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Chapter
Reproductive behavior Reproductive behaviors
Reproductive Constitute the most important category of social behaviors, because
Constitute
without them, most species would not survive
without
These behaviors (e.g. courting, mating, parental behavior) are
These
categories of sexually dimorphic behaviors, ii.e. behaviors that differ
.e.
sexually
in males and females
in
Hormones present both before and after birth play a role in the
Hormones
development and control of sexually dimorphic behaviors
development Sexual development
Sexual Production of gametes and
Production
fertilization
fertilization The production of gametes (ova and
The
gametes
sperm) is a special form of cell division:
produces cells that contain one member
of each of the 23 pairs of chromosomes;
the development of a human begins at
the time of fertilization, when a sperm
and ovum join, sharing their
chromosomes to create 23 pairs
chromosomes
The last pair of chromosomes
The
determines the sex of the offspring (sex
chromosomes; XX female, XY male)
chromosomes Sexual development
Sexual Development of the sex organs Only sex hormones are responsible for our sexual dimorphism
3 general categories of sex organs:
• Gonads – testes or ovaries
produce ova or sperm and secrete hormones
produce
the factor that controls their development into either testes or ovaries is
the
a single gene on the Y chromosome called Sry, which produces a
Sry which
protein called testis-determining factor; thus if not present, ovaries
testis-determining
thus
develop
develop Sexual development
Sexual Development of the sex organs (con’t) Gonads (con’t)
Internal sex organs
• Müllerian system – the embryonic precursors of the female internal sex
organs
organs
• Wolffian system – the embryonic precursors of the male internal sex organs
• Gender of fetus determined by presence or absence of hormones secreted
Gender
by the testes:
by Anti-Müllerian hormone prevents female system from developing (defeminizing
effect)
effect
Androgens – stimulates development of Wolffian system (masculinizing effect) • Once gonads have developed, a series of events directed by hormones
Once
occur that determine the gender of the offspring
occur
• Two effects: Organizational effects – the effect of a hormone on tissue differentiation and
development; occurs during prenatal development
development;
Activational effects – the effect of a hormone that occurs in the fully developed
organism; may depend on the organism’s prior exposure to the organizational
effects of hormones
effects Sexual development
Sexual Development of the sex organs (con’t) Internal sex organs (con’t)
• Two types of androgens Testosterone – secreted from testes; principle androgen found in males
Dihydrotestosterone – an androgen, produced from testosterone through the
action of the enzyme 5α reductase
action • The Wolffian system thus contains androgen receptors in order for the
The
androgens to take action
androgens
• Androgen insensitivity syndrome – a condition caused by a congenital lack
of functioning androgen receptors; in a person with XY sex chromosomes,
causes the development of a female with testes but no internal sex organs
causes
• Persistent Müllerian duct syndrome – condition caused by a congenital lck of
anti-Müllerian hormone or receptors for this hormone; in a male, causes
development of both male and female internal sex organs
development
• Tuner’s syndrome – the presence of only one sex chromosome (X);
characterized by lack of ovaries but otherwise normal female sex organs and
genitalia
genitalia Androgen Insensitivity
Syndrome
Syndrome Sexual development
Sexual Development of the sex organs (con’t) External genitalia
• Visible sex organs, including penis and scrotum in males and labia, clitoris, and the
Visible
outer part of the vagina in females
outer
• In the presence of dihyrdotestosterone the external genitalia will become male Sexual maturation Primary sex characteristics include gonads, internal sex organs, and external
Primary
genitalia; all present at birth
genitalia;
Secondary sex characteristics (e.g. enlarged hips and breasts, facial hair and
Secondary
deep voice) appear at puberty
deep
The onset of puberty occurs when the cells in the hypothalamus secrete
The
gonadotropin-releasing hormone (GnRH), which stimulates the production of 2
gonadotropin-releasing
gonadotropic hormones by the anterior pituitary gland
gonadotropic
• Follicle-stimulating hormone (FSH) – causes development of an ovarian follicle and the
maturation of an ovum
maturation
• Luteinizing hormone – causes ovulation and development of the ovarian follicle into a
corpus luteum
corpus
• Both hormones are also produced in males, to stimulate the testes to produce sperm
Both
and to secrete testosterone
and Sexual development
Sexual Sexual maturation (con’t) In response to the gonadotropins, the gonads secrete steroid sex
In
hormones
hormones
• e.g. the ovaries produce estradiol one of a class of hormones known as
e.g.
