lemurs and the most male-like of female mammals, the spotted hyena (Crocuta crocuta), and call attention to a potential hormonal mechanism of "masculinization" in female lemur development.

Non-invasive measurement of fecal estrogens in the spotted hyena (Crocuta crocuta)

Fecal hormone analysis is a useful tool for frequent, non-invasive sampling of free-living animals. Estrogens fluctuate throughout life among reproductive states in female animals, and intensive repetitive sampling can permit accurate assessment of female reproductive condition. This type of repetitive sampling is difficult in large carnivores, including the spotted hyena (Crocuta crocuta). Patterns of estrogen secretion in captive and free-living hyenas are virtually unknown. Here we present validation of an enzyme-immunoassay to measure fecal estrogen (fE) concentrations in wild and captive spotted hyenas. Results from high-performance liquid chromatography indicate that an antibody specific for estradiol exhibits high immunoreactivity with our extracted samples. Fecal extract displacement curves paralleled our estradiol standard curve within the range of 20-80% antibody binding. Additionally, animals treated with luteinizing hormone-releasing hormone showed a measurable rise in fE concentrations. Finally, once we controlled for effects of time of day of sample collection from wild hyenas, patterns in fE concentrations resembled those in plasma estradiol, including higher levels of fE in mature than immature females, and higher levels of fE during late than early pregnancy. Together, these results suggest that fE concentrations reflect circulating estrogens in spotted hyenas.

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.

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.

A Neuroanatomical Comparison of Humans and Spotted Hyena, a Natural Animal Model for Common Urogenital Sinus: Clinical Reflections on Feminizing Genitoplasty

PURPOSE

Surgical treatment of the common urogenital sinus phallus has been one of the most challenging areas in pediatric urology. To better understand the neuroanatomy of the common urogenital sinus phallus, we evaluated an animal model naturally having this condition, the spotted hyena, Crocuta crocuta. We compared the neuroanatomy of male and female humans and spotted hyenas using anatomical, immunohistochemical and 3D reconstruction techniques. We also examined the implications of the pattern of clitoral innervation for the unique challenges faced by female spotted hyenas, the only extant species of mammal that mates and gives birth through the clitoris.

MATERIALS AND METHODS

Three adult male and 3 female spotted hyenas were studied. With the animals under anesthesia gross anatomical examination was performed before and after artificial erection. Histological analysis was performed on one 95-day fetal male and female spotted hyena specimens, and on 18 human male and female fetal external genitalia specimens using antibodies raised against the neuronal marker S-100. Three-dimensional computer reconstruction using serial sections allowed analysis of the neuroanatomy of the penis, clitoris and common urogenital sinus of the fetal spotted hyena and human.

RESULTS

Compared to other mammals, the clitoris and penis of spotted hyenas were remarkably similar in size and configuration in the flaccid and erect states. Male and female hyenas had a single opening on the tip of the glans penis/clitoris. The basic anatomical structures of the corporeal bodies in both sexes of humans and spotted hyenas were similar. As in humans, the dorsal nerve distribution was unique in being devoid of nerves at the 12 o'clock position in the penis and clitoris of the spotted hyena. Dorsal nerves of the penis/clitoris in humans and male spotted hyenas tracked along both sides of the corporeal body to the corpus spongiosum at the 5 and 7 o'clock positions. The dorsal nerves penetrated the corporeal body and distally the glans in the hyena. In female hyenas the dorsal nerves fanned out laterally on the clitoral body. Glans morphology was different in appearance in both sexes, being wide and blunt in the female and tapered in the male.

CONCLUSIONS

The neuroanatomy of the male and female external genitalia in the spotted hyena, Crocuta crocuta, although grossly similar, has distinct anatomical and functional characteristics. The clitoris of the spotted hyena is a classic example of a natural animal model of a common urogenital sinus. The neuroanatomical characteristics of the spotted hyena may be a useful model to simulate the anatomy of common urogenital sinus anomaly in humans.

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.

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.

