Reproductive behavior Chapter 10 Reproductive behavior

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

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)

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

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

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 Congenital – present at birth; due to heredity

Androgen Insensitivity Syndrome

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

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

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)

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

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

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

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

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

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

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)