Sex Differences in Behavior: Sex Determination & Differentiation PDF

CHAPTER 3 Sex Differences in Behavior: Sex Determination and Differentiation What is Sex? Sex is a biological concept referring to the classification of organisms based on reproductive structures and functions. Determined at multiple levels: chromosomal, gonadal, hormonal, morphologi...

CHAPTER 3 Sex Differences in Behavior: Sex Determination and Differentiation What is Sex? Sex is a biological concept referring to the classification of organisms based on reproductive structures and functions. Determined at multiple levels: chromosomal, gonadal, hormonal, morphological, and behavioral. Often seen as a binary (male and female), but variations exist. Example: Individuals with Turner syndrome (XO) or Klinefelter syndrome (XXY) challenge the strict binary model.  Sex: Biological state with Typically binary (male distinct facets and female) (genetic, Intersex states (DSDs) chromosomal, gonadal, occur when facets Understand hormonal, conflict phenotypic) ing Sex and Gender Influenced by biological, Gender: Subjective, personal, and societal self-identified factors social Can vary widely (single, construct multiple, fluid)  Chromosomal Variations Chromosomal Explanation Physical Characteristics Psychological Characteristics Variation Turner Syndrome Missing one X Short stature, webbed neck, broad chest, Normal intelligence, but may have (45,X) chromosome. infertility. difficulties with spatial reasoning and math. Klinefelter One extra X chromosome Tall stature, reduced muscle mass, breast Learning difficulties, particularly with Syndrome in males. development, small testes, infertility. language and executive functioning. (47,XXY) Triple X Normal intelligence, but may have learning One extra X chromosome Tall stature, normal sexual development, Syndrome disabilities and delayed speech in females. sometimes mild developmental delays. (47,XXX) development. XYY Syndrome One extra Y chromosome Tall stature, acne, normal sexual Normal intelligence, but may have learning (47,XYY) in males. development. difficulties and behavioral issues. Two extra sex XXYY Syndrome Tall stature, dental issues, developmental Learning disabilities, ADHD, and autism chromosomes (one X and (48,XXYY) delays, infertility. spectrum disorders are more common. one Y). XXXX Syndrome Two extra X chromosomes Developmental delays, facial dysmorphisms, Intellectual disability, speech and language (48,XXXX) in females. short stature. delays. XXXXY Syndrome Three extra X Severe developmental delays, skeletal Significant intellectual disability, behavioral (49,XXXXY) chromosomes in males. abnormalities, infertility. issues. XXXXX Syndrome Three extra X Severe developmental delays, facial Significant intellectual disability, speech (49,XXXXX) chromosomes in females. dysmorphisms, short stature. and language delays. Ultimate Causes of Sex Differences Ultimate cause The evolutionary, adaptive reason for a behavior, focusing on why it evolved. Includes survival and reproductive advantages. Sexual Reproduction Most animal species engage in sexual reproduction, but some reproduce asexually. Asexual reproduction: Offspring are genetically identical to the mother. Sexual reproduction: Genetic diversity through recombination of chromosomes. Evolutionary advantage: Increased adaptability to environmental changes. Asexual Reproduction Parthenogenesis: Asexual reproduction where females produce identical offspring. Disadvantages of asexual reproduction: Lack of genetic variation reduces adaptability. Increased vulnerability to pathogens specialized to a single genotype. Example: Asexual species may thrive in stable environments but struggle in changing conditions.  Asexual Reproduction Asexual Reproduction Animal Group Example Species Notes Method Cnidarians Hydra (Hydra spp.) Budding Offspring grows directly from parent’s body. Fragmentation, Echinoderms Starfish (Asteroidea) Can regrow body parts; some can regenerate into new individuals. Regeneration Platyhelminth Fragmentation, Planarians (Platyhelminthes) Can regenerate entire organisms from body parts. es Regeneration Cnidarians Jellyfish (Aurelia aurita) Strobilation Polyp stage divides into multiple young medusae. Rotifers Bdelloid rotifers (Bdelloidea) Parthenogenesis Entire group reproduces asexually without males. Insects Aphids (Aphidoidea) Parthenogenesis Females clone themselves in favorable conditions. Insects Stick insects (Phasmatodea) Parthenogenesis Some species reproduce without fertilization. Whiptail lizards (Cnemidophorus Reptiles Parthenogenesis All-female species reproduce without males. spp.) Komodo dragons (Varanus Reptiles Parthenogenesis Can lay viable eggs without mating. komodoensis) Bonnethead shark (Sphyrna Fish Parthenogenesis Some female sharks reproduce asexually in captivity. tiburo) Certain salamanders (Ambystoma Sperm needed to trigger egg development but does not Amphibians Gynogenesis spp.) contribute genetically.  