Animal Experiments - The Case for Nature

Gender, Nature, and Nurture - Richard A. Lippa 2014

Animal Experiments
The Case for Nature

To the extent that fetal hormones affect brain chemistry and histology, I've got a male brain. But I was raised as a girl. If you were going to devise an experiment to measure the relative influences of nature versus nurture, you couldn't come up with anything better than my life. During my time at the Clinic nearly three decades ago, Dr. Luce ran me through a barrage of tests. I was given the Benton Visual Retention Test and the Bender Visual-Motor Gestalt Test. My verbal IQ was measured, and lots of other things, too. Luce even analyzed my prose style to see if I wrote in a linear, masculine way, or in a circular, feminine one.

Excerpt from "Matchmaking" from Middlesex by Jeffrey Eugenides.

Copyright (c) 2002 by Jeffrey Eugenides.

Reprinted by permission of Farrar, Straus and Giroux, LLC.

Calliope, the protagonist of the Pulitzer Prize winning novel, Middlesex, has a secret: she carries the gene that causes 5-alpha reductase deficiency, an enzymatic defect that prevents testosterone (the most famous of male hormones) from being converted into dihydrotestosterone (a less famous but nonetheless important male hormone). You may recall from Chapter 3 that a surge of testosterone in the second trimester of male fetal development masculinizes the brain; however, it is the related androgen, dihydrotestosterone, which masculinizes the genitals. Thus, XY individuals with 5-alpha reductase deficiency have male-typical chromosomes and normal male levels of testosterone, but at birth their genitals may appear to be female (or ambiguous), because of low levels of dihydrotestosterone

This is true of the newborn Calliope. The distracted doctor who delivers her fails to notice her somewhat enlarged and phallic-looking clitoris, arid so Calliope (who comes to be nicknamed "Callie" by her Greek-American family) is baptized and reared as a girl. But like most XY individuals, Callie is destined to experience a dramatic rise in testosterone levels at puberty, which will be sufficient to enlarge her clitoris into something approximating a penis and to lower her voice, enlarge her Adam's apple, and develop her muscles. Callie's alarmed parents take her to a famed doctor in New York City, an expert in gender identity and gender anomalies. He recommends that Callie maintain her female gender identity, that she take female hormones, and that her genitals be surgically corrected to appear more feminine. But Callie secretly knows that she is sexually attracted to girls and she fears being mutilated, so she runs away from her parents, hitchhikes across the United States, and assumes a male identity. Callie becomes Cal.

The heart-wrenching transformation of Calliope from female to male is the stuff of fiction ... but sometimes it is the stuff of real life too. How does it all turn out for Callie? You have to read Jeffrey Eugenides' brilliant novel to find out. For now, consider the following: Although Middlesex paints a richly textured portrait of Callie's environment—her family, friends, home, schools, and cultural heritage—It also acknowledges that, in many ways, biology is destiny for Callie. Unlike the famous British philosopher, John Locke, who thought that each individual is born a tabula rasa (a "blank slate"), Eugenides vividly shows us how Callie's slate was far from blank.

Biology is destiny. Can this be true of gender? What exactly does scientific research tell us about the biology of gender? Do biological factors cause sex differences in human behavior? Do they also contribute to individual differences in masculinity and femininity? How can we answer such questions? The novel Middlesex points to one possibility: We can study people who are exposed to unusual levels of sex hormones early in life because of genetic or hormonal abnormalities. In addition, we can investigate those remarkable individuals who are reared as one sex and gender, but who choose another as they grow older. There are still other ways to study the biology of gender. We can experiment on animals and probe the impact that sex hormones have on their nervous systems and on their sex-linked behaviors, such as aggression and mating styles. We can measure sex hormones in humans and see if they are related to gender-related behaviors, such as aggression, visual-spatial abilities, and sexual attractions to men and women. And we can contemplate tragic real-life events that provide information about the power of nature and nurture to influence gender, such as when a baby boy loses his penis because of a botched circumcision procedure and is subsequently raised as a girl or when a baby boy lacks a penis because of a profound birth defect and is surgically reassigned to be a girl.

