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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