Download Chapter 10 Reproductive Behavior

Chapter 10
Reproductive behavior
Reproductive behaviors
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Constitute the most important category of social behaviors, because
without them, most species would not survive
These behaviors (e.g. courting, mating, parental behavior) are
categories of sexually dimorphic behaviors, i.e. behaviors that differ
in males and females
Hormones present both before and after birth play a role in the
development and control of sexually dimorphic behaviors
Sexual development
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Production of gametes and
fertilization
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The production of gametes (ova and
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
The last pair of chromosomes
determines the sex of the offspring (sex
chromosomes; XX female, XY male)
Sexual development
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Development of the sex organs
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Only sex hormones are responsible for our sexual dimorphism
3 general categories of sex organs:
• Gonads –
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testes or ovaries
produce ova or sperm and secrete hormones
the factor that controls their development into either testes or ovaries is
a single gene on the Y chromosome called Sry, which produces a
protein called testis-determining factor; thus if not present, ovaries
develop
Sexual development
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Development of the sex organs (con’t)
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Gonads (con’t)
Internal sex organs
• Müllerian system – the embryonic precursors of the female internal sex
organs
• Wolffian system – the embryonic precursors of the male internal sex organs
• Gender of fetus determined by presence or absence of hormones secreted
by the testes:
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Anti-Müllerian hormone prevents female system from developing (defeminizing
effect)
Androgens – stimulates development of Wolffian system (masculinizing effect)
• Once gonads have developed, a series of events directed by hormones
occur that determine the gender of the offspring
• Two effects:
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Organizational effects – the effect of a hormone on tissue differentiation and
development; occurs during prenatal 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
Sexual development
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Development of the sex organs (con’t)
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Internal sex organs (con’t)
• Two types of androgens
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Testosterone – secreted from testes; principle androgen found in males
Dihydrotestosterone – an androgen, produced from testosterone through the
action of the enzyme 5α reductase
• The Wolffian system thus contains androgen receptors in order for the
androgens to take action
• 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
• 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
• Tuner’s syndrome – the presence of only one sex chromosome (X);
characterized by lack of ovaries but otherwise normal female sex organs and
genitalia
Androgen Insensitivity
Syndrome
Sexual development
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Development of the sex organs (con’t)
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External genitalia
• Visible sex organs, including penis and scrotum in males and labia, clitoris, and the
outer part of the vagina in females
• In the presence of dihyrdotestosterone the external genitalia will become male
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Sexual maturation
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Primary sex characteristics include gonads, internal sex organs, and external
genitalia; all present at birth
Secondary sex characteristics (e.g. enlarged hips and breasts, facial hair and
deep voice) appear at puberty
The onset of puberty occurs when the cells in the hypothalamus secrete
gonadotropin-releasing hormone (GnRH), which stimulates the production of 2
gonadotropic hormones by the anterior pituitary gland
• Follicle-stimulating hormone (FSH) – causes development of an ovarian follicle and the
maturation of an ovum
• Luteinizing hormone – causes ovulation and development of the ovarian follicle into a
corpus luteum
• Both hormones are also produced in males, to stimulate the testes to produce sperm
and to secrete testosterone
Sexual development
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Sexual maturation (con’t)
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In response to the gonadotropins, the gonads secrete steroid sex
hormones
• e.g. the ovaries produce estradiol one of a class of hormones known as
estrogens
Hormonal control of sexual
behavior
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Female reproductive cycles
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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
Other species have estrous cycles
Cycle begins with secretion of gonadotropins by the anterior pituitary,
which stimulate the growth of ovarian follicles, and ovulation (release of
ovum); the ruptured ovarian follicle becomes a corpus luteum and
produces estrodiol and progesterone (promotes pregnancy)
Hormonal control of sexual
behavior
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Sexual behavior of lab animals
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Males
• Male sexual behavior is quite varied, however, the essential features
(intromission, pelvic thrusting, and ejaculation) are characteristic of all male
mammals
• Rat sexual behaviors studied most
• After ejaculation, males enter a refractory period, during which they cannot
ejaculate again
• In behavioral studies, observe Coolidge effect – the restorative effect of
introducing a new female sex partner to a male that has apparently become
“exhausted” by sexual activity
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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
Hormonal control of sexual
behavior
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Masculinization and Defeminization
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If a rodent brain is exposed to androgens during development, two
phenomenon occur:
• Behavioral defeminization – the organizational effect of androgens that
prevents an animal from displaying female sexual behaviors in adulthood
• Behavioral masculinization – enables animals to engage in male sexual
behavior
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Effects of pheromones
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A chemical released by one animal that affects the behavior or
physiology of another animal; usually smelled or tasted
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
Detection of pheromones is mediated by the vomeronasal organ (VNO),
which projects to the olfactory accessory bulb
Olfactory accessory bulb then projects to the medial nucleus of the
amygdala, which then projects to the hypothalamus
Hormonal control of sexual
behavior
Human sexual behavior
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Activational effects of sex hormones in women
• In higher primates (including humans), ovarian hormones are not necessary
to have intercourse, as with other mammals
• However, ovarian hormones can have an influence on their sexual interest
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In men
• Levels of testosterone not only affect sexual activity, but is also affected by it
Sexual orientation
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Exclusive homosexuality appears to occur only in humans
A likely biological cause of homosexuality is a subtle difference in
brain structure cased by differences in the amount of prenatal
exposure to androgens
However, these are speculations and have not been supported by
human data
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
The sexual dimorphism of the brain (e.g. different sizes, more
sharing of functions in female brains) may be a result of differential
exposure to androgens during early postnatal life
Many studies have shown possible relations between sizes of
certain brain structures and homosexuality; however, no real
conclusive data
Neural control of sexual behavior
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Males
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Spinal mechanisms
• Some sexual response are controlled by neural circuits in the spinal
cord (e.g. erection and ejaculation)
• e.g. spinal nucleus of the bulbocavernosus in the male rat
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Brain mechanisms
• Both excitatory and inhibitory controls over the spinal mechanisms
• e.g. medial preoptic area – most critical for sexual behavior in males
• Sexually dimorphic nucleus – larger in males than in females; plays
a role in male sexual behavior
• Periaqueductal gray matter – region of the midbrain that surrounds
the cerebral aqueduct; plays an essential role in various species
typical behaviors
Neural control of sexual behavior
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Females
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Ventromedial nucleus of
the hypothalamus – plays
an essential role in female
sexual behaviors; injection
of ovarian hormones into
the nucleus will elicit
sexual behavior even in
females without ovaries
Parental behavior
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Maternal behavior of rodents
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Hormonal control of maternal behavior
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During gestation (pregnancy) female rats and mice build nests
At the time of parturition (giving birth), the female will prepare herself for labor,
and afterwards nurse the pups
Mother will lick the pups’ anogenital region in order to stimulate urination and
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
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The medial preoptic area plays a role in maternal behavior (lesions elicit
indifference towards pups in new mothers)
In some species, the male will care for the offspring (e.g in monogamous prairie
vols)