Chapter 11
Reproductive Behaviors
Sex and Hormones
Sexual reproduction between two individuals increases variation in the gene pool.
Variation in the gene pool of a species enables quick evolutionary adaptations to change in the
environment.
Also corrects genetic errors and disadvantageous mutations.
Sex and Hormones
Widespread communication throughout the body is accomplished through the release of hormones.
One class of hormones are steroid hormones.
Sex and Hormones
Steroid hormones are derived from cholesterol, contain four carbon rings and exert their effects in
three ways.
1. Binding to membrane receptors like neurotransmitters.
2. Entering cells and activate certain kinds of proteins in the cytoplasm.
3. Binding to chromosomes where they activate or inactivate certain genes.
Sex and Hormones
The sex hormones are a special kind of steroids, released mostly by the gonads and to a lesser degree
by the adrenal glands.
affect the brain, genital and other organs
Two types of sex hormones include:
1. Androgens
2. Estrogens
Both sexes have both hormones.
Sex and Hormones
Androgens are a groups of sex hormones that include testosterone and others - Generally referred to
as “male hormones” because men have higher levels than women.
Estrogens include estradiol and others and are referred to as “female hormones” because women
have higher levels.
Progesterone is a type of hormone that prepares the uterus for the implantation of a fertilized ovum
and promotes the maintenance of pregnancy.
Sex and Hormones
Sex limited genes are those activated by androgens or estrogens and control most of the differences
between male and female.
Example: estrogens activate the gene for breast growth; androgens activate the gene for the growth
of facial hair in men.
Sex hormones increase or decrease the rate of apoptosis in various regions of the brain.
Certain areas are slightly larger in males or females
Sex and Hormones
Sex hormones can have the following effects:
1. Organizing effects- occur mostly at sensitive stages of development.
-Determine whether the brain and body will develop male or female characteristics
1. Activating effects- occur at any time of life and temporarily activate a particular response.
The distinction between the activating and organizing effects of hormone is not absolute.
Example: hormones early in life can exert temporary effects; during puberty hormones can also
induce long-lasting structural changes
Sex and Hormones
Obvious differences can exist between the reproductive organs and the gonads of males and female.
Sexual differentiation begins with the chromosomes.
Female mammal has two X chromosomes and a male has an X and a Y.
During an early stage of prenatal development, both male and female have a set of Mullerian ducts
and a set of Wolffian ducts as well as primitive gonads.
Sex and Hormones
Wolffian ducts are the precursors to other male reproductive organs.
Develop into the vas deferens and seminal vesicles.
Mullerian ducts are precursors to the female’s oviducts, uterus, and upper vagina.
The male Y chromosome includes the SRY gene which causes the primitive gonads to develop into
testes, the sperm-producing organ around 6 weeks.
The developed testes produce the hormone testosterone.
Testosterone induces the development of the penis and scrotum.
Females are not exposed to high testosterone levels and their gonads develop into ovaries, the eggproducing organs.
Sex and Hormones
Sensitive periods are early periods when hormones have long-lasting effects.
Sexual differentiation depends mostly on the level of testosterone during a sensitive period.
The human sensitive period for genital formation is about the third and fourth month of pregnancy.
Sex and Hormones
Female rats exposed to testosterone shortly before or after birth are partly masculinized in anatomy
and behavior.
Clitoris grows larger than normal
At maturity, pituitary and ovaries produce steady levels of hormones instead of cycles
Parts of the hypothalamus appear more male
Sexual behavior becomes masculinized
Sex and Hormones
Extra estradiol does not determine whether the individual looks female or male.
Estradiol and other estrogens do modify various aspects of the development of the brain and the
internal sexual organs.
The absence of sex hormones generally leads to female-looking external genitalia
If a male rat lacks androgen receptors or is castrated, it develops female-like anatomy and behavior.
Sex and Hormones
Sex hormones early in life bind to receptors in specific areas of the hypothalamus, amygdala, and
other brain areas and produce anatomical and physiological differences.
The sexually dimorphic nucleus is an area in the anterior hypothalamus that is larger in the male and
contributes to control of male sexual behavior.
Parts of the female hypothalamus generate a cyclical pattern of hormone release; the hypothalamus
of a male cannot.
Sex and Hormones
During early development in rodents, testosterone is converted within certain brain cells to estradiol,
which masculinizes development – aromatization.
