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Chapter 10:
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

Sex and Hormones
Obvious differences can exist between the
reproductive organs and the gonads of
males and female.
 Sexual differentiation begins with the
chromosomes and
 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
Mullerian Ducts
 Are precursors to the female’s oviducts, uterus,
and upper vagina
Sex and Hormones
The male Y chromosome includes the SRY
gene which causes the primitive gonads to
develop into testes, the sperm-producing organ
 The developed testes produce the hormone
testosterone
 Testosterone induce the development of the
penis and scrotum
 Females are not exposed to high testosterone
levels and their gonads develop into ovaries,
the egg-producing organs

Sex and Hormones
Widespread communication throughout the body
is accomplished through the release of
hormones.
 Two kinds of hormones include:
 Steroid hormones
 Sex hormones

Sex and Hormones
Steroid Hormones
 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
Sex Hormones
 A special kind of steroids, released mostly by the
gonads and to a lesser degree by the adrenal
glands
 Affect the brain, genitals and other organs
 Two types of sex hormones include:
1. Androgens
2. Estrogens
 Both sexes have both hormones
Sex and Hormones
Androgens
 A groups of sex hormones that include
testosterone and others
Sex and Hormones
Estrogens
 Include estradiol and others and are referred to as
“female hormones” because women have higher
levels
Progesterone
 A type of hormone that prepares the uterus for the
implantation of a fertilized ovum and promotes the
maintenance of pregnancy
Organizing Effects of Sex 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
Activating Effects
2.

Occur at any time of life and temporarily activate a
particular response
Organizing Effects of Sex Hormones
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
Organizing Effects of Sex Hormones
Sensitive Periods
 Are early periods when hormones have longlasting 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
Organizing Effects of Sex 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
Organizing Effects of Sex 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 Differences in the Hypothalamus
Sexually Dimorphic Nucleus
 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 Differences in the Hypothalamus
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
Intellectual Sex Differences
Sex hormones have 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
See next slide 
Intellectual Gender Differences

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
Sex Hormone Levels: Animal Studies
Testosterone and estradiol trigger the release of
dopamine by the medial preoptic area (MPOA)
and other areas
 Moderate dopamine release is associated with
initial sexual arousal
 Higher concentrations of dopamine stimulate D2
receptors and leads to orgasm
 Serotonin activity decreases sexual activity by
blocking dopamine release

Sex Hormone Levels: Animal Studies

Sex hormones activate sexual behavior partly
by facilitating activity in areas of the brain

Estrogens increase the sensitivity of the
pudendal nerve, which transmits tactile
stimulation from the pubic area to the brain
Activating Effects of Sex Hormones in Humans
Humans are less dependent on current sex
hormones than other species but changes can
increase or decrease sexual arousal
 For males, sexual excitement is generally
highest when testosterone levels are highest
 The hormone oxytocin contributes to sexual
pleasure
 The body releases enormous amounts of
oxytocin during orgasm

Activating Effects of Sex Hormones
In adulthood, sex hormones can activate
behavior
 Behavior can also influence hormone secretion
 Hormones do not cause behavior but rather
alter the activity in various brain areas to
change the way the brain responds to certain
stimuli
 Hormones also change sensitivity in the penis,
vagina, and cervix

Activating Effects of Sex Hormones
Although most sex offenders have normal
testosterone levels, testosterone reduction has
sometimes been tried as a means of controlling
sex offenders.
 Cyproterone is a drug that blocks the binding of
testosterone to receptors
 Medroxyprogesterone inhibits gonadotropin, the
pituitary hormone that stimulates testosterone
production
Activating Effects of Sex Hormones
Decreases in testosterone levels generally
decrease male sexual activity and interest
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
Activating Effects of Sex 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

Activating Effects of Sex Hormones

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

Activating Effects of Sex 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

Activating Effects of Sex Hormones
Birth control pills prevent pregnancy by
interfering with the usual feedback cycle
between the ovaries and pituitary
 The “combination-pill” contains both estrogen
and progesterone and prevents the surge of
FSH and LH that would release an ovum

 Also thickens the mucus of the cervix making it
harder for the sperm to reach the egg
Activating Effects of Sex 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
Activating Effects of Sex Hormones
Miller, Tybur, & Jordan (2007)
Procedures
 Had lap dancers’ record the times of their
menstrual periods and the amount of tips that they
received
Results
 Lap dancers taking contraceptive pills earned the
same tips throughout the study
 Lap dancers not taking contraceptive pills received
the largest tips 9-15 days after menstruation
Activating Effects of Sex Hormones
Miller, Tybur, & Jordan (2007)
Interpretation
 The lap dancers felt and acted sexier when
estrogen levels were highest
Activating Effects of Sex Hormones
Gangestad, Simpson, Cousins, & Christensen (2004)
Participants
 Women around ovulation and women not around
ovulation
Procedure
 Women viewed videotapes of two men and were
asked to choose who they anted to go out with for
a short-term relationship
Results
 Women around the time of ovulation choose the
“athletic, competitive, assertive” type; women not
around ovulation choose the “nice guy” type
Activating Effects of Sex Hormones
Gangestad, Simpson, Cousins, & Christensen (2004)
Interpretation
 Hormones associated with fertility move women’s
mate preferences toward men who look and act
more masculine
Effects of Hormones on Parental Behavior
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

