Behavioral Embryology I: Hormones
and Sexual Development
PSC 113
Jeff Schank
Outline
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Introduction
Ontogenetic Transitions and Adaptations
Hormones and Sexual Development
Fetal Hormones and the Development of
Reproductive Organs
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Development of Sex Differences in the Brain
Perinatal Hormones and Behavioral Development
Gender and Sex
Is There a Difference in the Brains of Homosexuals and
Heterosexuals
Introduction
• Ontogenetically, all vertebrates develop from a single
celled zygote to an adult form
• But, they do it in different ways
• Amphibians go through a free living larval stage, which
is morphologically and behaviorally different from the
adult stage
• Fish and Reptiles emerge from their embryonic stages
typically similar to adult forms in both behavior and
morphology
• Mammals and Birds undergo much more gradual and
developmental transitions from a neonate (just born
mammal or hatched bird) to adult
Altricial and Precocial
• Definition: The degree of immaturity in neonatal mammals
and birds is distinguished in terms of how altricial or
precocial they are
– Altricial young immature in their sensory and/or motor
development requiring substantial parental
– Precocial young are more mature in their sensory and/or motor
development and require less parental care
• For example, humans, rats, mice, many nesting birds all
have relatively altricial young requiring considerable
parental care
• Ducks, deer, cattle, pigs are relatively more precocial at
birth and though they require parental care, they need less
of it
Ontogenetic Transitions and
Adaptations
• Behavioral Embryology focuses on the development of behavior in
young animals, and how these changes are related to morphology,
neuoanatomical changes, and hormonal changes
• Earlier, we talked a lot about the Umwelten of different organisms,
which is their sensorimotor environment that they perceive and
react to
• Mammals and Birds can go through a succession of Umwelten
during development
– Consider the difference in Umwelt between a newly born mouse or rat
and it more mature stages (blind and deaf, etc.)
• During each stage of development, an organism must be able to
perform behaviors adaptive to its Umwelt
– For example, mammals must be able to nurse and how they do this
can be quite different from one mammalian species to the next
Types of Transitions
• Ontogenetic Transitions: Mammals and birds,
must make behavioral transitions from neonatal
behaviors to adult behaviors
– Early behaviors may be in preparation for later stages
of development
• Ontogenetic Adaptations: Organisms must also
behave in ways that facilitate immediate
biological functions
– For example, nursing in young mammals or pecking on
a parent’s beak or opening the mouth in response to a
parent with food
Three Hypotheses about Fetal
Development
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Fetal Activity as an Epiphenomenon: Mammals including humans exhibit
spontaneous fetal activity: movement of developing limbs, spontaneous breathing
in and out of amniotic fluids
– The epiphenomenal view is that all these forms of spontaneous activity are merely the
consequences of the developing nervous systems, and are neither adaptive or preparations
for postnatal behavior
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Fetal Activity as Preparation for postnatal Behavior: On this view, fetal behaviors
prepare the way for the further development of behavior
– For example, spontaneous breathing may be important for the differentiation of lung tissue
and for correcting “bad” intercostal nerve connections with muscle segments (i.e. muscles
surrounding the ribs).
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Fetal Activity as Ontogenetic Adaptation: On this view, fetal activity may be
adaptations to specific environments
– For example, fetal rats swallow amniotic fluids
– The flow of these fluids over olfactory and taste receptors may help the neonatal rat identify
the mother’s body as a food source, e.g., injecting citral into the amniotic fluid of surrounding
prenatal rats
– Ultrasounds in rat pups?