Alpha-fetoprotein is found in the blood during early sensitive periods and binds to estrogen and
prevents it from entering developing cells.
Testosterone does not bind to alpha-fetoprotein and freely enters the cell.
Sex and Hormones
Differences in the cerebral cortex also exist between men and women:
Men tend to have more white matter.
Women tend to have a greater density of neurons in parts of the temporal lobe dedicated to
language.
The language-related areas are larger in the left than right hemisphere for both sexes, but it is more
pronounced in women.
Sex and Hormones
Sex hormones have also been shown to influence intellectual performance in specific domains:
Females typically do better in most school subjects than men, except for math and science.
Boys perform better at mental rotation tasks and line orientation tasks.
Performance on these tasks most likely is attributed to organizational effects.
Sex and Hormones
Men excel in tasks involving spatial reasoning, but performance depends on effectiveness of
directional strategy.
Men are more likely to use directional (north, south, etc.) orientations to navigate.
Women are more likely to use landmarks
Evolutionary explanations suggest that differences exist because the males of many species travel
over greater geographical areas than do females.
Sex and Hormones
Sex hormones facilitate sexual behavior.
Arousal also depends on previous sexual experience.
Sex hormones activate sexual behavior by enhancing sensations.
Estrogens increase the sensitivity of the pudendal nerve, which transmits tactile stimulation from the
pubic area to the brain.
Sex and Hormones
Sex hormones also increase responses of certain areas of the hypothalamus.
the ventromedial nucleus, the medial preoptic area (MPOA), and the anterior hypothalamus.
Stimulation of an area known as the sexually dimorphic nucleus (SDN) fpound in the anterior
hypothalamus is larger in males and increases sexual behavior in males of many species.
Sex and Hormones
Testosterone and estradiol trigger the release of dopamine by the MPOA and other areas.
Dopamine stimulation of D1 and D5 receptors is associated with sexual arousal.
Facilitates erection of the penis and sexually receptive postures in females
Higher concentrations of dopamine stimulate D2 receptors and lead to orgasm.
Serotonin activity decreases sexual activity by blocking dopamine release.
Sex and Hormones
Humans are less dependent on current sex hormones than other species but changes can increase or
decrease sexual arousal.
For males, levels of testosterone correlate positively with sexual arousal and the drive to seek out
partners.
Sex and Hormones
Decreases in testosterone levels generally decrease male sexual activity and interest.
Example: castration
Impotence is the inability to maintain an erection.
usually caused by impaired blood circulation, not low testosterone.
Erection partially depends on testosterone increasing the release of nitric oxide.
facilitates the hypothalamic neurons and increases blood flow to the penis.
Sex and Hormones
Most sex offenders have normal testosterone levels.
Hormone levels do not completely explain the behaviors.
Testosterone reduction has been tried as a means of controlling sex offenders as it reduces sexual
activities.
Results favorable for those who continue taking medication, but dropout rate is high.
Sex and Hormones
In women, the hypothalamus and pituitary interact with the ovaries to produce the menstrual cycle.
The menstrual cycle is the periodic variation in hormones and fertility over the course of about 28
days.
After the end of a menstrual period:
the anterior pituitary releases follicle-stimulating hormone (FSH)
FSH promotes the growth of a follicle in the ovary.
The follicle nurtures the ovum and produces estrogen.
Towards the middle of the menstrual cycle, the follicle builds up receptors to FSH.
As a result, the follicle produces increasing amounts of estradiol, a type of estrogen.
Sex and Hormones
Increased estradiol causes the anterior pituitary to increase release of FSH and luetinizing hormone
(LH).
FSH an LH cause the follicle to release an ovum.
The remnants of the follicle release the hormone progesterone.
prepares the uterus for implantation of a fertilized ovum
inhibits the further release of LH
Sex and Hormones
The periovulatory period is the time of maximum fertility and increased estrogen levels when
ovulation occurs.
Studies suggest that women become more sexually responsive during this time when estrogen levels
are high.
Show increased attention to sex-related stimuli.
show increased mate preference towards men who act and look more masculine.
Sex and Hormones
Oxytocin is a pituitary hormone also important for reproductive behavior.
Stimulates contraction of the uterus during delivery of a baby
Stimulates the mammary gland to release milk.