Effects of Hormones on Parental Behavior
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

Effects of Hormones on Parental Behavior

Hormones also increase activity in the medial
preoptic area and the anterior hypothalamus
Normal mouse
Genetically mutated mouse
Effects of Hormones on Parental Behavior

Vasopressin is a hormone synthesized by the
hypothalamus and secreted by the posterior
pituitary gland
 Associated with establishing long-term bonds in
some species

Mothers are also stimulated by the odors of
their babies
Effects of Hormones on Parental Behavior

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
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.
 One perspective of explaining differences
in behavior is from an evolutionary
perspective.

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
Mate Selection:
The Evolutionary Perspective

Men and women by nature must differ in their optimal
mating behaviors
 Women must be highly selective because they are
biologically limited in the number of children they can
bear and raise in a lifetime
○ This is often referred to as the parental investment
model
 Men can father an unlimited number of children and
ensure their reproductive success by inseminating many
women
Findings appear to support this idea…

Buss (1989)
 Women wanted to marry guys 3.4 years older
than they were

Kenrick & Keefe (1992)
 Analysis of personal ads:
 Males:
○ 20s: seek younger woman to slightly older
○ 30-40s: seek women 5-10 years younger
○ 50s: seek women 10-20 years younger
Findings appear to support this idea…
Kenrick & Keefe (1992)
 Male adolescents were most attracted to women
in their “fertile 20s”
Interest in Multiple Mates
Hardy (2000)
 Several benefits to women as well:
 Husband may be infertile
 Multiple mates increases resources available to
her child and herself
 Can leave first one for a better one
What men and women seek in a mate…
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

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

Although cultures vary in attitudes towards
infidelity, no culture exists where infidelity is
more acceptable for women
Gender Identity
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”
 Current evidence strongly suggests that
biological factors, especially prenatal
hormones, play a large role in gender identity

Gender-Differentiated Behaviors
Intersex
 These individuals 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
Gender-Differentiated Behaviors
Hermaphrodites
 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
Gender-Differentiated Behaviors
Congenital Adrenal Hyperplasia (CAH)
 An overdevelopment of the adrenal glands
from birth
 Caused by a genetic defect in which cortisol
production leads to overstimulation of the
adrenal gland
 Overstimulation of the adrenal gland leads to
extra testosterone production
 Seems to affect their choice of toys
See study on next slide 
CAH Toy Preferences
Pasterski et al. (2005)
Procedures

Children were observed in
a room full of toys; some
were “girl typical” toys and
others were “boy typical”
toys; also some “neutral”
toys
Results
 CAH girls played with
toys intermediate of nonCAH girls and boys
Gender-Differentiated Behaviors
Androgen Insensitivity (Testicular Feminization)
 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
Issues of Gender Assignment

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
Issues of Gender Assignment
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

Click on picture for video 
Caitlin Jenner
Issues of Gender Assignment
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

Click on picture for video 
David Reimer
Sexual Orientation
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

Sexual Orientation
Bailey & Pallard (1991)
 Twin study
 Homosexual men
 Advertised in gay or lesbian magazine
 Co-twin was more than twice as likely to be
homosexual if the twins were MZ
Bailey, Pallard, Neale, & Agyei (1993)
 Replicated earlier study using homosexual women
 Same results
Sexual Orientation
Langstrom, Rahman, Carlstrom , & Lichenstein (2010)




Twin study
All twins in Sweden between ages 20 & 47
Asked if they had a same sex partner
Concordance: MZs about 10%; DZs about 5%
Concordance for the three studies
Prenatal Influences
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

Prenatal Influences
The probability of homosexual orientation appears
to be 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

See study on next slide 
Prenatal Influences

Blanchard (2008)
 Slight link to gay men having older brothers
○ First son: 2%
○ Second son: 3%
○ Third son: 4%
Prenatal Influences
Ward, Ward, Winn, & Bielawski (1994)
Procedure
 Pregnant rats kept in tight Plexiglass tubes for more
than two hours each day under bright lights; some
were also given alcohol
Results
 Male rats whose mothers were subjected to either
prenatal stress or alcohol developed male sexual
behavior in addition to female sexual behaviors
 Male rats whose mothers were exposed to both stress
and alcohol during prenatal development had
decreased sexual behavior
Implication
 Prenatal stress can alter sexual development
Brain Anatomy
Research 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

Brain Anatomy
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)
Credits
Some slides prepared with the help of the
following websites:

http://learnsciencewithmrsg.weebly.com/uploads/1/2/7/2/12728010/sex
_hormones_long_ppt.ppt