Females are Genetic Mosaics

• Female mammals “turn off” one entire X chromosome in each cell early in development =XCI

• Not perfect up to 20% of X genes escape XCI, so some sex chromosome genes M>F, F>M

5-alpha-reductase deficiency

•Males are usually infertile due in part to genital malformation = hypospadias

Congenital Adrenal Hyperplasia (CAH)

Birds – Just Plain Weird

• ZZ males, WZ females • males “preferred”?• Mammalian rules don’t seem to fully apply

to them• Gonadal steroids don’t appear to be nearly

as important for sexual differentiation• Direct genetic effects more pronounced?

An example of a critical period

SNB motoneurons are more numerous in male than in female rats.

FEMALE MALE

400 um

Activational effects on the SNB

• Testosterone maintains:• Muscle size • Motoneuron soma size• Motoneuron dendritic length• Synaptic density on Motoneurons• Function of the reflexes

• Most of these happen because of AR in muscle!

posterodorsal medial amygdala – 85% larger in males•connected to POA•Activational effect!

Activational Dimorphisms

Keep in mind – much overlap between sexes in most behaviors•more variation within each sex, than between sexes

Rough and Tumble Play

Non-Sexual Sex Differences in Humans

Numerous differences observed cross-culturally A. Sensory Systems

1. Olfaction•women better at identifying odors at all ages

•women sense musk 1000x more easily than men•begins at puberty, is estrogen-dependent

•periovulatory and early pregnancy – better sensitivity•menstruating and late pregnancy – worse sensitivity

2. Taste•women higher sensitivity than men

•better at identifying tastes, and discriminating them•begins at puberty, highest at pregnancy/ovulation•in rats, due to lack of perinatal androgen and then

activated by estrogens

Non-Sexual Sex Differences in Humans

Numerous cognitive differences observed cross-culturally A. Sensory Systems

3. Audition•women more sensitive than men

•hear tones at lower thresholds•particularly true for higher pitches•tolerance for loudness of white noise less

•women have louder evoked otoacoustic emissions•cochlea makes faint echo-like noises•sex difference at birth, organized by hormones•females with male twin more male-like•gay or bisexual women more male-like

4. Vision•men higher visual acuity than women

•see objects more clearly at further distances•women tolerate higher light intensity than men•women undergo dark adaptation faster•visuospatial abilities better in men•not known how hormones affect

B. Lateralization of Cognitive Function•cerebral hemispheres somewhat specialized

•example – right side best at spatial processing, left side best at verbal processing

Non-Sexual Sex Differences in Humans

•females less lateralized than males•hemisphere size more similar in women•sex difference in corpus callosum•females’ cognitive function more distributed

•men slower and more mistakes in responding to auditory stimuli entering left ear

•women equally good for stimuli at either ear/eye•stroke on left side affects speech more in males•stroke on right side affects vision more in males

C. Verbal and Mathematical Skills•no sex difference in IQ•however, small sex differences in some skills

•females better at:•verbal skills

•language comp., acquisition, spelling, grammar•perceptual skills•fine motor skills•mathematical calculations

Non-Sexual Sex Differences in Humans

males better at:interception of projectile objectsquantitative tasksvisuospatial ability

maps, directional sense, mazes, math reasoning

Almost all of these sex difference arise after puberty, so activational hormone basis?

Not that simple – recall organizational limits on activational effects

CAH women – maybe enhanced visuospatial ability suggest organizational effects do happen

mathematical reasoning difference exists prior to puberty (but gender roles and early environment effects)

Again, recall that individual differences are more important than sex differences in this regard!

But Hormonally Based?

Brain Structuree.g. superior temporal cortex and Broca’s area are 20% larger in females BUT use can affect structure:

Brain activity fMRI patterns differ between sexes when asked to do same task

Source of Sex Differences in Cognitive Ability