Perhaps poor Caenis had overdosed on the stuff that Poseidon used to effect the metamorphosis. There is a proper amount of whatever it is that makes one male, the ancient Greeks recognized, and too much or too little can get you into trouble.
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The testicles of a sparrow are about a millimeter long and weigh about a milligram. (That’s one of the reasons you never hear that someone’s hung like a sparrow.) With testes intact, the scrappy birds enter into their mainly linear hierarchy, chase away other birds who invade their territory, and, if they’re high-ranking, make successful overtures to fertile females. But reach under those feathers, remove those two tiny organs, and, after the bird has recovered, all of these traits are lost, or nearly so. Aggressive birds become submissive, territorial birds become complacent about intruders, passionate birds lose interest in sex. Now inject a certain steroid molecule into the sparrow and it regains its plucky enthusiasm for sex, aggression, dominance, and territoriality.
Shortly after castration, male Japanese quails stop strutting, crowing, and copulating. They also fail to elicit the interest of female Japanese quails. Treat them with that same steroid and they’re back to strutting, crowing, and copulating, and the females find them irresistible once more. Castrate a young male fiddler crab and he will never develop his distinctive asymmetrical giant claw.
Humans have understood some of this for thousands of years. Captured warriors were castrated so they’d make no trouble. We still describe an ineffective leader as a “political eunuch.” Chieftains and emperors castrated men so they could guard the harems without succumbing to temptation (or at least—the compromise sometimes reached—without impregnating any of the residents); and so their loyalties to the leader would not be adulterated by family ties or other distracting affections and obligations. It is remarkable that almost exactly the same molecule should produce such fundamental changes in behavior in sparrows, quail, crabs, and humans.
The steroid molecule that works these transformations like some wizard’s potion is testosterone. Along with other, similar molecules, it’s called an androgen. It’s manufactured (from, of all things, cholesterol) mainly in the testicles,4 enters the bloodstream, and induces a complex set of behaviors that we recognize as characteristically male. Here too, the connection is acknowledged in the language, as in the expression “He’s got balls”—meaning he’s shown exemplary courage and independence, he’s not a coward or a sycophant.
In newly formed groups of male monkeys, the higher the rank in a forming dominance hierarchy, the more testosterone is found to be circulating in the blood. But when the hierarchy settles down to symbolic encounters, and betas are routinely submitting to alphas, the correlation vanishes. The more testosterone an animal has, the farther away he’s willing to roam to challenge and dominate potential rivals.6 With high testosterone levels there’s a cross-species tendency for dominance within the group to be extended to dominance over a piece of territory. The boss and the landlord become one.
In the brains of many animals are specific receptor sites to which the testosterone molecule and other sex hormones chemically bind, and which are in charge of hormone-induced behavior. There may be separate brain centers responsible for strutting, crowing, bullying, fighting, copulating, defending territory, and fitting into the dominance hierarchy; but each center has a button pushed by testosterone. The behavior is actuated once the testosterone migrates from the testicles through the blood to the brain. In the individual brain cells, the presence of testosterone activates otherwise untranscribed and ignored segments of the ACGT sequence, synthesizing a set of key enzymes. As with many hormones, testosterone is at the nexus of an array of positive and negative feedback loops that maintain the concentration of the molecule circulating in the blood.
Male animals don’t just endure, but seem to delight in, testosterone-mediated scuffles, intimidation, and combat. Mice will learn to run a complex maze when the only reward or reinforcement is the opportunity to have a tussle with another male. There are abundant similar examples in our species. Activities that are central to leaving many offspring tend to be entered into with enthusiasm. Sex itself is the most obvious example. Aggression is in the same category.
Even among animals with very short gestation periods, such as mice, the delay between conception and birth is too long for the animal to associate cause and effect. To leave it to mice to figure out the connection between copulation and the creation of the next generation is to condemn their genes to extinction. Instead there must be an absolutely overwhelming need for sex and—as a means of reinforcement—a delight in partaking of it. This is just the DNA creatively demonstrating its control in the most overt and clear-cut way.
A deal has been struck: The animal will forgo food, will conform to extreme postural indignities, will put its very life at risk so its strands of DNA can join up with the strands from some other animal of the same species. In exchange, there will be a few moments of sexual ecstasy, one of the currencies in which the DNA pays off the animal that carries it around and nurtures it. There are many other examples of DNA-mediated delight in activities tending toward adaptive fitness—including parental love for children, joy in exploration and discovery, courage, camaraderie, and altruism, as well as the standard array of testosterone-driven traits making bosses and landlords.
Hormones similar to testosterone play a central role in the development, of sexual organs and sexual behavior all the way down to the aquatic fungi. Steroids must have evolved very early to be so widely distributed today, perhaps going a fair way back to the invention of sex around a billion years ago.
