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Different between males and females.

Need to know things in textbook, like some sex development stuff.


Reproductive Cycles

§ Reproductive cycle of female mammals is called the estrous cycle (menstrual cycle in primates).
§ A female rat in estrous behaves differently which goads the male rat to behave differently as well.
§ The sexual behaviour of mammals is linked with ovulation (in primates, females can mate any time during their menstrual cycle).
§ Menstrual and estrous cycles are controlled by hormonal secretions of the pituitary gland and ovaries.


Hormonal Control of Sexual Behaviour in Laboratory Animals

§ Sexual behaviour in male rats depends on testosterone released by testes.
- A castrated male rat will cease sexual activity which can be reinstated with a injection of testosterone.
§ Sexual behaviour in female rats depends on estradiol and progesterone released by ovaries. Estrodial must follow estrogen as it primes it!
- Ovariectomised rats are not sexually receptive but it can be produced by administering estradiol followed by progesterone.

Sex less complicated in animals, no non ejaculations I think she said.


Sexual Behaviour in Female Laboratory Animals

§ When she is receptive, the female rat arches her back which elevates her hindquarters (lordosis).
§ Although a passive participant, she is active in initiating copulation; she may entice him by sniffing and nuzzling the males genitals and make quick, short hopping movements with rapid ear wiggles.
§ Male rats are most responsive to a female rat on ‘heat.’
§ Males will ignore females whose ovaries have been removed but injections of estradiol and progesterone will increase her attractiveness.


Sexual Behaviour in Male Laboratory Animals

§ Intromission (entry of penis into female vagina), pelvic thrusting (rhythmic movement of the hindquarters) and ejaculation (semen discharge) are characteristics of all male mammals.
§ When a male rat encounters a receptive female, he will spend more time sniffing, nuzzling and licking her genitals, as well as mount her and engage in pelvic thrusting. (Sim what female does in courting)
§ He will mount her several times and intromit on most occasions but will ejaculate only after about 8-15 intromissions.
§ After a refractory period, the male will return to copulate again.
§ If the male is exhausted by repeated copulation with the same female, he will respond more quickly if presented with a new female.
§ Successive introductions with new females keeps up his performance for long periods of time.


Behavioural Defeminization

§ Androgens are male sex steroid hormones which stimulate the
development of the male sex organs.
§ Androgens have a defeminizing effect; they inhibit the later development of anatomical or behavioural female characteristics
§ If a rodent brain is NOT exposed to androgens at a critical time during development (shortly after birth), and treated with
estradiol and progesterone as an adult, the rat will engage in female sexual behaviour.
§ If a male rat is castrated after birth and given injections of estradiol and progesterone as an adult, it will respond to the presence of another male by assuming lordosis, i.e. it acts as if it were a female.


Behavioural Masculinization

§ If a rodent brain IS exposed to androgens shortly after birth, behavioural defeminization will suppress the neural circuits controlling female sexual behaviour, and behavioural masculinization will allow the rat to engage in male sexual behaviour as an adult.
§ If a female rat is ovariectomised at birth and then treated with testosterone as an adult, she will attempt to mount and copulate with a receptive female. i.e. it acts as if it were a male.


Remember default is female

So critical period to get androgens there to become a male.



§ Pheromones are chemicals that carry messages from one
animal to another.
§ Groups of female rats when housed together show slowing
down of their estrous cycle (Lee-Boot effect).
§ Exposure to male odour (or urine) will begin the cycle again
(Whitten effect) and the cycles will synchronise.
§ A female rat will show accelerated puberty if she is exposed to
male odour (Vandenbergh effect).
§ These effects are caused by compounds present in the urine of
‘intact’ adult males; the urine of a castrated male has no effect.
§ Therefore, the production of pheromone requires the presence of testosterone.


Lee-Boot effect?

Groups of female rats when housed together show slowing
down of their estrous cycle (


Whitten effect)

Exposure to male odour (or urine) will begin the cycle again
(and the cycles will synchronise.


Vandenbergh effect).

A female rat will show accelerated puberty if she is exposed to
male odour (


The Bruce Effect

§ If a recently impregnated mouse encounters a male mouse other than the one she mated, the pregnancy will fail.
§ This effect is caused by a substance in the ‘intact’ male urine.
§ Thus, a male mouse is able to prevent the birth of infants carrying another male’s genes and subsequently impregnate the female himself.
§ This is an important survival advantage; the new male is likely to be more healthier and more vigorous thereby increasing the chances of passing genes to her offspring that are more likely to survive.


