Sex differences Flashcards
types of hormone action
organisational
activational
organisation effects
effects on tissue differentiation and
development
Permanent – long lasting
activational effects
effects that occur in fully developed organism; may depend on prior exposure to organisational effects of hormones
Transient – short term effects
sexual differentiation
Development of sexual dimorphisms
- differences in anatomy, physiology and behaviour between males and females
Sexual differentiation in mammals depends on organisational effects of hormones during development
- 2D-4D ratio
- Facial width to height
genetic basis of sex differentiation
Sex is genetically determined – due to genotype
Diploid human cells contain 46 chromosomes (23 matched pairs):
- 22 pairs of autosomes – don’t determine sex differences – common between genders
- 1 pair of sex chromosomes
gametes
are haploid: just one copy of each chromosome
fuse to create diploid offspring
what is sex determined by?
the male’s gametes (sperm)
SRY (sex-determining region Y) gene a.k.a. testis determining factor (TDF) gene
gonadal sex differentiation
during early embryonic development
- no SRY gene –> no protein = ovaries
- gene –> protein = testis
sets off chain of events that (usually) results in male v female indvs
ovary in development
ovary does not produce significant amounts of steroid hormones during embryonic development
each part of the body develops according to its own intrinsic programme
testis in development
various androgens, incl. testosterone –> masculinises many other tissues, incl. brain
anti-Müllerian hormone (AMH) –> masculinises internal genitalia (a.k.a. Müllerian regression factor)
testosterone
steroid hormone
- fat-soluble, passes readily through cell membrane
primary androgen
synthesised by Leydig cells in testes
Sertoli cells produce AMH
what does testosterone lead to?
dihydrotestosterone (DHT)
phenotypic sex differentiation
Genotypic sex determines gonadal sex
Gonadal sex determines phenotypic sex
Differential exposure to sex steroids during critical periods of development (e.g. foetal development, puberty) causes
- sexual differentiation of the body
- sexual differentiation of the brain and behaviour
These are permanent effects – organisational effects – may then affect how we respond behaviourally to hormones later in development
masculinisation and de-feminisation
Effect of hormone present early in development promotes later development of anatomical or behavioural characteristics typical of males
AMH –> internal genitalia
DHT –> external genitalia
testosterone –> rest of body (inc. brain)
puberty
Further organisational effects of hormones at puberty:
- anterior pituitary releases growth hormone, gonadotropic hormone and adrenocorticotropic hormone
- leads to development of secondary sexual characteristics (not present at birth)
- Primary sexual characteristics = present at birth
sex differences in behaviour
homicide rates
- men kill men much more frequently than women kill women
- most victims and offenders are young men
- this pattern is stable across cultures and over time
brain masculinisation
Female guinea pigs treated with testosterone when pregnant produce de-feminised, masculinised daughters with male-typical behaviour
Female rhesus macaques given testosterone as new-borns show male-typical behaviour as adults:
- pursuit and mounting of other females
- pelvic thrusting
- post-ejaculatory behaviours
- preference for female partners
sexually dimorphic brain structures
Song control region in zebra finches
- 5–6 times larger in males than in females
- gets bigger in females given testosterone as hatchlings – organisational effect
Rat hypothalamus: sexually dimorphic nucleus of pre-optic area (SDN-POA) – function not definitively known
- smaller in males castrated at birth
- bigger in females given testosterone at birth
- no effect of castration/testosterone treatment in adulthood
- Organisational effect of testosterone
brain size in humans
On average male brains are 120–160 g (10–15%) heavier
Also heavier relative to body size – men on average weigh more
Difference present at birth despite equal body weights
What does it mean?
- are female brains more “efficient”?
- is bigger necessarily better?
- is the extra weight due to extra neurons or supporting cells, or water?
- is there a correlation between brain weight & performance in specific domains?
brain lateralisation
Female brains less strongly lateralised with respect to various functions than male brains
Anatomical lateralisation of the cerebral hemispheres appears to be more marked in males
Subtle differences but statistically significant
connection between hemispheres
Parts of corpus callosum are bigger in right-handed than left-handed men; no such pattern for women
Posterior portion (splenium) perhaps more bulbous in women than men
Subtle differences
reproductive behaviour can be divided into 4 stages
Sexual attraction
appetitive behaviour
copulation
postcopulatory behaviour, inc parental behaviours in some species
gonadal steroids activates sexual behaviour
Alterations in circulating gonadal steroids after sexual behaviour in male and female mammals
Androgens act on a neural system for male reproductive behaviour and oestrogens and progesterone regulate a lordosis circuit that spans from brain to muscle
In the female rat, a steroid-sensitive lordosis circuit extends from the ventromedial hypothalamus (VMH) to the spinal cord, via the periaque-ductal gray and medullary reticular formation
In male rat, neurons of medial preoptic area (mPOA) exert descending control of sexual behaviour, integrating inputs from the media amygdala and vomeronasal organ (VNO)
Projections, via ventral midbrain and brainstem nuclei, terminate on motor neurons involved in copulation
diversity
Human copulatory behaviour varied
Most men show single copulatory pattern; women show much more varied sexual responses
Classic model of sexuality
Modern models
Male and female sexuality overlap, and both are heavily influenced by sociocultural factors
for many vertebrates, parental care determines offspring survival
Parental behaviour crucial aspect of reproduction and significantly influenced by hormones
Brain mechanisms for parental behaviour show considerable overlap with mechanisms implicated in sexual behaviour
gonadal hormones direct sexual differentiation of the body
In birds and mammals, genetic sex determines whether testes/ovaries develop, and hormonal secretions from the gonads determine whether the rest of the body, inc the brain, developed in a feminine/masculine fashion
In presence of testicular secretions, a male develops; in the absence, a female develops
dysfunctional androgen receptors can block male masculinisation
People can be classified on the basis of their sex chromosomes, genitalia/gender they identify with
Options within each of these categories complex and sometimes overlap, so attempts to classify all indvs into just 2 gender groups oversimplify real situation
Gonadal hormones direct sexual differentiation of the brain and behaviour
Brains of vertebrates masculinised by presence of testicular steroids during early development
Such organisational effects pf steroids permanently alter the structure and function of brain and therefore permanently alter the behaviour of the indv
The spinal cord in mammals
gonadal steroids have been shown to alter characteristics such as neuronal survival, structure and synaptic connections
do foetal hormones masculinise human behaviours in adulthood?
Several regions of human brain sexually dimorphic
Don’t know whether dimorphisms general by foetal steroid levels/sex differences in early social env
what determines a person’s sexual orientation?
Although no perfect animal model of sexual orientation has been developed, all research indicates sexual orientation determined early in life and esp. in men, not a matter of indv choice
what is the hallmark of human sexual behaviour?
diversity
what does the classic model of sexuality emphasise?
4 stages:
- Increasing excitement
- Plateau
- Orgasm
- Resolution
what do modern models of sexuality empahsise?
identify emotional factors and desire as crucial aspects of female sexuality, whereas male sexuality may involve feelings of power