This trans-species use of the same molecule for roughly the same sexual purpose has some bizarre consequences. For example, the chief sex pheromone in the pig is 5-alpha-androstenol—chemically similar to testosterone. It’s mixed in with the boar’s saliva (as testosterone is present in the spit of men). When a sow in heat smells this steroid on a slavering boar, she promptly adopts the come-hither mating posture. Oddly, truffles, the French culinary delight, produce exactly the same steroid and in a higher concentration than in boar spit. This seems to be why pigs are used by gastronomes to find and unearth truffles. (How strange it must seem to the sows, always falling in love with little black pieces of fungus, only to have them cruelly snatched away by humans.) Since truffles are fungi, in which steroids play key sexual roles, perhaps tormenting sows is just an accidental side-effect—or perhaps it serves the function of inciting pigs to dig so the spores are spread more widely and the Earth is covered with truffles.
Now in light of all this, what are we to make of the fact that 5-alpha-androstenol is also copiously produced in the underarm perspiration of men?7 Long ago—before institutionalized hygiene, before the present perfumed and deodorized age—might it have played a part in human and prehuman courtship and mating behavior? (The noses of women, we cannot help noting, are often at the same level as the armpits of men.*) Might this have something to do with the willingness of the rich to spend exorbitant sums on tiny pieces of a nearly tasteless cork-like substance?
A genetically male embryo deprived of testosterone and other androgens will emerge with what look very much like female genitals. Conversely, the genitalia of a genetically female embryo subjected to high levels of testosterone and other androgens will be masculinized: If smaller amounts of the steroid are present, perhaps she’s born with only a somewhat bigger clitoris; if larger amounts, her clitoris becomes a penis, and her labia majora fold over to become a scrotum. She may develop a normal-looking male penis and scrotum, although the scrotum will have no testicles within. (She’ll also have nonfunctioning ovaries.) Such girls as they grow up are found to prefer guns and cars to dolls and kitchen supplies, boy to girl playmates, and enjoy rough-housing and the outdoors; they may also find women sexually more attractive than men.8 (There’s no evidence for the converse—for example, that most tomboys have excessive amounts of androgens.)
The difference between m
ale and female, not genetically but on so fundamental a matter as which set of external genitalia you are to have, depends on how much male steroid you encountered in the first few weeks after conception. Leave that bit of developing embryonic tissue alone and it will become a female. Suffuse it with a little testosterone-like hormone and it will become a male, † The tissue is spring-loaded to respond to the androgen (the word literally means “male maker”), which serves as a means of internal communication. There are buttons on the developing embryo that only androgens can push. Once they are pushed, substantial machinery, whose existence you might otherwise never have guessed, takes over and works mythic transformations.
Across widely different animal species, another class of sex hormones, the estrogens, curbs aggressiveness in females, and yet another, progesterone, increases the feminine inclination to protect and care for the young. (The words signify, respectively, something like estrus-maker and gestation-promoter.) Mother rats, as all mammals, are attentive to their offspring: They build and defend nests, nurse the pups, lick them clean, retrieve them when they wander away, and teach them. None of this behavior is evident in virgin females, though, who studiously ignore newborn pups, or even make some efforts to avoid them. But prolonged treatment with the female hormones progesterone and estradiol—bringing the hormone levels of virgins up to those typical of late pregnancy—results in the emergence of marked maternal behavior. Rats with high levels of estrogen are also less anxious and fearful and less likely to engage in conflict.10
These female hormones are produced mainly in the ovaries. But when we see a calm, competent, and loving mother, most of us are not driven to exclaim “Man, has she got ovaries!” The reason doubtless has something to do with the ready accessibility of testicles for accidental or experimental removal, dangling as they do in vulnerable external sacs*—quite differently situated than the ovaries, which are locked away for safekeeping within the vault of the body. But clearly ovaries must equally be counted as among the family jewels.
The female hormones control the estrus cycle—which culminates when the females are ovulating and, usually, broadcasting olfactory and visual cues that they’re available for mating. In many species this doesn’t happen often and doesn’t last long; cows, for example, are interested in sex for about six hours every three weeks. Cows don’t date much. “For most species,” writes Mary Midgley,11 “a brief mating season and a simple instinctive pattern makes of it a seasonal disturbance with a definite routine, comparable to Christmas shopping.” In a wide variety of mammals, from guinea pigs to small monkeys, mating outside of estrus is not only discouraged by the female, it’s also made physically impossible by an organic chastity belt: The vagina is sealed by a membrane or plug grown specially for the purpose, or—even more decisive—it’s fused shut.
In contrast, among most humans and some apes, sex is not only possible but is equally probable at virtually any phase of the cycle. Some humans monitor the cycle (by measuring small changes in body temperature) and then avoid sex around the time of ovulation. This Church-condoned contraceptive technique is the mirror image of the practice of most animals—who garishly advertise ovulation and avoid sex at all other times. It is a reminder of how far from our ancestors our culture has taken us, and what fundamental changes in us are possible.
