Shadows of Forgotten Ancestors (27 page)

THE NEXT-TO-LAST REMEDY

O
nce organisms get really good at sex, once they evolve the plumbing and the passion for it, there gets to be a danger: So many competent, DNA-exchanging beings may be born that they will improvidently gobble up all the food or nutrients or prey, and then almost everyone, including their close relatives, will die. This must have occurred innumerable times in the history of life.

Take a being as modest as a bacterium, weighing in at a trillionth of a gram, and let it reproduce with no impediments. In the second generation there will be two bacteria; in the third generation, four; in the fourth generation, eight; and so on. If we imagine that none of those offspring die, then in 100 generations they will collectively weigh as much as a mountain; in 135 generations, as much as the Earth; in 150 generations, as much as the Sun; and in 185 generations, as much as the Milky Way galaxy.

Of course, such prodigious increases in mass are arithmetic exercises only. They could never occur in the real world. For one thing, the replicating microbes would soon run out of food. Your descendants cannot weigh as much as a mountain if there’s not a mountain’s worth of food to eat—much less an Earth’s worth or a Sun’s or a galaxy’s. There is only so much food available. Thus, your descendants will quite soon be in competition with one another for scarce resources. But because of the enormous power of exponential reproduction, an organism with even a slight advantage in finding or utilizing food rapidly supplants the competition (or at least its descendants do). Fast reproducers generate large populations, and competition for resources; they provide the raw material for a natural selection that efficiently magnifies small differences in fitness, differences that might be too small or subtle for even the most skilled naturalist to notice. This was the central argument of Darwin’s unpublished 1844 manuscript on evolution, and of his article in the
Proceedings of the Linnaean Society
of London for 1858.
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So what happens in fact when there’s too much crowding? Some responses seem to serve a larger purpose. Sibling shark embryos fight to the death in utero. In many nonhuman mammals, brothers and sisters of the same litter compete for access to nipples; often, there is a least competent infant, unsuccessful in elbowing its way to a nipple—the runt of the litter, who becomes progressively weaker with each failed attempt to nurse. The Virginia opossum has thirteen teats and, generally, more than thirteen pups per litter. Only those who regularly get to a teat live. Such competitions weed out the weak. Those species with more teats than pups permit weakling and unaggressive youngsters to reach adulthood. If they are unlikely to compete successfully as adults and pass their genes on, their mother has, from the point of view of her genes, been wasting her time nursing such pups. Those mothers with fewer teats or more pups have a selective advantage. Concern about cruelty and suffering doesn’t, so far as we know, enter into it.

Cities aside, we humans routinely experiment on crowding animals into confined enclosures. The institutions responsible are called zoos; some are much more pernicious than others. A well-known problem of zoos is that many of the inmates are somehow less able to “breed in captivity”; another problem is sustained and violent conflict, usually between males of the same species. Zookeepers have learned that if they wish to maintain their “inventories,” they must often separate the males. Experiments have also been performed in the laboratory to study overcrowding. In all of these cases it’s important to remember the artificiality of the circumstances. An option available in the wild is unachievable in captivity: No matter what the provocation, a caged animal cannot flee conflict and make a new start somewhere else.

Norway rats have been bred in scientific laboratories since the middle nineteenth century. Artificial selection has elicited—partly through unconscious choices by laboratory personnel—a strain of rats that is calmer, tamer, less aggressive, more fertile, and with significantly smaller brains than their wild ancestors. All this is a convenience for those experimenting on rats.
³

In a now-classic experiment,
⁴
the psychologist John B. Calhoun let Norway rats reproduce in an enclosure of fixed size until the number of occupants, and therefore the population density, was very high. He
made sure, however, to provide everyone with enough to eat. What happened?

As the population increased, a range of unusual behavior was noted. Nursing mothers became somehow distracted, rejecting and abandoning their infants, who would wither away and die. Despite the surplus of ordinary food, the bodies of the newborn would be greedily eaten by passersby. An adult female in heat or estrus would be pursued relentlessly, not by one, but by a pack of males. She had no hope of escape, or even sanctuary. Obstetrical and gynecological disorders proliferated, and many females died giving birth, or from complications soon after. When crowded together, the rats lost their inclination or ability to build nests for themselves and their young; their desultory constructions were amateurish and ineffective.

Among the males Calhoun distinguished four types: the dominant, highly aggressive ones who, although “the most normal,” would occasionally go “berserk”; the homosexuals who made sexual advances to adults and juveniles of both sexes (but, significantly, only to nonovulating females): their invitations were generally accepted, or at least tolerated, but they were frequently attacked by the dominant males; a wholly passive population that “moved through the community like somnambulists” with nearly complete social disorientation; and a subgroup Calhoun calls the “probers,” uninvolved in the struggle for status but hyperactive, hypersexual, bisexual, and cannibalistic.

If there were no differences between rats and people, we might conclude that among the consequences of crowding humans into cities—other things being equal—would be more outbreaks of street fighting and domestic violence, child abuse and neglect, soaring infant and maternal mortality, gang rape, psychosis, increased homosexuality and hypersexuality, gay bashing, alienation, social disorientation and rootlessness, and a decline in traditional domestic skills. It’s suggestive, surely. But people are not rats.

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.
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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.

——

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.
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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.