Animals may begin to live together for a variety of reasons—most obviously, safety in numbers. In one of his most engaging papers, Hamilton observed that a tight flock, herd, or shoal will readily appear if every animal tries to make itself safer by moving into the middle of the group—a phenomenon he termed the “selfish herd.” But protection from predators isn’t the only benefit of bunching together. A bird in a flock spends more time eating and less time looking about for danger than it does when on its own. Indeed, eating well is another common reason for group living. Some predatory animals— chimpanzees, spotted hyenas, and wild dogs, for example— have evolved to hunt together.
Many social animals thus live in huge flocks or herds, and not in family groups—or even if the nexus of social life is the family, the family group is itself part of a larger community. In species such as these, social behavior must extend beyond a simple “Be friendly and helpful to your family and hostile to everybody else” approach to the world. At the least, the evolution of social living requires limiting aggression so that neighbors can tolerate each other. And often, the evolution of larger social groupings is accompanied by an increase in the subtlety and complexity of the ways animals get along together.
Consider baboons. Baboons are monkeys, not apes, and are thus not nearly as closely related to us as chimpanzees are. Nonetheless, baboons have evolved complex social lives. They live in troops that can number from as few as eight to as many as 200. Females live with their sisters, mothers, aunts, and infants; males head off to find a new troop at adolescence (around age 4). Big troops typically contain several female family groups, along with some adult males. The relationships between members of a troop are varied and complex. Sometimes two or more males team up to defeat a dominant male in combat. Females often have a number of male “friends” that they associate with (friends may or may not also be sex partners). If a female is attacked or harassed, her friends will come bounding to the rescue; they will also protect her children, play with them, groom them, carry them, and sometimes share food with them. If the mother dies, they may even look after an infant in her place.
Yet friendliness and the associated small acts of affection and kindness—a bout of grooming here, a shared bite to eat there—seem like evolutionary curiosities. Small gestures like these don’t affect how many children you have. Or do they?
Among social animals, one potentially important cause of premature death is murder. Infanticide can be a problem for social mammals, from baboons and chimpanzees to lions and even squirrels. During one four-year study of Belding’s ground squirrels, for example, the main cause of death for juveniles was other Belding’s ground squirrels; at least 8 percent of the young were murdered before being weaned. Similarly, fighting between adults—particularly in species where animals are well armed with horns, tusks, or teeth—can be lethal, and even if it is not, it may result in severe injuries, loss of status, or eviction from the group.
The possibility of death by murder creates natural selection for traits that reduce this risk. For example, any animal that can appease an aggressor, or that knows when to advance and when to retreat, is more likely to leave descendants than an animal that leaps wildly into any fray. Which explains why, in many social-mammal species, you don’t see many murders, though you do see males engaging in elaborate rituals to see who’s bigger and stronger. Serious physical fights tend to break out only when both animals think they can win (that is, when they are about the same size).
Thus, among animals such as baboons, friendships mean more than a bit of mutual scratching; they play a fundamental role in an animal’s ability to survive and reproduce within the group. Friendships between males can be important in overcoming a dominant male—which may in turn lead to an improvement in how attractive the animals are to females. Similarly, females that have a couple of good male friends will be more protected from bullying—and their infants less likely to be killed. Why do the males do it? Males that are friends with a particular female are more likely to become her sex partners later on, if indeed they are not already. In other words, friendship may be as primal an urge as ferocity.
The lineage that became modern humans split off from the lineage that became chimpanzees around 6 million years ago. Eventually this new lineage produced the most socially versatile animal the planet has ever seen: us. How did we get to be this way?
One clue comes from chimpanzees. Chimpanzee society is the mirror image of baboon society, in that it’s the females that leave home at adolescence, and the males that stay where they were born. Chimpanzee communities can also be fairly large, comprising several different subcommunities and family groups. Males prefer to associate with their brothers and half-brothers on their mother’s side, but they also have friendships with unrelated males. Friends hang out together and hunt together—and gang up on other males.
However, unlike baboon troops, which roam around the savannah freely intermingling, chimpanzee communities are territorial. Bands of males patrol the edges of their community’s territory looking for strangers—and sometimes make deep incursions into neighboring terrain. Males on patrol move together in silence, often stopping to listen. If they run into a neighboring patrol, there may be some sort of skirmish, which may or may not be violent. But woe betide a lone animal that runs into the patrolling males. If they encounter a strange male on his own, they may well kill him. And sometimes, repeated and violent attacks by one community lead to the annihilation of another, usually smaller, one. Indeed, two of the three most-studied groups of chimpanzees have wiped out a neighboring community.
Chimpanzees have two important sources of premature death at the hands of other chimpanzees: They may be murdered by members of their own community, or they may be killed during encounters with organized bands of hostile neighbors.
Just like humans. Except that humans aren’t armed with big teeth and strong limbs. Humans carry weapons, and have done so for thousands of years.