For many animals the ovulation cycle is a few weeks in length. Not many species have periods almost exactly equal to the lunar cycle (the time from new moon to new moon). Whether this peculiarity of humans is more than a coincidence—and if so, why it should be—is unknown.
Mammals suckle their young, but only the females are appropriately endowed.* It’s one of the few cases where the definition of a major classification category in biology, or taxon, is determined by the characteristics of only one of the sexes. Giving milk is also hormonally mediated. Mother’s milk is essential for the young, who are born helpless, unable to digest the adult diet. This is another reason that females spend more time with, and therefore have a greater investment in, the young. The males are generally more interested in other things—dominance, aggression, territoriality, many sex partners.
The connection between steroids and aggression applies with surprising regularity across the animal kingdom. Remove the principal source of sex hormones and aggression declines, not just among the mammals and birds, but in lizards and even fish. Treat castrated males with testosterone and the aggression returns. Give estrogen to intact animals and aggression diminishes, again across all these species. The repeated use of these same steroids for the same functions, turning aggression on and off, for so many different animals, is a testament both to their effectiveness and to their antiquity.
Aggression is adaptive, but only in controlled amounts. The repertoire of aggressive behavior is on call, awaiting only to be disinhibited. The steroids, their production titrated by the social environment and the biological clocks, do the disinhibiting. This being the case, why is it that males are so often more aggressive than females? If the females can generate a little less estrogen and a little more testosterone, can’t they become as aggressive as males? Something like gender equality in aggression occurs in wolves, tree squirrels, laboratory mice and rats, short-tailed shrews, ring-tailed lemurs, and gibbons. In the southern flying squirrel, males are not territorial but females are, and most quarrels between the sexes are initiated by the females—and won by them.13 The clear fact that males are more aggressive than females among us humans (where blood plasma testosterone is about ten times greater in men than in women) by no means commits the rest of the animal kingdom, or even the rest of the primates, to the same arrangement.
As anyone knows who has seen a pet tomcat drag himself home after an absence of a day or two—with an eye closed, an ear torn, his fur matted and bloody—testosterone exacts a price. What happens if you take a male animal—let’s say, someone less combative than tomcats out for a night on the town—and equip him with an implant that keeps his testosterone blood levels high? When this is done to sparrows, hardy territorialists, there seems to be no significant increase in the sparrow murder rate. But when male cowbirds are implanted, their numbers markedly decrease;14 many birds are now observed with unusually serious injuries, clearly obtained in combat with their fellows. Unlike sparrows, cowbirds establish dominance hierarchies but do not have core territorial refuges into which they can flee. Bluff can escalate into serious fighting if you’re simultaneously charged up with testosterone and have no tradition of sanctuary. Another steroid deficit: Male birds with artificially high testosterone levels are less inclined to feed their hatchlings.15 Macho males tend to neglect their family responsibilities.
Sex hormones are now manufactured by pharmaceutical companies, and widely used—legally and illegally. We can learn something about their role in Nature by asking why people use them. Anabolic steroids are molecules very like, but usually not identical to, testosterone. They’re taken mainly by: (1) bodybuilders and athletes (who widely believe that certain feats of strength can be accomplished only by young men on steroids); (2) young men who wish to macho up, usually to attract women or other men; and (3) those who wish to disinhibit their meanness (nightclub bouncers, hit men in organized crime, prison guards, and so on).16 The enhanced musculature does not come about through steroids alone; it also requires vigorous and systematic exercise. One of the side effects is facial and back acne. Anabolic steroids don’t seem to grow hair. Large doses lead to dysfunction and atrophy of the testicles—perhaps the body’s response to excessive testosterone titers; too much testosterone is socially sufficiently dangerous that a mechanism may have evolved so that tendencies toward excessive production aren’t passed on to future generations.
Estrogen is taken by women, usually post-menopause or post-hysterectomy, to preserve sexual interest and lubrication, to slow loss of bone calcium, and to achieve a more youthful complexion. Bodybuilding and transsexual women may take anabolic steroids because they strikingly redistribute weight—from thighs
to chest and biceps, for example. Transsexual men taking estrogen redistribute weight the other way, grow breasts, and feminize the nipples and areolae; there’s also a general mellowing of temperament. Bearing in mind these consequences of taking sex hormones as an adult, and the much more profound influence they have on the embryo—actually determining which sexual organs will be present—it seems likely that far subtler changes in hormone levels might influence not just dominance, territoriality, aggression, care for the young, gentleness, anxiety level, and talent for conflict resolution, but also sexual appetite and preference.
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Bulls, stallions, and roosters are made into steers, geldings, and capons because humans find their machismo inconvenient—the very same male spirit that the castrators likely admire in themselves. One or two skilled motions of the blade—or a deft bite by a reindeer-herding Lapp woman—and the testosterone levels are down to manageable proportions for the rest of the animal’s life. Humans want their domestic animals to be submissive, easily controlled. Intact males are an awkward necessity; we want just enough of them to father a new generation of captives.