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Chapter 9
Reproductive Behavior
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Hormonal Control of Sexual Behavior
Hormones are responsible for sexual dimorphism in the structure
of the body and its organs.
Hormones have organizational and activational effects on the
internal sex organs, genitals, and secondary sex characteristics.
Naturally, all of these effects influence a person’s behavior.
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Hormonal Control of Sexual Behavior
Hormonal Control of Female Reproductive Cycles
menstrual cycle (men strew al)
The female reproductive cycle of most primates,
including humans; characterized by growth of the lining of
the uterus, ovulation, development of a corpus luteum, and
(if pregnancy does not occur) menstruation.
estrous cycle
The female reproductive cycle of mammals other than
primates.
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Hormonal Control of Sexual Behavior
Hormonal Control of Female Reproductive Cycles
The LH surge causes ovulation: The ovarian follicle ruptures,
releasing the ovum.
Under the continued influence of LH, the ruptured ovarian follicle
becomes a corpus luteum (“yellow body”), which produces
estradiol and progesterone.
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Hormonal Control of Sexual Behavior
Hormonal Control of Female Reproductive Cycles
corpus luteum (lew tee um)
A cluster of cells that develops from the ovarian follicle
after ovulation; secretes estradiol and progesterone.
progesterone (pro jess ter own)
A steroid hormone produced by the ovary that maintains
the endometrial lining of the uterus during the later part of
the menstrual cycle and during pregnancy; along with
estradiol it promotes receptivity in female mammals with
estrous cycles.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Human sexual behavior, like that of other mammals, is influenced
by activational effects of gonadal hormones and, almost certainly,
by organizational effects as well.
If hormones have organizational effects on human sexual
behavior, they must exert these effects by altering the
development of the brain.
Although there is good evidence that prenatal exposure to
androgens affects development of the human brain, the
consequences of this exposure are not yet fully understood.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Activational Effects of Sex Hormones in Women
As we saw, the sexual behavior of most female mammals other
than higher primates is controlled by the ovarian hormones
estradiol and progesterone.
As Wallen (1990) pointed out, the ovarian hormones control not
only the willingness (or even eagerness) of an estrous female to
mate but also her ability to mate.
That is, a male rat cannot copulate with a female rat that is not in
estrus.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Activational Effects of Sex Hormones in Women
In higher primates (including our own species) the ability to mate
is not controlled by ovarian hormones.
There are no physical barriers to sexual intercourse during any
part of the menstrual cycle.
If a woman or other female primate consents to sexual activity at
any time (or is forced to submit by a male), intercourse can
certainly take place.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Activational Effects of Sex Hormones in Women
A study by Van Goozen et al. (1997) supports this suggestion. The
investigators found that the sexual activity initiated by men and
women showed very different relations to the woman’s menstrual
cycle (and hence to her level of ovarian hormones).
Men initiated sexual activity at about the same rate throughout the
woman’s cycle, whereas sexual activity initiated by women
showed a distinct peak around the time of ovulation, when
estradiol levels are highest. (See Figure 9.9.)
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Activational Effects of Sex Hormones in Women
Gangestad and Thornhill (2008) suggest that women’s sexuality
changes across the menstrual cycle in a particular way: They do
not become indiscriminately more interested in sexual contact
during their fertile period, which occurs around the time of
ovulation.
Instead, because they are more likely to become pregnant if they
engage in unprotected sex at that time, they become more
choosy.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Activational Effects of Sex Hormones in Women
In particular, they become more attracted to male characteristics
that might indicate high genetic quality (or did so in the evolution
of our species).
For example, Gangestad and Thornhill note that studies have
shown that at mid-cycle, women’s preference increases for the
sight of facial and bodily masculinity, for masculine behavioral
displays, for masculine vocal qualities, for androgen-related
scents, and for body symmetry, which correlates with genetic
fitness.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Activational Effects of Sex Hormones in Men
Although women and mammals with estrous cycles differ in their
behavioral responsiveness to sex hormones, men resemble other
mammals in their behavioral responsiveness to testosterone.
With normal levels they can be potent and fertile; without
testosterone sperm production ceases, and sooner or later, so
does sexual potency.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Activational Effects of Sex Hormones in Men
In a double-blind study, Bagatell et al. (1994) gave a placebo or a
gonadotropin-releasing hormone (GnRH) antagonist to young
male volunteers to temporarily suppress secretion of testicular
androgens.
Within two weeks, the subjects who received the GnRH
antagonist reported a decrease in sexual interest, sexual fantasy,
and intercourse.
Men who received replacement doses of testosterone along with
the antagonist did not show these changes.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation
A large-scale study of several hundred male and female
homosexuals reported by Bell, Weinberg, and Hammersmith
(1981) attempted to assess the effects of these factors.
The researchers found no evidence that homosexuals had been
raised by domineering mothers or submissive fathers, as some
clinicians had suggested.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation
The best predictor of adult homosexuality was a self-report of
homosexual feelings, which usually preceded homosexual activity
by three years.
