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Chapter 10. Reproductive behavior. Reproductive behaviors. Constitute the most important category of social behaviors, because without them, most species would not survive

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Chapter 10

Reproductive behavior

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Reproductive behaviors

  • 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

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Sexual development

  • Production of gametes and fertilization

    • 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)

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Sexual development

  • 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

        • 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

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Sexual development

  • Development of the sex organs (con’t)

    • 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:

        • 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:

        • 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

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Sexual development

  • 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

      • 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

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Sexual development

  • Development of the sex organs (con’t)

    • 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

  • Sexual maturation

    • 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

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Sexual development

  • Sexual maturation (con’t)

    • 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

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Hormonal control of sexual 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

    • 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)

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Hormonal control of sexual behavior

  • Sexual behavior of lab animals

    • 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

    • 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

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Hormonal control of sexual behavior

  • Masculinization and Defeminization

    • 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

  • Effects of pheromones

    • 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

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Hormonal control of sexual behavior

Human sexual behavior

  • 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

  • In men

    • Levels of testosterone not only affect sexual activity, but is also affected by it

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Sexual orientation

  • 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

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Neural control of sexual behavior

  • Males

    • 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

    • 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

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Neural control of sexual behavior

  • Females

    • 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

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Parental behavior

  • Maternal behavior of rodents

    • 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

  • Hormonal control of maternal behavior

    • 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 indifference towards pups in new mothers)

    • In some species, the male will care for the offspring (e.g in monogamous prairie vols)