Extent and limits of sexual dimorphism of the brain
Everybody will agree that
there are biological differences between boys and girls and between men and
women. We all think of such things as
primary (sexual organs) and secondary sexual physiognomic traits (Adam's
apples, voice tones, body hair, etc.).
Unfortunately, many persons
interested in sex differences are unconcerned with biological sex differences
which are not immediately palpable or which cannot be seen directly. In mammalian species (the rat has been
studied most in this respect) including
humans, there are obvious and subtle
brain differences between the sexes. The
obvious brain differences can be deduced as follows: if the sexual and reproductive organs are
very different in men and women, and to
the extent that these organs operate on rigidly biologically determined
species-specific time-tables (the menstrual cycle, the gestational cycle, etc.),
and if their operation is minimally complex,
then the neurons of the brain which control them must somehow be
different in men and women. This has
been shown by thousands of empirical investigations to be so. Consider this
example. The hypothalamus is a small
phylogenetically ancient (primitive) structure (it is found in fish
brains) situated at the center of the
brain. It is a group of closely packed
neurons, tightly interconnected by synapses (physiologically active
connections). It is strongly linked to a
gland situated just beneath it in the brain called the pituitary gland (also
known as the hypophysis). The
hypothalamus controls the pituitary gland.
The pituitary gland controls other glands that are sex-specific
(ovaries, testicles) and secretes factors which regulate sex-specific hormones
(oxytocin, prolactin, progesterone, estrogen, testosterone). You can imagine that there has to be a
difference between the hypothalamus of a man and that of a woman. There are other examples of brain systems
that are sex-dimorphic for very obvious reasons. Though the clitoris does undergo a bit of
tumescence during sexual excitation, the
penis undergoes much more. Penile tumescence, like every behavior, is under neural control. So it would be only natural, and it should be
expected, that scientists should find that the part of the brain controlling
penile tumescence ought to be bigger in men than in women. The spinal nucleus of the bulbocavernosus
(SNB) is one of the control centers in question. It is a small nucleus (aggregate) of neurons
situated in the spinal cord which controls penile tumescence. There are also
select muscles which are more markedly developed in the adult man than adult
woman. The perineal muscle, also involved in penile erection, is an example of this and it is controlled by
a spinal nucleus called the dorsolateral nucleus (DLN). These two spinal cord nuclei are sexually
dimorphic in the rat and human. These
nuclei and the perineal muscles they innervate are present in males but reduced
or absent in females. The sex difference in motoneuron number in these nuclei
is due to an androgen-regulated motoneuron (neurons directly feeding muscles)
death. Developing females treated with the androgen testosterone propionate
(TP) have a fully masculine number of SNB and DLN motoneurons and retain the
perineal muscles they would normally have lost. Paradoxically, females treated
prenatally with the androgen dihydrotestosterone propionate (DHTP) also retain
the perineal musculature but as adults lack the SNB motoneurons which would
normally innervate them. The SNB target muscles retained by DHTP females are
anomalously innervated by motoneurons in the DLN. Counts of motoneurons and
degenerating cells in the developing SNB of DHTP-treated females showed that
their feminine number is the result of a failure of DHTP to prevent the death
of SNB motoneurons. Furthermore, the peak number of SNB motoneurons was below
that of normal females, suggesting that DHTP treatment may also have inhibited
motoneuronal migration. However, DHTP treatment fully masculinized both
motoneuron number and degenerating cell counts in the DLN of these females, and
it is this masculinized DLN that gives rise to the anomalous projection. Taken
together, these results suggest that the effects of different androgens during
development are specific and complex, involving the regulation of motoneuron
death, migration, and specification of peripheral projections. Very recent findings add a fascinating twist
to this story. As surprising as it may
seem to many developmentalists, sexual
experience may play a role in the development of sex differences in such brain
nuclei. Investigators looked at the
brains of sexually active and inactive male rats. They found motor neurons --
the cells in a hormone-sensitive part of the brain -- were smaller in the lover
than in the virgin rats (Breedlove, 1997).
