Hormones L4 Sexual mimicry - Google Docs PDF

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The strang case of sexual mimicry •Where the sex of one species resembles the other •Mimicry of primary sexual characteristics (penis and scrotum) by females. •Development of penis-like structure from the clitoris (the female homologue of the penis). Rare: mimic of females by male Clitoromegaly: The curious case of the spotted hyena A 3-month old male hyena flanked by two 4-month old females. All display erections, a sign of social excitement. Note the greater length and more slender male phallus. To achieve intromission the male has to engage in multiple flipping movements against the female abdomen Female spotted hyenas display a false penis The sexual mimicry within this species was first noted by Aristotle Consequences •Females mate and give birth through the “peniform” clitoris. •In pre-pubertal females, the urinogenital sinus is only slightly larger than the penis. •At puberty it enlarges and retractor muscles develop Sex Male has to flip the semi-erect penis against the abdomen of the female as he searches for the clitoral opening Theories for sexual mimicry •Genital sniffing and inspection is part of social greeting display. Hyenas have ferocious teeth, so a good deal of trust is required. Females re-enforce their position in hyena society by developing the most delicate appendages. But why have such a system with such high associated mortality? •Competition aggression hypothesis –Intense competition for food – selection for increased female size and aggression –This hypothesis requires that female rank, body size, aggression and reproductive success are closely linked and that variance in reproductive success is greater in females than males.

NB. Female rank is inherited – from their mothers. Females dominate males at the kill and in social encounters. 2 males wait their turn as the female feeds.

Ecology •Females much more aggressive than males – dominate males at social encounters. •Females 10% larger. Other hyenids (striped, brown, aardwolf) have normal anatomy and lack dimorphism •Spotted hyenas are the only true carnivores of the group •Female status is a key predictor of offspring survival. •High ranker produce more offspring, breed earlier, neonates survive better. •Neonates are very aggressive. •Born with fully functional teeth •Twins are often produced •Fighting commences within 1-5 minutes of birth •Often one of the cubs dies •Survival is in favour of females – ie the male cub often killed. •Juvenile females show male-like play patterns Problems at birth •In a normal mammal of this size, distance from cervix to external vagina is about 30 cm. •In hyenas, the distance is ca 60 cm. •Umbilical cord is only 12-18 cm long •Cord breaks before birth •In primiparous females, the neonate can lodge in clitoris. •Birth can only occur after urinary meatus has torn. A large proportion of first cubs die of anoxia. •Female cubs are larger than male cubs at birth and in adult hood. 9-18% of females die at the attempt of first birth

The hyena placenta converts ovarian androstenedione to testosterone

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During pregnancy, ovary produces high levels of androstenedione (AD; weak androgen).

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Normal mammal placenta convert to oestrogen via aromatase.

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In hyenas, placental 17β hydroxysteroid dehydrogenase is high, and converts AD to testosterone

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Early studies suggested that hyena placenta is deficient in aromatase Manipulations of sex steroids •Measures of umbilical cord steroid levels shows females exposed to similar “T” concentrations than males •Neonatal androgen blockers (AB) do not prevent peniform clitoris from forming •But, such clitorises are thicker and cannot be stretched as far in AB-treated females. Males are unable to mate as adults – so neonatal androgens are essential. Penis is too small to penetrate the female clitoris Impossible to completely block female “penis” development with androgen blockers

Androgen blockers modify the clitoris and improve neonatal survival •Shorter clitorises in blocked females •Neonatal mortality rates in primiparous females –Controls 12 pregnancies, 20 cubs, 11 stillborn

–Blocker treated 4 Pregnancies, 7 cubs, 0 mortality Male and female brains are still dimorphic as is behaviour Male hyenas have sexually dimorphic brains ●

Male dimorphic SDN of hypothalamus is larger than females by factor of 2.

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Cell numbers are also higher.

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This is not as much as in some mammals, but it is still dimorphic

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Thus, the hypothalamus may be partially masculinised.

Sex steroids and genomic actions •Neonatal androgen exposure partially masculinises the female •Dimorphism of body size and aggression favours the female •Androgen blockers do not reverse the phenotype •Male brains still dimorphic, despite similar T levels in neonatal males in females. Dimorphism therefore probably a consequence of both steroid and non-steroid factors

Reproduction is very costly to females Need to search for an advantage of a partially androgenised status. ●

Large females are dominant and become alpha lineages in hyena society.