Sexual dimorphism: Differences in appearance and behavior between sexes. Ultimate Evolutionary analysis compares sexually monomorphic species (similar- looking males and females) with sexually dimorphic species (distinct male and female traits). Causes of Monogamy and Polygamy Sex Monogamy: One mating partner, reduced sexual dimorphism. Polygamy: Multiple partners, stronger selection for exaggerated male traits. Differences Example: Prairie voles (monogamous, little sexual dimorphism) vs. elk (polygynous, strong sexual dimorphism with males having antlers). Ultimate Causes of Sex Differences Mating Systems and Sexual Selection Sexual selection: A subcategory of natural selection. Male competition and female choice: Males develop traits that enhance reproductive success. Example: Male elk develop antlers to fight for access to mates, while females remain smaller and antlerless.  Proximate Causes of Sex Differences Proximate cause The immediate, Introduction to mechanistic cause of a Proximate Causes behavior, focusing on how it occurs. Includes genetic, neural, hormonal, and environmental influences.  Proximate Causes of Sex Differences The Organizational/Activational Hypothesis Proposed ~50 years ago to explain hormonal influence on behavior. Organizational effects: Hormones act early in development to shape neural circuits. Activational effects: Hormones later in life trigger behaviors through previously organized circuits. Organizational Effects of Hormones Occur during critical periods of early development. Establish long-term structural and functional changes in the brain. Examples: Testosterone exposure in utero influences brain differentiation in mammals. Activational Effects of Hormones Occur later in life, often at puberty or adulthood. Temporary and reversible effects triggered by circulating hormones. Examples: Testosterone promoting aggressive or reproductive behaviors in males.  Proximate Causes of Sex Differences Disorders of Sex Behavioral Development Consequences of (DSD) DSD Congenital and acquired Examples: Androgen Clinical and experimental syndromes affecting Insensitivity studies provide insights sexual differentiation. into hormonal influences. May result from genetic Syndrome (AIS), DSD conditions help mutations, endocrine Congenital Adrenal researchers understand disruptions, or Hyperplasia (CAH). gender identity, sex roles, environmental influences. and sexual orientation. Animal models are used to simulate human conditions and study behavioral outcomes. Mammalian Sexual Differentiation Chromosomal Sex Defined by the sex chromosomes an individual inherits at fertilization. Homogametic sex: Possesses two similar sex chromosomes (XX in mammals, ZZ in birds). Heterogametic sex: Possesses two different sex chromosomes (XY in mammals, ZW in birds). Example: In mammals, XX individuals develop as female, and XY individuals develop as male. Gonadal Sex Determined by the presence of either ovaries or testes. The SRY gene on the Y chromosome initiates testicular development. In the absence of SRY, gonads develop into ovaries. Example: Androgen Insensitivity Syndrome (AIS) individuals have XY chromosomes but develop as female due to non-functional androgen receptors. Gonadal Sex  Typical development of the accessory sex organs occurs along two dimensions. Females must become feminized (Müllerian duct development) as well as demasculinized (Wolffian duct regression). Males must become masculinized (Wolffian duct development) as well as defeminized (Müllerian duct regression). What is Sex? Gametic Sex Hormonal Sex Refers to the type of Determined by the ratio gametes produced: of circulating steroid Ovaries produce large, hormones. immobile, resource-rich Females generally have eggs (ova). high estrogen-to- Testes produce androgen ratios; males numerous small, mobile have the opposite sperm. pattern. Essential for sexual Hormones influence reproduction and genetic secondary sex diversity. characteristics and Example: In most behavior. mammals, females Example: In some species produce fewer, high- (e.g., spotted hyenas), investment ova, while females have high males produce millions of androgen levels, leading sperm daily. to masculinized genitalia and behavior.  