The investigators concluded that their data did not support social
explanations for homosexuality but were consistent with the
possibility that homosexuality is at least partly biologically
determined.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation
If homosexuality does have a physiological cause, it certainly is
not variations in the levels of sex hormones during adulthood.
Many studies have examined the levels of sex steroids in male
homosexuals (Meyer-Bahlburg, 1984), and the vast majority of
them found these levels to be similar to those of heterosexuals.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation
A few studies suggest that about 30 percent of female
homosexuals have elevated levels of testosterone (but still lower
than those found in men).
Whether these differences are related to a biological cause of
female homosexuality or whether differences in lifestyles may
increase the secretion of testosterone is not yet known.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation
A more likely biological cause of homosexuality is a subtle
difference in brain structure caused by differences in the amount
of prenatal exposure to androgens.
Perhaps, then, the brains of male homosexuals are neither
masculinized nor defeminized, those of female homosexuals are
masculinized and defeminized, and those of bisexuals are
masculinized but not defeminized.
Of course, these are hypotheses, not conclusions.
They should be regarded as suggestions to guide future research.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Failure of Androgenization of Genetic Males
As we saw, genetic males with androgen insensitivity syndrome
develop as females, with female external genitalia—but also with
testes and without uterus or Fallopian tubes.
If an individual with this syndrome is raised as a girl, all is well.
Normally, the testes are removed because they often become
cancerous; but if they are not, the body will mature into that of a
woman at the time of puberty through the effects of the small
amounts of estradiol produced by the testes.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation and the Brain
The human brain is a sexually dimorphic organ.
This fact was long suspected, even before confirmation was
received from anatomical and functional imaging studies.
For example, neurologists discovered that the two hemispheres of
a woman’s brain appear to share functions more than those of a
man’s brain do.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation and the Brain
If a man sustains a stroke that damages the left side of the brain,
he is more likely to show impairments in language than will a
woman with similar damage.
Presumably, the woman’s right hemisphere shares language
functions with the left, so damage to one hemisphere is less
devastating than it is in men.
Also, men’s brains are, on the average, somewhat larger—
apparently because men’s bodies are generally larger than those
of women.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation and the Brain
In addition, the sizes of some specific regions of the
telencephalon and diencephalon are different in males and
females, and the shape of the corpus callosum may also be
sexually dimorphic.
Most investigators believe that the sexual dimorphism of the
human brain is a result of differential exposure to androgens
prenatally and during early postnatal life.
Of course, additional changes could occur at the time of puberty,
when another surge in androgens occurs.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation and the Brain
As we saw earlier, functional imaging studies have found that the
brains of heterosexual men and women reacted differently to the
odors of AND and EST, two chemicals that may serve as human
pheromones. Savic, Berglund, and Lindström (2005) found that
the response of brain regions of homosexual men to AND and
EST was similar to that of the heterosexual women.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Sexual Orientation and the Brain
Berglund, Lindström, and Savic (2006) found that the response of
brain regions of homosexual women to these substances was
similar to those of heterosexual men.
These studies suggest that a person’s sexual orientation affects
(or is affected by) his or her response pattern to these potential
sexual pheromones.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Heredity and Sexual Orientation
Another factor that may play a role in sexual orientation is
heredity.
Twin studies take advantage of the fact that identical twins have
identical genes, whereas the genetic similarity between fraternal
twins is, on the average, 50 percent.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Heredity and Sexual Orientation
Bailey and Pillard (1991) studied pairs of male twins in which at
least one member identified himself as homosexual.
If both twins are homosexual, they are said to be concordant for
this trait.
If only one is homosexual, the twins are said to be discordant.
Thus, if homosexuality has a genetic basis, the percentage of
monozygotic twins who are concordant for homosexuality should
be higher than that for dizygotic twins.
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Heredity and Sexual Orientation
This is exactly what Bailey and Pillard found: The concordance
rate was 52 percent for identical twins and only 22 percent for
fraternal twins—a difference of 30 percent. Other studies have
shown differences of up to 60 percent (Gooren, 2006).
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Hormonal Control of Sexual Behavior
Human Sexual Behavior
Heredity and Sexual Orientation
To summarize, evidence suggests that two biological factors—
prenatal hormonal exposure and heredity—may affect a person’s
sexual orientation.
These research findings certainly contradict the suggestion that a
person’s sexual orientation is a moral issue.
It appears that homosexuals are no more responsible for their
sexual orientation than heterosexuals are.
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Neural Control of Sexual Behavior
The control of sexual behavior—at least in laboratory animals—
involves different brain mechanisms in males and females. This
section describes these mechanisms.
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Neural Control of Sexual Behavior
Males
Erection and ejaculation are controlled by circuits of neurons that
reside in the spinal cord.
Coolen and her colleagues (Coolen et al., 2004; Coolen, 2005)
have identified a group of neurons in the lumbar region of the rat
spinal cord that appear to constitute a critical part of the spinal
ejaculation generator.
However, brain mechanisms have both excitatory and inhibitory
control of these circuits.
