Lecture 18 - Sex and Behavior II PDF

12 Many Vertebrates Depend on Their Parents for Survival Now, let’s talk about post-copulatory behavior involving parenting 12 Many Vertebrates Depend on Their Parents for Survival Many vertebrates, including mammals, require significant parenting for survival...

12 Many Vertebrates Depend on Their Parents for Survival Now, let’s talk about post-copulatory behavior involving parenting 12 Many Vertebrates Depend on Their Parents for Survival Many vertebrates, including mammals, require significant parenting for survival Precocial animals are born with well-developed sensory and motor systems – e.g. horses Altricial animals are less developed at birth – e.g. humans Both males and females may be involved in care of the young 12 Many Vertebrates Depend on Their Parents for Survival Rat mothers show four behaviors: nest building, crouching over pups, retrieving pups, and nursing During pregnancy, exposure to hormones will prepare her brain to display these maternal behaviors immediately after giving birth Virgin females do not normally show these behaviors toward pups Figure 12.11 Parental Behavior in Rats 12 Many Vertebrates Depend on Their Parents for Survival A parabiotic preparation shows the effects of maternal hormones (next slide shows preparation) A nonpregnant female exposed to the circulating hormones of a pregnant rat will display the same maternal behaviors. Figure 12.12 Parabiotic Exchange 12 Many Vertebrates Depend on Their Parents for Survival A nonpregnant female exposed to the circulating hormones of a pregnant rat will display the same maternal behaviors. Which hormone is responsible for this? No single one – combination of hormones (estrogens, progesterone, prolactin) 12 Many Vertebrates Depend on Their Parents for Survival Brain structures responsible for maternal behavior? Similar to sexual behavior – see a role for mPOA and the periaqueductal gray in different maternal behaviors 12 The Sex of an Individual Is Determined Early in Life Sexual Differentiation Sexual differentiation is the process by which individuals develop bodies and behaviors that are male or female We will first talk about “typical” development but there are many ways in which such sexual differentiation may follow a different path “Differences in Sexual Development” - DSD Begins in utero and continues into adulthood In some species, the sex is determined by the temperature at which the eggs incubate 12 The Sex of an Individual Is Determined Early in Life Sex determination is the early developmental event that decides if a fetus will be male or female. In mammals, this begins with the “sex” chromosomes – XX vs. XY The sperm carries either an X or a Y – sperm is the sex determining cell Why? Because the ovum always contains an X chromosome 12 The Sex of an Individual Is Determined Early in Life Every individual starts with indifferent gonads The early indifferent gonads begin to change into ovaries or testes in the first month. SRY gene–sex-determining region on the Y chromosome– is responsible for the development of testes – leads to domino effect in development of sex Without an SRY gene or the Y chromosome, an ovary forms. 12 The Sex of an Individual Is Determined Early in Life Hormones secreted by gonads, mainly from the testes, direct sexual differentiation  other tissue receiving testicular hormones become masculinized  ovaries do not secrete much hormone early on Result: Tissue not receiving testicular hormones becomes feminized Differential exposure to sex steroids  differences in gene expression throughout body 12 The Sex of an Individual Is Determined Early in Life Embryos have early tissues for both male and female structures. The wolffian ducts and the müllerian ducts connect the gonads to the body wall. Figure 12.13 Sexual Differentiation in Humans (Part 1) 12 The Sex of an Individual Is Determined Early in Life In females, the müllerian ducts develop into the fallopian tubes, the uterus, and vagina–only part of the wolffian ducts remains. In males, the wolffian ducts develop into the epididymis, vas deferens, and seminal vesicles, and the müllerian ducts shrink. Figure 12.13 Sexual Differentiation in Humans (Part 2) Figure 12.13 Sexual Differentiation in Humans (Part 3) 12 The Sex of an Individual Is Determined Early in Life How does this happen?? Two hormones from the testes make the system masculine: Testosterone promotes the development of the wolffian system. Anti-müllerian hormone (AMH) induces the regression of the müllerian system. 12 The Sex of an Individual Is Determined Early in Life Testosterone also masculinizes other structures: Other tissues form the prostate gland, scrotum, and penis. 5α-reductase (found in genital skin) is an enzyme that converts testosterone into the more powerful dihydrotestosterone (DHT), necessary to form genitalia Without local DHT production, genitalia become only partially masculinized – no testosterone whatsoever  genitals become labia and clitoris 12 The Sex of an Individual Is Determined Early in Life Describes typical process – many different kinds of disruptions to this process In Turner’s syndrome, a person only has one sex chromosome–a single X. The individual develops as a female–without the SRY gene, no masculinizing effects take place – though underdeveloped ovaries 12 The Sex of an Individual Is Determined Early in Life Sometimes, XX females are exposed to androgens in utero – may be slightly more masculinized as a result e.g. female rat sandwiched between two male siblings – anogenital distances will be greater (more like males) than that of female between two female siblings Figure 12.14 Sexual Differentiation and Gender Identity (Part 1) There are different “levels” where we can talk about the sex of the individual At each level, there are “differences” that can occur leading to mismatches 