Differences In Circulating Testosterone And Estrogen Levels Between Men And Women Hormone Men Women Testosterone Production and Levels: Production and Levels: - 15-70 ng/dL, produced in ovaries, adrenal glands, and peripheral - 300-1,000 ng/dL, primarily produced in testes. tissues. Physical Differences: Physical Differences: - Greater muscle mass and strength. - Less muscle mass. - Higher bone density. - Higher body fat percentage, subcutaneous fat distribution. - Lower body fat percentage. Psychological Differences: Psychological Differences: - Higher libido. - Libido and sexual function. - Increased aggression. - Mood swings. - Mood regulation. - Cognitive functions. Estrogen Production and Levels: Production and Levels: - 10-40 pg/mL, produced through conversion of - 15-350 pg/mL (varies with menstrual cycle), produced in ovaries. testosterone. Physical Differences: Physical Differences: - Contributes to sperm maturation. - Regulates menstrual cycle. - Bone health. - Maintains bone density. - Cardiovascular function. - Cardiovascular protection. Psychological Differences: Psychological Differences: - Affects mood. - Influences mood. - Cognitive functions. - Cognitive functions. - Libido. - Sexual health. Hormonal Differences and Their Effects on Men and Women Aspect Men Women Libido Hormonal Influence - Primarily influenced by testosterone. - Influenced by both estrogen and testosterone. - Higher levels of testosterone associated with - Higher levels of estrogen and testosterone associated increased sexual desire. with increased libido. - Libido decreases with lower levels of these hormones (e.g., menopause). Psychological and - More context-dependent, influenced by emotional - More visually stimulated and spontaneous. Social Factors intimacy and relationship quality. - Impacted by stress, anxiety, and mental health. - Shaped by social and cultural factors. Age-Related - Peaks in late teens and early twenties, declines - Peaks in late twenties to early forties, fluctuates with life Changes with age. events. Cognitive Functions Brain Structure and - Higher density of neurons in certain areas, better - Larger brain volumes overall. Function hemispheric connectivity. - Excel in spatial processing, sensorimotor speed, - Excel in verbal abilities, attention, word memory, social single-task performance. cognition, multitasking. Memory and - Better at verbal memory, fine-motor coordination, long- - Better at spatial memory and navigation. Learning term memory retrieval. - Focus on single tasks without distraction. - Oriented towards faces and social interactions. Emotional - More compartmentalized approach to emotional - More integrated approach to emotional processing. Processing processing. - Influences cognitive functions and decision- - Enhances social cognition and empathy. What is Sex? Morphological Sex Refers to physical differences between males and females: Size differences (sexual dimorphism). External genitalia. Secondary sexual characteristics (e.g., coloration, horns, antlers). Example: In peacocks, males have large, colorful plumage to attract females, while peahens are drab-colored for camouflage. Behavioral Sex Refers to sex-typical behaviors seen in different species. Examples: Parental care: Female mammals typically invest more in offspring (e.g., lactation in primates). Territorial defense: Male birds often sing and defend territories, while females do not. Mating behaviors: Male bowerbirds build elaborate structures to attract mates. Not all behaviors are strictly sex-dependent; social and environmental factors play a role. Chapter 3 Part 2 Mammalian Sexual Differentiation The Effects of Hormones on Sexually Dimorphic Behaviors The Effects of Hormones on Sexually Dimorphic Behaviors Hormonal Influences Hormonal influences lead to sexually dimorphic behaviors, which are distinct behaviors exhibited by males and females. Animal models help explain the neural, hormonal, and molecular mechanisms underlying these behaviors. The goal is to expand these findings to explain human behavioral differences. The Role of Hormones in Sexually Dimorphic Behaviors Hormones, particularly gonadal steroids, influence the development of sex-specific behaviors. These behaviors emerge from the interaction between genetics, hormones, and environmental factors.  