Urogenital system of the spotted hyena (Crocuta crocuta Erxleben): a functional histological study

The unique urogenital anatomy and histology of female spotted hyenas (Crocuta crocuta Erxleben) was reexamined to identify adaptations of "structure" that enable/facilitate urination, mating, and parturition through the clitoris. Unusual features of penile anatomy required for meeting ceremonies and successful mating through a clitoral point of insertion were also examined. As reported previously, the upper urogenital tract of the female spotted hyena is typical of other carnivores and consists of the oviducts, uterine horns, uterine body, and vagina. An anatomically defined cervix is absent, even though a histologically defined transition zone between the uterine body and vagina was demonstrated. Adaptive features of the upper genital tract were a helical-shaped uterine cavity, extensive smooth muscle in the uterus and vagina, and a newly discovered submucosal mucous urogenital gland (SMUG) located immediately caudal to the vagina. The extensive smooth muscle facilitates the expulsion of the large pups at parturition through the recurved birth canal. Secretions of the SMUG provide lubrication and protection for the urogenital mucosa during mating and parturition. Two types of "erections" are suggested by behavioral observations: the common hemodynamic erection required for insertion and thrusting by the male, and phallic "flipping" that commonly occurs earlier in the mating sequence and is sometimes seen during meeting ceremonies. Phallic "flipping" appears to be accomplished by the coordinated contractions of the large ischiocavernosus and retractor muscles acting on the semirigid organ. The extremely thick tunica albuginea and interstitial collagen of the common corporal body of the penis and clitoris gives the flaccid phallus some degree of rigidity even in the resting state in males and nulliparous females. Phallic "flipping" implies a hinge region in which flexibility is the key feature. Such a proximal hinge region of the male and female phallus was defined and was notable for its diminished collagen content. The urogenital sinus traversing the clitoris was specialized for distensibility, thus facilitating receipt of the penis during mating and for passage of the infant to the tip of the glans clitoris, where it emerges at parturition. The morphology of the glans penis is notable for the tapered common corporal body that extends to the distal tip of the glans. This adaptation is suggested to be required for a clitoral (as opposed to a vaginal) point of insertion during mating. Finally, additional segments of erectile tissue devoid of a thick collagenous capsule were demonstrated in the glans penis and glans clitoris, which appear to account for the "partially-locking" of the male into the female during the late stages of a mating sequence. Taken together, it is evident that the unusual sexual behaviors of the male and female spotted hyenas are facilitated by unique structural modifications of the relevant organs.

Evidence for a potential role of estrogen in the penis: detection of estrogen receptor-alpha and -beta messenger ribonucleic acid and protein

Body tissues are traditionally classified as estrogen targets based on both the response to the hormone and the presence of estrogen receptors (ERs). We undertook the study on expression of ERalpha and ERbeta in the penis to identify compartments/cells responsive to estrogen, using immunohistochemistry, Western blotting, in situ hybridization, and RT-PCR analyses. Expressions of ERalpha and ERbeta in the rat penis were age dependent at both mRNA and protein levels, with the most intense signals being observed during the perinatal period and declining thereafter with age. Initial signals (fetal d 17) of ERalpha were localized to the mesenchyme and subepithelial stroma and later (postnatal d 2) to the corpus spongiosus, corpus cavernosus, and urethral epithelia. ERbeta was initially detected by postnatal d 2 and was localized diffusely in corpus spongiosus and cavernosus in immature rats. In the adult, both ERs were concentrated largely to the urethral epithelia and vascular and neuronal structures. The present study provides the first evidence for ER expression in the penis. Thus, our data add the penis to the list of estrogen-responsive tissues in males and provide a base and insight for future studies aimed at investigating a functional role of estrogen in the penis, especially in development.

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.

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.

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.

Adaptation, Exaptation, and Constraint: A Hormonal Perspective

We approach conceptual issues in evolutionary biology from an endocrinological perspective, noting that single hormones typically act on several target tissues and thereby mediate suites of correlated phenotypic traits. When several components of such a suite are beneficial, an important evolutionary question is whether all are adaptations or some are exaptations. The answer may depend on whether the traits arose in response to selection on variation in systemic levels of the hormone on variation in responsiveness of target tissues to invariant levels of the hormone. If the former, selection probably acted directly on fewer than all traits; beneficial traits arising indirectly would be exaptations. In contrast, multiple beneficial traits that arose out of independent changes in target-tissue sensitivity to invariant hormone levels could all be adaptations. Knowledge of specific hormonal mechanisms as well as of historical selective regimes will be necessary to draw such distinctions. Endocrine constraints on evolution can be studied experimentally by applying hormones systemically and measuring interdependent responses of beneficial and detrimental traits to selection (phenotypic engineering with hormones). Supposing that alteration of one trait in isolation would enhance fitness, cases in which the net effect of endocrine alteration of multiple traits is to depress fitness provide evidence for constraints. We briefly report results of recent studies employing hormonal manipulations, stressing our own work on the dark-eyed junco (Junco hyemalis: Emberizidae).