Among social animals, inequality is a fact of life. Millions of ants do all the work for one reproducing queen. Troops of chimps form male-dominated hierarchies, males bossing females around and forming a pecking order with one highly aggressive alpha male on top. Poorly paid migrant workers pick grapes for $200-dollar bottles of wine enjoyed by royalty and corporate executives.
But sometimes the top dog gets toppled, inequality diminishes, and equality prevails. Looking at species other than our own shows that inequality (the subject of a special issue in Science this week) may not be inevitable. These animals provide glimpses of how humans got the way we are and what our potential may be for equality.
Forming coalitions is key to changing the power structure, whether it’s the French Revolution, the Arab Spring, or a particularly violent overthrow in chimps. On a peaceful Sunday afternoon in Mahale Mountains National Park in Tanzania, as tourists and their guides looked on, a fight broke out between Pimu, the alpha male, and four of his underlings—and they killed him. Chimps were known to form such large gangs and attack and kill chimps from other troops, but typically coalitions that challenge a chimp within a troop consist of just two or three individuals, and no one dies. A long time had passed since the males in this troop had had to cooperate to defend their territory, and with just six females for the group’s 10 males, competition for mates was fierce. Moreover, “Pimu was a particularly violent individual; therefore, the other males might not have liked him,” says Stefano Kaburu, a behavioral ecologist at the University of Kent in the United Kingdom. He and his colleagues described the attack in 2013. Though such uprisings are rare, he said in an email, “this event teaches us that whoever is in charge needs to balance their aggressiveness against their subordinates … with the ability to show them social support and cooperation.”
An aggressive male also roused the ire of a bonobo troop in the Democratic Republic of the Congo. Bonobos have been called the hippie apes because conflicts are rare and they have a lot of sex. Male bonobos are bigger than the females and have bigger teeth, but unlike male chimps they don’t boss the females around. Nor do males form coalitions. They don’t need to: They don’t fight with neighbors, and they can’t really tell when a female is fertile, so there’s little reason to bicker over them. Instead males spend a lot of time with females with the hope that she will mate with them when the time is right.
“But they are not as peaceful as has been described,” says Gottfried Hohmann, an anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Pecking orders do exist, and females do bond, enabling them to dominate the males. As Hohmann and his colleagues discovered one day, those bonds can be powerful enough to overthrow the power hierarchy.
Hohmann and his colleagues witnessed the head male try to attack a young female carrying an infant. Chimps often practice infanticide to get rid of rivals’ offspring and to hasten a female’s return to fertility, but this usually doesn’t happen in bonobos. This offending male was the son of the dominant female, but even so, the females immediately came to the target’s defense. They drove the offending male away—for good. “This is one lesson that humans can learn,” says Hohmann. “Our picture of dominant males over females is not always true.”
These two recent incidents in our closest relatives help “establish the fact that our human nature is set up so that we will resent authority and being bossed around, and we are able to form coalitions to do something about that,” says Christopher Boehm, a cultural anthropologist at the University of Southern California. Lions, meerkats, wolves, and dolphins can all work together, but they don’t seem to take the next step and turn on undesirable leaders. The fact that other primates do “helps explain modern democracy,” he proposes.
Even when inequality persists, as illustrated by a chance event in a group of baboons in Masai Mara National Reserve in Kenya, a lessening of top-down control can make low-ranking individuals less miserable.
One group of baboons lived near the garbage dump of a tourist lodge and foraged there; another group lived about a mile away. Males from the latter troop that were big and strong enough to fight their way past the rival group and into the garbage dump did so, forgoing daily grooming with their own troop members to get access to a rich food supply. But one day contaminated meat was left in the dump, and these males (along with most of the troop living near the dump) succumbed to tuberculosis. Their loss changed the atmosphere of their home troop.
“Baboons were literally the textbook example of a primate whose social structure was built around violence, male domination, inequalities,” says Robert Sapolsky of Stanford University, whose research group was observing the baboons. The animals engage in physically and psychologically menacing behavior. Higher ranking males, when beaten on by a superior, tend to turn on the next one down in the pecking order and beat him in turn.