Hormones and Sexual Development
• Fetal Hormones and the Development of Reproductive
Organs
– Sexual development in mammals begins at the time of
fertilization with the production of typically two kinds of zygotes
XX (female) and XY (male)
– It is an excellent example of how sex morphology and behavior
is not programmed by the genes
– In humans, at 6 weeks of after fertilization, a developing fetus,
regardless of its genetic sex has the same pair of primordial
gonads
– After 6 weeks, testis determining factor (TDF a protein) causes
the medulla to develop into a testi
– The production of TDF is one of the major function of the Y
chromosome
Fetal Hormones and the Development
of Reproductive Organs
• Both males and females have two compete
sets of reproductive ducts:
– The Wolffian system, which has the capacity
to develop into the male reproductive ducts,
e.g., seminal vesicles, vas deferens
– Müllerian system, which has the capacity to
develop into the female ducts e.g., uterus,
vagina, fallopian tubes
• In the absence of the TDF, the primordial
gonads automatically develop into ovaries
• In the third month of male fetal
development, the testes secret testosterone
and Müllarian-inhibiting substance
• The development of the Müllarian system
occurs in any fetus that is not exposed to
testosterone during early development
• If male or female gonads are removed, the
Müllarian system develops
Timing of Sexual Differentiation in the
Human Fetus
Disorders of Sexual Development
Normal Development
Androgen insensitivity syndrome (AIS)
• A developmental disorder of genetic males in which an insensitivity
to androgens causes them to develop female bodies, but not
internal female anatomy
• Testes develop and release Müllarian inhibiting substance
• The testes release lots of testosterone and enough estrogens to
feminize the body in the absence of any sensitivity to androgens
• AIS comes in degrees and is caused by is a set of disorders of sex
development caused by mutations of the gene encoding the
androgen receptor
• In complete AIS (CAIS), people generally women with internal
testes, genetically male, and normal female bodies by external
appearance with some exceptions
• There can also be partial AIS (PAIS): male or female body, with
slightly virilized genitalia or micropenis; testes in the abdomen;
sparse to normal androgenic hair
Androgen insensitivity syndrome (AIS)
CAIS
PAIS
Adrenogenital Syndrome
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A disorder characterized by a decrease
in the release of the hormone cortisol
from the adrenal cortex, which results in
the production of high levels of adrenal
androgens and masculinizes the bodies of
genetic females
It is any of several autosomal recessive
diseases resulting from mutations of genes
for enzymes mediating the biochemical steps
of production of cortisol from cholesterol by
the adrenal glands
Usually, the onset of this disorder is relatively late
in development, and thus does not stimulate the
development of the Wolffian system
Thus, there is often only partial development of secondary sex characteristic
– enlarged clitoris
– partially fused labia
– if left untreated, leads to unpredictable changes at puberty
Adrenogenital Syndrome
Perinatal Hormones and Behavioral
Development
• Most of the research on hormones and behavioral development has
focused on the role of hormones in sexual behaviors in laboratory animals
• Early research showed that the prenatal injection of testosterone both
maculinizes and defeminizes a genetic female’s copulatory behavior (Phoenix CH,
Goy RW, Gerall AA, Young WC 1959 Organizing action of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female
guinea pig. Endocrinology 65:369–382)
• In that study they
– injected pregnant guinea pigs with testosterone
– When the litters were born, they ovariectomized the female offspring
– When these ovariectomized female guinea pigs reached maturity, they
injected them with testosterone and assessed their copulatory behavior
– They found that the females exposed to prenatal testosterone displayed more
male like mounting behavior in adulthood than those who were injected with
testosterone but not exposed to testosterone prenatally
– When females were injected with estrogen and progesterone, females
prenatally exposed to testosterone displayed less lordosis
Perinatal Hormones and Behavioral
Development
• In a complementary study, they also found that the
lack of exposure to testosterone feminizes and
demasculinized copulatory behavior in adult males
• Male rats castrated shortly after birth failed to display
the normal male copulatory pattern of mounting
behavior when they were injected with testosterone
and given access to sexually receptive females.
• And when they were injected with estrogen and
progesterone, they displayed more lordosis than did
castrated control males
Gender and Sex
• The sexuality of a person or other mammal is not merely a matter of the
individual’s genotype (i.e., the presence of X or Y chromosomes).
• The hormones one is exposed to pre and postnatally determine to what
degree male or female secondary sexual characteristics develop
• In addition to biological sex and sex characteristics, there is also the notion
of gender
• Roughly, gender refers to social or cultural distinctions about masculine or
feminine traits, sex refers to the more biological aspects we have talked
about
• Are gender and sex distinct?
• Can the social environment of a person be arranged early in postnatal
development so that an individual, which is male or female based on
genetic sex, will develop the opposite gender?
• The theory that this can be done is the theory of gender neutrality, which
states that gender can be socially developed/constructed (perhaps with
the help of surgery) independent of the biological sex of the individual
The Boy Who was Turned into a Girl
Is There a Difference in the Brains of
Homosexuals and Heterosexuals?
• This question implies more than just a difference
• Is there a difference that causes a difference in behavior?
• Simon LeVay (Science, 1991), published the results of a postmortem study
that compared the neuroanatomy of three groups of subjects:
– heterosexual men
– homosexual men
– women (who were assumed to be heterosexual)
• Previous research reporting sex differences in the anterior and preoptic
areas of the hypothalamus of various species, focused LeVay’s research on
these areas
• And as we have seen, the hypothalamus is an important area for sex
differences in the brain
• Levay found a difference in the volume of the third interstitial nucleus of
the anterior hypothalamus (INAH 3)
Is There a Difference in the Brains of
Homosexuals and Heterosexuals?
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Is the study replicable? (Overlap of brain areas, problems of data mining)
What does it really mean?
What is the direction of causation?
Sexuality in humans has a cognitive component, so the hypothalamus does not hold all the answers
Nevertheless, because we do know that hormones effect behavior, changes in hormones can have a
variety of causes, hormones can restructure the brain at any developmental stage of the organism,
it would not be surprising if these or results like this turn out to be correct. How we are to interpret
them is another matter