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in behavior are commonly referred to as sexual dimor-
phisms. Predatory behavior is sexually dimorphic in
lions (females do more of it), urination posture is
sexually dimorphic in dogs, and olfactory signaling
is sexually dimorphic in moths. In both rats and
monkeys, young animals engage in play behavior that
differs between the sexes, at least in its frequency of
occurrence. Males tend to have more play fights than
females. These “fights” will typically begin with one
animal jumping onto the other, and they end with one
animal on top of the other. When testosterone is given
to a pregnant monkey, the play behavior of her female
offspring becomes more male-like (Abbott and Hearn,
1978).
Another behavioral sign of a sexually distinct
nervous system comes from the prevalence of certain
neurological and psychiatric diseases in males versus
females. For example, both dyslexia and schizophrenia
are more prominent in males (about 75% of cases),
while anorexia nervosa is exhibited primarily by
females (over 90% of cases). Many studies have also
focused on the cognitive abilities of normal adult
humans (Kimura, 1996). When presented with two
figures drawn at different orientations, males are
better able to “mentally rotate” the objects to deter-
mine whether the two figures are the same. In contrast,
when presented with a picture containing many
objects, females are better able to say which objects
have been moved in a second picture. While these
results tend to fascinate us, the challenge will be to
understand what exactly is being measured and what
its relevance is to behavior.
The male-female differences in complex behavior
patterns do raise a host of interesting questions: Are
these differences due to biology or environment? If
there is a biological signal, then is it genetic or hor-
monal? Are the differences irretrievably established at
birth, or are they modifiable throughout life? Certain
sexual characteristics emerge during embryonic devel-
opment, such as differentiation of the genitals and the
motor neurons that innervate them (see Chapter 7).
However, this is only the first step of a lifelong process.
The nervous system continues to respond to steroid
hormones throughout life, making it important to ask
whether a behavior is determined by early exposure to
a hormone or whether the behavior can be elicited in
adults of either sex merely by adjusting the amount of
circulating hormone. For example, one region of the
amygdala has a greater volume in male rats than in
females, but adult castration of males causes the
volume to shrink to female values, and androgen treat-
ment of adult females enlarges the structure to normal
male size (Cooke et al, 1999). Therefore, we will begin
by examining the early determinants of gender and
then explore determinants of behavior and brain
development.
GENETIC SEX
Animals seem to have two general ways of estab-
lishing gender. In fruit flies and other insects, the
genetic sex of each cell is the key determinant. If a cell
has a single X, then it is male. If it has two Xs, then the
cell expresses a protein called sex-lethal and becomes
female. The nematode, C. elegans , also comes in two
categories, but they are male and hermaphrodite (i.e.,
an animal with both types of gonad). As with fruit flies,
sex is determined by the ratio of X chromosomes to
autosomes, and XO-lethal is the gene product that is
activated in animals with a single X.
The genetic sex of each cell in a mammal is speci-
fied by the presence of either two X chromosomes
(female) or one X and one Y (male). However, the
genetic sex of most somatic cells is not thought to have
an immediate influence on their development. It is the
genetic sex of gonadal tissue that really matters.
Primary sex determination refers to differentiation of
the gonadal tissue, and this is determined by the SRY
gene on the Y chromosome, which encodes a tran-
scription factor (Goodfellow and Lovell-Badge, 1993).
If SRY is present, the gonads develop into testes and
secrete testosterone; if SRY is absent, the gonads
develop into ovaries. The SRY gene product is a DNA-
binding protein, and it probably controls the expres-
sion of downstream targets to prevent development
along the female pathway. For example, a locus on the
X chromosome, called Dax-1, is probably involved in
ovary determination. Thus, it is thought that SRY
represses Dax-1 in genetic males. Furthermore, SRY
specifies the male gonads by activating the insulin
receptor family of tyrosine kinases (Nef et al., 2003).
After primary sex determination is complete, all sex
differences, including those of the nervous system, are
thought to originate from the gonads. The possibility
remains that genetic sex of an individual somatic cell
plays a role in maturation, as will be discussed below.
HORMONAL SIGNALS
In most vertebrates, as the gonads differentiate and
begin to secrete hormones, tissues throughout the
body respond by adopting a male or female pheno-
type. This is called secondary sex determination . The
principal importance of gonadal hormones is power-
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