With the most aggressive males gone, however, there were far fewer confrontations among the remaining males. This much more benevolent culture persisted for more than a decade, even as new males joined the troop. The newcomers took their cues from those already there and maintained an unusually peaceful culture. “If that can occur in a troop of baboons, you don't have a leg to stand on when claiming the inevitabilities and unchangeability of human societies,” Sapolsky says.
Some animal species regularly maintain egalitarian societies. The reasons behind equal sharing of mates, food, or other resources vary. In some cases, social control of others just doesn’t work. In others, equality seems to arise as a way to keep a group strong.
In species that are cooperative breeders, females communally feed and care for infants. It may sound utopian, but in most cooperative breeders, the dominant female monopolizes the mating and kills any other female’s offspring—a necessary step because food is usually so sparse that a group can support only one litter’s worth at a time. This happens in meerkats. But female banded mongooses have better access to food. They synchronize their mating, and 85 percent of the females breed successfully. Because pups are born at the same time in a communal den and begin nursing any female, it’s hard to know which pup belongs to whom, so the dominant female allows all the young to survive.
Equality among female lions likely arises for a different reason. Although male lions are hierarchical, lionesses show a lot of mutual respect and don’t challenge one another over food or mates, says Craig Packer, a behavioral ecologist at the University of Minnesota, who has studied lions in the wild for 48 years. Tim Clutton-Brock, a behavioral ecologist from the University of Cambridge in the U.K., suggests that egalitarianism arose because it’s just too dangerous for females to fight each other. They are built for killing animals bigger than themselves, so a larger female trying to push a smaller one around could easily run into trouble. “Lionesses basically have the nuclear option,” says Clutton-Brock.
But the lions also really need one another. Each pride fends off other groups of lions that want to take over the few prime hunting grounds, such as watering holes. In addition, females raise their cubs together for almost two years. With multiple females around, they have enough hunting power to take out a water buffalo or large prey. “So they are co-equals,” says Packer.
Among northern muriqui monkeys in the Brazilian rainforest, both sexes are co-equals and there’s little conflict. Karen Strier started studying this species in 1982 after conducting field research on baboons, and she fully expected that higher-ranking monkeys would get better food, have more friends and family, and have more reproductive success. But she observed no spats over food, even though males hung out close to one another, and she was shocked to see that when a female was ready to mate, the males waited patiently in a line for their turn to copulate. No particular male got to go first, and a study of 22 youngsters showed that 13 different males had become fathers.
Over the past 30 years, Strier, a primatologist at the University of Wisconsin–Madison, has expanded the work to include multiple groups of muriquis, and the population at her study site has soared from 50 to 350 individuals in part because of protection from hunting and habitat loss and in part because for decades there have been many females born who have in turn reproduced. “Yet the low rates of aggression and absence of consistent [hierarchal] patterns have persisted despite the greater potential for competition,” she says.
How have these monkeys bucked the trend? “Having two sexes starts the whole game of inequality, especially in mammals,” says Hohmann. Males tend to evolve to attain larger sizes to dominate females and other males, but muriqui males and females are the same size. Strier thinks size differences are constrained by the need to be fast and agile as the monkeys hop among tree branches in search of fruit. This species also eats leaves, which are readily available rather than concentrated in certain locations, so females spread out more. Males are likely to be related because they stay with their natal group. (Females are the ones that disperse when they grow up.) Males work together to keep track of which females are ready to mate and to make sure males from other groups stay away. There’s plenty of food to go around, so the monkeys don’t fuss much over who is eating what.
Strier and her colleagues have found that males tend to associate with a large number of other males and that females develop a few close friendships. But when females forage, they tend to avoid those close friends, possibly to avoid conflict, Strier notes.
Muriquis provide an example of a peaceful, alternative way of life, she points out. “Just knowing that such alternatives are possible should give us humans something to which we can aspire.” If animals can work things out equitably, then so should humans, and perhaps muriquis may be an even better role models than bonobos.