Shadows of Forgotten Ancestors

Preadolescent female mice have a molecule in theirurine that induces testosterone production in males who get a whiff. In turn, the males’ urine now contains pheromones which, when sniffed by the immature female, quicken her sexual development. She matures early if there are males around, and late if there aren’t—a positive feedback loop that saves unnecessary effort. (As you might expect, female mice who can’t detect odors never come into heat.) What’s more, normal pregnant females who sniff the urine from males of a different strain of mice spontaneously abort their pregnancies; they resorb the embryos back into their bodies and quickly come into heat.²³

This is convenient for the alien males. If the resident males don’t like it, it’s up to them to stop strangers from coming around with their abortion-inducing aromas.

In mice, as for many other animals, testosterone begins to be manufactured in earnest at puberty, and that’s when serious aggression against other mice begins. In adult males, the more testosterone, the quicker will be the attack when a strange male appears at the territorial frontiers. Again, castrate the males and their aggressiveness declines. Again, deliver testosterone to the castrates and their aggressiveness increases. Male mice are given to “marking” their environment with tiny dribbles of urine—a practice they pursue with redoubled effort when other mice are around (or when they come upon some unfamiliar object, maybe a hairbrush). Because of embryo resorption, if the males are to leave progeny at all, they must be the chief urinators in their territory. Maybe marking is like nametags on luggage, “no trespassing” signs on private property, or heroic portraits of the national leader in public places. The doughty little mouse is singing “This land is my land” and “She belongs to me.” Even when he’s not physically present he wants passersby to take careful note of his proprietorship. As you might suspect, castrate the mouse and urinary marking declines strikingly; resupply testosterone and his compulsion to mark is rekindled.

Normal female mice are infrequent urinators. They are not inveterate markers. But what happens if anatomically normal female infants are jolted with testosterone? Then they begin marking often. (If a similar experiment is done in dogs, adult females who were given testosterone before birth adopt the urination posture of the males; they lift one leg and trickle the urine down the other—one more indignity visited at the hands of the scientists.) When female rats with ovaries surgically removed are supplied with testosterone, they become aggressive, alternating a masculine propensity for confrontation with distinctly feminine sexual behavior. But one thing about giving testosterone to normal females early in their lives: When they grow up, the males find them much less attractive.

While testosterone in the blood is intimately connected with the expression of aggression in male animals, it is by no means the whole story. There are, for example, molecules in the brain that repress aggression. Hereditary strains of rats that are unusually violent turn out to have less of these inhibitory brain chemicals than more peace-loving strains. Aggressive rats are calmed when there are more of these chemicals in their brains; peaceful rats are agitated when there is less of these chemicals. If you’re a rat, busy watching violence in other rats—mice-killing, say—your level of inhibiting brain chemicals drops.²⁴

You’re now more likely to be violent yourself, and not just toward mice. Your repressed aggressive tendencies have been disinhibited. And everybody else’s. Hostility can then rapidly spread through your group, expressed differently by different individuals. Perhaps that’s what happened with Calhoun’s rats, so confined that aggression and despair spread in waves, reflected and amplified from multiple foci through the community. Violence is contagious.

In experiments performed by Heidi Swanson and Richard Schuster,²⁵ rats were given a complex cooperative task to learn, having to run together over specific floor panels in a particular sequence. If they succeeded, they were rewarded with sugar water; if they didn’t, they found themselves racing around the experimental chamber for the fun of it. Nobody taught them what to do, or at least not directly. It was trial and error. The experiment was tried on pairs of males, pairs of females, pairs of castrated males, and pairs of castrated males with testosterone implants. Some of the rats had previously lived alone.

Here’s how it turned out: Females, as well as male castrates, learned fairly quickly. Normal males and castrates with administered testosterone learned much more slowly. Males who had previously lived alone did still worse. Some pairs of previously solitary male rats—pairs with intact testicles as well as pairs of testosterone-jolted castrates—never learned at all.