12 The Sex of an Individual Is Determined Early in Life So, how does gender identity develop? Figure 12.14 Sexual Differentiation and Gender Identity (Part 3) 12 The Sex of an Individual is Determined Early in Life Lots of ways in which “differences in sexual development” can occur So far, we’ve mentioned 5-alpha reductase deficiency and Turner syndrome Let’s look at some more 12 The Sex of an Individual is Determined Early in Life Congenital adrenal hyperplasia (CAH) is the result of female exposure to androgens before birth What happens? Adrenal glands – fail to produce sufficient corticosteroids – produce androgens instead  Newborn has an intersex appearance No testes are present and ovaries are normal, despite appearance of external genitalia. Figure 12.15 An Intersex Phenotype 12 The Sex of an Individual is Determined Early in Life No testes – makes it possible to determine that the newborn is an XX female with CAH Interestingly, CAH females  more likely to be described as “tomboys” and have enhanced spatial abilities on tests that usually favor males Also, CAH females – more likely to report homosexual orientation Is this due to masculinizing effects of testosterone during prenatal period? 12 The Sex of an Individual Is Determined Early in Life Cloacal exstrophy refers to genetic males born with testes but without penises How do we handle such cases? Best course: Probably letting child decide sex at puberty In past, sex reassignment surgery (to female) had been decided at birth – 8 out of 14 cases, though, later declared themselves to be boys – 5 out of 6 comfortable being female 12 The Sex of an Individual Is Determined Early in Life Androgen insensitivity syndrome (AIS) occurs in XY people whose androgen receptors do not respond to testosterone. Testes – produce testosterone and AMH – but no androgen receptors prevents development of male feature Testes remain internal, external genitals develop into female genitals, and they develop mainly as women Are infertile, no uterus or ovaries, no menstruation  usually when AIS is discovered 12 How Should We Define Gender–by Genes, Gonads, Genitals, or the Brain? So, how do we define sex and gender? Most people  there is a consistent pattern among chromosomes, gonads, external genitalia, internal structures, and self-identity Each is either masculine or feminine But there are MANY, MANY ways in which this does not break down into a simple dichotomy Behavior is more difficult to define as masculine or feminine – again, differences between sexes are typically much smaller than differences within a sex and can be socially/culturally driven 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior Sexual differentiation of the brain The same hormones that masculinize the developing genitalia also masculinize the brain E.g. female guinea pigs exposed to testosterone in utero will not show lordosis as adults This organizational effect permanently alters behavior, in contrast to a temporary activational effect. 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior Steroids have an organizational effect only when present during a sensitive period in early development Depending on the species and the behavior, this period may be before birth or just afterwards, in the neonatal period Puberty – second sensitive period for organizational effects on body 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior The organizational hypothesis explains sexual differentiation: A single steroid signal–androgen–masculinizes the body, the brain, and behavior. In rats, many behaviors can be explained: if exposed to androgens early in life, they behave as males; if not, then as females. 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior However, after exposing female newborn rats to estrogen– specifically estradiol, a female hormone–they failed to show lordosis as adults So, wait, what? Estradiol masculinized the female pups?? If that’s true, then why were female fetuses not masculinized by circulating maternal estrogen? 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior It was explained by the process of aromatization–the enzyme aromatase converts testosterone to estradiol The aromatization hypothesis says that testosterone enters the brain and is converted there to estrogens– these estrogens are what masculinize the developing rodent brain So, is estrogen a “female” hormone or a “male” hormone? 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior So, if estradiol masculinizes the brain, why don’t female rats have masculinized brains? The estrogen in developing females does not enter the brain It is kept in the blood stream by binding with α- fetoprotein, preventing aromatization and masculinization α-fetoprotein does not bind testosterone Figure 12.17 The Aromatization Hypothesis 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior So, why does giving estradiol to female newborn rats prevent lordosis as adults? The estradiol injection overwhelms the amount of α- fetoprotein in blood  estradiol reaches brain and masculinizes In primates, however, aromatization does not appear to be important for masculinization of the brain – it’s unknown how this occurs 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior Sexual dimorphism Sexual dimorphism refers to sex differences in behavior or in structure. In birds, the syrinx is a special muscle organ that controls song. It is five times larger in males (complex songs) than in females (simple songs), as are brain structures involved in song production 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior The Higher Vocal Center (HVC) and the Robustus Archistriatum (RA) – both involved in singing Expose hatchling female to testosterone or estradiol – makes HVC and RA larger 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior In rats, a nucleus in the pre-optic area (POA) is larger in males than in females. The nucleus is known as the sexually dimorphic nucleus of the POA (SDN-POA)–lesions in this area disrupt ovulatory and copulatory behaviors Testicular androgens during development – make SDN- POA larger Figure 12.18 A Sex Difference in the Hypothalamus Figure 12.19 Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA) 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior In rats, the spinal nucleus of the bulbocavernosus (SNB) is larger in males. Motoneurons in this nucleus innervate bulbocavernosus (BC) muscles that surround the base of the penis. Before birth there is an almost equal number of these neurons and their target muscles in both sexes. 