גר זאב עם־כבש ונמר עם־גדי ירבץ ועגל וכפיר ומריא יחדו ונער קטן נהג בםi
The wolf will live with the lamb, the leopard will lie down with the goat, the calf and the lion and the yearling together; and a little child will lead them.
Nature is supposed to be red in tooth and claw, and domestication an artificial process for making animals gentle. But it appears that some corners of the animal kingdom are becoming kinder, gentler places. Certain creatures may be domesticating themselves.
This possibility is most apparent in bonobos, a close cousin of chimpanzees. Unlike their violent cousins, bonobos are generally peaceful. And while many animals have evolved to be socially agreeable, bonobos — and possibly other species — seem to be experiencing something more precise and profound: the physical and behavioral changes specifically described in studies of domestication, but as a natural evolutionary process.
“Normally you think of domestication as something that happens at the hands of humans,” said Brian Hare, a Duke University evolutionary anthropologist . “The idea that a species domesticated itself is a bit crazy, but there are some species that outcompeted others by becoming nicer.”
The essence of domestication is a loss of aggression. Because this is such a basic trait, involving modifications to nervous and endocrine systems, and alterations of complex gene networks with multiple functions, it generates a variety of changes. Researchers call them a “domestication syndrome,” and while aspects are seen in all domesticated animals, the principles are distilled in a famous Russian experiment on foxes.
Starting in 1959 with 130 farm-bred but wild foxes and continuing until today, researchers allowed only those individuals most tolerant of human contact to breed. In less than 50 years, the fierce-tempered and untouchable foxes became playful, face-licking sweethearts who loved to be held. Those traits are typically seen in wild pups, but disappear as they grow up.
With juvenile behaviors came juvenile appearances: Even as adults, foxes in the experiment now have spotted coats, floppy ears, curly tails and short legs. They’re evolutionarily suspended in childhood — and that, said Hare, may explain bonobos. “I have a lot of bonobos who are ‘friends,’ and I look at them and say, ‘I don’t understand how you evolved. You are too goofy, too nice, too silly. How did you not get eaten?” he said. “But they are very successful.”
Bonobos are very different than chimpanzees, from whom they split taxonomically about one million years ago. Chimp males struggle constantly and violently for dominance; bonobo males almost never fight, and stage virility contests involving non-confrontational stick-dragging. Male chimps often coerce females into sex; bonobos ask for permission. At the group level, chimpanzees regularly engage in something like low-level warfare, with lethal consequences; bonobos don’t. Mostly they hang out, play, and exchange sexual favors with frequency so astounding they’ve become pop-culture tropes.
Lab tests back up in-the-wild observations. Relative to chimps, bonobos are stressed by competition, attentive to others’ needs, and eager to cooperate and share. Brain regions crucial to behavior and development, like the amygdala and occipital frontal cortex, are arranged differently. And in keeping with theories of domestication, bonobos play like juvenile chimpanzees, but throughout their lives. Their skulls also have smaller jawbones and teeth, or what anatomists call “paedomorphic” — child-shaped — features. They also have a white tail tuft and extra-pink lips, a possible analogue to the white spots often seen in, for example, cats and dogs.
According to Hare and study co-author Richard Wrangham, one of the world’s foremost primatologists, these are likely signs of domestication. But why and how could natural selection tame the bonobo? One possible narrative begins about 2.5 million years ago, when the last common ancestor of bonobos and chimpanzees lived both north and south of the Zaire River, as did gorillas, their ecological rivals. A massive drought drove gorillas from the south, and they never returned. That last common ancestor suddenly had the southern jungles to themselves.
As a result, competition for resources wouldn’t be as fierce as before. Aggression, such a costly habit, wouldn’t have been so necessary. And whereas a resource-limited environment likely made female alliances rare, as they are in modern chimpanzees, reduced competition would have allowed females to become friends. No longer would males intimidate them and force them into sex. Once reproduction was no longer traumatic, they could afford to be fertile more often, which in turn reduced competition between males.
“If females don’t let you beat them up, why should a male bonobo try to be dominant over all the other males?” said Hare. “In male chimps, it’s very costly to be on top. Often in primate hierarchies, you don’t stay on top very long. Everyone is gunning for you. You’re getting in a lot of fights. If you don’t have to do that, it’s better for everybody.” Chimpanzees had been caught in what Hare called “this terrible cycle, and bonobos have been able to break this cycle.” In doing so, they rose to primate supremacy in a region roughly the size of the United States east of the Mississippi River, and reigned unchallenged untilHomo sapiens came along.
All this, at least, is the hypothesis: It’s important to note that it’s a proposed rather than certain scenario. It’s at least conceivable, if highly unlikely, that bonobos started out peaceful and chimpanzees became more aggressive. Conclusive proof would require a time machine. Still, the evidence is suggestive and the scenario plausible.
Not that bonobos will soon be peeking out of cardboard boxes on Cute Overload. On the trajectory from wild to domestic, they’re something like certain wolves were tens of thousands of years ago, after reduced aggression allowed them to exploit a new ecological niche at the edges of growing human settlements, said Hare. At that time, people hadn’t yet started keeping and breeding dogs. Once they did, it accelerated a domestication already naturally underway.
But why stop with dogs and bonobos? Hare and Wrangham suspect self-domestication is happening elsewhere, and new niches around human habitation remain a likely place look. Large cities and suburbs are new to much of Earth’s surface, and represent opportunity to animals that can exploit them. A lack of aggression isn’t absolutely necessary — learning to hide quietly in brush by a sidewalk can represent wariness, not amiability — but it could help.
“I live in Chapel Hill, North Carolina, and we’re overrun by deer. I keep seeing deer in my neighborhood with the star mutation,” said Hare, referencing the Bambi-style spotting traditionally seen in young deer and domesticated animals. “I’d love to know whether, if you did a study where there’s no urbanization, would you see a lower rate of the star mutation?” One study of Florida Keys deer, an endangered subspecies of white-tailed deer, didn’t look at pigmentation, but did find that urbanized deer were less fearful and lived in larger groups than before.
(If it seems like such significant evolution shouldn’t happen so quickly, remember that many changes underlying the domestication syndrome don’t involve mutations to genes, but so-called epigenetic changes in the timing of gene activity. Epigenetics allows change to occur more rapidly than is possible through genetic mutation alone. “Evolution can happen very fast. It’s happening now,” Hare said.)
Another possible example is the Tonkean macaque, which is less aggressive than any other macaque species. They live in cohesive social groups and fight rarely. Unlike other macaques, their conflicts are followed by acts of reconciliation. They also have a habit of baring their teeth, but whereas in other macaques it signals submission, in Tonkeans it’s exchanged between equals. “This display is used like a smile,” said ethologist Bernard Thierry of the University of Strasbourg.
Thierry has tried to understand why Tonkean macaques are so exceptionally agreeable. It’s almost certainly the result of some evolutionary pressure, and that pressure could be self-domestication, though it hasn’t yet been demonstrated. “We still don’t know why some are nicer than others,” he said.
Self-domestication may also be favored on islands, where limited space and high population densities turn territorial defense into a constant fight. In such circumstances, aggression becomes self-destructive. “At very high densities, it becomes impossible to defend anything,” said Judy Stamps, the University of California, Davis evolutionary ecologist who studied how behaviors change on islands. “In that situation, the animals might as well just relax. Instead of competing aggressively, animals might begin to cooperate with each other.”
That seems to be happening in Panamanian island populations of Central American spiny rats, which are significantly less aggressive than their mainland brethren. Hare even wonders if historical tales of island animals treating the first human arrivals without fear reflect not inexperience, but possible self-domestication.