Environmental Behavior Boys/Men Girls/Women Genetic Factors Epigenetic Influences Key Hormones Influence Prenatal stress can alter SRY gene on the Y Testosterone (prenatal & Parents encourage rough Rough-and- More frequent, higher Less frequent, lower testosterone receptor chromosome initiates postnatal) increases activity play in boys, discourage it Tumble Play intensity physical play intensity physical play expression, affecting play testosterone production levels in girls behavior Sex-linked genes Maternal diet and Prefer mechanical & Prefer nurturing toys Testosterone in utero Gendered marketing and influence neural wiring endocrine disruptors can Toy Preferences moving toys (e.g., cars, (e.g., dolls, stuffed correlates with preference for social reinforcement related to object alter androgen receptor balls) animals) movement-based objects influence toy choices preference sensitivity Early childhood Slightly lower verbal Higher verbal fluency, FOXP2 gene (linked to Estrogen enhances verbal Girls receive more experiences can modify Verbal Abilities fluency, but stronger better language language development) is fluency and corpus callosum encouragement in FOXP2 expression via spatial reasoning processing more active in females connectivity reading/writing activities epigenetic changes Weaker in mental Environmental Stronger in mental X-linked genes influence Testosterone supports Boys engage more in rotation, but better at enrichment (e.g., puzzles, Spatial Abilities rotation and hippocampal structure, spatial reasoning, estrogen spatial problem-solving landmark-based video games) modifies navigation tasks affecting spatial skills may slightly hinder it activities navigation hippocampal plasticity Childhood trauma can Boys are encouraged to More physical More relational Testosterone increases Aggression & MAOA ("warrior gene") increase MAOA be assertive, girls aggression, higher risk- aggression, lower risk- competitiveness and Risk-Taking affects aggression regulation methylation, reducing socialized to be more taking taking impulsivity aggression inhibition cautious Higher empathy, Early caregiving affects Lower baseline OXTR (oxytocin receptor) Oxytocin & estrogen Girls are encouraged to Empathy & stronger ability to read oxytocin receptor empathy, focus on gene is more active in enhance emotional express emotions, boys Social Sensitivity facial expressions & methylation, influencing systemizing women processing discouraged emotions social bonding CRHR1 (cortisol receptor) Chronic stress can alter Cortisol & testosterone Women seek social More likely to respond More likely to use tend- gene affects stress cortisol receptor drive aggressive responses; Stress Response support more than men with fight-or-flight and-befriend strategies responses differently in men sensitivity via DNA oxytocin promotes social under stress vs. women methylation bonding Less immediate Fatherhood reduces Parental caregiving response, but Stronger immediate PRL (prolactin receptor) testosterone and Prolactin & oxytocin Social expectations shape Behavior involvement increases caregiving response gene more active in women increases oxytocin via enhance maternal bonding caregiving roles over time epigenetic changes More frequent sexual Prenatal stress can alter More selective, greater AR (androgen receptor) Testosterone drives libido in Cultural norms influence Sexual Behavior thoughts and desire, androgen receptor emphasis on emotional gene affects testosterone both sexes, estrogen & expectations of sexual & Desire higher interest in visual methylation, affecting connection sensitivity oxytocin enhance bonding behavior stimuli adult sex drive The Effects of Hormones on Sexually Dimorphic Behaviors Example of Sexually Dimorphic Mating Behaviors Female rodents exhibit lordosis posture (arched back) when mating, while males mount females. These behaviors are influenced by gonadal hormones. Lordosis is a feminine behavior, while mounting is masculine. Hormonal Control of Mating Behaviors Castration of males stops mounting behavior; testosterone replacement restores it. Injection of testosterone in adult females does not induce male-typical mounting behavior. Females lose the potential for male-typical behaviors during development (demasculinization). Males lose the potential to display female-typical behaviors (defeminization) during development. The Organizational/Activational Hypothesis suggests that early exposure to hormones determines the ability to express masculine or feminine behaviors. The Organizational/Activational Hypothesis The Organizational/Activational Hypothesis Proposed by William C. Young in 1959. The Organizational/Activational Hypothesis explains how hormones influence sexually dimorphic behaviors in mammals, including humans. It proposes that hormones act at two distinct periods in development: 1. Organizational Effects (Early Development) 1. Occur prenatally or perinatally (before or shortly after birth). 