Sexual Mimicry Flashcards
Describe the ecology of spotted hyenas
- Dimorphic – however is the male that is subdominant
- Female is around 10% heavier
- Females much more aggressive – even lowest ranking females displace the highest ranking males
- Female status is a key predictor of offspring survival → higher rank produce more offspring, breed earlier, neonates survive better
- Neonates are very aggressive, born with fully functional teeth
- Twins often produced – fighting begins within 1-5 minutes of birth. Often, one cub dies. Survival favours females.
- Juvenile females show male-like play patterns
Describe the external genitalia of female spotted hyenas
- most highly masculinsed extant female mammals → have a false penis
- Fused scrotum, rather than labia majora
- Hypetrophied clitoris, similar in form to male penis
- Erection as a form of meeting ceremony (both males and females do this)
- Clitoris is transversed by a central urogenital sinus, exiting through a meatus at the tip of the glans clitoris
- In the absence of an external vaginal opening, female hyenas urinate, receive the male and give birth through the peniform organ.
What are the mating problems of spotted hyenas?
- Penis is longer and thinner than the clitoris, with an angular rather than rounded glans
- During mating, the female retracts her clitoris, presenting the male the a small target area on the surface of the abdnomel
- Male has to flip the semi-erect penis against the abdnomen of the female has he searches for the opening
- Male also has to overcome fear of the female
What are the birth problems of spotted hyenas?
- Normally, distance from cervix to external vagina ~30cm
- Hyenas, distance ~60cm
- Umbilical cord only 12-18cm, so breaks before birth
- In primiparous females, neonate can lodge in the clitoris
- Birth can only occur after urinary meatus is torn → urogenital sinus surrounded by unconstrained connective tissue. Under influence of estrogen, the elasticity is increased, facilitating its expansion
- A large proportion of first cubs die of anoxia
- 9-18% of females die at attempt of first birth
Why was it thought androgens may play a role in masculinising female genitalia?
- In typical mammals, ontogeny of male genitalia is dependent on testosterone secreted by the fetal testits
- Genetic females develop as anatomical males when exposed to T and DHT
Is there a role of the female ovary in masculinising the female genitalia?
- Abundance of interstitial tissue and relative paucity of follicles → speculates that the hyena ovary may produce androgens
- However, a large genital tubercle with a urethra extending to the tip of the organ is in place when the fetal ovary and adrenal are still in an undifferentiated state – masculinized genitalia has already appeared
- So fetal ovary cannot be source of androgens
Is there a role for placental metabolism in masculinising female genitalia?
• during pregnancy, the ovary produces high androstenedione
• Mammalian placentas normally produce aromatase to protect fetus from androgens by converting them to estradiol
• Hyena placenta is deficient in aromatase
• Placental 17B-hydroxysteroid dehydrogenase is high, convert androstenedione to testosterone
• Testosterone delivered to embryo in levels comparable to that found in adult males.
− In humans, two clinical cases have been reported where women gave birth to daughters with masculinsed external genitalia
− In both cases, found that a mutation had inactivated aromatase
How did they test the theory of placental androgens masculinising female genitalia?
- Mixture of androgen blocker and finasteride (prevents conversion of T to DHT) administered to pregnant females
- Hypothesised that this treatment might produce the first females with an external vaginal opening, and a typical female clitoris
- Instead, a scrotum and prominent phallus were maintained in female offspring – just slightly more feminized.
- Male offspring had feminized external and internal phallic morphology (penis too short to achieve and maintain entry into the clitoris, and elimination of angular glans). Hormonal response to GnRH also feminized.
- The male configuration of the phallus (eliminated by anti-androgen treatment) is formed between 34-45 of gestation
- Seems likely that, as in other mammalian species, secretion of androgens by the fetal testes promotes the sex differences
- in females, anti-androgens enhanced the feminine characteristics of clitoris → decreases its length, increases size and elasticity of urogenital sinus.
- in males, anti-androgens feminize the phallus
- The fact androgen blockers still retain the prominent phallus (it is just in the female state) suggests that there are non-steroidal influences.
- Androgen blockers modify the clitoris in a way that improves neonate survival, as they are shorter:
- Controls → 12 pregnancies, 20 cubs, 11 stillborns
- Blocker treated → 4 pregnanies, 7 cubs, 0 mortalities
Describe the Mimicry hypothesis for female masculinisation
- Genital sniffing and inspection is part of social greeting display
- Suggested that inspection of the erect phallus serves to distract attention from teeth, allowing tension to subside
- May assist in frequent rapid transformations from solitary to social circumstances
- Proposed that the peniform clitoris evolved to allow the female to mimic the behavour display used by males → but this doesn’t account for the high mortality costs of the organ.
Describe the male infanticide hypothesis of female masculinisation
Suggested that female dominance evolved as a countermeasure to infanticide by males
Describe the siblicide hypothesis of female masculinisation
- Neonatal aggression may have originated as a side effect of androgen exposure
- Possible to reverse this theory – prenatal androgenisation may have originated as an adaptation for wimming, by increasing aggressiveness
- Genital masculinization would therefore follow as an unselected side effect
Describe the chastity belt hypothesis of female masculinisation
Postulated that the vagina fused to prevent forced copulation by males – but there is no evidence of this amongst hyena species
Describe the competition aggression hypothesis of female masculinsation
• Competition for food gives selection for increased female size and aggression
• Hypothesis requires that female rank, body size, aggression and reproductive success are closely linked
• Large females are dominant and become alpha lineages in the hyena society → selection operates to give reproductive advantage to large dominant females.
− High ranking females give birth at an earlier age, have shorter intervals between litters, and more of their offspring survive to adulthood
• Masculinisaiton of behaviour confers an advantage in competition for food – females will dominant when feeding on a carcass → Masculinised female genitalia may be a secondary consequence of selection for early exposure to testosterone
• Mutated aromatase gene is associated with the aggressive female phenotype
Is the social rank of the female associated with exposure of maternal hormones in utero?
Dloniak et al, 2006:
- Measure the fecal steroid concentrations
- Associate with social rank of the female
- Assess behavior of the offspring
➢ Dominant females produce higher levels of androgen in pregnancy
➢ Both male and female cups exposed to high uterine androgen levels in the second half of gestation (when behavioural imprint likely) have higher rates of mounting
➢ Female cubs exposed to high androgen levels in utero have higher rates of aggression
• So there is a link between rank, androgens and reproductive success
Describe the external appearance of elephant genitalia
- Difficult to sex an elephant from the appearance of the external genitalia
- male and female have identical anogenital distance, with the prepuce and vulva situated on the ventral aspect of the abdomen.
- Male has no scrotum (testis are intra-abdominal)
- Female has a hypertrophied clitoris that resembles a male penis, but it is not canalised by a urethra
In order for males to copulate successfully, female cooperation is essential:
• Female must retract the vagina caudally
• Anatomically impossible for a male elephant to rape a female
Compare hyena and elephant sexual mimicry
Hyena: Carnivore with intense competition for limited food resource Female slightly larger than male Extreme male mimicry by females → penile clitoris → pseudoscrotum Matriarchal social structure Adult females social dominant Mating impossible without cooperation from the female
Elephant: Herbivore Male much larger then female Some male mimicry by females →identical anogenital distance →enlarged clitoris Male has no scrotum and intrabdominal testes Matriarchal social structure Mating impossible without cooperation from the female