Released during orgasm and triggers a state of complete relaxation.
Facilitates formation of pair bonds between mating partners and mother and infant
Sex and Hormones
Hormones released around the time of giving birth facilitate maternal behavior in females.
Late in pregnancy, the female secretes large amounts of estradiol, prolactin, and oxytocin.
Prolactic is responsible for milk production.
Oxytocin is associated with maternal behavior and social attachment.
Sex and Hormones
Females also change patterns of hormone receptors.
Late in pregnancy, the brain increases its sensitivity to estradiol in areas responsible for maternal
behavior, but not for sexual behavior.
The hormonal changes increase the attention of the mother to the young after birth.
Hormones also increase activity in the medial preoptic area and the anterior hypothalamus.
Sex and Hormones
Mammals have two mechanisms for stimulating maternal behavior:
1. Hormones in the early phase compensate for the lack of familiarity with the young.
2. Later experience maintains the maternal behavior as hormones decline.
Although hormonal changes are necessary to nurse a baby, they are not necessary to elicit care for a
baby by humans.
Variations in Sexual Behavior
A wide degree of variation exists between people in terms of frequency of sexual behavior, preferred
types of sexual activity, and sexual orientation.
Evolutionary explanations are controversial in explaining mating behavior.
Variations in Sexual Behavior
Gender differences in sexual behavior include the following:
Men are more likely to seek multiple sex partners, especially for short-term encounters.
Women are more likely to be concerned about a mates earning potential: men are more likely to be
concerned about a mate’s youth.
Men usually show greater jealousy at indications of sexual infidelity.
Variations in Sexual Behavior
Buss (2000) argues that gender differences reflect past evolutionary pressures.
Men are interested in brief sexual relationships with multiple partners because such a strategy
increases the likelihood of his genes being passed along to the next generation.
Variations in Sexual Behavior
Research has also suggested that women can gain from having multiple sexual partners as well.
Multiple mates increases resources available to her child and herself.
No direct evidence suggests that specific genes influence whether people prefer single or multiple
mates.
Variations in Sexual Behavior
Both men and women prefer a mate that is healthy, intelligent, honest, and physically attractive.
In almost all cultures, women prefer mates who are likely to be good providers.
Evolutionary explanations suggest that choosing a father who is likely to be a good provider aids the
women while she is pregnant or caring for a small child.
Variations in Sexual Behavior
Men tend to prefer a young partner.
Evolutionary explanations suggest that this preference exists because younger women are more likely
to be fertile than older women.
Men remain fertile well into old age so preference for a young mate for women is not as pronounced.
Variations in Sexual Behavior
Evolutionary explanations of gender differences in jealousy suggest that men need to be sure that the
children he supports are his own.
Unfaithful wives threaten this certainty.
Women are always sure that the child is their own.
Although cultures vary in attitudes towards infidelity, no culture exists where infidelity is more
acceptable for women.
Variations in Sexual Behavior
Although mating habits of people can be explained in terms of increasing the probability of passing on
one’s genes, we can not assume a genetic basis.
Behaviors and preferences may be a product of learning.
More data, especially about the effects of particular genes, is needed to draw conclusions.
Variations in Sexual Behavior
Gender identity refers to how we identify sexually and what we call ourselves.
Biological differences are generally referred to as “sex differences”.
Differences that result from people’s thoughts about themselves as male or female are referred to as
“gender differences”.
Variations in Sexual Behavior
Most people have a gender identity that matches their external appearance.
Some people have a gender identity that is opposite their biological sex.
Psychologists and researchers once believed that gender identity was learned and more a product of
rearing and experience.
Current evidence strongly suggests that biological factors, especially prenatal hormones, play a large
role in gender identity.
Variations in Sexual Behavior
Intersex people are people are intermediate between being male or female.
Some XY males with a mutation of the SRY gene have poorly developed genitals.
Some are born with an XX chromosome pattern but an SRY gene that translocates from the father’s Y
chromosome causes ambiguous genitalia.
Can also occur because of an atypical hormone pattern or mutation of testosterone receptors before
birth.
Variations in Sexual Behavior
Hermaphrodites are individuals whose genitals do not match the usual development for their genetic
sex.
An estimated 1 out of 100 children is born with some degree of genital ambiguity.
1 in 2000 has enough genital ambiguity to make the sex uncertain.
