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Chapter 3: Sex Differences in Behavior: Sex Determination and Differentiation
Sexual differentiation – the developmental process leading to becoming a male or a female • Hormones and environmental experiences guide the development of physiological, morphological, and behavioral characteristics that are displayed later in life. • Sex Determination and Sex Differentiation • Chromosomal sex (fertilization) • Gonadal sex (ovaries or testes) • Gametic sex (which type of gametes are produced) • Hormonal sex (estrogen to androgen ratio) • Morphological sex (differences in body type) • Behavioral sex (male-typical vs female-typical) • Specifically in humans: • Gender identity (sex or gender a person feels themselves to be) • Sexual orientation (preference for a sex partner) • Legal sex (governmentally determined)
ULTIMATE CAUSES OF SEX DIFFERENCES Parthenogenesis – asexual reproduction in vertebrates… they will be female Polygamous – having multiple mating partners Sexual Selection – a subset of Natural Selection where selection pressures are based upon the competition for obtaining mating partners. PROXIMATE CAUSES OF SEX DIFFERENCES Organizational / activational hypothesis – behavioral sex differences result from a) differential exposure to hormones that act early in development to shape the brain and nervous system guiding these behaviors, and b) differential exposure to sex steroid hormones later in life (puberty through adulthood) activate the neural circuits shaped in (a)
3.4 Gonadal development of the human embryo All mammals start out being BIPOTENTIAL – they can develop into either the male or female sex. The germinal ridge will be able to develop into either primordial gonad once acted upon by proteins produced in the body.
Anlagen – a term relating to the dual primordial tissues relevant to the development of secondary sexual characteristics. Mullerian duct system – female accessory structures Wollfian duct system – male accessory structures
Female accessory sex structures are the default pattern in mammals. But, due to the anglen effect (presence) of both systems intitally, in rare circumstances, both systems can develop in a single individual… This leads to potential hermaphroditism – (now sometimes called intersexism) is a group of conditions where there is a discrepancy between the external genitals and the internal genitals (the testes and ovaries) or there are present both sexes in the same individual. Hermaphroditic Lamb
Female development is the default, but male development requires testosterone and MIH (Mullerian inhibitory hormone). The testosterone will activate growth of the Wolffian duct mophology. The MIH will cause regression of the Mullerian duct morphology. There are two pathways that occur simultaneously to guide sexual differentiation. These are referred to as: 1. The masculinization – demasculinization continuum – masculinization induces male traits while demasculinization is the removal of the potential for male traits. 2. The feminization – defeminization continuum - feminization induces female traits and defeminization is the removal of the potential for female traits.
Aromatase Inhibitors - a class of drugs used in the treatment of breast cancer and ovarian cancer in postmenopausal women. Some cancers require estrogen to grow. Aromatase is an enzyme that synthesizes estrogen. Aromatase inhibitors block the synthesis of estrogen. This lowers the estrogen level, and slows the growth of estrogen sensitive cancers. There are naturally occurring aromatase inhibitors in the body.
fertilization full maturity loss of bipotentialty puberty birth -20 -10 10 20 30 40 150 180 0 (birth) Rat Developmental Time Line
Study of the Pharmacological Agent ATD Receives Exposure to ATD (an aromatase enzyme inhibitor) Pregnant Female Rat Pups are born having been exposed to the ATD (the aromatase enzyme inhibitor) developmentally
3.15 Development of female copulatory behavior requires active hormonal secretion
3.18 Three types of male tree lizards Territorial males – Sedentary, non-territorial males – Nomadic non-territorial males -
3.20 Aggression in adult female geckos In this graph, you need to note: The high temperature range is a low-female population. The low temperature is an all-female population.
Freemartins - an infertile female mammal which has masculinized behavior and non-functioning ovaries. Genetically and externally the animal is female, but it is sterilized in the womb by hormones from a male twin, becoming an infertile partial intersex. Freemartinism is the normal outcome of mixed-sex twins in all cattle species that have been studied, and it also occurs occasionally in other mammals including sheep, goats and pigs. It is possible this may arise in dizygotic mixed sex twins in humans as well, although examples are limited
3.29 Rough-and-tumble play behavior is demasculinized in males by stress in utero