Chapter 3:
Sex Differences in Behavior: Sex
Determination and Differentiation
Stereotypes based on trends.
Sexual differentiation – the developmental process leading to becoming
a male or a female
Hormones and environmental experiences guide the development of
physiological, morphological, and behavioral characteristics that are
displayed later in life.
Sex Determination and Sex Differentiation
-Chromosomal sex (fertilization)
-Gonadal sex (ovaries or testes)
-Gametic sex (which type of gametes are produced)
-Hormonal sex (estrogen to androgen ratio)
-Morphological sex (differences in body type)
-Behavioral sex (male-typical vs female-typical)
Specifically in humans:
-Gender identity (sex or gender a person feels themselves to be)
-Sexual orientation (preference for a sex partner)
-Legal sex (governmentally determined)
3.3 Levels of sex determination
ULTIMATE CAUSES OF SEX DIFFERENCES
Parthenogenesis – asexual reproduction in vertebrates… they will be female
Polygamous – having multiple mating partners
Sexual Selection – a subset of Natural Selection where selection pressures
are based upon the competition for obtaining mating partners.
PROXIMATE CAUSES OF SEX DIFFERENCES
Organizational / activational hypothesis – behavioral sex differences result
from a) differential exposure to hormones that act early in development to
shape the brain and nervous system guiding these behaviors, and b)
differential exposure to sex steroid hormones later in life (puberty through
adulthood) activate the neural circuits shaped in (a)
3.4 Gonadal development of the human embryo
All mammals start
out being
BIPOTENTIAL –
they can develop
into either the
male or female
sex.
The germinal
ridge will be able
to develop into
either primordial
gonad once acted
upon by proteins
produced in the
body.
Anlagen – a term relating to the dual primordial tissues relevant to the
development of secondary sexual characteristics.
Mullerian duct system – female accessory structures
Wollfian duct system – male accessory structures
3.5 The Müllerian and Wolffian duct systems
Female accessory sex structures are the default pattern in mammals.
But, due to the anglen effect (presence) of both systems intitally, in rare
circumstances, both systems can develop in a single individual…
This leads to potential hermaphroditism – (now sometimes called
intersexism) is a group of conditions where there is a discrepancy
between the external genitals and the internal genitals (the testes and
ovaries) or there are present both sexes in the same individual.
Hermaphroditic Lamb
Female development is the default, but male development requires
testosterone and MIH (Mullerian inhibitory hormone).
The testosterone will activate growth of the Wolffian duct mophology.
The MIH will cause regression of the Mullerian duct morphology.
There are two pathways that occur simultaneously to guide sexual
differentiation. These are referred to as:
1.
The masculinization – demasculinization continuum –
masculinization induces male traits while demasculinization is the
removal of the potential for male traits.
2.
The feminization – defeminization continuum - feminization induces
female traits and defeminization is the removal of the potential for
female traits.
3.6 Normal development of the accessory sex organs
3.7 Embryonic development of human external genitalia
3.8 Normal development of the external genitalia
3.9 Sexual differentiation in humans
Aromatase Inhibitors a class of drugs used in the treatment of breast cancer and ovarian
cancer in postmenopausal women.
Some cancers require estrogen to grow. Aromatase is an enzyme that
synthesizes estrogen. Aromatase inhibitors block the synthesis of
estrogen. This lowers the estrogen level, and slows the growth of
estrogen sensitive cancers.
There are naturally occurring aromatase inhibitors in the body.
Rat Developmental Time Line
fertilization
full maturity
loss of bipotentialty
puberty
birth
-20
-10
0
(birth)
10
20
30
40
150
180
Study of the Pharmacological Agent ATD
Pregnant Female Rat
Pups are born
having been
exposed to the
ATD (the
aromatase
enzyme inhibitor)
developmentally
Receives
Exposure to
ATD (an
aromatase
enzyme
inhibitor)
Reproductive Behaviors
3.15 Development of female copulatory behavior requires active hormonal secretion
3.16 Ovary and oviduct of a chicken
3.18 Three types of male tree lizards
Territorial males –
Sedentary, nonterritorial males –
Nomadic non-territorial
males -
3.19 Temperature-dependent sex determination in reptiles
3.20 Aggression in adult female geckos
In this graph, you need to note:
The high temperature range is
a low-female population.
The low temperature is an allfemale population.
3.22 Freemartins
Freemartins an infertile female mammal which has masculinized behavior
and non-functioning ovaries.
Genetically and externally the animal is female, but it is
sterilized in the womb by hormones from a male twin,
becoming an infertile partial intersex.
Freemartinism is the normal outcome of mixed-sex twins in
all cattle species that have been studied, and it also occurs
occasionally in other mammals including sheep, goats and
pigs.
It is possible this may arise in dizygotic mixed sex twins in
humans as well, although examples are limited
3.23 LH profiles of female and male rats
3.26 Experimental protocol
3.27 Testosterone is a prohormone
3.28 Rat pups gestating in utero
3.29 Rough-and-tumble play behavior is demasculinized in males by stress in utero
3.24 Positive feedback and the control of ovulation
3.25 The surge and pulse centers of the hypothalamus