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior Androgens act on the BC muscles in males to prevent their breakdown. The innervating SNB motoneurons also survive, perhaps due to ciliary neurotrophic factor (CNTF). If the CNTF receptor is knocked out in mice, SNB neurons die. 12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior In nonrodent mammals, the BC motoneurons are found in Onuf’s nucleus in the spinal cord. Most female mammals retain BC muscles into adulthood–the BC surrounds the vagina. The BC is larger in men than in women and men have more Onuf’s motoneurons. 12 Social Influences Affect Social Differentiation of the Nervous System Environmental and social influences can modulate masculinization. A mother rat cares for males differently than for females, helping to masculinize the developing nervous system – greater urogenital licking -- important for SNB cell survival Maternal attention has a different effect on females– greater licking will predispose them to being more or less attentive mothers. 12 Social Influences Affect Social Differentiation of the Nervous System What about humans? Socialization vs. biology? Small differences determined by biology may be amplified by socialization e.g. male vs. female babies  may be slightly different (on average) but this produces different responses by other people – these responses in turn may influence the sexual differentiation process, including gender 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? Behavioral differences between men and women  suggest there should be differences in the brain Many parts of the human brain are sexually dimorphic. The questions arises–are these dimorphisms caused by prenatal hormone exposure or by social influence? Table 12.2 Sex Differences in Human Cognition 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? The questions arises–are these dimorphisms caused by prenatal hormone exposure or by social influence? Reality is: Very difficult to know How can we separate the two? Even with CAH females – play more like boys, more likely to be lesbians BUT, their ambiguous genitals could have subtly influenced parental behavior toward them 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? A genetic mutation of the enzyme 5α-reductase results in incomplete masculinization of the genitalia -- normally, this enzyme converts testosterone to DHT, leading to masculinized external genitals Babies are usually raised as females. Figure 12.22 Guevedoces 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? At puberty, masculinization progresses– testosterone leads to development of a (small) penis generally speaking, body becomes masculinized Known in the Dominican Republic as guevedoces, or “eggs (testes) at 12 (years).” -- usually sexually interested in women and, generally speaking, behave as men 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? Two explanations: People raised as girls who later behave as boys may have brains masculinized by prenatal testosterone, that proceeds with development at puberty Or, early hormones have no effect–the Dominican culture recognizes that people can start out as girls and become boys, providing a social influence on gender development. Or both Note that this differs from Caster Semanya’s experience 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? What about sexual orientation? Two classes of possible influence on sexual orientation: Society’s instruction on how one should behave when grown up The biological factor–so strong in animal models– varying levels of fetal androgen direct future behavior 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? Brain structures may differ in homosexuals. Simon LeVay examined the INAH-3 nucleus of the POA. The nucleus was larger in men than in women, and larger in heterosexual men than in homosexual men. It is not clear if the size difference is a result or a cause of being homosexual Figure 12.23A Interstitial Nuclei of the Anterior Hypothalamus 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? Fetal androgen exposure markers include: otoacoustic emissions, finger length patterns, eyeblinks, and skeletal features. Measures of these indicate that lesbians as a group were exposed to slightly more androgens than heterosexual women were BUT considerable overlap between the two groups – can’t possibly fully explain how sexual orientation develops 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? Fetal androgen exposure is not as well understood in homosexual men. One influence on the probability of homosexual behavior is the number of brothers born before Not related to growing up with brothers – rather, appears to be dependent on the number of brothers born before from same mother – roughly 1 in 7 homosexual men in North America can be ascribed to this effect Figure 12.25 Fraternal Birth Order and Orientation 12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood? Research indicates that sexual orientation in both sexes is determined early in life Appears to be a significant genetic component – that is, sexual orientation is, in part, heritable monozygotic twins do not always have same sexual orientation