2. Hormones (especially testosterone) permanently shape the brain and body. 3. These changes establish sex-specific neural circuits that later respond to hormones in adulthood. 2. Activational Effects (Adulthood) 1. Occur later in life when hormones (e.g., testosterone, estrogen) fluctuate. 2. These hormones temporarily activate behaviors by acting on pre- existing neural structures. 3. Examples include puberty, mating behaviors, and parental care. In summary: Organizational effects = Permanent structural changes (occur early). Activational effects = Temporary influences on behavior (occur later). Young’s Experiment and the Organizational/Activational Hypothesis Background and Rationale Professor Young and his team wanted to understand how hormones influence sexually dimorphic behaviors. They hypothesized that hormonal exposure during early development organizes the brain, shaping future sexual behavior. They distinguished between: Organizational effects (permanent changes in neural structures due to early hormone exposure). Activational effects (temporary influences of hormones in adulthood). Young’s Experiment and the Organizational/Activational Hypothesis Experimental Design Phase 1: Prenatal Hormonal Manipulation Testosterone propionate was administered to pregnant guinea pigs during most of their 69-day gestation. Two groups of female offspring were created based on hormone exposure: "Hermaphrodite" females → Given high doses of testosterone → Developed male-like external genitalia. "Unmodified" females → Given low doses of testosterone → Appeared externally normal. Control male and female guinea pigs were also included for comparison. Phase 2: Hormone Administration in Adulthood Once mature, all animals were gonadectomized to eliminate natural hormone influences. Two hormone treatments were administered: Estrogen + progesterone → To test for female-typical sexual behavior (lordosis). Testosterone → To test for male-typical sexual behavior (mounting).  Theoretical Implications and the Organizational/Activational Hypothesis 1. Clear distinction between organization and activation: 1. Prenatal hormones permanently shape (organize) the nervous system. 2. Adult hormones activate pre-existing circuits rather than creating new behaviors. 2. Critical periods for hormonal effects: Young’s Experiment 1. There is a sensitive window during perinatal and the development when hormones can irreversibly shape the brain. Organizational/Activa 3. Neural organization mirrors genital differentiation: tional Hypothesis 1. Just as testosterone masculinizes external genitalia, it also masculinizes the brain during development. 4. Subtle changes in neural function, not just structure: 1. Even unmodified females (without visible genital changes) exhibited masculinized behavior, showing that early hormones alter brain function. 5. Implications for primates and humans: 1. These findings suggest that prenatal hormone exposure could influence human sexual behavior and brain organization. The Organizational/Activational Hypothesis  The experiment involved four key manipulations: Removing gonads before day 10 (prevents natural hormone exposure). Injecting testosterone before day 10 (to test organizational effects). Removing gonads in adulthood (eliminates endogenous adult hormone effects). Injecting testosterone in adulthood (to test activational effects). Findings from the Figure: Experiment Control females (no testosterone exposure early or in adulthood) → Show no male-typical behavior. Summary Females injected with testosterone before day 10 → Develop male-like organization but still require testosterone in adulthood to show male-typical behavior. Control males (natural testosterone exposure) → Show male- typical behavior even if gonads are removed in adulthood. Males castrated early (before day 10) and not given testosterone later → Do not show male-typical behavior, indicating the importance of early organizational effects. Males castrated early but given testosterone in adulthood → Still do not show male-typical behavior because they were not exposed to testosterone during the critical period.  Testosterone must be present early in development (before day 10) to organize male-typical behavior. Take aways Adult testosterone exposure alone cannot induce male-typical behavior if the brain from the was not organized by early testosterone. experiment Once the brain is organized in a male- typical way, adult testosterone is required to activate male behaviors. Early hormone exposure has permanent effects, while adult hormones are transient activators.