Crowding in cats leads to a nightmarish tableau of incessant hissing and squalling, fur standing on end, remorseless fighting, and the designation of pariahs who are attacked by all. But people are not cats either.
Crowding in our nearer relatives, the baboons, can lead to bloodshed and social disorder at least on the scale of rats and cats, as we treat later. In many animals overcrowding also leads to increased susceptibility to disease, and smaller adult stature. But as vervet monkeys become more and more crowded together, the inmates begin studiously avoiding one another, inspecting with great interest the ground on which they sit and the motion of clouds in the sky above. In chimpanzees, crowding does tend to make everybody a little edgy. There is more aggression. But not much more. As the population density increases, chimps make concerted efforts at appeasing one another, at peacemaking.5 They have neural machinery and a social idiom to compensate for overcrowding. Are we not more like chimps than like rats?
The rat response to overcrowding, even at its most pathological, might be viewed as making sense in a remorseless evolutionary way. If the population density becomes too high, then mechanisms are set into motion to reduce it. Huge numbers of socially disinterested adults, illness, increased homosexuality, and soaring infant and maternal mortality, all serve this purpose. Eventually, the population crashes, overcrowding is relieved, and the next generation is back to business as usual—until the population pressures build up again. Some of the behavioral responses to high population density in Calhoun’s rats, and in many other species, might be looked on not as barbarous and unfeeling, but as a calamitous necessity, the capability for which has been painstakingly evolved.
We’ve phrased this in terms of group selection, but interpretations in the idiom of kin selection are also possible. We could, instead, have stressed that overcrowding is, almost invariably in Nature, a prelude to famine, so it makes a desperate kind of sense to abandon or eat nursing infants, or to cease building nests for the young, or to arrange that babies be stillborn or not conceived at all.6
In many animals—howler monkeys, for example—high population density leads to takeovers by alien males and the wholesale slaughter of resident infants. This behavior is especially vivid in animals where dominant males keep harems or try to prevent other males from reproducing.7 But is it fundamentally due to overcrowding, or to the evolutionary strategy of the new dominant male? It benefits the proliferation of his complement of genes to remove all distractions from the females as quickly as possible, move them into ovulation (which killing their young accomplishes), and impregnate them before he’s overthrown by the next usurper.* The more crowding there is, the more challenges from sexual rivals and the more such infanticides. Whether all of the anomalous behavior of Calhoun’s rats can be understood in these ways is still unclear; but surely some of it can.
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If, sympathizing with the rats, cats, and baboons in these experiments, we wished to help them, what could we do? We might be tempted to organize a jailbreak and return them to their natural environments. We would eliminate the overcrowding and—assuming the animals could fend for themselves—hope they would revert to their normal behavior and social organization. But then shouldn’t evolution also have invented mechanisms for dispersing competing organisms so they’re not in each other’s way—especially the most flagrantly aggressive variety, usually the young adult males? This would be to the advantage of both the individual and the species.
In fact, Nature provides such a safety valve: Instead of staying on to fight to the death, the potential losers—those who estimate that they would be vanquished if they continued fighting, or those who judge that the probable benefits of fighting are not worth the risk—may simply pick up and leave. There is an escape clause in their contract, a get-out-of-jail-free card, which precipitously reduces the incidence of mutilation and murder. A few formalities and they’re gone. But lock them up in a zoo or a laboratory apartment house for rats and all possibility of escape is denied them. That’s when they go crazy.
Some kind of mutual repulsion is needed, like that provided by electrical charges of the same sign or polarity. When two electrons are far from one another, they hardly feel each other’s influence. But bring them close together and a powerful force of electrical repulsion is brought into play, the force being stronger the closer together the electrons are. Something similar is true for magnets. Opportunistic animals able, under favorable conditions, to reproduce exponentially need a similar mutual repulsion, increasing quickly as the animals are brought into systematic close contact. There is such a force in Nature: intraspecific aggression, aggression within, internal to, a given species.
Most competition in animals is with members of the same species. How could it be otherwise? They have almost precisely the same habitat, the same tastes in food, the same erotic aesthetic, the same nesting and sleeping places, the same foraging and hunting grounds. If the animals are spread out, there’s enough food and other resources for everyone, while they can still remain near enough so they can find each other when it’s time to mate. If they’re crowded together, conflict escalates and even the strongest animals run an increased risk of lethal combat.
Spreading out is accomplished by aggression, but aggression is not the same as violence and rarely goes as far as violence.8 Often it’s enough to announce menacingly to all within earshot that this is your territory and no intruders will be tolerated. You might patrol the frontiers, spraying your urine or depositing your feces in prominent, strategic locations—or leaving, through special scent glands and much dragging and rubbing, an aromatic token of your proprietary interest. If you’re a grizzly bear, you might try marking a pine tree as high up as you can reach; when potential poachers grasp how big you must be to mark so high, they’ll give you wide berth.
About 80% of the different orders of mammals are armed with specialized scent glands. Gazelles have them in front of their eyes, camels on their feet and neck, sheep on their bellies, some pigs on the wrist, chamois behind the horns, pronghorns on the jaw, peccaries on the back, musk deer in front of the genitals, and goats on the tail. Water voles rub their hind feet over their flank gland and rhythmically drum them on the ground. Gerbils and woodrats rub their bellies directly on the ground, secreting their scent mark from a ventral gland. Some animals have five or six different kinds of scent glands in various places on their bodies, each conveying a different chemical proclamation. Cats spray carefully titrated amounts of urine on the drapes and upholstery, in case some presumptuous alien feline might enter the living room and curl up before the fire. Rabbits meticulously deposit piles of feces, each pellet coated by the anal scent gland, at crossroads in the warren—like the altars of Hecate on the highways of ancient Greece.
Some animals mark others with these scents, and rats urinate on their partners’ bodies—perhaps as a sign of proprietorship over individuals as well as territories. Animals can distinguish male and female, their own group or strain from others, age, individual identity, and the sexual receptivity of females, all by odor alone.9 Scientists have begun to decipher the stock phrases of their chemical communications—maybe just “foreigners keep out: this means you,” or “single male, well-bred, wishes to meet attractive single female …,” or “for a good time, follow this scent trail.” Sometimes it seems to be something much more subtle. Animals are busy filling the olfactory communications channels with a richness and fineness of discrimination long ago lost to humans. With all our instruments, we have not yet learned how to reenter that world.