Pheromones require



The Vomeronasal Organ

§ The effects that pheromones have on the reproductive cycle is mediated by the vomeronasal organ (VNO).
§ The VNO projects to the olfactory bulb which detects odours.
§ The VNO responds primarily to non-volatile compounds found in urine
§ Removal of the accessory olfactory bulb disrupts the Lee- Boot, Whitten, Vandenbergh and Bruce effects.
§ Neurons of the vomeronasal system respond only when a mouse actively investigates the mouth or anogenital region or another mouse.


Deleting a pheromone receptor from the VNO causes male sexual behaviour in females.



Medial Nucleus of the Amygdala

§ The olfactory bulbs sends axons to the medial nucleus of the amygdala.
§ Lesions of the medial amygdala abolish sexual behaviour in male hamsters.
§ The medial amygdala is part of a system that mediates the effects of pheromones
§ The amygdala projects to the preoptic area and the ventromedial hypothalamic nuclei.


Pheromone-(related) phenomena in Humans

§ The menstrual cycle of women attending an all female college have synchronised cycles. Women who spend more time in the company of men have shorter cycles.
§ Compounds present in human sweat have different effects. Androstadienone increases alertness and positive mood in women but decreases positive mood in men.
§ The familiar odour of a sex partner has a positive effect on sexual arousal.
§ Men and women can learn to be attracted to their partners characteristic odour, e.g. the smell of their clothes.

Respond to sensory cue!

Mic sensitive to male or female experimentor. Has effect!


Sexual Orientation

§ Sexual orientation is the gender of the preferred sex partner.
§ A social explanation?
- There is no evidence that childhood experiences affect sexual
social explanation? orientation.

§ A biological explanation
- Levels of sex steroids in homosexual males are equivalent to
heterosexual males.
- 30% of female homosexuals have elevated levels of testosterone
(but still lower than found in men).

- Differences in prenatal exposure to androgens might cause differences in brain structure.
> Male homosexual brains are neither masculinized nor defeminized.
> Female homosexuals brains are masculinized and defeminized.
> The brains of bisexuals are masculinized but not defeminized
These are speculations
biological explanation?


Prenatal Androgenization of Genetic Females

§ Prenatal androgens can affect human sexual orientation.
§ In congenital adrenal hyperplesia (CAH), the adrenal glands
secrete abnormal amounts of androgen (starts prenatally). High abnormal amounts of male hormone androgens
§ This causes prenatal masculinization. Therefore, boys born with CAH develop as boys.
§ Girls with CAH will be born with an enlarged clitoris and the labia might be fused together. Sometimes, surgery is needed to correct them.
§ Females with CAH may be attracted to other women (or are bisexual), but several women with CAH consider themselves heterosexual.

Neat drawing. Sexual dimorphism Behaviour ex. Difference between males and females.

Sex typical toys, socially determined.


Failure of Androgenization of Genetic Males

§ Genetic males with androgen insensitivity syndrome develop as females, with female external genitalia, but also with testes and without a uterus or Fallopian tubes.
§ If raised as a girl, all is well (the testes are surgically removed and she is given estradiol).
§ There are no reports of sexual orientation towards women or reports of bisexuality. Thus, lack of androgen prevents masculinization and defeminizing effects on a persons sexual interest.


Sexual Orientation and the Brain

§ The human brain is sexually dimorphic: the brains of men and
women are different.

Male brain is larger on average.
§ A man’s sexual orientation appears to be associated! with:
- the suprachiasmatic nucleus (SCN) of the hypothalamus, is larger
- the anterior commissure, larger.

Domestic Rams show this as well.


Bed Nucleus of the Stria Terminalis (BNST).
Sexual difference due to sexual identity

§ The bed nucleus of the stria terminalis (BNST) is larger in males than females. Extended amygdala and hpa axis
§ The BNST is small in male transsexuals (same size as females) but large in male homosexuals (same size as male heterosexuals).

NB. We cannot conclude that any part of the brain is directly involved in sexual orientation or sexual identity.