Sex differences

biological sex

• differences due to sex chromosomes (XX vs XY) & the cascade of developmental changes that follow in the vast majority of people 

atypical genotypes

• e.g. Turner syndrome (XO), Klinefelter syndrome (XXY)

• people with XX/XY genotype but atypical development, often due to mutations

  • e.g. androgen insensitivity syndrome, 5α-reductase deficiency

• people who have undergone medical procedures to alter sex/gender phenotype

  • e.g. hormone therapy, gender reassignment surgery

Sexual dimorphisms

• differences between sexes in anatomy, physiology, psychology and behaviour

• Many traits, particularly psychological & behavioural ones, there is more variations within each sex than between them

• Overlapping traits  

types of hormone action

• organisational effects

• activational effects

organisational effects

• effects on tissue differentiation and development

• Permanent

  • Undifferentiated cell (e.g 1 week old embryo)

    • Differentiated cells e.g brain cells in foetus 

activation effects

• effects that occur in fully developed organism

  • may depend on prior exposure to organisational effects of hormones

• transient

Sexual differentiation

 in mammals

• depends on organisational effects of hormones during development

types of sexual differentiation

• genetic

• gonadal

• phenotypic

genetic sexual differentiation

• DNA is present in nucleus of every cell & packaged in chromosomes

• sex is genetically determined

  • diploid cells contain 46 chromosomes (23 matched pairs) → 22 pairs, 1 pair of sex chromosomes

• determined randomly by the type of male’s gametes (sperm cell)

  • Gametes = haploid (one copy of each chromosome → 23)

    1. Parent’s diploid cells = XX or XY

    2. Gametes are a single X (females) or X or Y (males)

    3. gametes fuse to create diploid offspring resulting in gender

importance of the presence or absence of the Y chromosome in sperm cell

• SRY (Sex-determining Region Y) gene

  • Testis-determining factor (TDF)

1. At 6 weeks the baby is undifferentiated, not yet either a boy or a girl 

   • It has undifferentiated ‘primordial’ gonads (not yet ovaries or testes) 

2. During early embryonic development

   • If no SRY gene = no SRY protein = ovary formed (default)

   • If SRY gene = SRY protein = testes formed

3. Sets of a chain of events that (usually) result in biological male or female individuals 

Gonadal sex differentiation

• during early embryonic development 

  • Ovary does not produce significant amounts of steroid hormones

  • body will develop according to its own intrinsic programme

• OR

  • Testes produce various androgens including testosterone

    • Masculinises many other tissues including effects on the brain 

  • Anti-mullerian hormone (AMH)

    • Masculine internal genitalia  → mullerian regression factor

• Wolffian system which contains the developing testis or ovaries

  • For the developing testes, AMH increases = mullerian ducts regress

Testosterone

• Steroid hormone 

  • Fat soluble, passes readily through cell membrane

• Primary androgen

• Synthetised by leydig cells in testes

• Sertoli cells produce AMH

sex-determining cascade

1. Genotypic sex 

   1. XX = no SRY 

   2. XY = SRY gene -> resulting in SRY protein 

2. Gonadal sex

   1. XX = ovaries 

   2. XY = testes

3. Phenotypic sex 

   1. Ovaries = feminisation (default)

   2. Masculinising hormones (AMH + andorgens) = masculinisation

phenotypic sexual differentiation

• Genotypic sex determined gonadal sex

• Gonadal sex determined phenotypic sex

• Differential exposure to sex steroids (e.g testorone, DHT) during critical periods of development (foetal development, pinery) causes

  • Sexual differentiation of the body

  • Sexual differentiation of the brain & behaviour 

• these are permanent effects

phenotypic sex differentiation - masculinisation & defeminisation

• Effect of hormones present early in development promotes later development of anatomical or behavioural characteristics typical of males

  • AMH → internal gentiala 

  • DHT → external genitalia

  • Testosterone → rest of the body including brain

organisational effects of horemones at puberty

• Hormones released by the anterior pituitary (growth hormone, gonadotropic hormone and adrenocorticotropic hormone)

