ovulation (see PBS sex video for graphic discussion

a pre-existing egg becomes mature

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. Or they turned to Behaviorism offering a similar hope.)

Each female has at least 500 eggs.

sperm competition to fertilize the egg

Males have millions of sperm constantly being produced. There is a race amongst all sperm to be the first to reach the ovum and fertilize it. Sperm from one or more males mating with the female around ovulation "compete" to win that race. See online PBS video.

Here is a graphic outline of the entire process based on Money & Ehrhardt (1972).


one sperm is allowed to enter egg

cell division

cell differentiation

form fetal gonad

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 brain-brain relationships.

Hormones, primarily androgens, shapes these basic common parts into mammalian males and females. The actual effects in a particular individual or species depends on the level of hormones, and especially the timing of secretion. Primates and humans, in particular, seem less defined in this way than other mammals -- I would guess because of their slower development leaving more time for environmental impact. See EHE.

(See EHE, p.93 for a photo of the "adrenogenital syndrome" where androgens from overactive adrenal glands shapes female genitals into male form. The linked drawing is from Dixon (1998) Primate Sexuality.)

The mammalian fetus of both XX and XY individuals begins with undifferentiated sexual parts. A gene (SRY) on the Y chromosome has been identified which triggers a cascade of events causing the asexual gonads to develop as testes.

Lacking this gene the gonads become ovaries. The sex chromosomes have little more to do with sex differentiation which hereafter is driven by hormones produced in the now sex-specific gonads. The testes produce testosterone (T) during gestation, and production peaks again a month or two after birth, then declines by six months of age to the low range seen in later childhood (Winter et al. 1976). T and other testicular secretions cause the external genitalia to form into penis and scrotum rather than clitoris and labia, and internal ducts take the male form. A genetic male without androgens will develop into the body form of a female as this photo illustrates, also from Dixon, 1998).

The central nervous system is masculinized. All this prenatal differentiation precludes effective later so-called "sex-change" procedures in young children. Tragic cases of penis loss during circumcision were at one time dealt with by surgically removing testicles, creating a vagina, and later giving exogenous female sex hormones. Despite treating the infants as girls, they still developed a self-identity as boys. (Holden, C. (1997) Science, 275, p.1745) Most of these probably fail clinically because the brain is masculinized prenatally unlike in the sistuation where a genetic male has never had androgen "masculatiion." (above photo).

hormone input determined by M/F genes

develop genetic sex- appropriate anatomy

differentiate female reproductive anatomy

(differentiate MALE anatomy ONLY if male hormones are present)

"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.)

brain develops toward end of fetal period

brain differentiation

overproduction of neurons and synapses

birth (to about age two)

continued brain growth

Pelvic size limits pre-birth brain development. In humans this is means that the infant must be born at nine months prematurely with continued growth in relation to body size for another 12 months of intense maternal care.

begin mylination of neural fibers

These speed transmission of information; reduce errors. It is unlikely large mammal brains could have evolved without myelin to speed up the information processing.

begin pruning (loss) of surplus neurons and synapses

New synapses continue forming based on internal guidance (basically an unknown process) and on the basis of external feedback from receptors. This is why early experiences play a foundational role in shaping the brain.

primate growth rates: high, slow, high

Primates have a distinct pattern of growth. Growth rates are high until shortly after birth, then slow until adolescence when they accelerate. Growth patterns are regulated by hormones. (See EHE, p. 100)

form self-identity including gender identity

based on anatomy and neural "programming"

self observations and comparisons

behavior of others toward self (socialization)

adolescence (triggered by hormones)

Various hormones come into play again, growth hormone from the pituitary, sex hormones, and hormones from the hypothalamus regulating the others.

gender identity established

erotic imagery established

secondary sex characteristics emerging

While female hips are widening in aid of child-birth, male shoulders are broadening in aid of greater strength, e.g. for fighting and throwing.

Females are more advanced in maturity at any age but males grow somewhat larger due to longer growth period.

Similar processes must occur across all primates. Orangutans are particularly interesting in that full maturity including full size, cheek flanges, etc. appears to be controlled by a social stimulus -- namely the presence of a large dominant male.

sexual activities increase

length of estrus in nonhumans varies greatly

(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 and humans.

In humans, ovulation is concealed and the female is more or less continuously interested in sex.

sex differences maximized during periods of high fertility

(these differences may have unforseen consequences in the life of contemporary humans)

(of course this is the same process that enables adaptive change and speciation!)

some examples of "unforeseen" differences

greater chance of ruptured ACL (knee ligament) in females playing soccer/basketball

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 later -- pregnancies. The !Kung women, for example, have half the exposure to hormones as do contemporary urban women. This is due to their greater number of pregnancies and longer nursing after each one.