estradiol
estrogens
estrogens Hormonal control of sexual
behavior
behavior Female reproductive cycles Menstrual cycle – primate species; characterized by the growth of the
lining of the uterus, ovulation, development of a corpus luteum, and (if
pregnancy does not occur), menstruation
pregnancy
Other species have estrous cycles
Other
estrous
Cycle begins with secretion of gonadotropins by the anterior pituitary,
Cycle
which stimulate the growth of ovarian follicles, and ovulation (release of
ovarian
and
ovum); the ruptured ovarian follicle becomes a corpus luteum and
corpus
produces estrodiol and progesterone (promotes pregnancy)
estrodiol
progesterone Hormonal control of sexual
behavior
behavior Sexual behavior of lab animals Males
• Male sexual behavior is quite varied, however, the essential features
Male
(intromission, pelvic thrusting, and ejaculation) are characteristic of all male
mammals
mammals
• Rat sexual behaviors studied most
• After ejaculation, males enter a refractory period, during which they cannot
After
refractory
during
ejaculate again
ejaculate
• In behavioral studies, observe Coolidge effect – the restorative effect of
In
Coolidge
introducing a new female sex partner to a male that has apparently become
“exhausted” by sexual activity
“exhausted” Females
• Lordosis – a spinal sexual reflex seen in many four-legged female mammals;
arching of the back in response to approach of a male or to touching the
flanks, which elevates the hindquarters
flanks, Hormonal control of sexual
behavior
behavior Masculinization and Defeminization If a rodent brain is exposed to androgens during development, two
If
phenomenon occur:
phenomenon
• Behavioral defeminization – the organizational effect of androgens that
Behavioral
prevents an animal from displaying female sexual behaviors in adulthood
prevents
• Behavioral masculinization – enables animals to engage in male sexual
Behavioral
behavior
behavior Effects of pheromones A chemical released by one animal that affects the behavior or
chemical
physiology of another animal; usually smelled or tasted
physiology
Whitten effect – the synchronization of the menstrual or estrous cycles
of a group of females, which occurs only in the presence of a
pheromone in a male’s urine
pheromone
Detection of pheromones is mediated by the vomeronasal organ (VNO),
Detection
vomeronasal
which projects to the olfactory accessory bulb
which
Olfactory accessory bulb then projects to the medial nucleus of the
Olfactory
amygdala, which then projects to the hypothalamus
amygdala Hormonal control of sexual
behavior
behavior
Human sexual behavior Activational effects of sex hormones in women
• In higher primates (including humans), ovarian hormones are not necessary
In
to have intercourse, as with other mammals
to
• However, ovarian hormones can have an influence on their sexual interest In men
• Levels of testosterone not only affect sexual activity, but is also affected by it Sexual orientation
Sexual Exclusive homosexuality appears to occur only in humans
A llikely biological cause of homosexuality is a subtle difference in
ikely
brain structure cased by differences in the amount of prenatal
exposure to androgens
exposure
However, these are speculations and have not been supported by
However,
speculations
human data
human
Congenital adrenal hyperplasia – a condition characterized by
hypersecretion of androgens by the adrenal cortex; in females,
causes masculinization of the external genitalia; studies have seen
a higher proportion of homosexual women with this disorder
higher
The sexual dimorphism of the brain (e.g. different sizes, more
The
sharing of functions in female brains) may be a result of differential
exposure to androgens during early postnatal life
exposure
Many studies have shown possible relations between sizes of
Many
certain brain structures and homosexuality; however, no real
conclusive data
conclusive Neural control of sexual behavior
Neural Males Spinal mechanisms
• Some sexual response are controlled by neural circuits in the spinal
Some
cord (e.g. erection and ejaculation)
cord
• e.g. spinal nucleus of the bulbocavernosus in the male rat
e.g. spinal Brain mechanisms
• Both excitatory and inhibitory controls over the spinal mechanisms
• e.g. medial preoptic area – most critical for sexual behavior in males
e.g. medial
• Sexually dimorphic nucleus – larger in males than in females; plays
a role in male sexual behavior
role
• Periaqueductal gray matter – region of the midbrain that surrounds
the cerebral aqueduct; plays an essential role in various species
typical behaviors
typical Neural control of sexual behavior
Neural Females Ventromedial nucleus of
Ventromedial
the hypothalamus – plays
the
an essential role in female
sexual behaviors; injection
of ovarian hormones into
the nucleus will elicit
sexual behavior even in
females without ovaries
females Parental behavior
Parental Maternal behavior of rodents Hormonal control of maternal behavior During gestation (pregnancy) female rats and mice build nests
At the time of parturition (giving birth), the female will prepare herself for labor,
At
parturition
and afterwards nurse the pups
and
Mother will lick the pups’ anogenital region in order to stimulate urination and
Mother
defecation
defecation
Will retrieve pups that may leave the nest
Continue to nurse the pups until weaning
No evidence that organizational effects of hormones play a role
Although hormones may affect maternal behavior, they do not control them Neural control of maternal and paternal behaviors The medial preoptic area plays a role in maternal behavior (lesions elicit
The
indifference towards pups in new mothers)
indifference
In some species, the male will care for the offspring (e.g in monogamous prairie
In
vols)
vols) ...
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- Fall '08
- Williams
- Sexual intercourse, Sex organ, Internal Sex Organs