Animal Experiments

Because it is possible to do experiments on animals that would be unethical to do on people, we have more detailed knowledge about the effects of sex hormones on animals than on humans. Decades of research and hundreds of experiments show that sex hormones affect animals' nerve cells, which are the building blocks of their nervous system. Sex hormones influence the growth of nerve cells, the selective death of nerve cells, the tissues that nerve cells enter into, the density of nerve cells in various regions of the brain and spinal cord, the connections nerve cells make with one another, and levels of neurotransmitters (Breedlove, 1994; Hines, 2004; MacLusky & Naftolin, 1981). All of these effects may lead to sex differences in the nervous systems, both in lower animals and in humans.

Consider the following example. Both lower animals (e.g., rats) and humans have a collection of nerve cells in the lower spine called the spinal nucleus of the bulbocavernosus. In humans, these cells control (in men) a muscle that wraps around the base of the penis and contracts during ejaculation and (in women) a muscle that wraps around the opening of the vagina and controls vaginal contraction. In both rats and people, males have more nerve cells in the spinal nucleus of the bulbocavernosus than do females. Sex hormones, particularly prenatal or perinatal (around the time of birth) testosterone, affect the development and death rate of these nerve cells (Forger, Hodges, Roberts, & Breedlove, 1992; Nordeen, Nordeen, Sengelaub, & Arnold, 1985).

Sex hormones affect animals' behaviors as well as their nervous systems. Indeed, the behavioral effects of hormones were shown before their physiological effects were proven (Phoenix, Goy, Gerall, & Young, 1959). Experiments on rats and other rodents show that early exposure to androgens masculinizes behavior. Females exposed to androgens (as well as normal males, who are exposed to androgens in the course of their development) show male-typical behaviors such as rough-and-tumble play and sexual mounting. Males who have the effects of androgen stopped, either through castration or through chemicals that block its action, show female-typical behavior such as the female sexual posture (called lordosis), as do normal females.

Experiments on primates also demonstrate that early exposure to sex hormones influences later behaviors. Rhesus monkeys consistently show sex differences in rough-and-tumble play and foot-clasp mounting (the sexual posture that males use when mating). Exposing females to early androgens increases these masculine behaviors (Wallen, 1996). Other behaviors that show sex differences in rhesus monkeys, such as sexual presentation of the rump, aggression, and submissive postures, seem to depend more on the social rearing of monkeys: whether monkeys are raised in same-sex or mixed-sex environments, or whether they are reared by their mother or are separated from her. Nonetheless, these behaviors often show sex differences in natural settings, and they too are influenced by early exposure to testosterone.

A particularly fascinating example of the effects of hormones on brain structures and behavior comes from research on songbirds (Cooke, Hegstrom, Villeneuve. & Breedlove, 1998). In a classic study, Nottebohm and Arnold (1976) showed that in zebra finches, the brain region that controls the production of song is more than five times larger in males than in females. Male finches sing much more and produce more complex and elaborate songs than females, and thus this brain difference is matched by a behavioral difference.

The difference between the song regions of male and female finches brains results from the effects of early exposure to sex hormones. In birds, testosterone often acts on brain cells by first being converted into estrogen (a process called dramatization). Experiments show that female finches exposed early in life to elevated levels of estrogen show masculinized brains and sing like male finches as adults, as long as they are given androgens as adults to activate their song production (Gurney & Konishi, 1979). Thus sex hormones show both organizational and activational effects in songbirds (see Chapter 3). The songs of male songbirds are molded by the environment as well as by hormones; male birds must be exposed to the songs of their species while growing up in order to show well-formed songs as adults. Thus biological factors work in concert with, not in opposition to, learning.