BIO372 : Behavior : Hormones : Lesson

I. Redwing blackbirds

1. Observations
   • Birds aggressively defend territories

2. Hypotheses
   • Testosterone levels can affect aggressiveness
   • Alternative hypothesis

3. Evidence
   • Experimentally add testosterone
   • Experimentally block testosterone

II. Ground squirrels

1. Hypotheses
   • Testosterone "organizes" the male brain for dispersal
   • Testosterone "activates" the male brain for dispersal
   • Testosterone has no effect on the male brain

2. Evidence
   • Observe testosterone levels in the blood
   • Experimentally remove testosterone, add testosterone

Note: both testes and ovaries secrete both male and female sex hormones. Cholesterol is a precursor to both male and female sex hormones.

Some of the information in the outline below can be found in Reading No. 9. The rest of the outline is based on several sources that you may find yourself either in printed or electronic form.

III. Anatomical dimorphism in vertebrates

1. Sexual differentiation in mammals
   • Male and female anatomy begins identically
   • Genetic signals promote testes in males, ovaries in females
   • Testosterone from testes guides development toward the male, lack of testosterone allows development toward the female

2. Brain dimorphism
   • Gerbils: there is a nucleus of cells in the male brain for mounting behavior, but this nucleus is absent in the female brain
   • Leopard gecko: the temperature at which the eggs develop determines the sex of the offspring, but can be overridden by estrogen
   • Other examples: whiptail lizards

IV. Humans

1. Congenital adrenal hyperplasia:
   • Human females exposed to large concentrations of androgens at the time of birth exhibit more male behaviors and sometimes male-like anatomical structures

Brain anatomy and physiology Overall brain size is greater in males (Hypothalamus is 5 times larger in male rats than in female rats) Corpus callosumCorpus callosum (nerve fibers connecting the two cerebral hemispheres) is larger in human females The right cerebral hemisphere is thicker in males Metabolic rate is 20% greater in females A nucleus of cells in the brain of homosexual males is smaller than it is in heterosexual males but the same size as in heterosexual females A nucleus of cells in the brain of transexual genetic males is smaller than it is in heterosexual males

3. Behavior
   • Males are more aggressive
   • Males are better at visual-spatial skills and at mathematics
   • Females are better at verbal and social skills

4. Hormones
   • Testosterone organizes the male anatomy and brain (through conversion in the brain to estradiol)
   • DES, a synthetic estrogen, masculinizes the brain but not the body

5. Hypotheses
   • Hormones organize the sexes
   • The environment, including culture, influences sexual (and other) behavior
   • Both hormones and environment determine behavior

When you have completed this lesson, go on to Review Questions