• lead to development of secondary sexual characteristics (not present at birth

sex differences in behaviour

• Sex differences in behaviour

  • Female typical behaviours

    • Caring for young

  • Male typical

    • aggression

    • competitiveness

Reproductive behaviour in humans

• Masters & Johnson (1966)

• Detailed lab observations of physiology & behaviour during intercourse

• 4 stages of physiological response to sexual stimulation

  • excitement, plateau, orgasm , resolution

• Similarities between males & females

aggressive behaviour

• Homicide rates among males is high compared to females (68% compared to 2%)

• over 9000 same-sex homicides in males (US homicide reports, 1976–1999)

• victims of male homicide/offenders normally in 20s

aggressive behaviour - homicide rates

• men severe violence to men much more frequently than women severe violence to women

  • most victims and offenders are young men

• this pattern is stable across cultures & over time

  • suggests the involvement of testosterone in aggressive behaviour

  • although the evidence is not so convincing for human populations

    • e.g. influence of environment, social aspects

female hormones - oestrous cycle

Mammalian oestrous cycle

• Cyclic patterns of gonadotropin secretion (FSH, LH), present only in females

• prompted by surge in GnRH (from hypothalamus) above tonic levels

• Pro-oestrus - follicular development and ovulation

• Oestrus - receptive period during which fertilisation is most likely to lead to pregnancy

  • Most female mammals are only sexually active during oestrous phase

female hormones - Oestrus period in non-human mammals

• generally obvious

  • Female cats ‘call’ at night, roll and tread the carpet and ‘stand’ firm when pressure is placed on the pelvic region (lordosis response)

  • Many primates have conspicuous sexual swellings, often red or pink in colour

female hormones - Oestrus period in human mammals

• ovulation is concealed

• Fertile window 

  • conception is only likely if sperm is present in the reproductive tract when ovulation takes place

  • Behavioural and physiological changes

female hormones - Is ovulation really cryptic

Beall & Tracy (2013)

• N = 124 normally ovulating women, aged 17–47 (undergrad + community samples)

• asked on-line what colour shirt they were wearing

• classified as high fertility or low fertility based on reported time since last period

• women are more likely to wear red or pink at peak fertility

poor evidence of differences in sexual activity across different phases of menstrual cycle

female hormones - Premenstrual dysphoric disorder

• Similar to premenstrual disorder, but more severe

• 3-8% menstruating women

• Within one week before period starts (luteal phase)

• Mood swings, depression, anxiety, irritability

• Difficulty concentrating, Sleep problems

• Treatment based on hormone therapy, antidepressants, surgery.

Sex differences in the brain - Sexually dimorphic brains structure 

• Song control region in zebra finches

  • 5–6 times larger in males than in females

  • gets bigger in females given testosterone as hatchlings

• Rat hypothalamus → sexually dimorphic nucleus of pre-optic area

  • smaller in males castrated at birth

  • bigger in females given testosterone at birth

  • no effect of castration/testosterone treatment in adulthood

Sex differences in the human brain

• Brain size

  • On average male brains are 120-160g (10-15%) heavier

• Brain lateralization

  • Female brains are less strongly lateralised with respect to various functions than male brains

    • Anatomical lateralisation of the cerebral hemispheres more marked in males

key points

• Hormones may have organisational effects on

  • tissue differentiation and development

  • activational effects on behaviour

• Genotypic sex (XX vs XY) determines gonadal sex, which through the production of sex steroids determines phenotypic sex

• Production of SRY protein in genotypic males initiates a cascade of processes that masculinise (& de-feminise) the developing embryo

• Further organisational effects of hormones at puberty lead to the development of secondary sexual characteristics

• Men and women are more similar than different, but (like in other mammals) there are some sex differences in body, brain and behaviour

• The functional significance of sex differences in the brain is still unclear