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Selection therefore operates to give reproductive advantage to larger dominant females.

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Masculinisation of behaviour confers an advantage in the ferociously competitive hyena society

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Perhaps masculinisation of the female genitalia is a secondary consequence of selection for early exposure to testosterone

Hyenas have a mutation of the aromatase gene, but clearly sexual differentiation can proceed in its absence. This mutation is associated with the aggressive female phenotype.

Social rank and exposure to maternal hormones in utero What is the adaptive significance of high female androgens? •Measure faecal steroid concentrations •Associate with social rank •Assess behaviour of offspring

Take home message: Dominant females produce higher levels of androgens in pregnancy

Take home message: Cubs exposed to high uterine androgen levels in the second half of gestation (when behavioural imprint likely) are more aggressive Hyenas and sexual mimcry •Link between maternal androgens and offspring behaviour –Rank determines success, selection drives maternal androgen •Not so clear for sexual mimicry –Androgen blockers do not reverse the phenotype –Suggests that primary mechanisms are involved Sexual simorphism in mammals: hypothesis •Development of external genitalia in female eutherian mammals is the default state •Occurs in absence of the potent virilising androgen (5 α dihydrotestosterone; DHT) •Does masculinization of female require an alternative source of androgen?

Strongly recommended review for you to read on Blackboard •Mammalian sexual differentiation: lessons from the spotted hyena Stephen E. Glickman, Gerald R. Cunha, Christine M. Drea, Alan J. Conley and Ned J. Place Elephants

External appearances ●

The male and female have an identical anogenital distance, with the prepuce and the vulva being situated on the ventral aspect of the abdomen, just behind the umbilicus, and opening anteriorly. The male has no scrotum, and the female has a hypertrophied clitoris that resembles the male's penis, except that it is not canalized by the urethra and cannot be extrude

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In order for a male elephant to copulate successfully, female cooperation is essential. The upturned tip of the penis has to engage with the vagina, which the female must retract caudally, presumably using the well-developed levator clitoris muscles that arise from the pelvis and insert via a common tendon near the tip of the clitoris. It would be anatomically impossible for a male elephant to rape a female

Matrilineal societies The social structure is matriarchal, centered around an elderly female and her related offspring, to form a tight-knit stable family group. The males leave or are driven out from the family group at puberty and have few social bonds, pursuing a solitary existence or occasionally forming short-lived bachelor groups Origins (and speculations) •Nearest relatives are marine sea-cows (Manatees) •Like marine mammals, they grow throughout life •Testis are intra-abdominal •The trunk acts as a snorkel for aquatic life •Little opportunity in marine mammals for evolution of overt sexually dimorphic structures

Summary of ancestry, reproductive anatomy and behavior of the spotted hyena, elephant The spotted hyena Carnivore with, during some periods, intense competition for limited food resource Body size: female slightly larger than male Extreme male mimicry by female, including a penile clitoris and pseudoscrotum Matriarchal social structure, with sexually differentiated roles in the mating system Adult females socially dominant to adult immigrant males Mating impossible without cooperation of female The elephant Eutherian herbivore with aquatic ancestry Male much larger than female

Some male mimicry by female a. Identical male and female anogenital distance b. Female has enlarged clitoris Male has no scrotum and intra-abdominal testes Matriarchal social structure Mating impossible without cooperation of the female European moles •European moles are very hard to sex •Testis are intra-abdominal •Females possess bilateral ovotestes •Ovarian tissue of normal histological appearance and develops in spring Also a histologically defined interstitial testicular region which develops in the autumn Mole ovotestes ●

Ovarian follicles are limited to a distinct pole in the gonad

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Ovarian interstitial tissue gland (OIG) is found on opposite pole

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Follicular pole enlarges at sexual maturity and beginning of each subsequent breeding season (spring) while interstitial tissue regresses

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This situation is reversed at the end of the breeding season

Stuctural differences in the mouse developing gonad

Mole ovo-testis in the female Talpa occidentalis Testosterone production in male and female moles •Sexual mimicry is quite widespread in the mole family in both New and Old-World moles BUT in New World moles, there is little evidence for ovo-testes formation and ovaries are of normal appearance Another source of testosterone – or steroid-independent primary genetic mechanism?...