Wednesday October 19^th

Our discussion of hormones leads us to a discussion of how hormones are responsible for sex differences in body morphology, brain, and behavior.

First morphology—male and female bodies are different, and this is largely under the control of the organizational effects of androgens during development.

Despite chromosomal sex, the developing fetus is bipotential—it has the internal structures that can become either male or female reproductive system, and a body that can take on either male or female form.

Y chromosome present in males—SRY gene cause the undifferentiated gonads to become testes. Testes release testosterone, which masculinizes the external genitalia to develop in the male pattern, and promotes the development of the male reproductive structures. This occurs during an early critical period of development, and the changes are permanent (thus, an organizational effect). Also, testes release MIS, which inhibits the development of the female internal reproductive structures. In the absence of T (and MIS), the female pattern develops…the undifferentiated gonads become ovaries, and the reproductive system develops in the female pattern.

What happens when this process goes awry? Androgen Insensitivity Syndrome—XY male who is insensitive to T. Masculinization of body does not occur, but internal reproductive structures do not develop because MIS inhibits them. Internal, undescended testes in an individual who appears as (and identifies with) female gender.

Congenital Adrenal Hypersensitivity (CAH) occurs when the adrenal gland releases a large amount of sex hormones during development. Excess T partially masculinizes female fetuses (but has no effect on male fetuses).

Other sex differences are also under the control of organizational effects of hormones.

IN brain—sexually dimorphic structures in rodents are caused by exposure to T during the first postnatal week (the critical period for neural sexual differentiation).

Other neural sex differences are caused by both organizational and activational effects of sex hormones. Ovulatory cycle in females—the ability of the hypothalamus to release GnRH cyclically is caused by the absence of T during development (if T is exposed during development, the hypothalamus may not properly cycle in its GnRH release). However, the release of GnRH after puberty is entirely dependent on the activational effects of hormones—if hormone levels are manipulated (say by the use of oral contraceptives), GnRH release is altered.

Some behaviors that are sexually dimorphic in humans. One example:

Proximal senses—females are more sensitive to stimuli in the environment than males. Possibly a result of division of labor in the ancestral environment—females cared for non-verbal infants, must develop a sensitivity to immediate surroundings.

Cognitive/motor skills—no difference in overall intelligence (and even for skills that show differences, the differences are on average population scores—no predicting any individual’s response based solely on gender).

-tasks that favor males:

-2D and 3D rotation—examples of spatial skills

-abstract mathematics

-gross motor skills--targeting

-tasks that favor females

-language fluency

-perceptual speed (picking out objects in an array)

-detail memory

-fine motor skills, manual dexterity

Males and females also have different strategies for “way-finding” (navigating)…females rely more on landmarks, males, perhaps to their superior spatial skills rely on directional orientation.