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Motivation IV
Sexual Motivation
Sexual Reproduction
Reproduction is necessary for the survival of the
species.
Some organisms (e.g., bacteria) reproduce
asexually.
Sexual reproduction allows the genes of two
individuals to be mixed to produce individuals
with new genetic combinations.
This increases the chances that at least some
combinations will be found that will survive
under given conditions.
Genetics and Sex
Genes carried on the chromosomes code for
proteins that ultimately determine bodily
characteristics and function.
Chromosomes come in pairs. One member of
each pair comes from the male, one from the
female.
The sex of the individual is specified by the sex
chromosomes,, which come in X and Y types.
chromosomes
Females have two X chromosomes, males have
one X and one Y chromosome.
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Sources of the X and Y
Chromosomes
Females produce eggs
eggs,, whereas males produce
sperm..
sperm
In either case, during production, each egg or
sperm receives only one member of each gene
pair, yielding only half the normal complement of
chromosomes.
Because females have only X- X chromosome
pairs, all eggs have only an X chromosome.
Because males have X- Y chromosome pairs,
half of the sperm have X chromosomes and half
have Y chromosomes.
Fertilization and Genetic Sex
During fertilization, egg and sperm
combine, each contributing their half of the
total chromosome complement.
Genetic sex is determined by the sperm:
n
n
Sperm X plus egg X = X- X = female
Sperm Y plus egg X = X- Y = male
However, this is not the whole story.
Genotype Versus Phenotype
The genotype is the characteristic
programmed by the genes.
The phenotype is the characteristic
actually expressed in the individual.
They are not always the same, because of
the environment in which the phenotype
develops (including other genes as well as
hormones, drugs, infections, nutrition,
etc.).
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Sexual Development
The sex chromosomes specify how the
gonads develop – whether into ovaries or
testes .
Other than that, early in fetal development,
males and females have the same “proto
sex--organs,” including external genitalia.
sex
In genetic males, the testes begin to
secrete testosterone and other “male” sex
hormones (androgens).
Organizing Influence of Sex
Hormones in Early Development
In genetic males, sex hormones suppress further
development of female internal organs and
promote the development of male internal
organs (seminal vesicles, prostate, etc.).
External genitalia develop further into penis and
scrotum. Brain development is also affected.
In genetic females, absence of testosterone
suppresses further development of male internal
organs and allows female internal organs to
develop. External genitalia develop further into
clitoris, labia, and part of vagina.
Sex Hormones and SexSex-typical
Behavior
Presence or absence of high levels of
testosterone during fetal development changes
the “wiring” of the brain in some areas (e.g., prepreoptic area of the hypothalamus).
Female rats exposed to testosterone late in
development (brain is still developing) behave
sexually like males after puberty.
Male rats deprived of testosterone late in
development behave sexually like females after
puberty.
During development, testosterone biases the
brain to favor malemale- typical as opposed to
female-- typical behavior.
female
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Androgen Insensitivity Syndrome
Genetic error renders cells of body unable to
respond to androgens.
Consequently, genetic males develop externally
as females, but lack internal female organs.
(They do, however, have a shallow vagina.)
Typically, these individuals develop sexual
interest in the phenotypically opposite sex (i.e.,
males). Thus, sexual preference is not directly
influenced by the genes.
Early Exposure to Androgens in
Females
Genetic females are sometimes exposed
to relatively high levels of androgens
during fetal development.
Depending on how soon and how much,
this can change development toward the
male “plan.”
Girls with this condition generally develop
heterosexual interests at puberty, but in
childhood tend to be described as
“tomboys.”
Sexual Orientation and Hormones
In humans, determination of sexual orientation is
complex and poorly understood at present.
In all probability, genetic, hormonal (during fetal
development), and environmental factors all
contribute.
At puberty, when sex hormone levels rise,
sexual interest awakens. However, sexual
orientation may already have been determined
(not a matter of “choice”).
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Sexual Motivation
Sexual reproduction carries with it a
downside – unlike bacteria, sexual
reproducers must find a partner of the
opposite sex.
Sexual attraction to members of the
opposite sex fosters this.
Differences in reproductive equipment
yield different needs in the two sexes.
Female Needs
Females are able to produce only a few
offspring, and human children are extremely
dependent for a relatively long period.
It is difficult to provide for many highly
dependent children while at the same time
caring for them, especially while pregnant.
Females must live in a world inhabited by
generally larger and more powerful males.
Must be able to attract a mate.
Female Strategies
Select a mate based on his apparent abilities to provide
and protect. Often these will be somewhat older than
herself. Power is more important than physical
attractiveness.
Be concerned about her own physical appearance as
this is important in attracting a mate.
Find a mate willing to stick around and help (long(long-term
commitment).
Be concerned about “the relationship” and work to
maintain it.
Maintain strong ties with maternal side. (Receive
support and protection.)
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Male Needs
Males are capable of having a great number of
children. However, to do so they must
impregnate females. This places them in
competition with other males.
Reproductive success is more likely with young,
healthy adult females who can bear children and
have the energy to care for them.
Successful reproduction requires that at least
some of his children reach adulthood.
Males need to know that the children they invest
their resources in are theirs.
Male Strategies
Look for females of reproductive age who are
relatively young and healthy (attraction to
physical beauty).
Successfully compete with other males for
access to females, resources. Survival and
success may depend on forming alliances with
other males, being seen as a valuable
contributor.
Best reproductive strategy will depend on how
difficult it is for his offspring to survive without
him.
Prevent other males from having sexual
relations with his mate.
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