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HEN
I SAW THIS SEX DIFFERENCE, it blew my socks up!" my graduate adviser,
Geert De Vries, has been known to say. While not always quite up to speed
on his English idioms, De Vries is fluent when it comes to communicating the
excitement of research, and it was this comically worded phrase that first
convinced me I wanted to work with the tall Dutch psychologist who discovered,
back in 1981, the first documented sex difference in a neurotransmitter system.
The exploded-socks allusion describes a high point in De Vries' career: the moment he observed the tremendous differences between the brains of male and female rats. Though he calls it "pure, dumb luck," it was knowledge of what questions to ask that made the discovery possible. De Vries had been asked to trace the development of a neurotransmitter called "vasopressin" in the brains of rats. Not realizing that the sex of his subjects could interfere with his data, he used both males and females in his sample. To his dismay, he encountered tremendous variability. Some animals had abundant vasopressin, others had meager traces.
When he organized the data by sex, however,
the variation disappeared. Males had two to three times the number of vasopressin
cells that females had! Subsequent studies correlated the difference with
higher levels of testosterone in the males. And the difference is not genetically
fixed, since it can be erased by changing the hormones the rats are exposed
to as babies and as adults.
At the time of De Vries' discovery, the list of known sex differences in
brain structure was very short. Today, we know that sex differences in
brain structure are pervasive among mammals including ourselves. Various
brain nuclei, for example, are larger in men than in women; the mass that
connects the two hemispheres of the brain is larger in women than in men.
What
is not clear, however, is why these differences exist. It was the desire to
understand the functional importance of sex differences in the brain that
led De Vries to team up, in 1989, with Melinda Novak and Betty McGuire, fellow
members of the psychology faculty who were working with a species of chestnut-brown,
mouse-like creatures called prairie voles. Normally found inhabiting the Midwestern
United States, prairie voles are distinguished by an appealing tendency to
form stable pair-bonds what we would call marriages. Pair-bonded prairie voles
are virtually inseparable; they groom each other, huddle together, and nest
together until death separates them. When pups are born, Mr. and Mrs. Vole
share the work of keeping them warm, safe, and clean. While this behavior
may sound commonplace, or at least ideal, by human standards, it is quite
unusual; among mammalian species, few males remain with their mates after
copulation, and even fewer are involved in the care of offspring.
Traditionally, researchers who study sex
differences in the brain associate differences in structure with differences
in behavior . For example, if we humans had nucleii in our brains that
determined how much we like to watch Monday night football, tradition would
dictate that such nucleii would be larger or more active, on average, in
men than in women. Conversely, nucleii controlling behavior that is the
same between men and women say, reading or walking should be indistinguishable
between the sexes.
Following this logic, male and female prairie voles should have virtually
indistinguishable brains. Yet Maryam Bamshad, who pioneered the work on
voles in De Vries' lab, has shown that the male and female voles not only
differ in brain structure, but that the differences are even more pronounced
than among promiscuous, non-paternal rodents.
Differences
in brain structure may become important in the context of differences in hormonal
activity. The popular terms "premenstrual syndrome" and "postpartum
depression" have made us all aware of the impact hormones have on behavior.
While these conditions denote discomfort, irritability and misery, others
denote enhanced sexual arousal, aggression, or euphoria. And of particular
interest in connection with the prairie voles the hormones of pregnancy can
promote bonding between mother and infant, an intense will to protect and
nurture the young.
In essence, it is hormones that trigger
female mammals to become maternal. But only the female parent can experience
this parenting elixir. How then, does nature produce daddies?
De Vries now proposes that differences in brain structure may in fact be
what allow prairie vole parents to behave so similarly : that neurological
systems that are larger and more active in the male than in the female
may compensate for his dearth of pregnancy hormones, and prompt him to
be as fatherly as if he'd carried the pups himself. If so, it's the very
difference between male and female prairie voles that allows them to be
equal!
Voles are probably not the only mammal
whose brains use difference to promote parity. Bennett Shaywitz at Yale
has shown that men and women performing equally well on language-processing
tasks use their brains differently to do so. Women tend to use both the
right and the left hemispheres of the brain when processing language; men
tend to use only the left. Language may thus be an area in which we use
our brains differently to accomplish the same goal.
Author and graduate student Connie Villalba adjusts for sex-related differences in height with her adviser, Geert De Vries.
Photo by Thomas Kendall '93
Here's a more homely example. My kitchen
was apparently designed for an Amazon woman. While most men would have
no problem reaching the cereal boxes in the cupboard above the stove or
the French cookbook on the top shelf, these objects are far beyond the
reach provided by my five-foot, two-inch stature. Ingeniously, I employ
a collapsible step ladder: instant man height! What is to prevent our brains
from providing us with internal balances biological step ladders, as it
were that unfold without our conscious intercession?
If, as De Vries proposes, non-gender-specific behavior is actually promoted by sex-specific differences, we must change the way we think about brain function. For instance, strokes that damage the left side of the brain leave men barely capable of speech; the same damage to a woman's brain is far less debilitating, since she still has much of her language apparatus intact. Similarly, drugs we assume are affecting men and women similarly may actually have very different effects on behavior. My own recent research has shown, for example, that the drug Prozac which countless Americans are taking for behavioral disorders has different behavioral effects in male and female prairie voles. "By gaining an understanding of how male and female brains are different," says my adviser, "and understanding how drugs like Prozac may be affecting males and females differently, we may eventually be able to tailor our approaches to address specific needs."
