(Recall not until Weismann, 1885, was it known that egg and sperm cells are
independent of influences from other cells. This was the death of Lamarckian
inheritance of acquired traits though it was not fully accepted until about
1920 or so. Many psychologists and biologists educated around 1900--Freud,
Watson, Piaget-- seemed to hang onto the Lamarckian idea for the progress to
the human race it promised but could not deliver. Behaviorism in the
USA took up the need for "progress" -- discounting heredity now that
it could not be readily changed.)
Each female has about 500 eggs.
Males have millions of sperm constantly being produced. Only one cell can
fertilize an egg --hence there's a race to be first. Among publically
promiscuous primates such as chimps, sperm competition takes on even more
significance when a fertile female mates with perhaps dozens of males in a
short time period.
Remember male and female anatomy must come from the same "plan" hence we should
expect there are corresponding parts in both sexes anatomy, e.g.
penis-clitoris, nipples on both male and female, etc. including corresponding
Hormones, primarily androgens, shapes these basic common parts into mammalian
males and females.
"The results reviewed here prompt the consideration of feminization as a
process which functions parallel to masculinization. The two processes are
qualitatively different and operate during different developmental periods. In
order for the brain to become sexually differentiated, males need exposure to
testicular androgens during the perinatal period (roughly from embryonic day 17
through postnatal day 10 in rats), and females need exposure to ovarian
secretions including, but not necessarily limited to, estrogen, during a later
period that may extend to or even beyond puberty."
(psycoloquy.95.6.05.sex-brain.1.fitch; A ROLE FOR OVARIAN HORMONES IN SEXUAL
DIFFERENTIATION OF THE BRAIN by Fitch & Denneberg.)
These speed transmission of information; reduce errors.
New synapses continue forming based on internal guidance (basically an unknown
process) and on the basis of external feedback from receptors
(See overhead based on deWaal (1995.)
One day in gorillas to several weeks in bonobos.
This typically coincides with maximum fertility with the exception of bonobos
In humans, ovulation is concealed and the female is more or less
continuously interested in sex.
(of course this is the same process that enables change and speciation!)
greater chance of ruptured ACL (knee ligament) in females playing
greater chance of permanent damage to language centers from strokes in males.
greater incidence of reading disorders in males
perhaps increased risk of breast cancer in modern females due to greater
"exposure" to their own hormones due to early sexual maturity and relatively
few --often late -- pregnancies.
- A role
for ovarian hormones in sexual differentiation of the brain
erone and dominance in men