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Neural Control of Sexual Behavior
Males
medial preoptic area (MPA)
An area of cell bodies just rostral to the hypothalamus;
plays an essential role in male sexual behavior.
The medial preoptic area (MPA), located just rostral to the
hypothalamus, is the forebrain region most critical for male sexual
behavior.
Electrical stimulation of this region elicits male copulatory
behavior.
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Neural Control of Sexual Behavior
Males
The organizational effects of androgens are responsible for sexual
dimorphisms in brain structure. Gorski et al. (1978) discovered a
nucleus within the MPA of the rat that is three to seven times
larger in males than in females.
This area is called (appropriately enough) the sexually dimorphic
nucleus (SDN) of the preoptic area.
sexually dimorphic nucleus
A nucleus in the preoptic area that is much larger in
males than in females; first observed in rats; plays a role in
male sexual behavior.
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Neural Control of Sexual Behavior
Males
The motor neurons that innervate the pelvic organs involved in
copulation are located in the lumbar region of the spinal cord
(Coolen et al., 2004).
Anatomical tracing studies suggest that the most important
connections between the MPA and the motor neurons of the
spinal cord are accomplished through the periaqueductal gray
matter (PAG) of the midbrain and the nucleus
paragigantocellularis (nPGI) of the medulla.
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Neural Control of Sexual Behavior
Males
periaqueductal gray matter (PAG)
The region of the midbrain that surrounds the cerebral
aqueduct; plays an essential role in various speciestypical behaviors, including female sexual behavior.
nucleus paragigantocellularis (nPGi)
A nucleus of the medulla that receives input from the
medial preoptic area and contains neurons whose
axons form synapses with motor neurons in the spinal
cord that participate in sexual reflexes in males.
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Neural Control of Sexual Behavior
Males
The inhibitory connections between neurons of the nPGI and
those of the ejaculation generator are serotonergic.
As Marson and McKenna (1996) showed, application of serotonin
(5-HT) to the spinal cord suppresses ejaculation.
This connection may explain a well-known side effect of selective
serotonin reuptake inhibitors (SSRIs).
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Neural Control of Sexual Behavior
Males
Men who take SSRIs as a treatment for depression often report
that they have no trouble attaining an erection but have difficulty
achieving an ejaculation.
Presumably, the action of the drug as an agonist at serotonergic
synapses in the spinal cord increases the inhibitory influence of
nPGi neurons on spinal neurons responsible for ejaculation.
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Neural Control of Sexual Behavior
Females
Just as the MPA plays an essential role in male sex behavior,
another region in the ventral forebrain plays a similar role in
female sexual behavior: the ventromedial nucleus of the
hypothalamus (VMH).
A female rat with bilateral lesions of the ventromedial nuclei will
not display lordosis, even if she is treated with estradiol and
progesterone.
Conversely, electrical stimulation of the ventromedial nucleus
facilitates female sexual behavior.
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Neural Control of Sexual Behavior
Females
ventromedial nucleus of the hypothalamus (VMH)
A large nucleus of the hypothalamus located near the walls
of the third ventricle; plays an essential role in female
sexual behavior.
As we saw earlier, sexual behavior of female rats is activated by a
priming dose of estradiol, followed by progesterone.
The estrogen sets the stage, so to speak, and the progesterone
stimulates the sexual behavior.
Injections of these hormones directly into the VMH will stimulate
sexual behavior even in females whose ovaries have been
removed.
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Neural Control of Sexual Behavior
Females
Daniels, Miselis, and Flanagan-Cato (1999) injected a
transneuronal retrograde tracer, pseudorabies virus, in the
muscles responsible for the lordosis response in female rats.
They found that the pathway innervating these muscles was as
previous studies predicted: VMHPAGnPGimotor neurons in
the ventral horn of the lumbar region of the spinal cord.
Figure 9.13 summarizes the evidence I have presented so far in
this section. (See Figure 9.13.)
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Neural Control of Sexual Behavior
Formation of Pair Bonds
In approximately 5 percent of mammalian species, heterosexual
couples form monogamous, long-lasting bonds. In humans, such
bonds can be formed between members of homosexual couples
as well.
As naturalists and anthropologists have pointed out, monogamy is
not always exclusive: In many species of animals, humans
included, individuals sometimes cheat on their partners.
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Neural Control of Sexual Behavior
Formation of Pair Bonds
Several studies have revealed a relation between monogamy and
the levels of two peptides in the brain: vasopressin and oxytocin.
These compounds are both released as hormones by the
posterior pituitary gland and as neurotransmitters by neurons in
the brain.
In males, vasopressin appears to play the more important role.
Monogamous voles have a higher level of vasopressin receptors
in the ventral forebrain than do polygamous voles.
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Neural Control of Sexual Behavior
Formation of Pair Bonds
Many investigators believe that oxytocin and vasopressin may
play a role in the formation of pair bonding in humans.
For example, after intercourse, at a time when blood levels of
oxytocin are increased, people report feelings of calmness and
well being, which are certainly compatible with the formation of
bonds with one’s partner.
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