A leading researcher in this field,
Breedlove, proposes that
"Somehow the extensive sexual experience affected the morphology of these
neurons." It is possible, he says,
that differences in sexual behavior cause,
rather than are caused by, differences in brain structure. Another recent study also investigated the
effects of sexual behaviorial manipulation on brain plasticity in adult male
rats. Adult male Sprague-Dawley rats
were divided into four groups: control male; gonadectomized (Gdx) male;
sexually active male; and sexually nonactive male. Female animals were used as
an additional control group. At the end of a 12-week experimental period, the
animals were again tested for male sexual behavior and tested for sexual
motivation. Sexual behavior manipulations over the 12-week period resulted in
significant differences in mount latency, mount frequency, intromission latency,
intromission frequency, ejaculation latency, and the postejaculation
interval. In the motivation test,
significant differences in the number of approaches, contacts, and crossings of
an electrified grid separating the test animal from a receptive female were
also observed. The sexually dimorphic
nucleus of the preoptic area (SDN-POA) volume in sexually nonactive males was
significantly smaller than in control males or sexually active males.
Anteroventral periventricular nucleus (AVPV) volumes in the male groups were
not significantly altered by sexual behavioral manipulations and did not differ
from female volumes (Prince et al, 1998).
Men's brains weigh, on the average, 200 grams more
than women's brains: about 1500 grams
for men and 1300 grams for women. But
what does this mean ? Large men's
brains weigh more than small men's brains.
In fact, the body-size difference
between men and women is believed sufficient,
by most commentators, to explain the difference in brain weight. Besides,
brain weight does not correlate well at all with intelligence. Einstein had a rather small brain, but he wasn't linebacker format either, was
he ? Whales have huge brains, much larger than humans, but they are not as smart. If a well trained neuroanatomist (not
specifically trained in the anatomy of sex differences though) is handed
several normal brains and asked whether they were extracted from the skulls of
a) a large woman, b) an ordinary woman, c) an ordinary man, d) a small man, he or she will have absolutely no idea, no matter how many hours he or she gazes at
it and no matter how many slices he or she cuts. In short,
men's and women's brains are remarkably similar at the macroscopic level
(at the level of gross anatomy).
Nevertheless,
there may be a subtle difference in overall brain anatomy between
normal men and women. The
neuroendocrinologist Helmuth Nyborg recently published a book chapter in which
he carefully analyzed the evidence. He
concluded, apparently justifiably as far as I can judge, that there remains a
difference in brain weight between the
sexes, after adjustment for body weight, of about 100 grams. Others have independently come to the exact
same conclusion. Furthermore, pubescent
boys, who weigh less than their female
counterparts have 20% heavier brains.
Finally, within a given sex, body weight barely correlates with brain
volume. In fact, a consensus is building in neuroscience to
the effect that the human male has 100 milligrams of extra brain, but it still not clear whether he has more
neurons than the human female: Sandra
Witelson found that neuronal packing is significantly tighter in women in one
specific area of cortex (the planum temporale). However, scandinavian researchers have
recently published a paper reporting that they estimated normal men to possess
about 19 million more neurons than women.
Since there are about a thousand billion neurons in the average
brain, this difference is roughly
commensurable with Nyborg’s proposal of a 100 gram brain weight difference
between men and women. However, the reader is advised to withhold judgement
on this issue until exhaustive research has been published.
Besides, more
does not always mean better. Excessive
brain tissue can be caused by an error of genetic programs controlling brain
development in utero (before birth). Some of these conditions are so severe that
they are called macrocrania or macrocephaly (an excessively large head) and are
typically associated with mental deficiency. Sotos syndrome (cerebral
gigantism) is one of the better known of these conditions. I am not suggesting that the male sex is more
at risk for macrocrania. However, the male sex could indeed be subject to a
subtle delay (and even a disadvantage) in prenatal pruning (natural cell death)
in the brain. A neuroanatomist named
Sandra Witelson has eloquently argued that this may indeed be the case, and I review some of her arguments to this
effect a few sections down.
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