Variations in Sexual Behavior
Androgen insensitivity or testicular feminization is a condition in which individuals with an XY
chromosome pattern have the genital appearance of a female.
Production of androgens remains normal but they lack the androgen receptor that enables it to
activate genes in a cell’s nucleus.
Condition occurs in various degrees from a smaller than average penis to genitals that develop a
female appearance.
Variations in Sexual Behavior
Physicians have traditionally recommended that intersex people be reared as girls.
surgery was often conducted to make them look more feminine.
Assumed that children consistently raised as female would accept that identity.
Many intersex people protest against such surgery and suggest that an informed consent would have
been preferred.
Variations in Sexual Behavior
Some genetic males fail to produce an enzyme that converts testosterone to dihydrotestosterone.
Most look female at birth but a penis develops during adolescence and puberty.
Most then accept a male gender identity.
Brain is exposed to testosterone during early development.
Variations in Sexual Behavior
Many genetic males born without a penis or who had the penis accidentally removed and who were
raised as a girl ask to be reassigned as males.
Many who remain female feel discontent or conflict with being female.
Such cases indicate that although hormones do not determine gender identity, they do play an
important role.
Variations in Sexual Behavior
Subtle average behavioral and anatomical differences between heterosexual and homosexual include:
Length of bones in the arms, legs and hands
Percentage of hair whorl direction
Use of landmarks versus distances to give directions
Implies sexual orientation is an integral part of the individual.
But shifting of behavior to the opposite sex does not equally apply to both sexes.
Variations in Sexual Behavior
Studies of twins suggest sexual orientation is influenced by genetic factors.
Probability of homosexuality is highest in monozygotic twins and lower in dizygotic twins, and even
lower in siblings and adopted brothers or sisters.
No single particular gene has been identified.
Each of several genes may have an influence.
Variations in Sexual Behavior
Studies also suggest a higher incidence of homosexuality among the maternal relatives of homosexual
men.
These results suggest a gene on the X chromosome that a man receives from his mother may play a
role.
Other studies have not replicated this result and thus the findings are inconclusive.
Variations in Sexual Behavior
Sexual orientation may be influenced by testosterone levels during sensitive periods of brain
development.
Studies of male animals deprived of testosterone early in life show sexual interest in other males as
adults.
Studies of female animals exposed to testosterone during early development show an increased
likelihood of mounting behavior.
Variations in Sexual Behavior
Subtle physiological differences between heterosexual and homosexual men and women include:
Length of the arms, legs, and hands.
Ratio of index finger length to ring finger.
Homosexual males and females do not differ in terms of adult hormone levels.
Homosexual men are partly “feminized” and homosexual women are partly “masculinized”, but not
all homosexuals correspond to this trend.
Variations in Sexual Behavior
Research also suggests that certain brain structures differ in size between heterosexual and
homosexual men and women.
On average, the male homosexual brain is shifted towards a female development in some (but not all)
ways; the female is shifted in some ways towards male development.
Studies emphasize the role of testosterone at certain times of development as certain areas of the
brain have altered sensitivities to testosterone.
Variations in Sexual Behavior
The probability of homosexual orientation is also higher among men with older brothers.
Number of previous sisters has no effect nor do these effects apply to females.
Results suggest that a mother’s immune system may react against a protein in a son and attacks
subsequent sons to alter development.
Variations in Sexual Behavior
Prenatal exposure to stress and alcohol may also play a role.
Stress releases endorphins
antagonizes the effects of testosterone on the hypothalamus.
Stress elevates adrenal hormone corticosterone
decreases testosterone release.
Both result in changes in the structure of the nervous system.
makes the male anatomy closer to female
Variations in Sexual Behavior
Laboratory research has also shown that prenatal stress can alter sexual development.
Male subjects subjected to either prenatal stress or alcohol developed male sexual behavior in
addition to female sexual behaviors.
Male subjects exposed to both stress and alcohol during prenatal development had decreased sexual
behavior.
Variations in Sexual Behavior
On average, differences in brain anatomy exist between heterosexual and homosexuals.
Homosexual men tend to have:
Larger anterior commissure and suprachiasmatic nucleus
Smaller neurons in the third interstitial nucleus of the anterior hypothalamus (INAH-3)
Difficult to decipher why differences exist.